hibernate學習手冊

Hibernate.orgCommunity Documentation

HIBERNATE - 符合Java習慣的關係數據庫持久化

Hibernate參考文檔

由 Gavin King、Christian Bauer、MaxRydahl Andersen、EmmanuelBernard和Steve Ebersole

and thanks to James Cobb (Graphic Design)、Cheyenne Weaver (Graphic Design)和Cao Xiaogang

3.3.2.GA

版權 © 2004 Red Hat Middleware, LLC.

Legal Notice

June 24, 2009

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前言

Working with object-oriented software and a relational database can be cumbersome and time consuming in today's enterprise environments. Hibernate is an Object/Relational Mapping tool for Java environments. The term Object/Relational Mapping (ORM) refers to the technique of mapping a data representation from an object model to a relational data model with a SQL-based schema.

Hibernate not only takes care of the mapping from Java classes to database tables (and from Java data types to SQL data types), but also provides data query and retrieval facilities. It can also significantly reduce development time otherwise spent with manual data handling in SQL and JDBC.

Hibernate's goal is to relieve the developer from 95 percent of common data persistence related programming tasks. Hibernate may not be the best solution for data-centric applications that only use stored-procedures to implement the business logic in the database, it is most useful with object-oriented domain models and business logic in the Java-based middle-tier. However, Hibernate can certainly help you to remove or encapsulate vendor-specific SQL code and will help with the common task of result set translation from a tabular representation to a graph of objects.

如果你對Hibernate和對象/關係數據庫映射還是個新手，或者甚至對Java也不熟悉，請按照下面的步驟來學習。

1. 閱讀

1. 第 1 章Tutorial，這是一篇包含詳細的逐步指導的指南。本指南的源代碼包含在發行包中，你可以在doc/reference/tutorial/目錄下找到。

2. 閱讀第 2 章體系結構(Architecture)來理解Hibernate可以使用的環境。

3. View the eg/ directory in the Hibernate distribution. It contains a simple standalone application. Copy your JDBC driver to thelib/ directory and edit etc/hibernate.properties, specifying correct values for your database. From a command prompt in the distribution directory, typeant eg (using Ant), or under Windows, type build eg.

4. Use this reference documentation as your primary source of information. Consider reading [JPwH] if you need more help with application design, or if you prefer a step-by-step tutorial. Also visithttp://caveatemptor.hibernate.org and download the example application from [JPwH].

5. 在Hibernate 的網站上可以找到經常提問的問題與解答(FAQ)。

6. Links to third party demos, examples, and tutorials are maintained on the Hibernate website.

7. Hibernate網站的“社區(Community Area)”是討論關於設計模式以及很多整合方案(Tomcat, JBoss AS, Struts, EJB,等等)的好地方。

If you have questions, use the user forum linked on the Hibernate website. We also provide a JIRA issue tracking system for bug reports and feature requests. If you are interested in the development of Hibernate, join the developer mailing list. If you are interested in translating this documentation into your language, contact us on the developer mailing list.

商業開發、產品支持和Hibernate培訓可以通過JBoss Inc.獲得。（請查閱：http://www.hibernate.org/SupportTraining/）。 Hibernate是一個專業的開放源代碼項目(Professional Open Source project)，也是JBoss Enterprise Middleware System(JEMS),JBoss企業級中間件系統的一個核心組件。

1. Feedback

Use

Hibernate JIRA to report errors or request enhacements to this documentation.

第 1 章 Tutorial

Intended for new users, this chapter provides an step-by-step introduction to Hibernate, starting with a simple application using an in-memory database. The tutorial is based on an earlier tutorial developed by Michael Gloegl. All code is contained in thetutorials/web directory of the project source.

重要

This tutorial expects the user have knowledge of both Java and SQL. If you have a limited knowledge of JAVA or SQL, it is advised that you start with a good introduction to that technology prior to attempting to learn Hibernate.

注意

The distribution contains another example application under the tutorial/eg project source directory.

1.1. 第一部分 － 第一個Hibernate應用程序

For this example, we will set up a small database application that can store events we want to attend and information about the host(s) of these events.

注意

Although you can use whatever database you feel comfortable using, we will use

HSQLDB (an in-memory, Java database) to avoid describing installation/setup of any particular database servers.

1.1.1. Setup

The first thing we need to do is to set up the development environment. We will be using the "standard layout" advocated by alot of build tools such as

Maven. Maven, in particular, has a good resource describing thislayout. As this tutorial is to be a web application, we will be creating and making use ofsrc/main/java, src/main/resources andsrc/main/webapp directories.

We will be using Maven in this tutorial, taking advantage of its transitive dependency management capabilities as well as the ability of many IDEs to automatically set up a project for us based on the maven descriptor.

<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">

    <modelVersion>4.0.0</modelVersion>

    <groupId>org.hibernate.tutorials</groupId>
    <artifactId>hibernate-tutorial</artifactId>
    <version>1.0.0-SNAPSHOT</version>
    <name>First Hibernate Tutorial</name>

    <build>
         <!-- we dont want the version to be part of the generated war file name -->
         <finalName>${artifactId}</finalName>
    </build>

    <dependencies>
        <dependency>
            <groupId>org.hibernate</groupId>
            <artifactId>hibernate-core</artifactId>
        </dependency>

        <!-- Because this is a web app, we also have a dependency on the servlet api. -->
        <dependency>
            <groupId>javax.servlet</groupId>
            <artifactId>servlet-api</artifactId>
        </dependency>

        <!-- Hibernate uses slf4j for logging, for our purposes here use the simple backend -->
        <dependency>
            <groupId>org.slf4j</groupId>
            <artifactId>slf4j-simple</artifactId>
        </dependency>

        <!-- Hibernate gives you a choice of bytecode providers between cglib and javassist -->
        <dependency>
            <groupId>javassist</groupId>
            <artifactId>javassist</artifactId>
        </dependency>
    </dependencies>

</project>

提示

It is not a requirement to use Maven. If you wish to use something else to build this tutoial (such as Ant), the layout will remain the same. The only change is that you will need to manually account for all the needed dependencies. If you use something like Ivy providing transitive dependency management you would still use the dependencies mentioned below. Otherwise, you'd need to graball dependencies, both explicit and transitive, and add them to the project's classpath. If working from the Hibernate distribution bundle, this would meanhibernate3.jar, all artifacts in the lib/required directory and all files from either the lib/bytecode/cglib or lib/bytecode/javassist directory; additionally you will need both the servlet-api jar and one of the slf4j logging backends.

Save this file as pom.xml in the project root directory.

1.1.2. 第一個class

Next, we create a class that represents the event we want to store in the database; it is a simple JavaBean class with some properties:

package org.hibernate.tutorial.domain;

import java.util.Date;

public class Event {
    private Long id;

    private String title;
    private Date date;

    public Event() {}

    public Long getId() {
        return id;
    }

    private void setId(Long id) {
        this.id = id;
    }

    public Date getDate() {
        return date;
    }

    public void setDate(Date date) {
        this.date = date;
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }
}

This class uses standard JavaBean naming conventions for property getter and setter methods, as well as private visibility for the fields. Although this is the recommended design, it is not required. Hibernate can also access fields directly, the benefit of accessor methods is robustness for refactoring.

The id property holds a unique identifier value for a particular event. All persistent entity classes (there are less important dependent classes as well) will need such an identifier property if we want to use the full feature set of Hibernate. In fact, most applications, especially web applications, need to distinguish objects by identifier, so you should consider this a feature rather than a limitation. However, we usually do not manipulate the identity of an object, hence the setter method should be private. Only Hibernate will assign identifiers when an object is saved. Hibernate can access public, private, and protected accessor methods, as well as public, private and protected fields directly. The choice is up to you and you can match it to fit your application design.

The no-argument constructor is a requirement for all persistent classes; Hibernate has to create objects for you, using Java Reflection. The constructor can be private, however package or public visibility is required for runtime proxy generation and efficient data retrieval without bytecode instrumentation.

Save this file to the src/main/java/org/hibernate/tutorial/domain directory.

1.1.3. 映射文件

Hibernate需要知道怎樣去加載（load）和存儲（store）持久化類的對象。這正是Hibernate映射文件發揮作用的地方。映射文件告訴Hibernate它，應該訪問數據庫(database)裏面的哪個表（table）及應該使用表裏面的哪些字段（column）。

一個映射文件的基本結構看起來像這樣：

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping package="org.hibernate.tutorial.domain">
[...]
</hibernate-mapping>

Hibernate DTD is sophisticated. You can use it for auto-completion of XML mapping elements and attributes in your editor or IDE. Opening up the DTD file in your text editor is the easiest way to get an overview of all elements and attributes, and to view the defaults, as well as some comments. Hibernate will not load the DTD file from the web, but first look it up from the classpath of the application. The DTD file is included inhibernate-core.jar (it is also included in the hibernate3.jar, if using the distribution bundle).

重要

We will omit the DTD declaration in future examples to shorten the code. It is, of course, not optional.

Between the two hibernate-mapping tags, include aclass element. All persistent entity classes (again, there might be dependent classes later on, which are not first-class entities) need a mapping to a table in the SQL database:

<hibernate-mapping package="org.hibernate.tutorial.domain">

    <class name="Event" table="EVENTS">

    </class>

</hibernate-mapping>

So far we have told Hibernate how to persist and load object of class Event to the table EVENTS. Each instance is now represented by a row in that table. Now we can continue by mapping the unique identifier property to the tables primary key. As we do not want to care about handling this identifier, we configure Hibernate's identifier generation strategy for a surrogate primary key column:

<hibernate-mapping package="org.hibernate.tutorial.domain">

    <class name="Event" table="EVENTS">
        <id name="id" column="EVENT_ID">
            <generator class="native"/>
        </id>
    </class>

</hibernate-mapping>

The id element is the declaration of the identifier property. Thename="id" mapping attribute declares the name of the JavaBean property and tells Hibernate to use thegetId() and setId() methods to access the property. The column attribute tells Hibernate which column of theEVENTS table holds the primary key value.

The nested generator element specifies the identifier generation strategy (aka how are identifier values generated?). In this case we choosenative, which offers a level of portability depending on the configured database dialect. Hibernate supports database generated, globally unique, as well as application assigned, identifiers. Identifier value generation is also one of Hibernate's many extension points and you can plugin in your own strategy.

提示

native is no longer consider the best strategy in terms of portability. for further discussion, see

第 25.4 節 “Identifier generation”

Lastly, we need to tell Hibernate about the remaining entity class properties. By default, no properties of the class are considered persistent:

<hibernate-mapping package="org.hibernate.tutorial.domain">

    <class name="Event" table="EVENTS">
        <id name="id" column="EVENT_ID">
            <generator class="native"/>
        </id>
        <property name="date" type="timestamp" column="EVENT_DATE"/>
        <property name="title"/>
    </class>

</hibernate-mapping>

Similar to the id element, the name attribute of the property element tells Hibernate which getter and setter methods to use. In this case, Hibernate will search forgetDate(), setDate(), getTitle() and setTitle() methods.

注意

Why does the date property mapping include the column attribute, but the title does not? Without thecolumn attribute, Hibernate by default uses the property name as the column name. This works fortitle, however, date is a reserved keyword in most databases so you will need to map it to a different name.

The title mapping also lacks a type attribute. The types declared and used in the mapping files are not Java data types; they are not SQL database types either. These types are calledHibernate mapping types, converters which can translate from Java to SQL data types and vice versa. Again, Hibernate will try to determine the correct conversion and mapping type itself if thetype attribute is not present in the mapping. In some cases this automatic detection using Reflection on the Java class might not have the default you expect or need. This is the case with thedate property. Hibernate cannot know if the property, which is ofjava.util.Date, should map to a SQL date, timestamp, or time column. Full date and time information is preserved by mapping the property with atimestamp converter.

提示

Hibernate makes this mapping type determination using reflection when the mapping files are processed. This can take time and resources, so if startup performance is important you should consider explicitly defining the type to use.

Save this mapping file as src/main/resources/org/hibernate/tutorial/domain/Event.hbm.xml.

1.1.4. Hibernate配置

At this point, you should have the persistent class and its mapping file in place. It is now time to configure Hibernate. First let's set up HSQLDB to run in "server mode"

注意

We do this do that the data remains between runs.

We will utilize the Maven exec plugin to launch the HSQLDB server by running: mvn exec:java -Dexec.mainClass="org.hsqldb.Server" -Dexec.args="-database.0 file:target/data/tutorial" You will see it start up and bind to a TCP/IP socket; this is where our application will connect later. If you want to start with a fresh database during this tutorial, shutdown HSQLDB, delete all files in the target/data directory, and start HSQLDB again.

Hibernate will be connecting to the database on behalf of your application, so it needs to know how to obtain connections. For this tutorial we will be using a standalone connection pool (as opposed to ajavax.sql.DataSource). Hibernate comes with support for two third-party open source JDBC connection pools:

c3p0 and proxool. However, we will be using the Hibernate built-in connection pool for this tutorial.

小心

The built-in Hibernate connection pool is in no way intended for production use. It lacks several features found on any decent connection pool.

For Hibernate's configuration, we can use a simple hibernate.properties file, a more sophisticatedhibernate.cfg.xml file, or even complete programmatic setup. Most users prefer the XML configuration file:

<?xml version='1.0' encoding='utf-8'?>
<!DOCTYPE hibernate-configuration PUBLIC
        "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
        "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">

<hibernate-configuration>

    <session-factory>

        <!-- Database connection settings -->
        <property name="connection.driver_class">org.hsqldb.jdbcDriver</property>
        <property name="connection.url">jdbc:hsqldb:hsql://localhost</property>
        <property name="connection.username">sa</property>
        <property name="connection.password"></property>

        <!-- JDBC connection pool (use the built-in) -->
        <property name="connection.pool_size">1</property>

        <!-- SQL dialect -->
        <property name="dialect">org.hibernate.dialect.HSQLDialect</property>

        <!-- Enable Hibernate's automatic session context management -->
        <property name="current_session_context_class">thread</property>

        <!-- Disable the second-level cache  -->
        <property name="cache.provider_class">org.hibernate.cache.NoCacheProvider</property>

        <!-- Echo all executed SQL to stdout -->
        <property name="show_sql">true</property>

        <!-- Drop and re-create the database schema on startup -->
        <property name="hbm2ddl.auto">update</property>

        <mapping resource="org/hibernate/tutorial/domain/Event.hbm.xml"/>

    </session-factory>

</hibernate-configuration>

注意

Notice that this configuration file specifies a different DTD

You configure Hibernate's SessionFactory. SessionFactory is a global factory responsible for a particular database. If you have several databases, for easier startup you should use several<session-factory> configurations in several configuration files.

The first four property elements contain the necessary configuration for the JDBC connection. The dialectproperty element specifies the particular SQL variant Hibernate generates.

提示

In most cases, Hibernate is able to properly determine which dialect to use. See第 25.3 節 “Dialect resolution” for more information.

Hibernate's automatic session management for persistence contexts is particularly useful in this context. Thehbm2ddl.auto option turns on automatic generation of database schemas directly into the database. This can also be turned off by removing the configuration option, or redirected to a file with the help of theSchemaExport Ant task. Finally, add the mapping file(s) for persistent classes to the configuration.

Save this file as hibernate.cfg.xml into the src/main/resources directory.

1.1.5. Building with Maven

We will now build the tutorial with Maven. You will need to have Maven installed; it is available from the

Maven download page. Maven will read the/pom.xml file we created earlier and know how to perform some basic project tasks. First, lets run thecompile goal to make sure we can compile everything so far:

[hibernateTutorial]$ mvn compile
[INFO] Scanning for projects...
[INFO] ------------------------------------------------------------------------
[INFO] Building First Hibernate Tutorial
[INFO]    task-segment: [compile]
[INFO] ------------------------------------------------------------------------
[INFO] [resources:resources]
[INFO] Using default encoding to copy filtered resources.
[INFO] [compiler:compile]
[INFO] Compiling 1 source file to /home/steve/projects/sandbox/hibernateTutorial/target/classes
[INFO] ------------------------------------------------------------------------
[INFO] BUILD SUCCESSFUL
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 2 seconds
[INFO] Finished at: Tue Jun 09 12:25:25 CDT 2009
[INFO] Final Memory: 5M/547M
[INFO] ------------------------------------------------------------------------

1.1.6. 啓動和輔助類

It is time to load and store some Event objects, but first you have to complete the setup with some infrastructure code. You have to startup Hibernate by building a globalorg.hibernate.SessionFactory object and storing it somewhere for easy access in application code. Aorg.hibernate.SessionFactory is used to obtainorg.hibernate.Session instances. A org.hibernate.Session represents a single-threaded unit of work. The org.hibernate.SessionFactory is a thread-safe global object that is instantiated once.

We will create a HibernateUtil helper class that takes care of startup and makes accessing theorg.hibernate.SessionFactory more convenient.

package org.hibernate.tutorial.util;

import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;

public class HibernateUtil {

    private static final SessionFactory sessionFactory = buildSessionFactory();

    private static SessionFactory buildSessionFactory() {
        try {
            // Create the SessionFactory from hibernate.cfg.xml
            return new Configuration().configure().buildSessionFactory();
        }
        catch (Throwable ex) {
            // Make sure you log the exception, as it might be swallowed
            System.err.println("Initial SessionFactory creation failed." + ex);
            throw new ExceptionInInitializerError(ex);
        }
    }

    public static SessionFactory getSessionFactory() {
        return sessionFactory;
    }

}

Save this code as src/main/java/org/hibernate/tutorial/util/HibernateUtil.java

This class not only produces the global org.hibernate.SessionFactory reference in its static initializer; it also hides the fact that it uses a static singleton. We might just as well have looked up theorg.hibernate.SessionFactory reference from JNDI in an application server or any other location for that matter.

If you give the org.hibernate.SessionFactory a name in your configuration, Hibernate will try to bind it to JNDI under that name after it has been built. Another, better option is to use a JMX deployment and let the JMX-capable container instantiate and bind a HibernateService to JNDI. Such advanced options are discussed later.

You now need to configure a logging system. Hibernate uses commons logging and provides two choices: Log4j and JDK 1.4 logging. Most developers prefer Log4j: copylog4j.properties from the Hibernate distribution in theetc/ directory to your src directory, next tohibernate.cfg.xml. If you prefer to have more verbose output than that provided in the example configuration, you can change the settings. By default, only the Hibernate startup message is shown on stdout.

The tutorial infrastructure is complete and you are now ready to do some real work with Hibernate.

1.1.7. 加載並存儲對象

We are now ready to start doing some real worjk with Hibernate. Let's start by writing anEventManager class with a main() method:

package org.hibernate.tutorial;

import org.hibernate.Session;

import java.util.*;

import org.hibernate.tutorial.domain.Event;
import org.hibernate.tutorial.util.HibernateUtil;

public class EventManager {

    public static void main(String[] args) {
        EventManager mgr = new EventManager();

        if (args[0].equals("store")) {
            mgr.createAndStoreEvent("My Event", new Date());
        }

        HibernateUtil.getSessionFactory().close();
    }

    private void createAndStoreEvent(String title, Date theDate) {
        Session session = HibernateUtil.getSessionFactory().getCurrentSession();
        session.beginTransaction();

        Event theEvent = new Event();
        theEvent.setTitle(title);
        theEvent.setDate(theDate);
        session.save(theEvent);

        session.getTransaction().commit();
    }

}

In createAndStoreEvent() we created a new Event object and handed it over to Hibernate. At that point, Hibernate takes care of the SQL and executes anINSERT on the database.

A org.hibernate.Session is designed to represent a single unit of work (a single atmoic piece of work to be performed). For now we will keep things simple and assume a one-to-one granularity between a Hibernateorg.hibernate.Session and a database transaction. To shield our code from the actual underlying transaction system we use the Hibernateorg.hibernate.Transaction API. In this particular case we are using JDBC-based transactional semantics, but it could also run with JTA.

What does sessionFactory.getCurrentSession() do? First, you can call it as many times and anywhere you like once you get hold of yourorg.hibernate.SessionFactory. The getCurrentSession() method always returns the "current" unit of work. Remember that we switched the configuration option for this mechanism to "thread" in oursrc/main/resources/hibernate.cfg.xml? Due to that setting, the context of a current unit of work is bound to the current Java thread that executes the application.

重要

Hibernate offers three methods of current session tracking. The "thread" based method is not intended for production use; it is merely useful for prototyping and tutorials such as this one. Current session tracking is discussed in more detail later on.

A org.hibernate.Session begins when the first call togetCurrentSession() is made for the current thread. It is then bound by Hibernate to the current thread. When the transaction ends, either through commit or rollback, Hibernate automatically unbinds theorg.hibernate.Session from the thread and closes it for you. If you callgetCurrentSession() again, you get a new org.hibernate.Session and can start a new unit of work.

Related to the unit of work scope, should the Hibernate org.hibernate.Session be used to execute one or several database operations? The above example uses oneorg.hibernate.Session for one operation. However this is pure coincidence; the example is just not complex enough to show any other approach. The scope of a Hibernateorg.hibernate.Session is flexible but you should never design your application to use a new Hibernateorg.hibernate.Session for every database operation. Even though it is used in the following examples, considersession-per-operation an anti-pattern. A real web application is shown later in the tutorial which will help illustrate this.

See

第 11 章Transactions and Concurrency for more information about transaction handling and demarcation. The previous example also skipped any error handling and rollback.

To run this, we will make use of the Maven exec plugin to call our class with the necessary classpath setup:mvn exec:java -Dexec.mainClass="org.hibernate.tutorial.EventManager" -Dexec.args="store"

注意

You may need to perform mvn compile first.

You should see Hibernate starting up and, depending on your configuration, lots of log output. Towards the end, the following line will be displayed:

[java] Hibernate: insert into EVENTS (EVENT_DATE, title, EVENT_ID) values (?, ?, ?)

This is the INSERT executed by Hibernate.

To list stored events an option is added to the main method:

        if (args[0].equals("store")) {
            mgr.createAndStoreEvent("My Event", new Date());
        }
        else if (args[0].equals("list")) {
            List events = mgr.listEvents();
            for (int i = 0; i < events.size(); i++) {
                Event theEvent = (Event) events.get(i);
                System.out.println(
                        "Event: " + theEvent.getTitle() + " Time: " + theEvent.getDate()
                );
            }
        }

A new listEvents() method is also added:

    private List listEvents() {
        Session session = HibernateUtil.getSessionFactory().getCurrentSession();
        session.beginTransaction();
        List result = session.createQuery("from Event").list();
        session.getTransaction().commit();
        return result;
    }

Here, we are using a Hibernate Query Language (HQL) query to load all existingEvent objects from the database. Hibernate will generate the appropriate SQL, send it to the database and populateEvent objects with the data. You can create more complex queries with HQL. See第 14 章 HQL: Hibernate查詢語言 for more information.

Now we can call our new functionality, again using the Maven exec plugin: mvn exec:java -Dexec.mainClass="org.hibernate.tutorial.EventManager" -Dexec.args="list"

1.2. 第二部分 － 關聯映射

So far we have mapped a single persistent entity class to a table in isolation. Let's expand on that a bit and add some class associations. We will add people to the application and store a list of events in which they participate.

1.2.1. 映射Person類

The first cut of the Person class looks like this:

package org.hibernate.tutorial.domain;

public class Person {

    private Long id;
    private int age;
    private String firstname;
    private String lastname;

    public Person() {}

    // Accessor methods for all properties, private setter for 'id'

}

Save this to a file named src/main/java/org/hibernate/tutorial/domain/Person.java

Next, create the new mapping file as src/main/resources/org/hibernate/tutorial/domain/Person.hbm.xml

<hibernate-mapping package="org.hibernate.tutorial.domain">

    <class name="Person" table="PERSON">
        <id name="id" column="PERSON_ID">
            <generator class="native"/>
        </id>
        <property name="age"/>
        <property name="firstname"/>
        <property name="lastname"/>
    </class>

</hibernate-mapping>

最後，把新的映射加入到Hibernate的配置中：

<mapping resource="events/Event.hbm.xml"/>
<mapping resource="events/Person.hbm.xml"/>

Create an association between these two entities. Persons can participate in events, and events have participants. The design questions you have to deal with are: directionality, multiplicity, and collection behavior.

1.2.2. 單向Set-based的關聯

By adding a collection of events to the Person class, you can easily navigate to the events for a particular person, without executing an explicit query - by callingPerson#getEvents. Multi-valued associations are represented in Hibernate by one of the Java Collection Framework contracts; here we choose ajava.util.Set because the collection will not contain duplicate elements and the ordering is not relevant to our examples:

public class Person {

    private Set events = new HashSet();

    public Set getEvents() {
        return events;
    }

    public void setEvents(Set events) {
        this.events = events;
    }
}

Before mapping this association, let's consider the other side. We could just keep this unidirectional or create another collection on theEvent, if we wanted to be able to navigate it from both directions. This is not necessary, from a functional perspective. You can always execute an explicit query to retrieve the participants for a particular event. This is a design choice left to you, but what is clear from this discussion is the multiplicity of the association: "many" valued on both sides is called amany-to-many association. Hence, we use Hibernate's many-to-many mapping:

<class name="Person" table="PERSON">
    <id name="id" column="PERSON_ID">
        <generator class="native"/>
    </id>
    <property name="age"/>
    <property name="firstname"/>
    <property name="lastname"/>

    <set name="events" table="PERSON_EVENT">
        <key column="PERSON_ID"/>
        <many-to-many column="EVENT_ID" class="Event"/>
    </set>

</class>

Hibernate supports a broad range of collection mappings, a set being most common. For a many-to-many association, or n:m entity relationship, an association table is required. Each row in this table represents a link between a person and an event. The table name is decalred using thetable attribute of the set element. The identifier column name in the association, for the person side, is defined with thekey element, the column name for the event's side with thecolumn attribute of the many-to-many. You also have to tell Hibernate the class of the objects in your collection (the class on the other side of the collection of references).

因而這個映射的數據庫schema是：

    _____________        __________________
   |             |      |                  |       _____________
   |   EVENTS    |      |   PERSON_EVENT   |      |             |
   |_____________|      |__________________|      |    PERSON   |
   |             |      |                  |      |_____________|
   | *EVENT_ID   | <--> | *EVENT_ID        |      |             |
   |  EVENT_DATE |      | *PERSON_ID       | <--> | *PERSON_ID  |
   |  TITLE      |      |__________________|      |  AGE        |
   |_____________|                                |  FIRSTNAME  |
                                                  |  LASTNAME   |
                                                  |_____________|
 

1.2.3. 使關聯工作

Now we will bring some people and events together in a new method in EventManager:

    private void addPersonToEvent(Long personId, Long eventId) {
        Session session = HibernateUtil.getSessionFactory().getCurrentSession();
        session.beginTransaction();

        Person aPerson = (Person) session.load(Person.class, personId);
        Event anEvent = (Event) session.load(Event.class, eventId);
        aPerson.getEvents().add(anEvent);

        session.getTransaction().commit();
    }

After loading a Person and an Event, simply modify the collection using the normal collection methods. There is no explicit call toupdate() or save(); Hibernate automatically detects that the collection has been modified and needs to be updated. This is calledautomatic dirty checking. You can also try it by modifying the name or the date property of any of your objects. As long as they are inpersistent state, that is, bound to a particular Hibernateorg.hibernate.Session, Hibernate monitors any changes and executes SQL in a write-behind fashion. The process of synchronizing the memory state with the database, usually only at the end of a unit of work, is calledflushing. In our code, the unit of work ends with a commit, or rollback, of the database transaction.

You can load person and event in different units of work. Or you can modify an object outside of aorg.hibernate.Session, when it is not in persistent state (if it was persistent before, this state is calleddetached). You can even modify a collection when it is detached:

    private void addPersonToEvent(Long personId, Long eventId) {
        Session session = HibernateUtil.getSessionFactory().getCurrentSession();
        session.beginTransaction();

        Person aPerson = (Person) session
                .createQuery("select p from Person p left join fetch p.events where p.id = :pid")
                .setParameter("pid", personId)
                .uniqueResult(); // Eager fetch the collection so we can use it detached
        Event anEvent = (Event) session.load(Event.class, eventId);

        session.getTransaction().commit();

        // End of first unit of work

        aPerson.getEvents().add(anEvent); // aPerson (and its collection) is detached

        // Begin second unit of work

        Session session2 = HibernateUtil.getSessionFactory().getCurrentSession();
        session2.beginTransaction();
        session2.update(aPerson); // Reattachment of aPerson

        session2.getTransaction().commit();
    }

The call to update makes a detached object persistent again by binding it to a new unit of work, so any modifications you made to it while detached can be saved to the database. This includes any modifications (additions/deletions) you made to a collection of that entity object.

This is not much use in our example, but it is an important concept you can incorporate into your own application. Complete this exercise by adding a new action to the main method of theEventManager and call it from the command line. If you need the identifiers of a person and an event - thesave() method returns it (you might have to modify some of the previous methods to return that identifier):

        else if (args[0].equals("addpersontoevent")) {
            Long eventId = mgr.createAndStoreEvent("My Event", new Date());
            Long personId = mgr.createAndStorePerson("Foo", "Bar");
            mgr.addPersonToEvent(personId, eventId);
            System.out.println("Added person " + personId + " to event " + eventId);
        }

This is an example of an association between two equally important classes : two entities. As mentioned earlier, there are other classes and types in a typical model, usually "less important". Some you have already seen, like anint or a java.lang.String. We call these classesvalue types, and their instances depend on a particular entity. Instances of these types do not have their own identity, nor are they shared between entities. Two persons do not reference the samefirstname object, even if they have the same first name. Value types cannot only be found in the JDK , but you can also write dependent classes yourself such as anAddress or MonetaryAmount class. In fact, in a Hibernate application all JDK classes are considered value types.

You can also design a collection of value types. This is conceptually different from a collection of references to other entities, but looks almost the same in Java.

1.2.4. 值類型的集合

Let's add a collection of email addresses to the Person entity. This will be represented as ajava.util.Set of java.lang.String instances:

    private Set emailAddresses = new HashSet();

    public Set getEmailAddresses() {
        return emailAddresses;
    }

    public void setEmailAddresses(Set emailAddresses) {
        this.emailAddresses = emailAddresses;
    }

The mapping of this Set is as follows:

        <set name="emailAddresses" table="PERSON_EMAIL_ADDR">
            <key column="PERSON_ID"/>
            <element type="string" column="EMAIL_ADDR"/>
        </set>

The difference compared with the earlier mapping is the use of the element part which tells Hibernate that the collection does not contain references to another entity, but is rather a collection whose elements are values types, here specifically of typestring. The lowercase name tells you it is a Hibernate mapping type/converter. Again thetable attribute of the set element determines the table name for the collection. Thekey element defines the foreign-key column name in the collection table. Thecolumn attribute in the element element defines the column name where the email address values will actually be stored.

Here is the updated schema:

  _____________        __________________
 |             |      |                  |       _____________
 |   EVENTS    |      |   PERSON_EVENT   |      |             |       ___________________
 |_____________|      |__________________|      |    PERSON   |      |                   |
 |             |      |                  |      |_____________|      | PERSON_EMAIL_ADDR |
 | *EVENT_ID   | <--> | *EVENT_ID        |      |             |      |___________________|
 |  EVENT_DATE |      | *PERSON_ID       | <--> | *PERSON_ID  | <--> |  *PERSON_ID       |
 |  TITLE      |      |__________________|      |  AGE        |      |  *EMAIL_ADDR      |
 |_____________|                                |  FIRSTNAME  |      |___________________|
                                                |  LASTNAME   |
                                                |_____________|
 

You can see that the primary key of the collection table is in fact a composite key that uses both columns. This also implies that there cannot be duplicate email addresses per person, which is exactly the semantics we need for a set in Java.

You can now try to add elements to this collection, just like we did before by linking persons and events. It is the same code in Java:

    private void addEmailToPerson(Long personId, String emailAddress) {
        Session session = HibernateUtil.getSessionFactory().getCurrentSession();
        session.beginTransaction();

        Person aPerson = (Person) session.load(Person.class, personId);
        // adding to the emailAddress collection might trigger a lazy load of the collection
        aPerson.getEmailAddresses().add(emailAddress);

        session.getTransaction().commit();
    }

This time we did not use a fetch query to initialize the collection. Monitor the SQL log and try to optimize this with an eager fetch.

1.2.5. 雙向關聯

Next you will map a bi-directional association. You will make the association between person and event work from both sides in Java. The database schema does not change, so you will still have many-to-many multiplicity.

注意

A relational database is more flexible than a network programming language, in that it does not need a navigation direction; data can be viewed and retrieved in any possible way.

First, add a collection of participants to the Event class:

    private Set participants = new HashSet();

    public Set getParticipants() {
        return participants;
    }

    public void setParticipants(Set participants) {
        this.participants = participants;
    }

Now map this side of the association in Event.hbm.xml.

        <set name="participants" table="PERSON_EVENT" inverse="true">
            <key column="EVENT_ID"/>
            <many-to-many column="PERSON_ID" class="events.Person"/>
        </set>

These are normal set mappings in both mapping documents. Notice that the column names inkey and many-to-many swap in both mapping documents. The most important addition here is theinverse="true" attribute in the set element of the Event's collection mapping.

What this means is that Hibernate should take the other side, the Person class, when it needs to find out information about the link between the two. This will be a lot easier to understand once you see how the bi-directional link between our two entities is created.

1.2.6. 使雙向連起來

First, keep in mind that Hibernate does not affect normal Java semantics. How did we create a link between aPerson and an Event in the unidirectional example? You add an instance ofEvent to the collection of event references, of an instance ofPerson. If you want to make this link bi-directional, you have to do the same on the other side by adding aPerson reference to the collection in an Event. This process of "setting the link on both sides" is absolutely necessary with bi-directional links.

Many developers program defensively and create link management methods to correctly set both sides (for example, inPerson):

    protected Set getEvents() {
        return events;
    }

    protected void setEvents(Set events) {
        this.events = events;
    }

    public void addToEvent(Event event) {
        this.getEvents().add(event);
        event.getParticipants().add(this);
    }

    public void removeFromEvent(Event event) {
        this.getEvents().remove(event);
        event.getParticipants().remove(this);
    }

The get and set methods for the collection are now protected. This allows classes in the same package and subclasses to still access the methods, but prevents everybody else from altering the collections directly. Repeat the steps for the collection on the other side.

What about the inverse mapping attribute? For you, and for Java, a bi-directional link is simply a matter of setting the references on both sides correctly. Hibernate, however, does not have enough information to correctly arrange SQL INSERT and UPDATE statements (to avoid constraint violations). Making one side of the associationinverse tells Hibernate to consider it a mirror of the other side. That is all that is necessary for Hibernate to resolve any issues that arise when transforming a directional navigation model to a SQL database schema. The rules are straightforward: all bi-directional associations need one side as inverse. In a one-to-many association it has to be the many-side, and in many-to-many association you can select either side.

1.3. 第三部分 - EventManager web應用程序

A Hibernate web application uses Session and Transaction almost like a standalone application. However, some common patterns are useful. You can now write anEventManagerServlet. This servlet can list all events stored in the database, and it provides an HTML form to enter new events.

1.3.1. 編寫基本的servlet

First we need create our basic processing servlet. Since our servlet only handles HTTPGET requests, we will only implement the doGet() method:

package org.hibernate.tutorial.web;

// Imports

public class EventManagerServlet extends HttpServlet {

    protected void doGet(
            HttpServletRequest request,
            HttpServletResponse response) throws ServletException, IOException {

        SimpleDateFormat dateFormatter = new SimpleDateFormat( "dd.MM.yyyy" );

        try {
            // Begin unit of work
            HibernateUtil.getSessionFactory().getCurrentSession().beginTransaction();

            // Process request and render page...

            // End unit of work
            HibernateUtil.getSessionFactory().getCurrentSession().getTransaction().commit();
        }
        catch (Exception ex) {
            HibernateUtil.getSessionFactory().getCurrentSession().getTransaction().rollback();
            if ( ServletException.class.isInstance( ex ) ) {
                throw ( ServletException ) ex;
            }
            else {
                throw new ServletException( ex );
            }
        }
    }

}

Save this servlet as src/main/java/org/hibernate/tutorial/web/EventManagerServlet.java

The pattern applied here is called session-per-request. When a request hits the servlet, a new HibernateSession is opened through the first call to getCurrentSession() on the SessionFactory. A database transaction is then started. All data access occurs inside a transaction irrespective of whether the data is read or written. Do not use the auto-commit mode in applications.

我們稱這裏應用的模式爲每次請求一個session(session-per-request)。當有請求到達這個servlet的時候，通過對SessionFactory的第一次調用，打開一個新的HibernateSession。然後啓動一個數據庫事務-所有的數據訪問都是在事務中進行，不管是讀還是寫（我們在應用程序中不使用auto-commit模式）。

Next, the possible actions of the request are processed and the response HTML is rendered. We will get to that part soon.

Finally, the unit of work ends when processing and rendering are complete. If any problems occurred during processing or rendering, an exception will be thrown and the database transaction rolled back. This completes thesession-per-request pattern. Instead of the transaction demarcation code in every servlet, you could also write a servlet filter. See the Hibernate website and Wiki for more information about this pattern calledOpen Session in View. You will need it as soon as you consider rendering your view in JSP, not in a servlet.

1.3.2. 最後，當處理與渲染都結束的時候，這個工作單元就結束了。假若在處理或渲染的時候有任何錯誤發生，會拋出一個異常，回滾數據庫事務。這樣，session-per-request模式就完成了。爲了避免在每個servlet中都編寫事務邊界界定的代碼，可以考慮寫一個servlet 過濾器（filter）來更好地解決。關於這一模式的更多信息，請參閱Hibernate網站和Wiki，這一模式叫做Open Session in View-只要你考慮用JSP來渲染你的視圖（view），而不是在servlet中，你就會很快用到它。

Now you can implement the processing of the request and the rendering of the page.

        // Write HTML header
        PrintWriter out = response.getWriter();
        out.println("<html><head><title>Event Manager</title></head><body>");

        // Handle actions
        if ( "store".equals(request.getParameter("action")) ) {

            String eventTitle = request.getParameter("eventTitle");
            String eventDate = request.getParameter("eventDate");

            if ( "".equals(eventTitle) || "".equals(eventDate) ) {
                out.println("<b><i>Please enter event title and date.</i></b>");
            }
            else {
                createAndStoreEvent(eventTitle, dateFormatter.parse(eventDate));
                out.println("<b><i>Added event.</i></b>");
            }
        }

        // Print page
       printEventForm(out);
       listEvents(out, dateFormatter);

       // Write HTML footer
       out.println("</body></html>");
       out.flush();
       out.close();

This coding style, with a mix of Java and HTML, would not scale in a more complex application-keep in mind that we are only illustrating basic Hibernate concepts in this tutorial. The code prints an HTML header and a footer. Inside this page, an HTML form for event entry and a list of all events in the database are printed. The first method is trivial and only outputs HTML:

    private void printEventForm(PrintWriter out) {
        out.println("<h2>Add new event:</h2>");
        out.println("<form>");
        out.println("Title: <input name='eventTitle' length='50'/><br/>");
        out.println("Date (e.g. 24.12.2009): <input name='eventDate' length='10'/><br/>");
        out.println("<input type='submit' name='action' value='store'/>");
        out.println("</form>");
    }

listEvents()方法使用綁定到當前線程的Hibernate Session來執行查詢：

    private void listEvents(PrintWriter out, SimpleDateFormat dateFormatter) {

        List result = HibernateUtil.getSessionFactory()
                .getCurrentSession().createCriteria(Event.class).list();
        if (result.size() > 0) {
            out.println("<h2>Events in database:</h2>");
            out.println("<table border='1'>");
            out.println("<tr>");
            out.println("<th>Event title</th>");
            out.println("<th>Event date</th>");
            out.println("</tr>");
            Iterator it = result.iterator();
            while (it.hasNext()) {
                Event event = (Event) it.next();
                out.println("<tr>");
                out.println("<td>" + event.getTitle() + "</td>");
                out.println("<td>" + dateFormatter.format(event.getDate()) + "</td>");
                out.println("</tr>");
            }
            out.println("</table>");
        }
    }

最後，store動作會被導向到createAndStoreEvent()方法，它也使用當前線程的Session:

    protected void createAndStoreEvent(String title, Date theDate) {
        Event theEvent = new Event();
        theEvent.setTitle(title);
        theEvent.setDate(theDate);

        HibernateUtil.getSessionFactory()
                .getCurrentSession().save(theEvent);
    }

The servlet is now complete. A request to the servlet will be processed in a singleSession and Transaction. As earlier in the standalone application, Hibernate can automatically bind these objects to the current thread of execution. This gives you the freedom to layer your code and access the SessionFactory in any way you like. Usually you would use a more sophisticated design and move the data access code into data access objects (the DAO pattern). See the Hibernate Wiki for more examples.

1.3.3. 部署與測試

To deploy this application for testing we must create a Web ARchive (WAR). First we must define the WAR descriptor assrc/main/webapp/WEB-INF/web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.4"
    xmlns="http://java.sun.com/xml/ns/j2ee"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">

    <servlet>
        <servlet-name>Event Manager</servlet-name>
        <servlet-class>org.hibernate.tutorial.web.EventManagerServlet</servlet-class>
    </servlet>

    <servlet-mapping>
        <servlet-name>Event Manager</servlet-name>
        <url-pattern>/eventmanager</url-pattern>
    </servlet-mapping>
</web-app>

To build and deploy call mvn package in your project directory and copy thehibernate-tutorial.war file into your Tomcat webapps directory.

注意

If you do not have Tomcat installed, download it from

http://tomcat.apache.org/ and follow the installation instructions. Our application requires no changes to the standard Tomcat configuration.

在部署完，啓動Tomcat之後，通過http://localhost:8080/hibernate-tutorial/eventmanager進行訪問你的應用，在第一次servlet 請求發生時，請在Tomcat log中確認你看到Hibernate被初始化了（HibernateUtil的靜態初始化器被調用），假若有任何異常拋出，也可以看到詳細的輸出。

1.4. 總結

This tutorial covered the basics of writing a simple standalone Hibernate application and a small web application. More tutorials are available from the Hibernate

website.

第 2 章 體系結構(Architecture)

2.1. 概況(Overview)

The diagram below provides a high-level view of the Hibernate architecture:

We do not have the scope in this document to provide a more detailed view of all the runtime architectures available; Hibernate is flexible and supports several different approaches. We will, however, show the two extremes: "minimal" architecture and "comprehensive" architecture.

This next diagram illustrates how Hibernate utilizes database and configuration data to provide persistence services, and persistent objects, to the application.

The "minimal" architecture has the application provide its own JDBC connections and manage its own transactions. This approach uses a minimal subset of Hibernate's APIs:

The "comprehensive" architecture abstracts the application away from the underlying JDBC/JTA APIs and allows Hibernate to manage the details.

Here are some definitions of the objects depicted in the diagrams:

SessionFactory (org.hibernate.SessionFactory)

A threadsafe, immutable cache of compiled mappings for a single database. A factory forSession and a client of ConnectionProvider,SessionFactory can hold an optional (second-level) cache of data that is reusable between transactions at a process, or cluster, level.

Session (org.hibernate.Session)

A single-threaded, short-lived object representing a conversation between the application and the persistent store. It wraps a JDBC connection and is a factory forTransaction. Session holds a mandatory first-level cache of persistent objects that are used when navigating the object graph or looking up objects by identifier.

持久的對象及其集合

Short-lived, single threaded objects containing persistent state and business function. These can be ordinary JavaBeans/POJOs. They are associated with exactly oneSession. Once the Session is closed, they will be detached and free to use in any application layer (for example, directly as data transfer objects to and from presentation).

瞬態(transient)和脫管(detached)的對象及其集合

Instances of persistent classes that are not currently associated with a Session. They may have been instantiated by the application and not yet persisted, or they may have been instantiated by a closedSession.

事務Transaction (org.hibernate.Transaction)

(Optional) A single-threaded, short-lived object used by the application to specify atomic units of work. It abstracts the application from the underlying JDBC, JTA or CORBA transaction. ASession might span several Transactions in some cases. However, transaction demarcation, either using the underlying API orTransaction, is never optional.

ConnectionProvider (org.hibernate.connection.ConnectionProvider)

(Optional) A factory for, and pool of, JDBC connections. It abstracts the application from underlyingDatasource or DriverManager. It is not exposed to application, but it can be extended and/or implemented by the developer.

TransactionFactory (org.hibernate.TransactionFactory)

(Optional) A factory for Transaction instances. It is not exposed to the application, but it can be extended and/or implemented by the developer.

擴展接口

Hibernate offers a range of optional extension interfaces you can implement to customize the behavior of your persistence layer. See the API documentation for details.

Given a "minimal" architecture, the application bypasses the Transaction/TransactionFactory and/or ConnectionProvider APIs to communicate with JTA or JDBC directly.

2.2. 實例狀態

An instance of a persistent class can be in one of three different states. These states are defined in relation to apersistence context. The Hibernate Session object is the persistence context. The three different states are as follows:

瞬態（transient）

The instance is not associated with any persistence context. It has no persistent identity or primary key value.

持久化(persistent)

The instance is currently associated with a persistence context. It has a persistent identity (primary key value) and can have a corresponding row in the database. For a particular persistence context, Hibernateguarantees that persistent identity is equivalent to Java identity in relation to the in-memory location of the object.

脫管(detached)

The instance was once associated with a persistence context, but that context was closed, or the instance was serialized to another process. It has a persistent identity and can have a corresponding row in the database. For detached instances, Hibernate does not guarantee the relationship between persistent identity and Java identity.

2.3. JMX整合

JMX is the J2EE standard for the management of Java components. Hibernate can be managed via a JMX standard service. AN MBean implementation is provided in the distribution:org.hibernate.jmx.HibernateService.

For an example of how to deploy Hibernate as a JMX service on the JBoss Application Server, please see the JBoss User Guide. JBoss AS also provides these benefits if you deploy using JMX:

• Session Management: the Hibernate Session's life cycle can be automatically bound to the scope of a JTA transaction. This means that you no longer have to manually open and close theSession; this becomes the job of a JBoss EJB interceptor. You also do not have to worry about transaction demarcation in your code (if you would like to write a portable persistence layer use the optional HibernateTransaction API for this). You call the HibernateContext to access a Session.

• HAR deployment: the Hibernate JMX service is deployed using a JBoss service deployment descriptor in an EAR and/or SAR file, as it supports all the usual configuration options of a HibernateSessionFactory. However, you still need to name all your mapping files in the deployment descriptor. If you use the optional HAR deployment, JBoss will automatically detect all mapping files in your HAR file.

這些選項更多的描述，請參考JBoss 應用程序用戶指南。

Another feature available as a JMX service is runtime Hibernate statistics. See

第 3.4.6 節 “Hibernate的統計(statistics)機制” for more information.

2.4. 對JCA的支持

Hibernate can also be configured as a JCA connector. Please see the website for more information. Please note, however, that at this stage Hibernate JCA support is under development.

2.5. Contextual sessions

Most applications using Hibernate need some form of "contextual" session, where a given session is in effect throughout the scope of a given context. However, across applications the definition of what constitutes a context is typically different; different contexts define different scopes to the notion of current. Applications using Hibernate prior to version 3.0 tended to utilize either home-grownThreadLocal-based contextual sessions, helper classes such asHibernateUtil, or utilized third-party frameworks, such as Spring or Pico, which provided proxy/interception-based contextual sessions.

Starting with version 3.0.1, Hibernate added the SessionFactory.getCurrentSession() method. Initially, this assumed usage ofJTA transactions, where the JTA transaction defined both the scope and context of a current session. Given the maturity of the numerous stand-aloneJTA TransactionManager implementations, most, if not all, applications should be usingJTA transaction management, whether or not they are deployed into aJ2EE container. Based on that, the JTA-based contextual sessions are all you need to use.

However, as of version 3.1, the processing behind SessionFactory.getCurrentSession() is now pluggable. To that end, a new extension interface,org.hibernate.context.CurrentSessionContext, and a new configuration parameter,hibernate.current_session_context_class, have been added to allow pluggability of the scope and context of defining current sessions.

See the Javadocs for the org.hibernate.context.CurrentSessionContext interface for a detailed discussion of its contract. It defines a single method,currentSession(), by which the implementation is responsible for tracking the current contextual session. Out-of-the-box, Hibernate comes with three implementations of this interface:

• org.hibernate.context.JTASessionContext: current sessions are tracked and scoped by aJTA transaction. The processing here is exactly the same as in the older JTA-only approach. See the Javadocs for details.

• org.hibernate.context.ThreadLocalSessionContext:current sessions are tracked by thread of execution. See the Javadocs for details.

• org.hibernate.context.ManagedSessionContext: current sessions are tracked by thread of execution. However, you are responsible to bind and unbind aSession instance with static methods on this class: it does not open, flush, or close aSession.

The first two implementations provide a "one session - one database transaction" programming model. This is also also known and used assession-per-request. The beginning and end of a Hibernate session is defined by the duration of a database transaction. If you use programmatic transaction demarcation in plain JSE without JTA, you are advised to use the Hibernate Transaction API to hide the underlying transaction system from your code. If you use JTA, you can utilize the JTA interfaces to demarcate transactions. If you execute in an EJB container that supports CMT, transaction boundaries are defined declaratively and you do not need any transaction or session demarcation operations in your code. Refer to

第 11 章Transactions and Concurrency for more information and code examples.

The hibernate.current_session_context_class configuration parameter defines whichorg.hibernate.context.CurrentSessionContext implementation should be used. For backwards compatibility, if this configuration parameter is not set but aorg.hibernate.transaction.TransactionManagerLookup is configured, Hibernate will use theorg.hibernate.context.JTASessionContext. Typically, the value of this parameter would just name the implementation class to use. For the three out-of-the-box implementations, however, there are three corresponding short names: "jta", "thread", and "managed".

第 3 章 配置

Hibernate is designed to operate in many different environments and, as such, there is a broad range of configuration parameters. Fortunately, most have sensible default values and Hibernate is distributed with an examplehibernate.properties file in etc/ that displays the various options. Simply put the example file in your classpath and customize it to suit your needs.

3.1. 可編程的配置方式

An instance of org.hibernate.cfg.Configuration represents an entire set of mappings of an application's Java types to an SQL database. Theorg.hibernate.cfg.Configuration is used to build an immutableorg.hibernate.SessionFactory. The mappings are compiled from various XML mapping files.

You can obtain a org.hibernate.cfg.Configuration instance by instantiating it directly and specifying XML mapping documents. If the mapping files are in the classpath, useaddResource(). For example:

Configuration cfg = new Configuration()
    .addResource("Item.hbm.xml")
    .addResource("Bid.hbm.xml");

An alternative way is to specify the mapped class and allow Hibernate to find the mapping document for you:

Configuration cfg = new Configuration()
    .addClass(org.hibernate.auction.Item.class)
    .addClass(org.hibernate.auction.Bid.class);

Hibernate will then search for mapping files named /org/hibernate/auction/Item.hbm.xml and/org/hibernate/auction/Bid.hbm.xml in the classpath. This approach eliminates any hardcoded filenames.

A org.hibernate.cfg.Configuration also allows you to specify configuration properties. For example:

Configuration cfg = new Configuration()
    .addClass(org.hibernate.auction.Item.class)
    .addClass(org.hibernate.auction.Bid.class)
    .setProperty("hibernate.dialect", "org.hibernate.dialect.MySQLInnoDBDialect")
    .setProperty("hibernate.connection.datasource", "java:comp/env/jdbc/test")
    .setProperty("hibernate.order_updates", "true");

This is not the only way to pass configuration properties to Hibernate. Some alternative options include:

1. Pass an instance of java.util.Properties to Configuration.setProperties().

2. Place a file named hibernate.properties in a root directory of the classpath.

3. 通過java -Dproperty=value來設置系統 (System)屬性.

4. Include <property> elements in hibernate.cfg.xml (this is discussed later).

If you want to get started quicklyhibernate.properties is the easiest approach.

The org.hibernate.cfg.Configuration is intended as a startup-time object that will be discarded once aSessionFactory is created.

3.2. 獲得SessionFactory

When all mappings have been parsed by the org.hibernate.cfg.Configuration, the application must obtain a factory fororg.hibernate.Session instances. This factory is intended to be shared by all application threads:

SessionFactory sessions = cfg.buildSessionFactory();

Hibernate does allow your application to instantiate more than one org.hibernate.SessionFactory. This is useful if you are using more than one database.

3.3. JDBC連接

It is advisable to have the org.hibernate.SessionFactory create and pool JDBC connections for you. If you take this approach, opening aorg.hibernate.Session is as simple as:

Session session = sessions.openSession(); // open a new Session

Once you start a task that requires access to the database, a JDBC connection will be obtained from the pool.

Before you can do this, you first need to pass some JDBC connection properties to Hibernate. All Hibernate property names and semantics are defined on the classorg.hibernate.cfg.Environment. The most important settings for JDBC connection configuration are outlined below.

Hibernate will obtain and pool connections using java.sql.DriverManager if you set the following properties:

表 3.1. Hibernate JDBC屬性

屬性名	用途
hibernate.connection.driver_class	jdbc驅動類
hibernate.connection.url	jdbc URL
hibernate.connection.username	數據庫用戶
hibernate.connection.password	數據庫用戶密碼
hibernate.connection.pool_size	連接池容量上限數目

Hibernate's own connection pooling algorithm is, however, quite rudimentary. It is intended to help you get started and isnot intended for use in a production system, or even for performance testing. You should use a third party pool for best performance and stability. Just replace thehibernate.connection.pool_size property with connection pool specific settings. This will turn off Hibernate's internal pool. For example, you might like to use c3p0.

C3P0 is an open source JDBC connection pool distributed along with Hibernate in thelib directory. Hibernate will use its org.hibernate.connection.C3P0ConnectionProvider for connection pooling if you sethibernate.c3p0.* properties. If you would like to use Proxool, refer to the packagedhibernate.properties and the Hibernate web site for more information.

The following is an example hibernate.properties file for c3p0:

hibernate.connection.driver_class = org.postgresql.Driver
hibernate.connection.url = jdbc:postgresql://localhost/mydatabase
hibernate.connection.username = myuser
hibernate.connection.password = secret
hibernate.c3p0.min_size=5
hibernate.c3p0.max_size=20
hibernate.c3p0.timeout=1800
hibernate.c3p0.max_statements=50
hibernate.dialect = org.hibernate.dialect.PostgreSQLDialect

For use inside an application server, you should almost always configure Hibernate to obtain connections from an application serverjavax.sql.Datasource registered in JNDI. You will need to set at least one of the following properties:

表 3.2. Hibernate數據源屬性

屬性名	用途
hibernate.connection.datasource	數據源JNDI名字
hibernate.jndi.url	URL of the JNDI provider (optional)
hibernate.jndi.class	class of the JNDI InitialContextFactory (optional)
hibernate.connection.username	database user (optional)
hibernate.connection.password	database user password (optional)

Here is an example hibernate.properties file for an application server provided JNDI datasource:

hibernate.connection.datasource = java:/comp/env/jdbc/test
hibernate.transaction.factory_class = \
    org.hibernate.transaction.JTATransactionFactory
hibernate.transaction.manager_lookup_class = \
    org.hibernate.transaction.JBossTransactionManagerLookup
hibernate.dialect = org.hibernate.dialect.PostgreSQLDialect

從JNDI數據源獲得的JDBC連接將自動參與到應用程序服務器中容器管理的事務(container-managed transactions)中去.

Arbitrary connection properties can be given by prepending "hibernate.connection" to the connection property name. For example, you can specify acharSet connection property using hibernate.connection.charSet.

You can define your own plugin strategy for obtaining JDBC connections by implementing the interfaceorg.hibernate.connection.ConnectionProvider, and specifying your custom implementation via thehibernate.connection.provider_class property.

3.4. 可選的配置屬性

There are a number of other properties that control the behavior of Hibernate at runtime. All are optional and have reasonable default values.

警告

Some of these properties are "system-level" only. System-level properties can be set only viajava -Dproperty=value or hibernate.properties. Theycannot be set by the other techniques described above.

表 3.3. Hibernate配置屬性

屬性名	用途
hibernate.dialect	The classname of a Hibernate org.hibernate.dialect.Dialect which allows Hibernate to generate SQL optimized for a particular relational database. e.g. full.classname.of.Dialect In most cases Hibernate will actually be able to choose the correct org.hibernate.dialect.Dialect implementation based on the JDBC metadata returned by the JDBC driver.
hibernate.show_sql	Write all SQL statements to console. This is an alternative to setting the log categoryorg.hibernate.SQL to debug. e.g. true |false
hibernate.format_sql	Pretty print the SQL in the log and console. e.g. true |false
hibernate.default_schema	Qualify unqualified table names with the given schema/tablespace in generated SQL. e.g. SCHEMA_NAME
hibernate.default_catalog	Qualifies unqualified table names with the given catalog in generated SQL. e.g. CATALOG_NAME
hibernate.session_factory_name	The org.hibernate.SessionFactory will be automatically bound to this name in JNDI after it has been created. e.g. jndi/composite/name
hibernate.max_fetch_depth	Sets a maximum "depth" for the outer join fetch tree for single-ended associations (one-to-one, many-to-one). A0 disables default outer join fetching. e.g. recommended values between 0 and 3
hibernate.default_batch_fetch_size	Sets a default size for Hibernate batch fetching of associations. e.g. recommended values 4, 8, 16
hibernate.default_entity_mode	Sets a default mode for entity representation for all sessions opened from thisSessionFactory 取值dynamic-map,dom4j, pojo
hibernate.order_updates	Forces Hibernate to order SQL updates by the primary key value of the items being updated. This will result in fewer transaction deadlocks in highly concurrent systems. e.g. true |false
hibernate.generate_statistics	If enabled, Hibernate will collect statistics useful for performance tuning. e.g. true |false
hibernate.use_identifer_rollback	If enabled, generated identifier properties will be reset to default values when objects are deleted. e.g. true |false
hibernate.use_sql_comments	If turned on, Hibernate will generate comments inside the SQL, for easier debugging, defaults tofalse. e.g. true |false

表 3.4. Hibernate JDBC和連接(connection)屬性

屬性名	用途
hibernate.jdbc.fetch_size	A non-zero value determines the JDBC fetch size (calls Statement.setFetchSize()).
hibernate.jdbc.batch_size	A non-zero value enables use of JDBC2 batch updates by Hibernate. e.g. recommended values between 5 and 30
hibernate.jdbc.batch_versioned_data	Set this property to true if your JDBC driver returns correct row counts fromexecuteBatch(). Iit is usually safe to turn this option on. Hibernate will then use batched DML for automatically versioned data. Defaults tofalse. e.g. true |false
hibernate.jdbc.factory_class	Select a custom org.hibernate.jdbc.Batcher. Most applications will not need this configuration property. e.g. classname.of.BatcherFactory
hibernate.jdbc.use_scrollable_resultset	Enables use of JDBC2 scrollable resultsets by Hibernate. This property is only necessary when using user-supplied JDBC connections. Hibernate uses connection metadata otherwise. e.g. true |false
hibernate.jdbc.use_streams_for_binary	Use streams when writing/reading binary or serializable types to/from JDBC. *system-level property* e.g. true |false
hibernate.jdbc.use_get_generated_keys	Enables use of JDBC3 PreparedStatement.getGeneratedKeys() to retrieve natively generated keys after insert. Requires JDBC3+ driver and JRE1.4+, set to false if your driver has problems with the Hibernate identifier generators. By default, it tries to determine the driver capabilities using connection metadata. e.g. true|false
hibernate.connection.provider_class	The classname of a custom org.hibernate.connection.ConnectionProvider which provides JDBC connections to Hibernate. e.g. classname.of.ConnectionProvider
hibernate.connection.isolation	Sets the JDBC transaction isolation level. Check java.sql.Connection for meaningful values, but note that most databases do not support all isolation levels and some define additional, non-standard isolations. e.g. 1, 2, 4, 8
hibernate.connection.autocommit	Enables autocommit for JDBC pooled connections (it is not recommended). e.g. true |false
hibernate.connection.release_mode	Specifies when Hibernate should release JDBC connections. By default, a JDBC connection is held until the session is explicitly closed or disconnected. For an application server JTA datasource, useafter_statement to aggressively release connections after every JDBC call. For a non-JTA connection, it often makes sense to release the connection at the end of each transaction, by usingafter_transaction. auto will chooseafter_statement for the JTA and CMT transaction strategies andafter_transaction for the JDBC transaction strategy. e.g. auto (default) |on_close | after_transaction |after_statement This setting only affects Sessions returned from SessionFactory.openSession. For Sessions obtained throughSessionFactory.getCurrentSession, the CurrentSessionContext implementation configured for use controls the connection release mode for thoseSessions. See 第 2.5 節 “Contextual sessions”
hibernate.connection.<propertyName>	Pass the JDBC property <propertyName> toDriverManager.getConnection().
hibernate.jndi.<propertyName>	Pass the property <propertyName> to the JNDIInitialContextFactory.

表 3.5. Hibernate緩存屬性

屬性名	用途
hibernate.cache.provider_class	The classname of a custom CacheProvider. e.g. classname.of.CacheProvider
hibernate.cache.use_minimal_puts	Optimizes second-level cache operation to minimize writes, at the cost of more frequent reads. This setting is most useful for clustered caches and, in Hibernate3, is enabled by default for clustered cache implementations. e.g. true|false
hibernate.cache.use_query_cache	Enables the query cache. Individual queries still have to be set cachable. e.g. true|false
hibernate.cache.use_second_level_cache	Can be used to completely disable the second level cache, which is enabled by default for classes which specify a<cache> mapping. e.g. true|false
hibernate.cache.query_cache_factory	The classname of a custom QueryCache interface, defaults to the built-inStandardQueryCache. e.g. classname.of.QueryCache
hibernate.cache.region_prefix	A prefix to use for second-level cache region names. e.g. prefix
hibernate.cache.use_structured_entries	Forces Hibernate to store data in the second-level cache in a more human-friendly format. e.g. true|false

表 3.6. Hibernate事務屬性

屬性名	用途
hibernate.transaction.factory_class	The classname of a TransactionFactory to use with HibernateTransaction API (defaults to JDBCTransactionFactory). e.g. classname.of.TransactionFactory
jta.UserTransaction	A JNDI name used by JTATransactionFactory to obtain the JTAUserTransaction from the application server. e.g. jndi/composite/name
hibernate.transaction.manager_lookup_class	The classname of a TransactionManagerLookup. It is required when JVM-level caching is enabled or when using hilo generator in a JTA environment. e.g. classname.of.TransactionManagerLookup
hibernate.transaction.flush_before_completion	If enabled, the session will be automatically flushed during the before completion phase of the transaction. Built-in and automatic session context management is preferred, see第 2.5 節 “Contextual sessions”. e.g. true |false
hibernate.transaction.auto_close_session	If enabled, the session will be automatically closed during the after completion phase of the transaction. Built-in and automatic session context management is preferred, see第 2.5 節 “Contextual sessions”. e.g. true |false

表 3.7. 其他屬性

屬性名	用途
hibernate.current_session_context_class	Supply a custom strategy for the scoping of the "current" Session. See 第 2.5 節 “Contextual sessions” for more information about the built-in strategies. e.g. jta |thread | managed | custom.Class
hibernate.query.factory_class	Chooses the HQL parser implementation. e.g. org.hibernate.hql.ast.ASTQueryTranslatorFactory ororg.hibernate.hql.classic.ClassicQueryTranslatorFactory
hibernate.query.substitutions	Is used to map from tokens in Hibernate queries to SQL tokens (tokens might be function or literal names, for example). e.g. hqlLiteral=SQL_LITERAL, hqlFunction=SQLFUNC
hibernate.hbm2ddl.auto	Automatically validates or exports schema DDL to the database when the SessionFactory is created. With create-drop, the database schema will be dropped when theSessionFactory is closed explicitly. e.g. validate |update | create | create-drop
hibernate.cglib.use_reflection_optimizer	Enables the use of CGLIB instead of runtime reflection (System-level property). Reflection can sometimes be useful when troubleshooting. Hibernate always requires CGLIB even if you turn off the optimizer. You cannot set this property inhibernate.cfg.xml. e.g. true |false

3.4.1. SQL方言

Always set the hibernate.dialect property to the correctorg.hibernate.dialect.Dialect subclass for your database. If you specify a dialect, Hibernate will use sensible defaults for some of the other properties listed above. This means that you will not have to specify them manually.

表 3.8. Hibernate SQL方言 (hibernate.dialect)

RDBMS	Dialect
DB2	org.hibernate.dialect.DB2Dialect
DB2 AS/400	org.hibernate.dialect.DB2400Dialect
DB2 OS390	org.hibernate.dialect.DB2390Dialect
PostgreSQL	org.hibernate.dialect.PostgreSQLDialect
MySQL	org.hibernate.dialect.MySQLDialect
MySQL with InnoDB	org.hibernate.dialect.MySQLInnoDBDialect
MySQL with MyISAM	org.hibernate.dialect.MySQLMyISAMDialect
Oracle (any version)	org.hibernate.dialect.OracleDialect
Oracle 9i	org.hibernate.dialect.Oracle9iDialect
Oracle 10g	org.hibernate.dialect.Oracle10gDialect
Sybase	org.hibernate.dialect.SybaseDialect
Sybase Anywhere	org.hibernate.dialect.SybaseAnywhereDialect
Microsoft SQL Server	org.hibernate.dialect.SQLServerDialect
SAP DB	org.hibernate.dialect.SAPDBDialect
Informix	org.hibernate.dialect.InformixDialect
HypersonicSQL	org.hibernate.dialect.HSQLDialect
Ingres	org.hibernate.dialect.IngresDialect
Progress	org.hibernate.dialect.ProgressDialect
Mckoi SQL	org.hibernate.dialect.MckoiDialect
Interbase	org.hibernate.dialect.InterbaseDialect
Pointbase	org.hibernate.dialect.PointbaseDialect
FrontBase	org.hibernate.dialect.FrontbaseDialect
Firebird	org.hibernate.dialect.FirebirdDialect

3.4.2. 外連接抓取(Outer Join Fetching)

If your database supports ANSI, Oracle or Sybase style outer joins, outer join fetching will often increase performance by limiting the number of round trips to and from the database. This is, however, at the cost of possibly more work performed by the database itself. Outer join fetching allows a whole graph of objects connected by many-to-one, one-to-many, many-to-many and one-to-one associations to be retrieved in a single SQLSELECT.

Outer join fetching can be disabled globally by setting the propertyhibernate.max_fetch_depth to 0. A setting of1 or higher enables outer join fetching for one-to-one and many-to-one associations that have been mapped withfetch="join".

參見

第 19.1 節 “抓取策略(Fetching strategies)”獲得更多信息.

3.4.3. 二進制流 (Binary Streams)

Oracle limits the size of byte arrays that can be passed to and/or from its JDBC driver. If you wish to use large instances ofbinary or serializable type, you should enablehibernate.jdbc.use_streams_for_binary. This is a system-level setting only.

3.4.4. 二級緩存與查詢緩存

The properties prefixed by hibernate.cache allow you to use a process or cluster scoped second-level cache system with Hibernate. See the

第 19.2 節 “二級緩存（The Second Level Cache）” for more information.

3.4.5. 查詢語言中的替換

You can define new Hibernate query tokens using hibernate.query.substitutions. For example:

hibernate.query.substitutions true=1, false=0

This would cause the tokens true and false to be translated to integer literals in the generated SQL.

hibernate.query.substitutions toLowercase=LOWER

This would allow you to rename the SQL LOWER function.

3.4.6. Hibernate的統計(statistics)機制

If you enable hibernate.generate_statistics, Hibernate exposes a number of metrics that are useful when tuning a running system viaSessionFactory.getStatistics(). Hibernate can even be configured to expose these statistics via JMX. Read the Javadoc of the interfaces inorg.hibernate.stats for more information.

3.5. 日誌

Hibernate utilizes

Simple Logging Facade for Java (SLF4J) in order to log various system events. SLF4J can direct your logging output to several logging frameworks (NOP, Simple, log4j version 1.2, JDK 1.4 logging, JCL or logback) depending on your chosen binding. In order to setup logging you will need slf4j-api.jar in your classpath together with the jar file for your preferred binding -slf4j-log4j12.jar in the case of Log4J. See the SLF4Jdocumentation for more detail. To use Log4j you will also need to place alog4j.properties file in your classpath. An example properties file is distributed with Hibernate in thesrc/ directory.

It is recommended that you familiarize yourself with Hibernate's log messages. A lot of work has been put into making the Hibernate log as detailed as possible, without making it unreadable. It is an essential troubleshooting device. The most interesting log categories are the following:

表 3.9. Hibernate日誌類別

類別	功能
org.hibernate.SQL	在所有SQL DML語句被執行時爲它們記錄日誌
org.hibernate.type	爲所有JDBC參數記錄日誌
org.hibernate.tool.hbm2ddl	在所有SQL DDL語句執行時爲它們記錄日誌
org.hibernate.pretty	在session清洗(flush)時，爲所有與其關聯的實體(最多20個)的狀態記錄日誌
org.hibernate.cache	爲所有二級緩存的活動記錄日誌
org.hibernate.transaction	爲事務相關的活動記錄日誌
org.hibernate.jdbc	爲所有JDBC資源的獲取記錄日誌
org.hibernate.hql.AST	在解析查詢的時候,記錄HQL和SQL的AST分析日誌
org.hibernate.secure	爲JAAS認證請求做日誌
org.hibernate	Log everything. This is a lot of information but it is useful for troubleshooting

在使用Hibernate開發應用程序時, 你應當總是爲org.hibernate.SQL 開啓debug級別的日誌記錄,或者開啓hibernate.show_sql屬性。

3.6. 實現NamingStrategy

org.hibernate.cfg.NamingStrategy接口允許你爲數據庫中的對象和schema 元素指定一個“命名標準”.

You can provide rules for automatically generating database identifiers from Java identifiers or for processing "logical" column and table names given in the mapping file into "physical" table and column names. This feature helps reduce the verbosity of the mapping document, eliminating repetitive noise (TBL_ prefixes, for example). The default strategy used by Hibernate is quite minimal.

You can specify a different strategy by calling Configuration.setNamingStrategy() before adding mappings:

SessionFactory sf = new Configuration()
    .setNamingStrategy(ImprovedNamingStrategy.INSTANCE)
    .addFile("Item.hbm.xml")
    .addFile("Bid.hbm.xml")
    .buildSessionFactory();

org.hibernate.cfg.ImprovedNamingStrategy是一個內建的命名策略, 對 一些應用程序而言，可能是非常有用的起點.

3.7. XML配置文件

另一個配置方法是在hibernate.cfg.xml文件中指定一套完整的配置. 這個文件可以當成hibernate.properties的替代。 若兩個文件同時存在，它將覆蓋前者的屬性.

The XML configuration file is by default expected to be in the root of your CLASSPATH. Here is an example:

<?xml version='1.0' encoding='utf-8'?>
<!DOCTYPE hibernate-configuration PUBLIC
    "-//Hibernate/Hibernate Configuration DTD//EN"
    "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">

<hibernate-configuration>

    <!-- a SessionFactory instance listed as /jndi/name -->
    <session-factory
        name="java:hibernate/SessionFactory">

        <!-- properties -->
        <property name="connection.datasource">java:/comp/env/jdbc/MyDB</property>
        <property name="dialect">org.hibernate.dialect.MySQLDialect</property>
        <property name="show_sql">false</property>
        <property name="transaction.factory_class">
            org.hibernate.transaction.JTATransactionFactory
        </property>
        <property name="jta.UserTransaction">java:comp/UserTransaction</property>

        <!-- mapping files -->
        <mapping resource="org/hibernate/auction/Item.hbm.xml"/>
        <mapping resource="org/hibernate/auction/Bid.hbm.xml"/>

        <!-- cache settings -->
        <class-cache class="org.hibernate.auction.Item" usage="read-write"/>
        <class-cache class="org.hibernate.auction.Bid" usage="read-only"/>
        <collection-cache collection="org.hibernate.auction.Item.bids" usage="read-write"/>

    </session-factory>

</hibernate-configuration>

The advantage of this approach is the externalization of the mapping file names to configuration. Thehibernate.cfg.xml is also more convenient once you have to tune the Hibernate cache. It is your choice to use eitherhibernate.properties or hibernate.cfg.xml. Both are equivalent, except for the above mentioned benefits of using the XML syntax.

With the XML configuration, starting Hibernate is then as simple as:

SessionFactory sf = new Configuration().configure().buildSessionFactory();

You can select a different XML configuration file using:

SessionFactory sf = new Configuration()
    .configure("catdb.cfg.xml")
    .buildSessionFactory();

3.8. J2EE應用程序服務器的集成

針對J2EE體系,Hibernate有如下幾個集成的方面:

• Container-managed datasources: Hibernate can use JDBC connections managed by the container and provided through JNDI. Usually, a JTA compatibleTransactionManager and a ResourceManager take care of transaction management (CMT), especially distributed transaction handling across several datasources. You can also demarcate transaction boundaries programmatically (BMT), or you might want to use the optional Hibernate Transaction API for this to keep your code portable.

• 自動JNDI綁定: Hibernate可以在啓動後將 SessionFactory綁定到JNDI.

• JTA Session binding: the Hibernate Session can be automatically bound to the scope of JTA transactions. Simply lookup theSessionFactory from JNDI and get the current Session. Let Hibernate manage flushing and closing the Session when your JTA transaction completes. Transaction demarcation is either declarative (CMT) or programmatic (BMT/UserTransaction).

• JMX deployment: if you have a JMX capable application server (e.g. JBoss AS), you can choose to deploy Hibernate as a managed MBean. This saves you the one line startup code to build yourSessionFactory from a Configuration. The container will startup yourHibernateService and also take care of service dependencies (datasource has to be available before Hibernate starts, etc).

如果應用程序服務器拋出"connection containment"異常, 根據你的環境，也許該將配置屬性 hibernate.connection.release_mode設爲after_statement.

3.8.1. 事務策略配置

The Hibernate Session API is independent of any transaction demarcation system in your architecture. If you let Hibernate use JDBC directly through a connection pool, you can begin and end your transactions by calling the JDBC API. If you run in a J2EE application server, you might want to use bean-managed transactions and call the JTA API andUserTransaction when needed.

爲了讓你的代碼在兩種(或其他)環境中可以移植，我們建議使用可選的Hibernate Transaction API, 它包裝並隱藏了底層系統. 你必須通過設置Hibernate配置屬性hibernate.transaction.factory_class來指定 一個Transaction實例的工廠類.

There are three standard, or built-in, choices:

org.hibernate.transaction.JDBCTransactionFactory

委託給數據庫(JDBC)事務（默認）

org.hibernate.transaction.JTATransactionFactory

delegates to container-managed transactions if an existing transaction is underway in this context (for example, EJB session bean method). Otherwise, a new transaction is started and bean-managed transactions are used.

org.hibernate.transaction.CMTTransactionFactory

委託給容器管理的JTA事務

You can also define your own transaction strategies (for a CORBA transaction service, for example).

Some features in Hibernate (i.e., the second level cache, Contextual Sessions with JTA, etc.) require access to the JTATransactionManager in a managed environment. In an application server, since J2EE does not standardize a single mechanism, you have to specify how Hibernate should obtain a reference to theTransactionManager:

表 3.10. JTA TransactionManagers

Transaction工廠類	應用程序服務器
org.hibernate.transaction.JBossTransactionManagerLookup	JBoss
org.hibernate.transaction.WeblogicTransactionManagerLookup	Weblogic
org.hibernate.transaction.WebSphereTransactionManagerLookup	WebSphere
org.hibernate.transaction.WebSphereExtendedJTATransactionLookup	WebSphere 6
org.hibernate.transaction.OrionTransactionManagerLookup	Orion
org.hibernate.transaction.ResinTransactionManagerLookup	Resin
org.hibernate.transaction.JOTMTransactionManagerLookup	JOTM
org.hibernate.transaction.JOnASTransactionManagerLookup	JOnAS
org.hibernate.transaction.JRun4TransactionManagerLookup	JRun4
org.hibernate.transaction.BESTransactionManagerLookup	Borland ES

3.8.2. JNDI綁定的SessionFactory

A JNDI-bound Hibernate SessionFactory can simplify the lookup function of the factory and create newSessions. This is not, however, related to a JNDI boundDatasource; both simply use the same registry.

If you wish to have the SessionFactory bound to a JNDI namespace, specify a name (e.g.java:hibernate/SessionFactory) using the property hibernate.session_factory_name. If this property is omitted, the SessionFactory will not be bound to JNDI. This is especially useful in environments with a read-only JNDI default implementation (in Tomcat, for example).

在將SessionFactory綁定至JNDI時, Hibernate將使用hibernate.jndi.url, 和hibernate.jndi.class的值來實例化初始環境(initial context). 如果它們沒有被指定, 將使用默認的InitialContext.

Hibernate will automatically place the SessionFactory in JNDI after you callcfg.buildSessionFactory(). This means you will have this call in some startup code, or utility class in your application, unless you use JMX deployment with theHibernateService (this is discussed later in greater detail).

If you use a JNDI SessionFactory, an EJB or any other class, you can obtain theSessionFactory using a JNDI lookup.

It is recommended that you bind the SessionFactory to JNDI in a managed environment and use astatic singleton otherwise. To shield your application code from these details, we also recommend to hide the actual lookup code for aSessionFactory in a helper class, such as HibernateUtil.getSessionFactory(). Note that such a class is also a convenient way to startup Hibernatesee chapter 1.

3.8.3. 在JTA環境下使用Current Session context (當前session上下文)管理

The easiest way to handle Sessions and transactions is Hibernate's automatic "current"Session management. For a discussion of contextual sessions see

第 2.5 節 “Contextual sessions”. Using the"jta" session context, if there is no Hibernate Session associated with the current JTA transaction, one will be started and associated with that JTA transaction the first time you callsessionFactory.getCurrentSession(). The Sessions retrieved via getCurrentSession() in the"jta" context are set to automatically flush before the transaction completes, close after the transaction completes, and aggressively release JDBC connections after each statement. This allows the Sessions to be managed by the life cycle of the JTA transaction to which it is associated, keeping user code clean of such management concerns. Your code can either use JTA programmatically through UserTransaction, or (recommended for portable code) use the HibernateTransaction API to set transaction boundaries. If you run in an EJB container, declarative transaction demarcation with CMT is preferred.

3.8.4. JMX部署

The line cfg.buildSessionFactory() still has to be executed somewhere to get aSessionFactory into JNDI. You can do this either in astatic initializer block, like the one in HibernateUtil, or you can deploy Hibernate as a managed service.

Hibernate is distributed with org.hibernate.jmx.HibernateService for deployment on an application server with JMX capabilities, such as JBoss AS. The actual deployment and configuration is vendor-specific. Here is an examplejboss-service.xml for JBoss 4.0.x:

<?xml version="1.0"?>
<server>

<mbean code="org.hibernate.jmx.HibernateService"
    name="jboss.jca:service=HibernateFactory,name=HibernateFactory">

    <!-- Required services -->
    <depends>jboss.jca:service=RARDeployer</depends>
    <depends>jboss.jca:service=LocalTxCM,name=HsqlDS</depends>

    <!-- Bind the Hibernate service to JNDI -->
    <attribute name="JndiName">java:/hibernate/SessionFactory</attribute>

    <!-- Datasource settings -->
    <attribute name="Datasource">java:HsqlDS</attribute>
    <attribute name="Dialect">org.hibernate.dialect.HSQLDialect</attribute>

    <!-- Transaction integration -->
    <attribute name="TransactionStrategy">
        org.hibernate.transaction.JTATransactionFactory</attribute>
    <attribute name="TransactionManagerLookupStrategy">
        org.hibernate.transaction.JBossTransactionManagerLookup</attribute>
    <attribute name="FlushBeforeCompletionEnabled">true</attribute>
    <attribute name="AutoCloseSessionEnabled">true</attribute>

    <!-- Fetching options -->
    <attribute name="MaximumFetchDepth">5</attribute>

    <!-- Second-level caching -->
    <attribute name="SecondLevelCacheEnabled">true</attribute>
    <attribute name="CacheProviderClass">org.hibernate.cache.EhCacheProvider</attribute>
    <attribute name="QueryCacheEnabled">true</attribute>

    <!-- Logging -->
    <attribute name="ShowSqlEnabled">true</attribute>

    <!-- Mapping files -->
    <attribute name="MapResources">auction/Item.hbm.xml,auction/Category.hbm.xml</attribute>

</mbean>

</server>

This file is deployed in a directory called META-INF and packaged in a JAR file with the extension.sar (service archive). You also need to package Hibernate, its required third-party libraries, your compiled persistent classes, as well as your mapping files in the same archive. Your enterprise beans (usually session beans) can be kept in their own JAR file, but you can include this EJB JAR file in the main service archive to get a single (hot-)deployable unit. Consult the JBoss AS documentation for more information about JMX service and EJB deployment.

第 4 章 持久化類(Persistent Classes)

Persistent classes are classes in an application that implement the entities of the business problem (e.g. Customer and Order in an E-commerce application). Not all instances of a persistent class are considered to be in the persistent state. For example, an instance can instead be transient or detached.

Hibernate works best if these classes follow some simple rules, also known as the Plain Old Java Object (POJO) programming model. However, none of these rules are hard requirements. Indeed, Hibernate3 assumes very little about the nature of your persistent objects. You can express a domain model in other ways (using trees of Map instances, for example).

4.1. 一個簡單的POJO例子

Most Java applications require a persistent class representing felines. For example:

package eg;
import java.util.Set;
import java.util.Date;

public class Cat {
    private Long id; // identifier

    private Date birthdate;
    private Color color;
    private char sex;
    private float weight;
    private int litterId;

    private Cat mother;
    private Set kittens = new HashSet();

    private void setId(Long id) {
        this.id=id;
    }
    public Long getId() {
        return id;
    }

    void setBirthdate(Date date) {
        birthdate = date;
    }
    public Date getBirthdate() {
        return birthdate;
    }

    void setWeight(float weight) {
        this.weight = weight;
    }
    public float getWeight() {
        return weight;
    }

    public Color getColor() {
        return color;
    }
    void setColor(Color color) {
        this.color = color;
    }

    void setSex(char sex) {
        this.sex=sex;
    }
    public char getSex() {
        return sex;
    }

    void setLitterId(int id) {
        this.litterId = id;
    }
    public int getLitterId() {
        return litterId;
    }

    void setMother(Cat mother) {
        this.mother = mother;
    }
    public Cat getMother() {
        return mother;
    }
    void setKittens(Set kittens) {
        this.kittens = kittens;
    }
    public Set getKittens() {
        return kittens;
    }
    
    // addKitten not needed by Hibernate
    public void addKitten(Cat kitten) {
            kitten.setMother(this);
        kitten.setLitterId( kittens.size() ); 
        kittens.add(kitten);
    }
}

The four main rules of persistent classes are explored in more detail in the following sections.

4.1.1. 實現一個默認的（即無參數的）構造方法（constructor）

Cat has a no-argument constructor. All persistent classes must have a default constructor (which can be non-public) so that Hibernate can instantiate them usingConstructor.newInstance(). It is recommended that you have a default constructor with at leastpackage visibility for runtime proxy generation in Hibernate.

4.1.2. 提供一個標識屬性（identifier property）（可選）

Cat has a property called id. This property maps to the primary key column of a database table. The property might have been called anything, and its type might have been any primitive type, any primitive "wrapper" type, java.lang.String or java.util.Date. If your legacy database table has composite keys, you can use a user-defined class with properties of these types (see the section on composite identifiers later in the chapter.)

標識符屬性是可選的。可以不用管它，讓Hibernate內部來追蹤對象的識別。 但是我們並不推薦這樣做。

In fact, some functionality is available only to classes that declare an identifier property:

• Transitive reattachment for detached objects (cascade update or cascade merge) - see

• 第 10.11 節 “傳播性持久化(transitive persistence)”

• Session.saveOrUpdate()

• Session.merge()

We recommend that you declare consistently-named identifier properties on persistent classes and that you use a nullable (i.e., non-primitive) type.

4.1.3. 使用非final的類 (可選)

代理（proxies）是Hibernate的一個重要的功能，它依賴的條件是，持久 化類或者是非final的，或者是實現了一個所有方法都聲明爲public的接口。

You can persist final classes that do not implement an interface with Hibernate. You will not, however, be able to use proxies for lazy association fetching which will ultimately limit your options for performance tuning.

你也應該避免在非final類中聲明 public final的方法。如果你想使用一 個有public final方法的類，你必須通過設置lazy="false" 來明確地禁用代理。

4.1.4. 爲持久化字段聲明訪問器(accessors)和是否可變的標誌(mutators)(可選)

Cat declares accessor methods for all its persistent fields. Many other ORM tools directly persist instance variables. It is better to provide an indirection between the relational schema and internal data structures of the class. By default, Hibernate persists JavaBeans style properties and recognizes method names of the formgetFoo, isFoo and setFoo. If required, you can switch to direct field access for particular properties.

屬性不需要要聲明爲public的。Hibernate可以持久化一個有 default、protected或private的get/set方法對 的屬性進行持久化。

4.2. 實現繼承（Inheritance）

A subclass must also observe the first and second rules. It inherits its identifier property from the superclass,Cat. For example:

package eg;

public class DomesticCat extends Cat {
        private String name;

        public String getName() {
                return name;
        }
        protected void setName(String name) {
                this.name=name;
        }
}

4.3. 實現equals()和hashCode()

You have to override the equals() and hashCode() methods if you:

• intend to put instances of persistent classes in a Set (the recommended way to represent many-valued associations);and

• 想重用脫管實例

Hibernate guarantees equivalence of persistent identity (database row) and Java identity only inside a particular session scope. When you mix instances retrieved in different sessions, you must implementequals() and hashCode() if you wish to have meaningful semantics forSets.

The most obvious way is to implement equals()/hashCode() by comparing the identifier value of both objects. If the value is the same, both must be the same database row, because they are equal. If both are added to a Set, you will only have one element in theSet). Unfortunately, you cannot use that approach with generated identifiers. Hibernate will only assign identifier values to objects that are persistent; a newly created instance will not have any identifier value. Furthermore, if an instance is unsaved and currently in a Set, saving it will assign an identifier value to the object. Ifequals() and hashCode() are based on the identifier value, the hash code would change, breaking the contract of theSet. See the Hibernate website for a full discussion of this problem. This is not a Hibernate issue, but normal Java semantics of object identity and equality.

It is recommended that you implement equals() andhashCode() using Business key equality. Business key equality means that theequals() method compares only the properties that form the business key. It is a key that would identify our instance in the real world (anatural candidate key):

public class Cat {

    ...
    public boolean equals(Object other) {
        if (this == other) return true;
        if ( !(other instanceof Cat) ) return false;

        final Cat cat = (Cat) other;

        if ( !cat.getLitterId().equals( getLitterId() ) ) return false;
        if ( !cat.getMother().equals( getMother() ) ) return false;

        return true;
    }

    public int hashCode() {
        int result;
        result = getMother().hashCode();
        result = 29 * result + getLitterId();
        return result;
    }

}

A business key does not have to be as solid as a database primary key candidate (see

第 11.1.3 節 “關注對象標識(Considering object identity)”). Immutable or unique properties are usually good candidates for a business key.

4.4. 動態模型(Dynamic models)

Note

The following features are currently considered experimental and may change in the near future.

Persistent entities do not necessarily have to be represented as POJO classes or as JavaBean objects at runtime. Hibernate also supports dynamic models (usingMaps of Maps at runtime) and the representation of entities as DOM4J trees. With this approach, you do not write persistent classes, only mapping files.

By default, Hibernate works in normal POJO mode. You can set a default entity representation mode for a particularSessionFactory using the default_entity_mode configuration option (see

表 3.3 “Hibernate配置屬性”).

The following examples demonstrate the representation using Maps. First, in the mapping file an entity-name has to be declared instead of, or in addition to, a class name:

<hibernate-mapping>

    <class entity-name="Customer">

        <id name="id"
            type="long"
            column="ID">
            <generator class="sequence"/>
        </id>

        <property name="name"
            column="NAME"
            type="string"/>

        <property name="address"
            column="ADDRESS"
            type="string"/>

        <many-to-one name="organization"
            column="ORGANIZATION_ID"
            class="Organization"/>

        <bag name="orders"
            inverse="true"
            lazy="false"
            cascade="all">
            <key column="CUSTOMER_ID"/>
            <one-to-many class="Order"/>
        </bag>

    </class>
    
</hibernate-mapping>

Even though associations are declared using target class names, the target type of associations can also be a dynamic entity instead of a POJO.

After setting the default entity mode to dynamic-map for theSessionFactory, you can, at runtime, work with Maps of Maps:

Session s = openSession();
Transaction tx = s.beginTransaction();
Session s = openSession();

// Create a customer
Map david = new HashMap();
david.put("name", "David");

// Create an organization
Map foobar = new HashMap();
foobar.put("name", "Foobar Inc.");

// Link both
david.put("organization", foobar);

// Save both
s.save("Customer", david);
s.save("Organization", foobar);

tx.commit();
s.close();

One of the main advantages of dynamic mapping is quick turnaround time for prototyping, without the need for entity class implementation. However, you lose compile-time type checking and will likely deal with many exceptions at runtime. As a result of the Hibernate mapping, the database schema can easily be normalized and sound, allowing to add a proper domain model implementation on top later on.

實體表示模式也能在每個Session的基礎上設置：

Session dynamicSession = pojoSession.getSession(EntityMode.MAP);

// Create a customer
Map david = new HashMap();
david.put("name", "David");
dynamicSession.save("Customer", david);
...
dynamicSession.flush();
dynamicSession.close()
...
// Continue on pojoSession

Please note that the call to getSession() using anEntityMode is on the Session API, not theSessionFactory. That way, the new Session shares the underlying JDBC connection, transaction, and other context information. This means you do not have to callflush() and close() on the secondarySession, and also leave the transaction and connection handling to the primary unit of work.

關於XML表示能力的更多信息可以在第 18 章XML映射中找到。

4.5. 元組片斷映射(Tuplizers)

org.hibernate.tuple.Tuplizer, and its sub-interfaces, are responsible for managing a particular representation of a piece of data given that representation'sorg.hibernate.EntityMode. If a given piece of data is thought of as a data structure, then a tuplizer is the thing that knows how to create such a data structure and how to extract values from and inject values into such a data structure. For example, for the POJO entity mode, the corresponding tuplizer knows how create the POJO through its constructor. It also knows how to access the POJO properties using the defined property accessors.

There are two high-level types of Tuplizers, represented by the org.hibernate.tuple.entity.EntityTuplizer and org.hibernate.tuple.component.ComponentTuplizer interfaces.EntityTuplizers are responsible for managing the above mentioned contracts in regards to entities, whileComponentTuplizers do the same for components.

Users can also plug in their own tuplizers. Perhaps you require that a java.util.Map implementation other than java.util.HashMap be used while in the dynamic-map entity-mode. Or perhaps you need to define a different proxy generation strategy than the one used by default. Both would be achieved by defining a custom tuplizer implementation. Tuplizer definitions are attached to the entity or component mapping they are meant to manage. Going back to the example of our customer entity:

<hibernate-mapping>
    <class entity-name="Customer">
        <!--
            Override the dynamic-map entity-mode
            tuplizer for the customer entity
        -->
        <tuplizer entity-mode="dynamic-map"
                class="CustomMapTuplizerImpl"/>

        <id name="id" type="long" column="ID">
            <generator class="sequence"/>
        </id>

        <!-- other properties -->
        ...
    </class>
</hibernate-mapping>


public class CustomMapTuplizerImpl
        extends org.hibernate.tuple.entity.DynamicMapEntityTuplizer {
    // override the buildInstantiator() method to plug in our custom map...
    protected final Instantiator buildInstantiator(
            org.hibernate.mapping.PersistentClass mappingInfo) {
        return new CustomMapInstantiator( mappingInfo );
    }

    private static final class CustomMapInstantiator
            extends org.hibernate.tuple.DynamicMapInstantitor {
        // override the generateMap() method to return our custom map...
            protected final Map generateMap() {
                    return new CustomMap();
            }
    }
}

4.6. EntityNameResolvers

The org.hibernate.EntityNameResolver interface is a contract for resolving the entity name of a given entity instance. The interface defines a single methodresolveEntityName which is passed the entity instance and is expected to return the appropriate entity name (null is allowed and would indicate that the resolver does not know how to resolve the entity name of the given entity instance). Generally speaking, an org.hibernate.EntityNameResolver is going to be most useful in the case of dynamic models. One example might be using proxied interfaces as your domain model. The hibernate test suite has an example of this exact style of usage under the org.hibernate.test.dynamicentity.tuplizer2. Here is some of the code from that package for illustration.

/**
 * A very trivial JDK Proxy InvocationHandler implementation where we proxy an interface as
 * the domain model and simply store persistent state in an internal Map.  This is an extremely
 * trivial example meant only for illustration.
 */
public final class DataProxyHandler implements InvocationHandler {
        private String entityName;
        private HashMap data = new HashMap();

        public DataProxyHandler(String entityName, Serializable id) {
                this.entityName = entityName;
                data.put( "Id", id );
        }

        public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
                String methodName = method.getName();
                if ( methodName.startsWith( "set" ) ) {
                        String propertyName = methodName.substring( 3 );
                        data.put( propertyName, args[0] );
                }
                else if ( methodName.startsWith( "get" ) ) {
                        String propertyName = methodName.substring( 3 );
                        return data.get( propertyName );
                }
                else if ( "toString".equals( methodName ) ) {
                        return entityName + "#" + data.get( "Id" );
                }
                else if ( "hashCode".equals( methodName ) ) {
                        return new Integer( this.hashCode() );
                }
                return null;
        }

        public String getEntityName() {
                return entityName;
        }

        public HashMap getData() {
                return data;
        }
}

/**
 *
 */
public class ProxyHelper {
    public static String extractEntityName(Object object) {
        // Our custom java.lang.reflect.Proxy instances actually bundle
        // their appropriate entity name, so we simply extract it from there
        // if this represents one of our proxies; otherwise, we return null
        if ( Proxy.isProxyClass( object.getClass() ) ) {
            InvocationHandler handler = Proxy.getInvocationHandler( object );
            if ( DataProxyHandler.class.isAssignableFrom( handler.getClass() ) ) {
                DataProxyHandler myHandler = ( DataProxyHandler ) handler;
                return myHandler.getEntityName();
            }
        }
        return null;
    }

    // various other utility methods ....

}

/**
 * The EntityNameResolver implementation.
 * IMPL NOTE : An EntityNameResolver really defines a strategy for how entity names should be
 * resolved.  Since this particular impl can handle resolution for all of our entities we want to
 * take advantage of the fact that SessionFactoryImpl keeps these in a Set so that we only ever
 * have one instance registered.  Why?  Well, when it comes time to resolve an entity name,
 * Hibernate must iterate over all the registered resolvers.  So keeping that number down
 * helps that process be as speedy as possible.  Hence the equals and hashCode impls
 */
public class MyEntityNameResolver implements EntityNameResolver {
    public static final MyEntityNameResolver INSTANCE = new MyEntityNameResolver();

    public String resolveEntityName(Object entity) {
        return ProxyHelper.extractEntityName( entity );
    }

    public boolean equals(Object obj) {
        return getClass().equals( obj.getClass() );
    }

    public int hashCode() {
        return getClass().hashCode();
    }
}

public class MyEntityTuplizer extends PojoEntityTuplizer {
        public MyEntityTuplizer(EntityMetamodel entityMetamodel, PersistentClass mappedEntity) {
                super( entityMetamodel, mappedEntity );
        }

        public EntityNameResolver[] getEntityNameResolvers() {
                return new EntityNameResolver[] { MyEntityNameResolver.INSTANCE };
        }

    public String determineConcreteSubclassEntityName(Object entityInstance, SessionFactoryImplementor factory) {
        String entityName = ProxyHelper.extractEntityName( entityInstance );
        if ( entityName == null ) {
            entityName = super.determineConcreteSubclassEntityName( entityInstance, factory );
        }
        return entityName;
    }

    ...
}
        

In order to register an org.hibernate.EntityNameResolver users must either:

1. Implement a custom

1. Tuplizer, implementing thegetEntityNameResolvers method.

2. Register it with the org.hibernate.impl.SessionFactoryImpl (which is the implementation class fororg.hibernate.SessionFactory) using the registerEntityNameResolver method.

第 5 章 對象/關係數據庫映射基礎(Basic O/R Mapping)

5.1. 映射定義（Mapping declaration）

Object/relational mappings are usually defined in an XML document. The mapping document is designed to be readable and hand-editable. The mapping language is Java-centric, meaning that mappings are constructed around persistent class declarations and not table declarations.

Please note that even though many Hibernate users choose to write the XML by hand, a number of tools exist to generate the mapping document. These include XDoclet, Middlegen and AndroMDA.

Here is an example mapping:

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
      "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
          "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping package="eg">

        <class name="Cat"
            table="cats"
            discriminator-value="C">

                <id name="id">
                        <generator class="native"/>
                </id>

                <discriminator column="subclass"
                     type="character"/>

                <property name="weight"/>

                <property name="birthdate"
                    type="date"
                    not-null="true"
                    update="false"/>

                <property name="color"
                    type="eg.types.ColorUserType"
                    not-null="true"
                    update="false"/>

                <property name="sex"
                    not-null="true"
                    update="false"/>

                <property name="litterId"
                    column="litterId"
                    update="false"/>

                <many-to-one name="mother"
                    column="mother_id"
                    update="false"/>

                <set name="kittens"
                    inverse="true"
                    order-by="litter_id">
                        <key column="mother_id"/>
                        <one-to-many class="Cat"/>
                </set>

                <subclass name="DomesticCat"
                    discriminator-value="D">

                        <property name="name"
                            type="string"/>

                </subclass>

        </class>

        <class name="Dog">
                <!-- mapping for Dog could go here -->
        </class>

</hibernate-mapping>

We will now discuss the content of the mapping document. We will only describe, however, the document elements and attributes that are used by Hibernate at runtime. The mapping document also contains some extra optional attributes and elements that affect the database schemas exported by the schema export tool (for example, the not-null attribute).

5.1.1. Doctype

All XML mappings should declare the doctype shown. The actual DTD can be found at the URL above, in the directoryhibernate-x.x.x/src/org/hibernate , or in hibernate3.jar. Hibernate will always look for the DTD in its classpath first. If you experience lookups of the DTD using an Internet connection, check the DTD declaration against the contents of your classpath.

5.1.1.1. EntityResolver

Hibernate will first attempt to resolve DTDs in its classpath. It does this is by registering a customorg.xml.sax.EntityResolver implementation with the SAXReader it uses to read in the xml files. This customEntityResolver recognizes two different systemId namespaces:

• a hibernate namespace is recognized whenever the resolver encounters a systemId starting withhttp://hibernate.sourceforge.net/. The resolver attempts to resolve these entities via the classloader which loaded the Hibernate classes.

• a user namespace is recognized whenever the resolver encounters a systemId using aclasspath:// URL protocol. The resolver will attempt to resolve these entities via (1) the current thread context classloader and (2) the classloader which loaded the Hibernate classes.

The following is an example of utilizing user namespacing:

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd" [
    <!ENTITY types SYSTEM "classpath://your/domain/types.xml">
]>

<hibernate-mapping package="your.domain">
    <class name="MyEntity">
        <id name="id" type="my-custom-id-type">
            ...
        </id>
    <class>
    &types;
</hibernate-mapping>

Where types.xml is a resource in the your.domain package and contains a custom

typedef.

5.1.2. Hibernate-mapping

This element has several optional attributes. The schema andcatalog attributes specify that tables referred to in this mapping belong to the named schema and/or catalog. If they are specified, tablenames will be qualified by the given schema and catalog names. If they are missing, tablenames will be unqualified. The default-cascade attribute specifies what cascade style should be assumed for properties and collections that do not specify acascade attribute. By default, the auto-import attribute allows you to use unqualified class names in the query language.

<hibernate-mapping
         schema="schemaName"                          
         catalog="catalogName"                        
         default-cascade="cascade_style"              
         default-access="field|property|ClassName"    
         default-lazy="true|false"                    
         auto-import="true|false"                     
         package="package.name"                       
 />

	schema (optional): the name of a database schema.
	catalog (optional): the name of a database catalog.
	default-cascade (optional - defaults to none): a default cascade style.
	default-access (optional - defaults to property): the strategy Hibernate should use for accessing all properties. It can be a custom implementation ofPropertyAccessor.
	default-lazy (optional - defaults to true): the default value for unspecified lazy attributes of class and collection mappings.
	auto-import (optional - defaults to true): specifies whether we can use unqualified class names of classes in this mapping in the query language.
	package (optional): specifies a package prefix to use for unqualified class names in the mapping document.

If you have two persistent classes with the same unqualified name, you should setauto-import="false". An exception will result if you attempt to assign two classes to the same "imported" name.

The hibernate-mapping element allows you to nest several persistent<class> mappings, as shown above. It is, however, good practice (and expected by some tools) to map only a single persistent class, or a single class hierarchy, in one mapping file and name it after the persistent superclass. For example, Cat.hbm.xml, Dog.hbm.xml, or if using inheritance,Animal.hbm.xml.

5.1.3. Class

You can declare a persistent class using the class element. For example:

<class
        name="ClassName"                              
        table="tableName"                             
        discriminator-value="discriminator_value"     
        mutable="true|false"                          
        schema="owner"                                
        catalog="catalog"                             
        proxy="ProxyInterface"                        
        dynamic-update="true|false"                   
        dynamic-insert="true|false"                   
        select-before-update="true|false"             
        polymorphism="implicit|explicit"              
        where="arbitrary sql where condition"         
        persister="PersisterClass"                    
        batch-size="N"                                
        optimistic-lock="none|version|dirty|all"      
        lazy="true|false"                             (16)
        entity-name="EntityName"                      (17)
        check="arbitrary sql check condition"         (18)
        rowid="rowid"                                 (19)
        subselect="SQL expression"                    (20)
        abstract="true|false"                         (21)
        node="element-name"
/>

	name (optional): the fully qualified Java class name of the persistent class or interface. If this attribute is missing, it is assumed that the mapping is for a non-POJO entity.
	table (optional - defaults to the unqualified class name): the name of its database table.
	discriminator-value (optional - defaults to the class name): a value that distinguishes individual subclasses that is used for polymorphic behavior. Acceptable values includenull and not null.
	mutable (optional - defaults to true): specifies that instances of the class are (not) mutable.
	schema (optional): overrides the schema name specified by the root<hibernate-mapping> element.
	catalog (optional): overrides the catalog name specified by the root<hibernate-mapping> element.
	proxy (optional): specifies an interface to use for lazy initializing proxies. You can specify the name of the class itself.
	dynamic-update (optional - defaults to false): specifies that UPDATE SQL should be generated at runtime and can contain only those columns whose values have changed.
	dynamic-insert (optional - defaults to false): specifies that INSERT SQL should be generated at runtime and contain only the columns whose values are not null.
	select-before-update (optional - defaults to false): specifies that Hibernate should never perform an SQLUPDATE unless it is certain that an object is actually modified. Only when a transient object has been associated with a new session usingupdate(), will Hibernate perform an extra SQL SELECT to determine if an UPDATE is actually required.
	polymorphism (optional - defaults to implicit): determines whether implicit or explicit query polymorphism is used.
	where (optional): specifies an arbitrary SQL WHERE condition to be used when retrieving objects of this class.
	persister (optional): specifies a custom ClassPersister.
	batch-size (optional - defaults to 1): specifies a "batch size" for fetching instances of this class by identifier.
	optimistic-lock (optional - defaults to version): determines the optimistic locking strategy.
(16)	lazy (optional): lazy fetching can be disabled by settinglazy="false".
(17)	entity-name (optional - defaults to the class name): Hibernate3 allows a class to be mapped multiple times, potentially to different tables. It also allows entity mappings that are represented by Maps or XML at the Java level. In these cases, you should provide an explicit arbitrary name for the entity. See第 4.4 節 “動態模型(Dynamic models)” and 第 18 章 XML映射 for more information.
(18)	check (optional): an SQL expression used to generate a multi-rowcheck constraint for automatic schema generation.
(19)	rowid (optional): Hibernate can use ROWIDs on databases. On Oracle, for example, Hibernate can use therowid extra column for fast updates once this option has been set torowid. A ROWID is an implementation detail and represents the physical location of a stored tuple.
(20)	subselect (optional): maps an immutable and read-only entity to a database subselect. This is useful if you want to have a view instead of a base table. See below for more information.
(21)	abstract (optional): is used to mark abstract superclasses in<union-subclass> hierarchies.

It is acceptable for the named persistent class to be an interface. You can declare implementing classes of that interface using the<subclass> element. You can persist any static inner class. Specify the class name using the standard form i.e.e.g.Foo$Bar.

Immutable classes, mutable="false", cannot be updated or deleted by the application. This allows Hibernate to make some minor performance optimizations.

The optional proxy attribute enables lazy initialization of persistent instances of the class. Hibernate will initially return CGLIB proxies that implement the named interface. The persistent object will load when a method of the proxy is invoked. See "Initializing collections and proxies" below.

Implicit polymorphism means that instances of the class will be returned by a query that names any superclass or implemented interface or class, and that instances of any subclass of the class will be returned by a query that names the class itself. Explicit polymorphism means that class instances will be returned only by queries that explicitly name that class. Queries that name the class will return only instances of subclasses mapped inside this <class> declaration as a <subclass> or <joined-subclass>. For most purposes, the defaultpolymorphism="implicit" is appropriate. Explicit polymorphism is useful when two different classes are mapped to the same table This allows a "lightweight" class that contains a subset of the table columns.

The persister attribute lets you customize the persistence strategy used for the class. You can, for example, specify your own subclass oforg.hibernate.persister.EntityPersister, or you can even provide a completely new implementation of the interfaceorg.hibernate.persister.ClassPersister that implements, for example, persistence via stored procedure calls, serialization to flat files or LDAP. Seeorg.hibernate.test.CustomPersister for a simple example of "persistence" to aHashtable.

The dynamic-update and dynamic-insert settings are not inherited by subclasses, so they can also be specified on the<subclass> or <joined-subclass> elements. Although these settings can increase performance in some cases, they can actually decrease performance in others.

Use of select-before-update will usually decrease performance. It is useful to prevent a database update trigger being called unnecessarily if you reattach a graph of detached instances to aSession.

如果你打開了dynamic-update，你可以選擇幾種樂觀鎖定的策略：

• version: check the version/timestamp columns

• all: check all columns

• dirty: check the changed columns, allowing some concurrent updates

• none: do not use optimistic locking

It is strongly recommended that you use version/timestamp columns for optimistic locking with Hibernate. This strategy optimizes performance and correctly handles modifications made to detached instances (i.e. whenSession.merge() is used).

There is no difference between a view and a base table for a Hibernate mapping. This is transparent at the database level, although some DBMS do not support views properly, especially with updates. Sometimes you want to use a view, but you cannot create one in the database (i.e. with a legacy schema). In this case, you can map an immutable and read-only entity to a given SQL subselect expression:

<class name="Summary">
    <subselect>
        select item.name, max(bid.amount), count(*)
        from item
        join bid on bid.item_id = item.id
        group by item.name
    </subselect>
    <synchronize table="item"/>
    <synchronize table="bid"/>
    <id name="name"/>
    ...
</class>

Declare the tables to synchronize this entity with, ensuring that auto-flush happens correctly and that queries against the derived entity do not return stale data. The<subselect> is available both as an attribute and a nested mapping element.

5.1.4. id

被映射的類必須定義對應數據庫表主鍵字段。大多數類有一個JavaBeans風格的屬性， 爲每一個實例包含唯一的標識。<id> 元素定義了該屬性到數據庫表主鍵字段的映射。

<id
        name="propertyName"                                          
        type="typename"                                              
        column="column_name"                                         
        unsaved-value="null|any|none|undefined|id_value"             
        access="field|property|ClassName">                           
        node="element-name|@attribute-name|element/@attribute|."

        <generator class="generatorClass"/>
</id>

	name (optional): the name of the identifier property.
	type (可選): 一個Hibernate類型的名字。
	column (optional - defaults to the property name): the name of the primary key column.
	unsaved-value (optional - defaults to a "sensible" value): an identifier property value that indicates an instance is newly instantiated (unsaved), distinguishing it from detached instances that were saved or loaded in a previous session.
	access (optional - defaults to property): the strategy Hibernate should use for accessing the property value.

如果 name屬性不存在，會認爲這個類沒有標識屬性。

unsaved-value 屬性在Hibernate3中幾乎不再需要。

There is an alternative <composite-id> declaration that allows access to legacy data with composite keys. Its use is strongly discouraged for anything else.

5.1.4.1. Generator

可選的<generator>子元素是一個Java類的名字， 用來爲該持久化類的實例生成唯一的標識。如果這個生成器實例需要某些配置值或者初始化參數， 用<param>元素來傳遞。

<id name="id" type="long" column="cat_id">
        <generator class="org.hibernate.id.TableHiLoGenerator">
                <param name="table">uid_table</param>
                <param name="column">next_hi_value_column</param>
        </generator>
</id>

All generators implement the interface org.hibernate.id.IdentifierGenerator. This is a very simple interface. Some applications can choose to provide their own specialized implementations, however, Hibernate provides a range of built-in implementations. The shortcut names for the built-in generators are as follows:

increment

用於爲long, short或者int類型生成 唯一標識。只有在沒有其他進程往同一張表中插入數據時才能使用。在集羣下不要使用。

identity

對DB2,MySQL, MS SQL Server, Sybase和HypersonicSQL的內置標識字段提供支持。 返回的標識符是long,short 或者int類型的。

sequence

在DB2,PostgreSQL, Oracle, SAP DB, McKoi中使用序列（sequence)， 而在Interbase中使用生成器(generator)。返回的標識符是long,short或者 int類型的。

hilo

使用一個高/低位算法高效的生成long,short 或者 int類型的標識符。給定一個表和字段（默認分別是hibernate_unique_key 和next_hi）作爲高位值的來源。 高/低位算法生成的標識符只在一個特定的數據庫中是唯一的。

seqhilo

使用一個高/低位算法來高效的生成long, short 或者int類型的標識符，給定一個數據庫序列（sequence)的名字。

uuid

uses a 128-bit UUID algorithm to generate identifiers of type string that are unique within a network (the IP address is used). The UUID is encoded as a string of 32 hexadecimal digits in length.

guid

在MS SQL Server 和 MySQL 中使用數據庫生成的GUID字符串。

native

selects identity, sequence orhilo depending upon the capabilities of the underlying database.

assigned

lets the application assign an identifier to the object before save() is called. This is the default strategy if no <generator> element is specified.

select

retrieves a primary key, assigned by a database trigger, by selecting the row by some unique key and retrieving the primary key value.

foreign

uses the identifier of another associated object. It is usually used in conjunction with a<one-to-one> primary key association.

sequence-identity

a specialized sequence generation strategy that utilizes a database sequence for the actual value generation, but combines this with JDBC3 getGeneratedKeys to return the generated identifier value as part of the insert statement execution. This strategy is only supported on Oracle 10g drivers targeted for JDK 1.4. Comments on these insert statements are disabled due to a bug in the Oracle drivers.

5.1.4.2. 高/低位算法（Hi/Lo Algorithm）

The hilo and seqhilo generators provide two alternate implementations of the hi/lo algorithm. The first implementation requires a "special" database table to hold the next available "hi" value. Where supported, the second uses an Oracle-style sequence.

<id name="id" type="long" column="cat_id">
        <generator class="hilo">
                <param name="table">hi_value</param>
                <param name="column">next_value</param>
                <param name="max_lo">100</param>
        </generator>
</id>

<id name="id" type="long" column="cat_id">
        <generator class="seqhilo">
                <param name="sequence">hi_value</param>
                <param name="max_lo">100</param>
        </generator>
</id>

Unfortunately, you cannot use hilo when supplying your ownConnection to Hibernate. When Hibernate uses an application server datasource to obtain connections enlisted with JTA, you must configure thehibernate.transaction.manager_lookup_class.

5.1.4.3. UUID算法（UUID Algorithm ）

The UUID contains: IP address, startup time of the JVM that is accurate to a quarter second, system time and a counter value that is unique within the JVM. It is not possible to obtain a MAC address or memory address from Java code, so this is the best option without using JNI.

5.1.4.4. 標識字段和序列（Identity columns and Sequences）

For databases that support identity columns (DB2, MySQL, Sybase, MS SQL), you can useidentity key generation. For databases that support sequences (DB2, Oracle, PostgreSQL, Interbase, McKoi, SAP DB) you can usesequence style key generation. Both of these strategies require two SQL queries to insert a new object. For example:

<id name="id" type="long" column="person_id">
        <generator class="sequence">
                <param name="sequence">person_id_sequence</param>
        </generator>
</id>

<id name="id" type="long" column="person_id" unsaved-value="0">
        <generator class="identity"/>
</id>

For cross-platform development, the native strategy will, depending on the capabilities of the underlying database, choose from theidentity, sequence and hilo strategies.

5.1.4.5. 程序分配的標識符（Assigned Identifiers）

If you want the application to assign identifiers, as opposed to having Hibernate generate them, you can use theassigned generator. This special generator uses the identifier value already assigned to the object's identifier property. The generator is used when the primary key is a natural key instead of a surrogate key. This is the default behavior if you do not specify a <generator> element.

The assigned generator makes Hibernate use unsaved-value="undefined". This forces Hibernate to go to the database to determine if an instance is transient or detached, unless there is a version or timestamp property, or you defineInterceptor.isUnsaved().

5.1.4.6. 觸發器實現的主鍵生成器（Primary keys assigned by triggers）

Hibernate does not generate DDL with triggers. It is for legacy schemas only.

<id name="id" type="long" column="person_id">
        <generator class="select">
                <param name="key">socialSecurityNumber</param>
        </generator>
</id>

In the above example, there is a unique valued property named socialSecurityNumber. It is defined by the class, as a natural key and a surrogate key namedperson_id, whose value is generated by a trigger.

5.1.5. Enhanced identifier generators

Starting with release 3.2.3, there are 2 new generators which represent a re-thinking of 2 different aspects of identifier generation. The first aspect is database portability; the second is optimization Optimization means that you do not have to query the database for every request for a new identifier value. These two new generators are intended to take the place of some of the named generators described above, starting in 3.3.x. However, they are included in the current releases and can be referenced by FQN.

The first of these new generators is org.hibernate.id.enhanced.SequenceStyleGenerator which is intended, firstly, as a replacement for thesequence generator and, secondly, as a better portability generator thannative. This is because native generally chooses betweenidentity and sequence which have largely different semantics that can cause subtle issues in applications eyeing portability.org.hibernate.id.enhanced.SequenceStyleGenerator, however, achieves portability in a different manner. It chooses between a table or a sequence in the database to store its incrementing values, depending on the capabilities of the dialect being used. The difference between this and native is that table-based and sequence-based storage have the same exact semantic. In fact, sequences are exactly what Hibernate tries to emulate with its table-based generators. This generator has a number of configuration parameters:

• sequence_name (optional, defaults to hibernate_sequence): the name of the sequence or table to be used.

• initial_value (optional, defaults to 1): the initial value to be retrieved from the sequence/table. In sequence creation terms, this is analogous to the clause typically named "STARTS WITH".

• increment_size (optional - defaults to 1): the value by which subsequent calls to the sequence/table should differ. In sequence creation terms, this is analogous to the clause typically named "INCREMENT BY".

• force_table_use (optional - defaults to false): should we force the use of a table as the backing structure even though the dialect might support sequence?

• value_column (optional - defaults to next_val): only relevant for table structures, it is the name of the column on the table which is used to hold the value.

• optimizer (optional - defaults to none): See

• 第 5.1.6 節 “Identifier generator optimization”

The second of these new generators is org.hibernate.id.enhanced.TableGenerator, which is intended, firstly, as a replacement for thetable generator, even though it actually functions much more likeorg.hibernate.id.MultipleHiLoPerTableGenerator, and secondly, as a re-implementation oforg.hibernate.id.MultipleHiLoPerTableGenerator that utilizes the notion of pluggable optimizers. Essentially this generator defines a table capable of holding a number of different increment values simultaneously by using multiple distinctly keyed rows. This generator has a number of configuration parameters:

• table_name (optional - defaults to hibernate_sequences): the name of the table to be used.

• value_column_name (optional - defaults to next_val): the name of the column on the table that is used to hold the value.

• segment_column_name (optional - defaults to sequence_name): the name of the column on the table that is used to hold the "segment key". This is the value which identifies which increment value to use.

• segment_value (optional - defaults to default): The "segment key" value for the segment from which we want to pull increment values for this generator.

• segment_value_length (optional - defaults to 255): Used for schema generation; the column size to create this segment key column.

• initial_value (optional - defaults to 1): The initial value to be retrieved from the table.

• increment_size (optional - defaults to 1): The value by which subsequent calls to the table should differ.

• optimizer (optional - defaults to ): See 第 5.1.6 節 “Identifier generator optimization”

5.1.6. Identifier generator optimization

For identifier generators that store values in the database, it is inefficient for them to hit the database on each and every call to generate a new identifier value. Instead, you can group a bunch of them in memory and only hit the database when you have exhausted your in-memory value group. This is the role of the pluggable optimizers. Currently only the two enhanced generators (

第 5.1.5 節 “Enhanced identifier generators” support this operation.

• none (generally this is the default if no optimizer was specified): this will not perform any optimizations and hit the database for each and every request.

• hilo: applies a hi/lo algorithm around the database retrieved values. The values from the database for this optimizer are expected to be sequential. The values retrieved from the database structure for this optimizer indicates the "group number". The increment_size is multiplied by that value in memory to define a group "hi value".

• pooled: as with the case of hilo, this optimizer attempts to minimize the number of hits to the database. Here, however, we simply store the starting value for the "next group" into the database structure rather than a sequential value in combination with an in-memory grouping algorithm. Here, increment_size refers to the values coming from the database.

5.1.7. composite-id

<composite-id
        name="propertyName"
        class="ClassName"
        mapped="true|false"
        access="field|property|ClassName">
        node="element-name|."

        <key-property name="propertyName" type="typename" column="column_name"/>
        <key-many-to-one name="propertyName class="ClassName" column="column_name"/>
        ......
</composite-id>

A table with a composite key can be mapped with multiple properties of the class as identifier properties. The<composite-id> element accepts <key-property> property mappings and <key-many-to-one> mappings as child elements.

<composite-id>
        <key-property name="medicareNumber"/>
        <key-property name="dependent"/>
</composite-id>

The persistent class must override equals() and hashCode() to implement composite identifier equality. It must also implementSerializable.

Unfortunately, this approach means that a persistent object is its own identifier. There is no convenient "handle" other than the object itself. You must instantiate an instance of the persistent class itself and populate its identifier properties before you can load() the persistent state associated with a composite key. We call this approach anembedded composite identifier, and discourage it for serious applications.

第二種方法我們稱爲mapped(映射式)組合標識符 (mapped composite identifier),<composite-id>元素中列出的標識屬性不但在持久化類出現，還形成一個獨立的標識符類。

<composite-id class="MedicareId" mapped="true">
        <key-property name="medicareNumber"/>
        <key-property name="dependent"/>
</composite-id>

In this example, both the composite identifier class, MedicareId, and the entity class itself have properties namedmedicareNumber and dependent. The identifier class must overrideequals() and hashCode() and implementSerializable. The main disadvantage of this approach is code duplication.

下面列出的屬性是用來指定一個映射式組合標識符的：

• mapped (optional - defaults to false): indicates that a mapped composite identifier is used, and that the contained property mappings refer to both the entity class and the composite identifier class.

• class (optional - but required for a mapped composite identifier): the class used as a composite identifier.

We will describe a third, even more convenient approach, where the composite identifier is implemented as a component class in

第 8.4 節 “組件作爲聯合標識符(Components as composite identifiers)”. The attributes described below apply only to this alternative approach:

• name (optional - required for this approach): a property of component type that holds the composite identifier. Please see chapter 9 for more information.

• access (optional - defaults to property): the strategy Hibernate uses for accessing the property value.

• class (optional - defaults to the property type determined by reflection): the component class used as a composite identifier. Please see the next section for more information.

The third approach, an identifier component, is recommended for almost all applications.

5.1.8. Discriminator

The <discriminator> element is required for polymorphic persistence using the table-per-class-hierarchy mapping strategy. It declares a discriminator column of the table. The discriminator column contains marker values that tell the persistence layer what subclass to instantiate for a particular row. A restricted set of types can be used:string, character, integer, byte, short,boolean, yes_no, true_false.

<discriminator
        column="discriminator_column"                      
        type="discriminator_type"                          
        force="true|false"                                 
        insert="true|false"                                
        formula="arbitrary sql expression"                 
/>

	column (optional - defaults to class): the name of the discriminator column.
	type (optional - defaults to string): a name that indicates the Hibernate type
	force (optional - defaults to false): "forces" Hibernate to specify the allowed discriminator values, even when retrieving all instances of the root class.
	insert (optional - defaults to true): set this to false if your discriminator column is also part of a mapped composite identifier. It tells Hibernate not to include the column in SQLINSERTs.
	formula (optional): an arbitrary SQL expression that is executed when a type has to be evaluated. It allows content-based discrimination.

鑑別器字段的實際值是根據<class>和<subclass>元素中 的discriminator-value屬性得來的。

The force attribute is only useful if the table contains rows with "extra" discriminator values that are not mapped to a persistent class. This will not usually be the case.

The formula attribute allows you to declare an arbitrary SQL expression that will be used to evaluate the type of a row. For example:

<discriminator
    formula="case when CLASS_TYPE in ('a', 'b', 'c') then 0 else 1 end"
    type="integer"/>

5.1.9. Version (optional)

The <version> element is optional and indicates that the table contains versioned data. This is particularly useful if you plan to uselong transactions. See below for more information:

<version
        column="version_column"                                      
        name="propertyName"                                          
        type="typename"                                              
        access="field|property|ClassName"                            
        unsaved-value="null|negative|undefined"                      
        generated="never|always"                                     
        insert="true|false"                                          
        node="element-name|@attribute-name|element/@attribute|."
/>

	column (optional - defaults to the property name): the name of the column holding the version number.
	name: the name of a property of the persistent class.
	type (optional - defaults to integer): the type of the version number.
	access (optional - defaults to property): the strategy Hibernate uses to access the property value.
	unsaved-value (optional - defaults to undefined): a version property value that indicates that an instance is newly instantiated (unsaved), distinguishing it from detached instances that were saved or loaded in a previous session.Undefined specifies that the identifier property value should be used.
	generated (optional - defaults to never): specifies that this version property value is generated by the database. See the discussion ofgenerated properties for more information.
	insert (optional - defaults to true): specifies whether the version column should be included in SQL insert statements. It can be set tofalse if the database column is defined with a default value of0.

Version numbers can be of Hibernate type long, integer, short, timestamp orcalendar.

A version or timestamp property should never be null for a detached instance. Hibernate will detect any instance with a null version or timestamp as transient, irrespective of what otherunsaved-value strategies are specified. Declaring a nullable version or timestamp property is an easy way to avoid problems with transitive reattachment in Hibernate. It is especially useful for people using assigned identifiers or composite keys.

5.1.10. Timestamp (optional)

The optional <timestamp> element indicates that the table contains timestamped data. This provides an alternative to versioning. Timestamps are a less safe implementation of optimistic locking. However, sometimes the application might use the timestamps in other ways.

<timestamp
        column="timestamp_column"                                    
        name="propertyName"                                          
        access="field|property|ClassName"                            
        unsaved-value="null|undefined"                               
        source="vm|db"                                               
        generated="never|always"                                     
        node="element-name|@attribute-name|element/@attribute|."
/>

	column (optional - defaults to the property name): the name of a column holding the timestamp.
	name: the name of a JavaBeans style property of Java typeDate or Timestamp of the persistent class.
	access (optional - defaults to property): the strategy Hibernate uses for accessing the property value.
	unsaved-value (optional - defaults to null): a version property value that indicates that an instance is newly instantiated (unsaved), distinguishing it from detached instances that were saved or loaded in a previous session.Undefined specifies that the identifier property value should be used.
	source (optional - defaults to vm): Where should Hibernate retrieve the timestamp value from? From the database, or from the current JVM? Database-based timestamps incur an overhead because Hibernate must hit the database in order to determine the "next value". It is safer to use in clustered environments. Not all Dialects are known to support the retrieval of the database's current timestamp. Others may also be unsafe for usage in locking due to lack of precision (Oracle 8, for example).
	generated (optional - defaults to never): specifies that this timestamp property value is actually generated by the database. See the discussion ofgenerated properties for more information.

Note

<Timestamp> is equivalent to <version type="timestamp">. And <timestamp source="db"> is equivalent to<version type="dbtimestamp">

5.1.11. Property

The <property> element declares a persistent JavaBean style property of the class.

<property
        name="propertyName"                                          
        column="column_name"                                         
        type="typename"                                              
        update="true|false"                                          
        insert="true|false"                                          
        formula="arbitrary SQL expression"                           
        access="field|property|ClassName"                            
        lazy="true|false"                                            
        unique="true|false"                                          
        not-null="true|false"                                        
        optimistic-lock="true|false"                                 
        generated="never|insert|always"                              
        node="element-name|@attribute-name|element/@attribute|."
        index="index_name"
        unique_key="unique_key_id"
        length="L"
        precision="P"
        scale="S"
/>

	name: 屬性的名字,以小寫字母開頭。
	column (optional - defaults to the property name): the name of the mapped database table column. This can also be specified by nested<column> element(s).
	type (可選): 一個Hibernate類型的名字。
	update, insert (optional - defaults to true): specifies that the mapped columns should be included in SQL UPDATE and/or INSERT statements. Setting both tofalse allows a pure "derived" property whose value is initialized from some other property that maps to the same column(s), or by a trigger or other application.
	formula (可選): 一個SQL表達式，定義了這個計算 （computed） 屬性的值。計算屬性沒有和它對應的數據庫字段。
	access (optional - defaults to property): the strategy Hibernate uses for accessing the property value.
	lazy (optional - defaults to false): specifies that this property should be fetched lazily when the instance variable is first accessed. It requires build-time bytecode instrumentation.
	unique (optional): enables the DDL generation of a unique constraint for the columns. Also, allow this to be the target of aproperty-ref.
	not-null (optional): enables the DDL generation of a nullability constraint for the columns.
	optimistic-lock (optional - defaults to true): specifies that updates to this property do or do not require acquisition of the optimistic lock. In other words, it determines if a version increment should occur when this property is dirty.
	generated (optional - defaults to never): specifies that this property value is actually generated by the database. See the discussion ofgenerated properties for more information.

typename可以是如下幾種：

1. The name of a Hibernate basic type: integer, string, character, date, timestamp, float, binary, serializable, object, blob etc.

2. The name of a Java class with a default basic type: int, float, char, java.lang.String, java.util.Date, java.lang.Integer, java.sql.Clob etc.

3. 一個可以序列化的Java類的名字。

4. The class name of a custom type: com.illflow.type.MyCustomType etc.

If you do not specify a type, Hibernate will use reflection upon the named property and guess the correct Hibernate type. Hibernate will attempt to interpret the name of the return class of the property getter using, in order, rules 2, 3, and 4. In certain cases you will need the type attribute. For example, to distinguish betweenHibernate.DATE and Hibernate.TIMESTAMP, or to specify a custom type.

The access attribute allows you to control how Hibernate accesses the property at runtime. By default, Hibernate will call the property get/set pair. If you specifyaccess="field", Hibernate will bypass the get/set pair and access the field directly using reflection. You can specify your own strategy for property access by naming a class that implements the interfaceorg.hibernate.property.PropertyAccessor.

A powerful feature is derived properties. These properties are by definition read-only. The property value is computed at load time. You declare the computation as an SQL expression. This then translates to aSELECT clause subquery in the SQL query that loads an instance:

<property name="totalPrice"
    formula="( SELECT SUM (li.quantity*p.price) FROM LineItem li, Product p
                WHERE li.productId = p.productId
                AND li.customerId = customerId
                AND li.orderNumber = orderNumber )"/>

You can reference the entity table by not declaring an alias on a particular column. This would becustomerId in the given example. You can also use the nested<formula> mapping element if you do not want to use the attribute.

5.1.12. Many-to-one

An ordinary association to another persistent class is declared using a many-to-one element. The relational model is a many-to-one association; a foreign key in one table is referencing the primary key column(s) of the target table.

<many-to-one
        name="propertyName"                                          
        column="column_name"                                         
        class="ClassName"                                            
        cascade="cascade_style"                                      
        fetch="join|select"                                          
        update="true|false"                                          
        insert="true|false"                                          
        property-ref="propertyNameFromAssociatedClass"               
        access="field|property|ClassName"                            
        unique="true|false"                                          
        not-null="true|false"                                        
        optimistic-lock="true|false"                                 
        lazy="proxy|no-proxy|false"                                  
        not-found="ignore|exception"                                 
        entity-name="EntityName"                                     
        formula="arbitrary SQL expression"                           
        node="element-name|@attribute-name|element/@attribute|."
        embed-xml="true|false"
        index="index_name"
        unique_key="unique_key_id"
        foreign-key="foreign_key_name"
/>

	name: the name of the property.
	column (optional): the name of the foreign key column. This can also be specified by nested<column> element(s).
	class (optional - defaults to the property type determined by reflection): the name of the associated class.
	cascade (optional): specifies which operations should be cascaded from the parent object to the associated object.
	fetch (optional - defaults to select): chooses between outer-join fetching or sequential select fetching.
	update, insert (optional - defaults to true): specifies that the mapped columns should be included in SQL UPDATE and/or INSERT statements. Setting both tofalse allows a pure "derived" association whose value is initialized from another property that maps to the same column(s), or by a trigger or other application.
	property-ref (optional): the name of a property of the associated class that is joined to this foreign key. If not specified, the primary key of the associated class is used.
	access (optional - defaults to property): the strategy Hibernate uses for accessing the property value.
	unique (optional): enables the DDL generation of a unique constraint for the foreign-key column. By allowing this to be the target of aproperty-ref, you can make the association multiplicity one-to-one.
	not-null (optional): enables the DDL generation of a nullability constraint for the foreign key columns.
	optimistic-lock (optional - defaults to true): specifies that updates to this property do or do not require acquisition of the optimistic lock. In other words, it determines if a version increment should occur when this property is dirty.
	lazy (optional - defaults to proxy): by default, single point associations are proxied. lazy="no-proxy" specifies that the property should be fetched lazily when the instance variable is first accessed. This requires build-time bytecode instrumentation.lazy="false" specifies that the association will always be eagerly fetched.
	not-found (optional - defaults to exception): specifies how foreign keys that reference missing rows will be handled.ignore will treat a missing row as a null association.
	entity-name (optional): the entity name of the associated class.
	formula (可選): SQL表達式，用於定義computed（計算出的）外鍵值。

Setting a value of the cascade attribute to any meaningful value other thannone will propagate certain operations to the associated object. The meaningful values are divided into three categories. First, basic operations, which include:persist, merge, delete, save-update, evict, replicate, lock and refresh; second, special values:delete-orphan; and third,all comma-separated combinations of operation names:cascade="persist,merge,evict" or cascade="all,delete-orphan". See

第 10.11 節 “傳播性持久化(transitive persistence)” for a full explanation. Note that single valued, many-to-one and one-to-one, associations do not support orphan delete.

Here is an example of a typical many-to-one declaration:

<many-to-one name="product" class="Product" column="PRODUCT_ID"/>

The property-ref attribute should only be used for mapping legacy data where a foreign key refers to a unique key of the associated table other than the primary key. This is a complicated and confusing relational model. For example, if the Product class had a unique serial number that is not the primary key. Theunique attribute controls Hibernate's DDL generation with the SchemaExport tool.

<property name="serialNumber" unique="true" type="string" column="SERIAL_NUMBER"/>

那麼關於OrderItem 的映射可能是：

<many-to-one name="product" property-ref="serialNumber" column="PRODUCT_SERIAL_NUMBER"/>

This is not encouraged, however.

如果被引用的唯一主鍵由關聯實體的多個屬性組成，你應該在名稱爲<properties>的元素 裏面映射所有關聯的屬性。

If the referenced unique key is the property of a component, you can specify a property path:

<many-to-one name="owner" property-ref="identity.ssn" column="OWNER_SSN"/>

5.1.13. One-to-one

持久化對象之間一對一的關聯關係是通過one-to-one元素定義的。

<one-to-one
        name="propertyName"                                          
        class="ClassName"                                            
        cascade="cascade_style"                                      
        constrained="true|false"                                     
        fetch="join|select"                                          
        property-ref="propertyNameFromAssociatedClass"               
        access="field|property|ClassName"                            
        formula="any SQL expression"                                 
        lazy="proxy|no-proxy|false"                                  
        entity-name="EntityName"                                     
        node="element-name|@attribute-name|element/@attribute|."
        embed-xml="true|false"
        foreign-key="foreign_key_name"
/>

	name: the name of the property.
	class (optional - defaults to the property type determined by reflection): the name of the associated class.
	cascade (optional): specifies which operations should be cascaded from the parent object to the associated object.
	constrained (optional): specifies that a foreign key constraint on the primary key of the mapped table and references the table of the associated class. This option affects the order in whichsave() and delete() are cascaded, and determines whether the association can be proxied. It is also used by the schema export tool.
	fetch (optional - defaults to select): chooses between outer-join fetching or sequential select fetching.
	property-ref (optional): the name of a property of the associated class that is joined to the primary key of this class. If not specified, the primary key of the associated class is used.
	access (optional - defaults to property): the strategy Hibernate uses for accessing the property value.
	formula (optional): almost all one-to-one associations map to the primary key of the owning entity. If this is not the case, you can specify another column, columns or expression to join on using an SQL formula. Seeorg.hibernate.test.onetooneformula for an example.
	lazy (optional - defaults to proxy): by default, single point associations are proxied. lazy="no-proxy" specifies that the property should be fetched lazily when the instance variable is first accessed. It requires build-time bytecode instrumentation.lazy="false" specifies that the association will always be eagerly fetched.Note that if constrained="false", proxying is impossible and Hibernate will eagerly fetch the association.
	entity-name (optional): the entity name of the associated class.

There are two varieties of one-to-one associations:

• 主鍵關聯

• 惟一外鍵關聯

Primary key associations do not need an extra table column. If two rows are related by the association, then the two table rows share the same primary key value. To relate two objects by a primary key association, ensure that they are assigned the same identifier value.

For a primary key association, add the following mappings to Employee and Person respectively:

<one-to-one name="person" class="Person"/>

<one-to-one name="employee" class="Employee" constrained="true"/>

Ensure that the primary keys of the related rows in the PERSON and EMPLOYEE tables are equal. You use a special Hibernate identifier generation strategy calledforeign:

<class name="person" table="PERSON">
    <id name="id" column="PERSON_ID">
        <generator class="foreign">
            <param name="property">employee</param>
        </generator>
    </id>
    ...
    <one-to-one name="employee"
        class="Employee"
        constrained="true"/>
</class>

A newly saved instance of Person is assigned the same primary key value as theEmployee instance referred with the employee property of that Person.

Alternatively, a foreign key with a unique constraint, from Employee to Person, can be expressed as:

<many-to-one name="person" class="Person" column="PERSON_ID" unique="true"/>

This association can be made bidirectional by adding the following to the Person mapping:

<one-to-one name="employee" class="Employee" property-ref="person"/>

5.1.14. Natural-id

<natural-id mutable="true|false"/>
        <property ... />
        <many-to-one ... />
        ......
</natural-id>

Although we recommend the use of surrogate keys as primary keys, you should try to identify natural keys for all entities. A natural key is a property or combination of properties that is unique and non-null. It is also immutable. Map the properties of the natural key inside the <natural-id> element. Hibernate will generate the necessary unique key and nullability constraints and, as a result, your mapping will be more self-documenting.

It is recommended that you implement equals() andhashCode() to compare the natural key properties of the entity.

This mapping is not intended for use with entities that have natural primary keys.

• mutable (optional - defaults to false): by default, natural identifier properties are assumed to be immutable (constant).

5.1.15. Component and dynamic-component

The <component> element maps properties of a child object to columns of the table of a parent class. Components can, in turn, declare their own properties, components or collections. See the "Component" examples below:

<component
        name="propertyName"                 
        class="className"                   
        insert="true|false"                 
        update="true|false"                 
        access="field|property|ClassName"   
        lazy="true|false"                   
        optimistic-lock="true|false"        
        unique="true|false"                 
        node="element-name|."
>

        <property ...../>
        <many-to-one .... />
        ........
</component>

	name: the name of the property.
	class (optional - defaults to the property type determined by reflection): the name of the component (child) class.
	insert: do the mapped columns appear in SQL INSERTs?
	update: do the mapped columns appear in SQL UPDATEs?
	access (optional - defaults to property): the strategy Hibernate uses for accessing the property value.
	lazy (optional - defaults to false): specifies that this component should be fetched lazily when the instance variable is first accessed. It requires build-time bytecode instrumentation.
	optimistic-lock (optional - defaults to true): specifies that updates to this component either do or do not require acquisition of the optimistic lock. It determines if a version increment should occur when this property is dirty.
	unique (optional - defaults to false): specifies that a unique constraint exists upon all mapped columns of the component.

其<property>子標籤爲子類的一些屬性與表字段之間建立映射。

<component>元素允許加入一個<parent>子元素，在組件類內部就可以有一個指向其容器的實體的反向引用。

The <dynamic-component> element allows a Map to be mapped as a component, where the property names refer to keys of the map. See

第 8.5 節 “動態組件 （Dynamic components）” for more information.

5.1.16. Properties

The <properties> element allows the definition of a named, logical grouping of the properties of a class. The most important use of the construct is that it allows a combination of properties to be the target of aproperty-ref. It is also a convenient way to define a multi-column unique constraint. For example:

<properties
        name="logicalName"                  
        insert="true|false"                 
        update="true|false"                 
        optimistic-lock="true|false"        
        unique="true|false"                 
>

        <property ...../>
        <many-to-one .... />
        ........
</properties>

	name: the logical name of the grouping. It is not an actual property name.
	insert: do the mapped columns appear in SQL INSERTs?
	update: do the mapped columns appear in SQL UPDATEs?
	optimistic-lock (optional - defaults to true): specifies that updates to these properties either do or do not require acquisition of the optimistic lock. It determines if a version increment should occur when these properties are dirty.
	unique (optional - defaults to false): specifies that a unique constraint exists upon all mapped columns of the component.

例如，如果我們有如下的<properties>映射:

<class name="Person">
    <id name="personNumber"/>

    ...
    <properties name="name"
            unique="true" update="false">
        <property name="firstName"/>
        <property name="initial"/>
        <property name="lastName"/>
    </properties>
</class>

You might have some legacy data association that refers to this unique key of thePerson table, instead of to the primary key:

<many-to-one name="person"
         class="Person" property-ref="name">
    <column name="firstName"/>
    <column name="initial"/>
    <column name="lastName"/>
</many-to-one>

The use of this outside the context of mapping legacy data is not recommended.

5.1.17. Subclass

Polymorphic persistence requires the declaration of each subclass of the root persistent class. For the table-per-class-hierarchy mapping strategy, the<subclass> declaration is used. For example:

<subclass
        name="ClassName"                              
        discriminator-value="discriminator_value"     
        proxy="ProxyInterface"                        
        lazy="true|false"                             
        dynamic-update="true|false"
        dynamic-insert="true|false"
        entity-name="EntityName"
        node="element-name"
        extends="SuperclassName">

        <property .... />
        .....
</subclass>

	name: the fully qualified class name of the subclass.
	discriminator-value (optional - defaults to the class name): a value that distinguishes individual subclasses.
	proxy (optional): specifies a class or interface used for lazy initializing proxies.
	lazy (optional - defaults to true): setting lazy="false" disables the use of lazy fetching.

Each subclass declares its own persistent properties and subclasses. <version> and <id> properties are assumed to be inherited from the root class. Each subclass in a hierarchy must define a uniquediscriminator-value. If this is not specified, the fully qualified Java class name is used.

For information about inheritance mappings see

第 9 章Inheritance mapping.

5.1.18. Joined-subclass

Each subclass can also be mapped to its own table. This is called the table-per-subclass mapping strategy. An inherited state is retrieved by joining with the table of the superclass. To do this you use the<joined-subclass> element. For example:

<joined-subclass
        name="ClassName"                    
        table="tablename"                   
        proxy="ProxyInterface"              
        lazy="true|false"                   
        dynamic-update="true|false"
        dynamic-insert="true|false"
        schema="schema"
        catalog="catalog"
        extends="SuperclassName"
        persister="ClassName"
        subselect="SQL expression"
        entity-name="EntityName"
        node="element-name">

        <key .... >

        <property .... />
        .....
</joined-subclass>

	name: the fully qualified class name of the subclass.
	table: the name of the subclass table.
	proxy (optional): specifies a class or interface to use for lazy initializing proxies.
	lazy (optional, defaults to true): setting lazy="false" disables the use of lazy fetching.

A discriminator column is not required for this mapping strategy. Each subclass must, however, declare a table column holding the object identifier using the<key> element. The mapping at the start of the chapter would then be re-written as:

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD//EN"
        "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping package="eg">

        <class name="Cat" table="CATS">
                <id name="id" column="uid" type="long">
                        <generator class="hilo"/>
                </id>
                <property name="birthdate" type="date"/>
                <property name="color" not-null="true"/>
                <property name="sex" not-null="true"/>
                <property name="weight"/>
                <many-to-one name="mate"/>
                <set name="kittens">
                        <key column="MOTHER"/>
                        <one-to-many class="Cat"/>
                </set>
                <joined-subclass name="DomesticCat" table="DOMESTIC_CATS">
                    <key column="CAT"/>
                    <property name="name" type="string"/>
                </joined-subclass>
        </class>

        <class name="eg.Dog">
                <!-- mapping for Dog could go here -->
        </class>

</hibernate-mapping>

For information about inheritance mappings see

第 9 章Inheritance mapping.

5.1.19. Union-subclass

A third option is to map only the concrete classes of an inheritance hierarchy to tables. This is called the table-per-concrete-class strategy. Each table defines all persistent states of the class, including the inherited state. In Hibernate, it is not necessary to explicitly map such inheritance hierarchies. You can map each class with a separate<class> declaration. However, if you wish use polymorphic associations (e.g. an association to the superclass of your hierarchy), you need to use the<union-subclass> mapping. For example:

<union-subclass
        name="ClassName"                    
        table="tablename"                   
        proxy="ProxyInterface"              
        lazy="true|false"                   
        dynamic-update="true|false"
        dynamic-insert="true|false"
        schema="schema"
        catalog="catalog"
        extends="SuperclassName"
        abstract="true|false"
        persister="ClassName"
        subselect="SQL expression"
        entity-name="EntityName"
        node="element-name">

        <property .... />
        .....
</union-subclass>

	name: the fully qualified class name of the subclass.
	table: the name of the subclass table.
	proxy (optional): specifies a class or interface to use for lazy initializing proxies.
	lazy (optional, defaults to true): setting lazy="false" disables the use of lazy fetching.

這種映射策略不需要指定辨別標誌(discriminator)字段。

For information about inheritance mappings see

第 9 章Inheritance mapping.

5.1.20. Join

Using the <join> element, it is possible to map properties of one class to several tables that have a one-to-one relationship. For example:

<join
        table="tablename"                        
        schema="owner"                           
        catalog="catalog"                        
        fetch="join|select"                      
        inverse="true|false"                     
        optional="true|false">                   

        <key ... />

        <property ... />
        ...
</join>

	table: the name of the joined table.
	schema (optional): overrides the schema name specified by the root<hibernate-mapping> element.
	catalog (optional): overrides the catalog name specified by the root<hibernate-mapping> element.
	fetch (optional - defaults to join): if set to join, the default, Hibernate will use an inner join to retrieve a<join> defined by a class or its superclasses. It will use an outer join for a<join> defined by a subclass. If set to select then Hibernate will use a sequential select for a <join> defined on a subclass. This will be issued only if a row represents an instance of the subclass. Inner joins will still be used to retrieve a<join> defined by the class and its superclasses.
	inverse (optional - defaults to false): if enabled, Hibernate will not insert or update the properties defined by this join.
	optional (optional - defaults to false): if enabled, Hibernate will insert a row only if the properties defined by this join are non-null. It will always use an outer join to retrieve the properties.

For example, address information for a person can be mapped to a separate table while preserving value type semantics for all properties:

<class name="Person"
    table="PERSON">

    <id name="id" column="PERSON_ID">...</id>

    <join table="ADDRESS">
        <key column="ADDRESS_ID"/>
        <property name="address"/>
        <property name="zip"/>
        <property name="country"/>
    </join>
    ...

This feature is often only useful for legacy data models. We recommend fewer tables than classes and a fine-grained domain model. However, it is useful for switching between inheritance mapping strategies in a single hierarchy, as explained later.

5.1.21. Key

The <key> element has featured a few times within this guide. It appears anywhere the parent mapping element defines a join to a new table that references the primary key of the original table. It also defines the foreign key in the joined table:

<key
        column="columnname"                      
        on-delete="noaction|cascade"             
        property-ref="propertyName"              
        not-null="true|false"                    
        update="true|false"                      
        unique="true|false"                      
/>

	column (optional): the name of the foreign key column. This can also be specified by nested<column> element(s).
	on-delete (optional - defaults to noaction): specifies whether the foreign key constraint has database-level cascade delete enabled.
	property-ref (optional): specifies that the foreign key refers to columns that are not the primary key of the original table. It is provided for legacy data.
	not-null (optional): specifies that the foreign key columns are not nullable. This is implied whenever the foreign key is also part of the primary key.
	update (optional): specifies that the foreign key should never be updated. This is implied whenever the foreign key is also part of the primary key.
	unique (optional): specifies that the foreign key should have a unique constraint. This is implied whenever the foreign key is also the primary key.

For systems where delete performance is important, we recommend that all keys should be definedon-delete="cascade". Hibernate uses a database-levelON CASCADE DELETE constraint, instead of many individualDELETE statements. Be aware that this feature bypasses Hibernate's usual optimistic locking strategy for versioned data.

The not-null and update attributes are useful when mapping a unidirectional one-to-many association. If you map a unidirectional one-to-many association to a non-nullable foreign key, youmust declare the key column using <key not-null="true">.

5.1.22. Column and formula elements

Mapping elements which accept a column attribute will alternatively accept a<column> subelement. Likewise, <formula> is an alternative to the formula attribute. For example:

<column
        name="column_name"
        length="N"
        precision="N"
        scale="N"
        not-null="true|false"
        unique="true|false"
        unique-key="multicolumn_unique_key_name"
        index="index_name"
        sql-type="sql_type_name"
        check="SQL expression"
        default="SQL expression"/>

<formula>SQL expression</formula>

column and formula attributes can even be combined within the same property or association mapping to express, for example, exotic join conditions.

<many-to-one name="homeAddress" class="Address"
        insert="false" update="false">
    <column name="person_id" not-null="true" length="10"/>
    <formula>'MAILING'</formula>
</many-to-one>

5.1.23. Import

If your application has two persistent classes with the same name, and you do not want to specify the fully qualified package name in Hibernate queries, classes can be "imported" explicitly, rather than relying uponauto-import="true". You can also import classes and interfaces that are not explicitly mapped:

<import class="java.lang.Object" rename="Universe"/>

<import
        class="ClassName"              
        rename="ShortName"             
/>

	class: the fully qualified class name of any Java class.
	rename (optional - defaults to the unqualified class name): a name that can be used in the query language.

5.1.24. Any

There is one more type of property mapping. The <any> mapping element defines a polymorphic association to classes from multiple tables. This type of mapping requires more than one column. The first column contains the type of the associated entity. The remaining columns contain the identifier. It is impossible to specify a foreign key constraint for this kind of association. This is not the usual way of mapping polymorphic associations and you should use this only in special cases. For example, for audit logs, user session data, etc.

The meta-type attribute allows the application to specify a custom type that maps database column values to persistent classes that have identifier properties of the type specified byid-type. You must specify the mapping from values of the meta-type to class names.

<any name="being" id-type="long" meta-type="string">
    <meta-value value="TBL_ANIMAL" class="Animal"/>
    <meta-value value="TBL_HUMAN" class="Human"/>
    <meta-value value="TBL_ALIEN" class="Alien"/>
    <column name="table_name"/>
    <column name="id"/>
</any>

<any
        name="propertyName"                      
        id-type="idtypename"                     
        meta-type="metatypename"                 
        cascade="cascade_style"                  
        access="field|property|ClassName"        
        optimistic-lock="true|false"             
>
        <meta-value ... />
        <meta-value ... />
        .....
        <column .... />
        <column .... />
        .....
</any>

	name: 屬性名
	id-type: 標識符類型
	meta-type (optional - defaults to string): any type that is allowed for a discriminator mapping.
	cascade (可選 -默認是none): 級聯的類型
	access (optional - defaults to property): the strategy Hibernate uses for accessing the property value.
	optimistic-lock (optional - defaults to true): specifies that updates to this property either do or do not require acquisition of the optimistic lock. It defines whether a version increment should occur if this property is dirty.

5.2. Hibernate types

5.2.1. 實體(Entities)和值(values)

In relation to the persistence service, Java language-level objects are classified into two groups:

An entity exists independently of any other objects holding references to the entity. Contrast this with the usual Java model, where an unreferenced object is garbage collected. Entities must be explicitly saved and deleted. Saves and deletions, however, can be cascaded from a parent entity to its children. This is different from the ODMG model of object persistence by reachability and corresponds more closely to how application objects are usually used in large systems. Entities support circular and shared references. They can also be versioned.

An entity's persistent state consists of references to other entities and instances ofvalue types. Values are primitives: collections (not what is inside a collection), components and certain immutable objects. Unlike entities, values in particular collections and components,are persisted and deleted by reachability. Since value objects and primitives are persisted and deleted along with their containing entity, they cannot be independently versioned. Values have no independent identity, so they cannot be shared by two entities or collections.

Until now, we have been using the term "persistent class" to refer to entities. We will continue to do that. Not all user-defined classes with a persistent state, however, are entities. Acomponent is a user-defined class with value semantics. A Java property of typejava.lang.String also has value semantics. Given this definition, all types (classes) provided by the JDK have value type semantics in Java, while user-defined types can be mapped with entity or value type semantics. This decision is up to the application developer. An entity class in a domain model will normally have shared references to a single instance of that class, while composition or aggregation usually translates to a value type.

We will revisit both concepts throughout this reference guide.

The challenge is to map the Java type system, and the developers' definition of entities and value types, to the SQL/database type system. The bridge between both systems is provided by Hibernate. For entities,<class>, <subclass> and so on are used. For value types we use<property>, <component>etc., that usually have atype attribute. The value of this attribute is the name of a Hibernatemapping type. Hibernate provides a range of mappings for standard JDK value types out of the box. You can write your own mapping types and implement your own custom conversion strategies.

With the exception of collections, all built-in Hibernate types support null semantics.

5.2.2. 基本值類型

The built-in basic mapping types can be roughly categorized into the following:

integer, long, short, float, double, character, byte, boolean, yes_no, true_false

這些類型都對應Java的原始類型或者其封裝類，來符合(特定廠商的)SQL 字段類型。boolean, yes_no 和true_false都是Java 中boolean 或者java.lang.Boolean的另外說法。

string

從java.lang.String 到 VARCHAR (或者 Oracle的VARCHAR2)的映射。

date, time, timestamp

從java.util.Date和其子類到SQL類型DATE,TIME 和TIMESTAMP (或等價類型)的映射。

calendar, calendar_date

從java.util.Calendar 到SQL 類型TIMESTAMP和DATE(或等價類型)的映射。

big_decimal, big_integer

從java.math.BigDecimal和java.math.BigInteger到NUMERIC (或者 Oracle 的NUMBER類型)的映射。

locale, timezone, currency

從java.util.Locale, java.util.TimeZone 和java.util.Currency 到VARCHAR (或者 Oracle 的VARCHAR2類型)的映射.Locale和 Currency 的實例被映射爲它們的ISO代碼。TimeZone的實例被影射爲它的ID。

class

從java.lang.Class 到 VARCHAR (或者 Oracle 的VARCHAR2類型)的映射。Class被映射爲它的全限定名。

binary

把字節數組(byte arrays)映射爲對應的 SQL二進制類型。

text

把長Java字符串映射爲SQL的CLOB或者TEXT類型。

serializable

Maps serializable Java types to an appropriate SQL binary type. You can also indicate the Hibernate typeserializable with the name of a serializable Java class or interface that does not default to a basic type.

clob, blob

Type mappings for the JDBC classes java.sql.Clob andjava.sql.Blob. These types can be inconvenient for some applications, since the blob or clob object cannot be reused outside of a transaction. Driver support is patchy and inconsistent.

imm_date, imm_time, imm_timestamp, imm_calendar, imm_calendar_date, imm_serializable, imm_binary

Type mappings for what are considered mutable Java types. This is where Hibernate makes certain optimizations appropriate only for immutable Java types, and the application treats the object as immutable. For example, you should not callDate.setTime() for an instance mapped as imm_timestamp. To change the value of the property, and have that change made persistent, the application must assign a new, nonidentical, object to the property.

Unique identifiers of entities and collections can be of any basic type exceptbinary, blob and clob. Composite identifiers are also allowed. See below for more information.

在org.hibernate.Hibernate中，定義了基礎類型對應的Type常量。比如，Hibernate.STRING代表string 類型。

5.2.3. 自定義值類型

It is relatively easy for developers to create their own value types. For example, you might want to persist properties of typejava.lang.BigInteger to VARCHAR columns. Hibernate does not provide a built-in type for this. Custom types are not limited to mapping a property, or collection element, to a single table column. So, for example, you might have a Java property getName()/setName() of typejava.lang.String that is persisted to the columns FIRST_NAME, INITIAL, SURNAME.

To implement a custom type, implement either org.hibernate.UserType ororg.hibernate.CompositeUserType and declare properties using the fully qualified classname of the type. Vieworg.hibernate.test.DoubleStringType to see the kind of things that are possible.

<property name="twoStrings" type="org.hibernate.test.DoubleStringType">
    <column name="first_string"/>
    <column name="second_string"/>
</property>

注意使用<column>標籤來把一個屬性映射到多個字段的做法。

CompositeUserType, EnhancedUserType,UserCollectionType, 和 UserVersionType 接口爲更特殊的使用方式提供支持。

You can even supply parameters to a UserType in the mapping file. To do this, yourUserType must implement the org.hibernate.usertype.ParameterizedType interface. To supply parameters to your custom type, you can use the<type> element in your mapping files.

<property name="priority">
    <type name="com.mycompany.usertypes.DefaultValueIntegerType">
        <param name="default">0</param>
    </type>
</property>

現在，UserType 可以從傳入的Properties對象中得到default 參數的值。

If you regularly use a certain UserType, it is useful to define a shorter name for it. You can do this using the<typedef> element. Typedefs assign a name to a custom type, and can also contain a list of default parameter values if the type is parameterized.

<typedef class="com.mycompany.usertypes.DefaultValueIntegerType" name="default_zero">
    <param name="default">0</param>
</typedef>

<property name="priority" type="default_zero"/>

也可以根據具體案例通過屬性映射中的類型參數覆蓋在typedef中提供的參數。

Even though Hibernate's rich range of built-in types and support for components means you will rarely need to use a custom type, it is considered good practice to use custom types for non-entity classes that occur frequently in your application. For example, a MonetaryAmount class is a good candidate for a CompositeUserType, even though it could be mapped as a component. One reason for this is abstraction. With a custom type, your mapping documents would be protected against changes to the way monetary values are represented.

5.3. 多次映射同一個類

It is possible to provide more than one mapping for a particular persistent class. In this case, you must specify anentity name to disambiguate between instances of the two mapped entities. By default, the entity name is the same as the class name. Hibernate lets you specify the entity name when working with persistent objects, when writing queries, or when mapping associations to the named entity.

<class name="Contract" table="Contracts"
        entity-name="CurrentContract">
    ...
    <set name="history" inverse="true"
            order-by="effectiveEndDate desc">
        <key column="currentContractId"/>
        <one-to-many entity-name="HistoricalContract"/>
    </set>
</class>

<class name="Contract" table="ContractHistory"
        entity-name="HistoricalContract">
    ...
    <many-to-one name="currentContract"
            column="currentContractId"
            entity-name="CurrentContract"/>
</class>

Associations are now specified using entity-name instead ofclass.

5.4. SQL中引號包圍的標識符

You can force Hibernate to quote an identifier in the generated SQL by enclosing the table or column name in backticks in the mapping document. Hibernate will use the correct quotation style for the SQLDialect. This is usually double quotes, but the SQL Server uses brackets and MySQL uses backticks.

<class name="LineItem" table="`Line Item`">
    <id name="id" column="`Item Id`"/><generator class="assigned"/></id>
    <property name="itemNumber" column="`Item #`"/>
    ...
</class>

5.5. 其他元數據(Metadata)

XML does not suit all users so there are some alternative ways to define O/R mapping metadata in Hibernate.

5.5.1. 使用 XDoclet 標記

Many Hibernate users prefer to embed mapping information directly in sourcecode using [e-mail]. We do not cover this approach in this reference guide since it is considered part of XDoclet. However, we include the following example of theCat class with XDoclet mappings:

package eg;
import java.util.Set;
import java.util.Date;

/**
 * @hibernate.class
 *  table="CATS"
 */
public class Cat {
    private Long id; // identifier
    private Date birthdate;
    private Cat mother;
    private Set kittens
    private Color color;
    private char sex;
    private float weight;

    /*
     * @hibernate.id
     *  generator-class="native"
     *  column="CAT_ID"
     */
    public Long getId() {
        return id;
    }
    private void setId(Long id) {
        this.id=id;
    }

    /**
     * @hibernate.many-to-one
     *  column="PARENT_ID"
     */
    public Cat getMother() {
        return mother;
    }
    void setMother(Cat mother) {
        this.mother = mother;
    }

    /**
     * @hibernate.property
     *  column="BIRTH_DATE"
     */
    public Date getBirthdate() {
        return birthdate;
    }
    void setBirthdate(Date date) {
        birthdate = date;
    }
    /**
     * @hibernate.property
     *  column="WEIGHT"
     */
    public float getWeight() {
        return weight;
    }
    void setWeight(float weight) {
        this.weight = weight;
    }

    /**
     * @hibernate.property
     *  column="COLOR"
     *  not-null="true"
     */
    public Color getColor() {
        return color;
    }
    void setColor(Color color) {
        this.color = color;
    }
    /**
     * @hibernate.set
     *  inverse="true"
     *  order-by="BIRTH_DATE"
     * @hibernate.collection-key
     *  column="PARENT_ID"
     * @hibernate.collection-one-to-many
     */
    public Set getKittens() {
        return kittens;
    }
    void setKittens(Set kittens) {
        this.kittens = kittens;
    }
    // addKitten not needed by Hibernate
    public void addKitten(Cat kitten) {
        kittens.add(kitten);
    }

    /**
     * @hibernate.property
     *  column="SEX"
     *  not-null="true"
     *  update="false"
     */
    public char getSex() {
        return sex;
    }
    void setSex(char sex) {
        this.sex=sex;
    }
}

See the Hibernate website for more examples of XDoclet and Hibernate.

5.5.2. 使用 JDK 5.0 的註解(Annotation)

JDK 5.0 introduced XDoclet-style annotations at the language level that are type-safe and checked at compile time. This mechanism is more powerful than XDoclet annotations and better supported by tools and IDEs. IntelliJ IDEA, for example, supports auto-completion and syntax highlighting of JDK 5.0 annotations. The new revision of the EJB specification (JSR-220) uses JDK 5.0 annotations as the primary metadata mechanism for entity beans. Hibernate3 implements theEntityManager of JSR-220 (the persistence API). Support for mapping metadata is available via theHibernate Annotations package as a separate download. Both EJB3 (JSR-220) and Hibernate3 metadata is supported.

這是一個被註解爲EJB entity bean 的POJO類的例子

@Entity(access = AccessType.FIELD)
public class Customer implements Serializable {

    @Id;
    Long id;

    String firstName;
    String lastName;
    Date birthday;

    @Transient
    Integer age;

    @Embedded
    private Address homeAddress;

    @OneToMany(cascade=CascadeType.ALL)
    @JoinColumn(name="CUSTOMER_ID")
    Set<Order> orders;

    // Getter/setter and business methods
}

Note

Support for JDK 5.0 Annotations (and JSR-220) is currently under development. Please refer to the Hibernate Annotations module for more details.

5.6. Generated properties

Generated properties are properties that have their values generated by the database. Typically, Hibernate applications needed torefresh objects that contain any properties for which the database was generating values. Marking properties as generated, however, lets the application delegate this responsibility to Hibernate. When Hibernate issues an SQL INSERT or UPDATE for an entity that has defined generated properties, it immediately issues a select afterwards to retrieve the generated values.

Properties marked as generated must additionally be non-insertable and non-updateable. Only

versions,timestamps, and simple properties, can be marked as generated.

never (the default): the given property value is not generated within the database.

insert: the given property value is generated on insert, but is not regenerated on subsequent updates. Properties like created-date fall into this category. Even thoughversion and timestamp properties can be marked as generated, this option is not available.

always: the property value is generated both on insert and on update.

5.7. Auxiliary database objects

Auxiliary database objects allow for the CREATE and DROP of arbitrary database objects. In conjunction with Hibernate's schema evolution tools, they have the ability to fully define a user schema within the Hibernate mapping files. Although designed specifically for creating and dropping things like triggers or stored procedures, any SQL command that can be run via ajava.sql.Statement.execute() method is valid (for example, ALTERs, INSERTS, etc.). There are essentially two modes for defining auxiliary database objects:

The first mode is to explicitly list the CREATE and DROP commands in the mapping file:

<hibernate-mapping>
    ...
    <database-object>
        <create>CREATE TRIGGER my_trigger ...</create>
        <drop>DROP TRIGGER my_trigger</drop>
    </database-object>
</hibernate-mapping>

The second mode is to supply a custom class that constructs the CREATE and DROP commands. This custom class must implement theorg.hibernate.mapping.AuxiliaryDatabaseObject interface.

<hibernate-mapping>
    ...
    <database-object>
        <definition class="MyTriggerDefinition"/>
    </database-object>
</hibernate-mapping>

Additionally, these database objects can be optionally scoped so that they only apply when certain dialects are used.

<hibernate-mapping>
    ...
    <database-object>
        <definition class="MyTriggerDefinition"/>
        <dialect-scope name="org.hibernate.dialect.Oracle9iDialect"/>
        <dialect-scope name="org.hibernate.dialect.Oracle10gDialect"/>
    </database-object>
</hibernate-mapping>

第 6 章 Collection mapping

6.1. 持久化集合類(Persistent collections)

Hibernate requires that persistent collection-valued fields be declared as an interface type. For example:

public class Product {
    private String serialNumber;
    private Set parts = new HashSet();
    
    public Set getParts() { return parts; }
    void setParts(Set parts) { this.parts = parts; }
    public String getSerialNumber() { return serialNumber; }
    void setSerialNumber(String sn) { serialNumber = sn; }
}

The actual interface might be java.util.Set, java.util.Collection, java.util.List, java.util.Map, java.util.SortedSet, java.util.SortedMap or anything you like ("anything you like" means you will have to write an implementation oforg.hibernate.usertype.UserCollectionType.)

Notice how the instance variable was initialized with an instance of HashSet. This is the best way to initialize collection valued properties of newly instantiated (non-persistent) instances. When you make the instance persistent, by callingpersist() for example, Hibernate will actually replace theHashSet with an instance of Hibernate's own implementation ofSet. Be aware of the following errors:

Cat cat = new DomesticCat();
Cat kitten = new DomesticCat();
....
Set kittens = new HashSet();
kittens.add(kitten);
cat.setKittens(kittens);
session.persist(cat);
kittens = cat.getKittens(); // Okay, kittens collection is a Set
(HashSet) cat.getKittens(); // Error!

The persistent collections injected by Hibernate behave like HashMap, HashSet, TreeMap,TreeSet or ArrayList, depending on the interface type.

Collections instances have the usual behavior of value types. They are automatically persisted when referenced by a persistent object and are automatically deleted when unreferenced. If a collection is passed from one persistent object to another, its elements might be moved from one table to another. Two entities cannot share a reference to the same collection instance. Due to the underlying relational model, collection-valued properties do not support null value semantics. Hibernate does not distinguish between a null collection reference and an empty collection.

Use persistent collections the same way you use ordinary Java collections. However, please ensure you understand the semantics of bidirectional associations (these are discussed later).

6.2. 集合映射（ Collection mappings ）

提示

There are quite a range of mappings that can be generated for collections that cover many common relational models. We suggest you experiment with the schema generation tool so that you understand how various mapping declarations translate to database tables.

The Hibernate mapping element used for mapping a collection depends upon the type of interface. For example, a<set> element is used for mapping properties of typeSet.

<class name="Product">
    <id name="serialNumber" column="productSerialNumber"/>
    <set name="parts">
        <key column="productSerialNumber" not-null="true"/>
        <one-to-many class="Part"/>
    </set>
</class>

除了<set>,還有<list>, <map>, <bag>, <array> 和<primitive-array> 映射元素。<map>具有代表性：

<map
    name="propertyName"                                         
    table="table_name"                                          
    schema="schema_name"                                        
    lazy="true|extra|false"                                     
    inverse="true|false"                                        
    cascade="all|none|save-update|delete|all-delete-orphan|delete-orphan"
    sort="unsorted|natural|comparatorClass"                     
    order-by="column_name asc|desc"                             
    where="arbitrary sql where condition"                       
    fetch="join|select|subselect"                               
    batch-size="N"                                              
    access="field|property|ClassName"                           
    optimistic-lock="true|false"                                
    mutable="true|false"                                        
    node="element-name|."
    embed-xml="true|false"
>

    <key .... />
    <map-key .... />
    <element .... />
</map>

	name: the collection property name
	table (optional - defaults to property name): the name of the collection table. It is not used for one-to-many associations.
	schema (optional): the name of a table schema to override the schema declared on the root element
	lazy (optional - defaults to true): disables lazy fetching and specifies that the association is always eagerly fetched. It can also be used to enable "extra-lazy" fetching where most operations do not initialize the collection. This is suitable for large collections.
	inverse (optional - defaults to false): marks this collection as the "inverse" end of a bidirectional association.
	cascade (optional - defaults to none): enables operations to cascade to child entities.
	sort (optional): specifies a sorted collection withnatural sort order or a given comparator class.
	order-by (optional, JDK1.4 only): specifies a table column or columns that define the iteration order of theMap, Set or bag, together with an optionalasc or desc.
	where (optional): specifies an arbitrary SQL WHERE condition that is used when retrieving or removing the collection. This is useful if the collection needs to contain only a subset of the available data.
	fetch (optional, defaults to select): chooses between outer-join fetching, fetching by sequential select, and fetching by sequential subselect.
	batch-size (optional, defaults to 1): specifies a "batch size" for lazily fetching instances of this collection.
	access (optional - defaults to property): the strategy Hibernate uses for accessing the collection property value.
	optimistic-lock (optional - defaults to true): specifies that changes to the state of the collection results in increments of the owning entity's version. For one-to-many associations you may want to disable this setting.
	mutable (optional - defaults to true): a value of false specifies that the elements of the collection never change. This allows for minor performance optimization in some cases.

6.2.1. 集合外鍵(Collection foreign keys)

Collection instances are distinguished in the database by the foreign key of the entity that owns the collection. This foreign key is referred to as thecollection key column, or columns, of the collection table. The collection key column is mapped by the<key> element.

There can be a nullability constraint on the foreign key column. For most collections, this is implied. For unidirectional one-to-many associations, the foreign key column is nullable by default, so you may need to specifynot-null="true".

<key column="productSerialNumber" not-null="true"/>

The foreign key constraint can use ON DELETE CASCADE.

<key column="productSerialNumber" on-delete="cascade"/>

對<key> 元素的完整定義，請參閱前面的章節。

6.2.2. 集合元素（Collection elements）

Collections can contain almost any other Hibernate type, including: basic types, custom types, components and references to other entities. This is an important distinction. An object in a collection might be handled with "value" semantics (its life cycle fully depends on the collection owner), or it might be a reference to another entity with its own life cycle. In the latter case, only the "link" between the two objects is considered to be a state held by the collection.

被包容的類型被稱爲集合元素類型（collection element type）。集合元素通過<element>或<composite-element>映射，或在其是實體引用的時候，通過<one-to-many> 或<many-to-many>映射。前兩種用於使用值語義映射元素，後兩種用於映射實體關聯。

6.2.3. 索引集合類(Indexed collections)

All collection mappings, except those with set and bag semantics, need an index column in the collection table. An index column is a column that maps to an array index, orList index, or Map key. The index of aMap may be of any basic type, mapped with <map-key>. It can be an entity reference mapped with <map-key-many-to-many>, or it can be a composite type mapped with <composite-map-key>. The index of an array or list is always of type integer and is mapped using the <list-index> element. The mapped column contains sequential integers that are numbered from zero by default.

<list-index 
        column="column_name"                
        base="0|1|..."/>

	column_name (required): the name of the column holding the collection index values.
	base (optional - defaults to 0): the value of the index column that corresponds to the first element of the list or array.

<map-key 
        column="column_name"                
        formula="any SQL expression"        
        type="type_name"                    
        node="@attribute-name"
        length="N"/>

	column (optional): the name of the column holding the collection index values.
	formula (optional): a SQL formula used to evaluate the key of the map.
	type (required): the type of the map keys.

<map-key-many-to-many
        column="column_name"                
        formula="any SQL expression"        
        class="ClassName"
/>

	column (optional): the name of the foreign key column for the collection index values.
	formula (optional): a SQ formula used to evaluate the foreign key of the map key.
	class (required): the entity class used as the map key.

If your table does not have an index column, and you still wish to use List as the property type, you can map the property as a Hibernate <bag>. A bag does not retain its order when it is retrieved from the database, but it can be optionally sorted or ordered.

6.2.4. 對於一個值集合, 我們使用<element>標籤。

Any collection of values or many-to-many associations requires a dedicated collection table with a foreign key column or columns, collection element column or columns, and possibly an index column or columns.

For a collection of values use the <element> tag. For example:

<element
        column="column_name"                     
        formula="any SQL expression"             
        type="typename"                          
        length="L"
        precision="P"
        scale="S"
        not-null="true|false"
        unique="true|false"
        node="element-name"
/>

	column (optional): the name of the column holding the collection element values.
	formula (optional): an SQL formula used to evaluate the element.
	type (required): the type of the collection element.

A many-to-many association is specified using the<many-to-many> element.

<many-to-many
        column="column_name"                               
        formula="any SQL expression"                       
        class="ClassName"                                  
        fetch="select|join"                                
        unique="true|false"                                
        not-found="ignore|exception"                       
        entity-name="EntityName"                           
        property-ref="propertyNameFromAssociatedClass"     
        node="element-name"
        embed-xml="true|false"
    />

	column (optional): the name of the element foreign key column.
	formula (optional): an SQL formula used to evaluate the element foreign key value.
	class (required): the name of the associated class.
	fetch (optional - defaults to join): enables outer-join or sequential select fetching for this association. This is a special case; for full eager fetching in a singleSELECT of an entity and its many-to-many relationships to other entities, you would enablejoin fetching,not only of the collection itself, but also with this attribute on the<many-to-many> nested element.
	unique (optional): enables the DDL generation of a unique constraint for the foreign-key column. This makes the association multiplicity effectively one-to-many.
	not-found (optional - defaults to exception): specifies how foreign keys that reference missing rows will be handled:ignore will treat a missing row as a null association.
	entity-name (optional): the entity name of the associated class, as an alternative toclass.
	property-ref (optional): the name of a property of the associated class that is joined to this foreign key. If not specified, the primary key of the associated class is used.

Here are some examples.

A set of strings:

<set name="names" table="person_names">
    <key column="person_id"/>
    <element column="person_name" type="string"/>
</set>

A bag containing integers with an iteration order determined by the order-by attribute:

<bag name="sizes" 
        table="item_sizes" 
        order-by="size asc">
    <key column="item_id"/>
    <element column="size" type="integer"/>
</bag>

An array of entities, in this case, a many-to-many association:

<array name="addresses" 
        table="PersonAddress" 
        cascade="persist">
    <key column="personId"/>
    <list-index column="sortOrder"/>
    <many-to-many column="addressId" class="Address"/>
</array>

一個組件的列表：（下一章討論）

<map name="holidays" 
        table="holidays" 
        schema="dbo" 
        order-by="hol_name asc">
    <key column="id"/>
    <map-key column="hol_name" type="string"/>
    <element column="hol_date" type="date"/>
</map>

A list of components (this is discussed in the next chapter):

<list name="carComponents" 
        table="CarComponents">
    <key column="carId"/>
    <list-index column="sortOrder"/>
    <composite-element class="CarComponent">
        <property name="price"/>
        <property name="type"/>
        <property name="serialNumber" column="serialNum"/>
    </composite-element>
</list>

6.2.5. 一對多關聯通過外鍵連接兩個類對應的表,而沒有中間集合表。 這個關係模型失去了一些Java集合的語義:

A one-to-many association links the tables of two classes via a foreign key with no intervening collection table. This mapping loses certain semantics of normal Java collections:

• An instance of the contained entity class cannot belong to more than one instance of the collection.

• An instance of the contained entity class cannot appear at more than one value of the collection index.

An association from Product to Part requires the existence of a foreign key column and possibly an index column to thePart table. A <one-to-many> tag indicates that this is a one-to-many association.

<one-to-many 
        class="ClassName"                                  
        not-found="ignore|exception"                       
        entity-name="EntityName"                           
        node="element-name"
        embed-xml="true|false"
    />

	class (required): the name of the associated class.
	not-found (optional - defaults to exception): specifies how cached identifiers that reference missing rows will be handled.ignore will treat a missing row as a null association.
	entity-name (optional): the entity name of the associated class, as an alternative toclass.

The <one-to-many> element does not need to declare any columns. Nor is it necessary to specify thetable name anywhere.

警告

If the foreign key column of a <one-to-many> association is declaredNOT NULL, you must declare the <key> mapping not-null="true" or use a bidirectional association with the collection mapping markedinverse="true". See the discussion of bidirectional associations later in this chapter for more information.

The following example shows a map of Part entities by name, wherepartName is a persistent property of Part. Notice the use of a formula-based index:

<map name="parts"
        cascade="all">
    <key column="productId" not-null="true"/>
    <map-key formula="partName"/>
    <one-to-many class="Part"/>
</map>

6.3. 高級集合映射（Advanced collection mappings）

6.3.1. 有序集合（Sorted collections）

Hibernate支持實現java.util.SortedMap和java.util.SortedSet的集合。 你必須在映射文件中指定一個比較器：

<set name="aliases" 
            table="person_aliases" 
            sort="natural">
    <key column="person"/>
    <element column="name" type="string"/>
</set>

<map name="holidays" sort="my.custom.HolidayComparator">
    <key column="year_id"/>
    <map-key column="hol_name" type="string"/>
    <element column="hol_date" type="date"/>
</map>

sort屬性中允許的值包括unsorted,natural和某個實現了java.util.Comparator的類的名稱。

分類集合的行爲事實上象java.util.TreeSet或者java.util.TreeMap。

If you want the database itself to order the collection elements, use the order-by attribute of set, bag or map mappings. This solution is only available under JDK 1.4 or higher and is implemented usingLinkedHashSet or LinkedHashMap. This performs the ordering in the SQL query and not in the memory.

<set name="aliases" table="person_aliases" order-by="lower(name) asc">
    <key column="person"/>
    <element column="name" type="string"/>
</set>

<map name="holidays" order-by="hol_date, hol_name">
    <key column="year_id"/>
    <map-key column="hol_name" type="string"/>
    <element column="hol_date type="date"/>
</map>

Note

The value of the order-by attribute is an SQL ordering, not an HQL ordering.

Associations can even be sorted by arbitrary criteria at runtime using a collectionfilter():

sortedUsers = s.createFilter( group.getUsers(), "order by this.name" ).list();

6.3.2. 雙向關聯（Bidirectional associations）

A bidirectional association allows navigation from both "ends" of the association. Two kinds of bidirectional association are supported:

一對多（one-to-many）

set or bag valued at one end and single-valued at the other

多對多（many-to-many）

兩端都是set或bag值

You can specify a bidirectional many-to-many association by mapping two many-to-many associations to the same database table and declaring one end asinverse. You cannot select an indexed collection.

Here is an example of a bidirectional many-to-many association that illustrates how each category can have many items and each item can be in many categories:

<class name="Category">
    <id name="id" column="CATEGORY_ID"/>
    ...
    <bag name="items" table="CATEGORY_ITEM">
        <key column="CATEGORY_ID"/>
        <many-to-many class="Item" column="ITEM_ID"/>
    </bag>
</class>

<class name="Item">
    <id name="id" column="ITEM_ID"/>
    ...

    <!-- inverse end -->
    <bag name="categories" table="CATEGORY_ITEM" inverse="true">
        <key column="ITEM_ID"/>
        <many-to-many class="Category" column="CATEGORY_ID"/>
    </bag>
</class>

Changes made only to the inverse end of the association are not persisted. This means that Hibernate has two representations in memory for every bidirectional association: one link from A to B and another link from B to A. This is easier to understand if you think about the Java object model and how a many-to-many relationship in Javais created:

category.getItems().add(item);          // The category now "knows" about the relationship
item.getCategories().add(category);     // The item now "knows" about the relationship

session.persist(item);                   // The relationship won't be saved!
session.persist(category);               // The relationship will be saved

非反向端用於把內存中的表示保存到數據庫中。

You can define a bidirectional one-to-many association by mapping a one-to-many association to the same table column(s) as a many-to-one association and declaring the many-valued endinverse="true".

<class name="Parent">
    <id name="id" column="parent_id"/>
    ....
    <set name="children" inverse="true">
        <key column="parent_id"/>
        <one-to-many class="Child"/>
    </set>
</class>

<class name="Child">
    <id name="id" column="child_id"/>
    ....
    <many-to-one name="parent" 
        class="Parent" 
        column="parent_id"
        not-null="true"/>
</class>

Mapping one end of an association with inverse="true" does not affect the operation of cascades as these are orthogonal concepts.

6.3.3. 雙向關聯，涉及有序集合類

A bidirectional association where one end is represented as a <list> or <map>, requires special consideration. If there is a property of the child class that maps to the index column you can useinverse="true" on the collection mapping:

<class name="Parent">
    <id name="id" column="parent_id"/>
    ....
    <map name="children" inverse="true">
        <key column="parent_id"/>
        <map-key column="name" 
            type="string"/>
        <one-to-many class="Child"/>
    </map>
</class>

<class name="Child">
    <id name="id" column="child_id"/>
    ....
    <property name="name" 
        not-null="true"/>
    <many-to-one name="parent" 
        class="Parent" 
        column="parent_id"
        not-null="true"/>
</class>

If there is no such property on the child class, the association cannot be considered truly bidirectional. That is, there is information available at one end of the association that is not available at the other end. In this case, you cannot map the collectioninverse="true". Instead, you could use the following mapping:

<class name="Parent">
    <id name="id" column="parent_id"/>
    ....
    <map name="children">
        <key column="parent_id"
            not-null="true"/>
        <map-key column="name" 
            type="string"/>
        <one-to-many class="Child"/>
    </map>
</class>

<class name="Child">
    <id name="id" column="child_id"/>
    ....
    <many-to-one name="parent" 
        class="Parent" 
        column="parent_id"
        insert="false"
        update="false"
        not-null="true"/>
</class>

Note that in this mapping, the collection-valued end of the association is responsible for updates to the foreign key.

6.3.4. 三重關聯（Ternary associations）

There are three possible approaches to mapping a ternary association. One approach is to use aMap with an association as its index:

<map name="contracts">
    <key column="employer_id" not-null="true"/>
    <map-key-many-to-many column="employee_id" class="Employee"/>
    <one-to-many class="Contract"/>
</map>

<map name="connections">
    <key column="incoming_node_id"/>
    <map-key-many-to-many column="outgoing_node_id" class="Node"/>
    <many-to-many column="connection_id" class="Connection"/>
</map>

A second approach is to remodel the association as an entity class. This is the most common approach.

A final alternative is to use composite elements, which will be discussed later.

6.3.5. 使用<idbag>

The majority of the many-to-many associations and collections of values shown previously all map to tables with composite keys, even though it has been have suggested that entities should have synthetic identifiers (surrogate keys). A pure association table does not seem to benefit much from a surrogate key, although a collection of composite valuesmight. It is for this reason that Hibernate provides a feature that allows you to map many-to-many associations and collections of values to a table with a surrogate key.

The <idbag> element lets you map a List (or Collection) with bag semantics. For example:

<idbag name="lovers" table="LOVERS">
    <collection-id column="ID" type="long">
        <generator class="sequence"/>
    </collection-id>
    <key column="PERSON1"/>
    <many-to-many column="PERSON2" class="Person" fetch="join"/>
</idbag>

An <idbag> has a synthetic id generator, just like an entity class. A different surrogate key is assigned to each collection row. Hibernate does not, however, provide any mechanism for discovering the surrogate key value of a particular row.

The update performance of an <idbag> supersedes a regular<bag>. Hibernate can locate individual rows efficiently and update or delete them individually, similar to a list, map or set.

在目前的實現中，還不支持使用identity標識符生成器策略來生成<idbag>集合的標識符。

6.4. 集合例子（Collection example）

This section covers collection examples.

The following class has a collection of Child instances:

package eg;
import java.util.Set;

public class Parent {
    private long id;
    private Set children;

    public long getId() { return id; }
    private void setId(long id) { this.id=id; }

    private Set getChildren() { return children; }
    private void setChildren(Set children) { this.children=children; }

    ....
    ....
}

If each child has, at most, one parent, the most natural mapping is a one-to-many association:

<hibernate-mapping>

    <class name="Parent">
        <id name="id">
            <generator class="sequence"/>
        </id>
        <set name="children">
            <key column="parent_id"/>
            <one-to-many class="Child"/>
        </set>
    </class>

    <class name="Child">
        <id name="id">
            <generator class="sequence"/>
        </id>
        <property name="name"/>
    </class>

</hibernate-mapping>

在以下的表定義中反應了這個映射關係：

create table parent ( id bigint not null primary key )
create table child ( id bigint not null primary key, name varchar(255), parent_id bigint )
alter table child add constraint childfk0 (parent_id) references parent

如果父親是必須的, 那麼就可以使用雙向one-to-many的關聯了：

<hibernate-mapping>

    <class name="Parent">
        <id name="id">
            <generator class="sequence"/>
        </id>
        <set name="children" inverse="true">
            <key column="parent_id"/>
            <one-to-many class="Child"/>
        </set>
    </class>

    <class name="Child">
        <id name="id">
            <generator class="sequence"/>
        </id>
        <property name="name"/>
        <many-to-one name="parent" class="Parent" column="parent_id" not-null="true"/>
    </class>

</hibernate-mapping>

請注意NOT NULL的約束:

create table parent ( id bigint not null primary key )
create table child ( id bigint not null
                     primary key,
                     name varchar(255),
                     parent_id bigint not null )
alter table child add constraint childfk0 (parent_id) references parent

Alternatively, if this association must be unidirectional you can declare the NOT NULL constraint on the <key> mapping:

<hibernate-mapping>

    <class name="Parent">
        <id name="id">
            <generator class="sequence"/>
        </id>
        <set name="children">
            <key column="parent_id" not-null="true"/>
            <one-to-many class="Child"/>
        </set>
    </class>

    <class name="Child">
        <id name="id">
            <generator class="sequence"/>
        </id>
        <property name="name"/>
    </class>

</hibernate-mapping>

On the other hand, if a child has multiple parents, a many-to-many association is appropriate:

<hibernate-mapping>

    <class name="Parent">
        <id name="id">
            <generator class="sequence"/>
        </id>
        <set name="children" table="childset">
            <key column="parent_id"/>
            <many-to-many class="Child" column="child_id"/>
        </set>
    </class>

    <class name="Child">
        <id name="id">
            <generator class="sequence"/>
        </id>
        <property name="name"/>
    </class>

</hibernate-mapping>

表定義：

create table parent ( id bigint not null primary key )
create table child ( id bigint not null primary key, name varchar(255) )
create table childset ( parent_id bigint not null,
                        child_id bigint not null,
                        primary key ( parent_id, child_id ) )
alter table childset add constraint childsetfk0 (parent_id) references parent
alter table childset add constraint childsetfk1 (child_id) references child

For more examples and a complete explanation of a parent/child relationship mapping, see

第 21 章示例：父子關係(Parent Child Relationships) for more information.

Even more complex association mappings are covered in the next chapter.

第 7 章 關聯關係映射

7.1. 介紹

Association mappings are often the most difficult thing to implement correctly. In this section we examine some canonical cases one by one, starting with unidirectional mappings and then bidirectional cases. We will usePerson and Address in all the examples.

Associations will be classified by multiplicity and whether or not they map to an intervening join table.

Nullable foreign keys are not considered to be good practice in traditional data modelling, so our examples do not use nullable foreign keys. This is not a requirement of Hibernate, and the mappings will work if you drop the nullability constraints.

7.2. 單向關聯（Unidirectional associations）

7.2.1. Many-to-one

單向many-to-one關聯是最常見的單向關聯關係。

<class name="Person">
    <id name="id" column="personId">
        <generator class="native"/>
    </id>
    <many-to-one name="address" 
        column="addressId"
        not-null="true"/>
</class>

<class name="Address">
    <id name="id" column="addressId">
        <generator class="native"/>
    </id>
</class>

create table Person ( personId bigint not null primary key, addressId bigint not null )
create table Address ( addressId bigint not null primary key )
        

7.2.2. One-to-one

基於外鍵關聯的單向一對一關聯和單向多對一關聯幾乎是一樣的。唯一的不同就是單向一對一關聯中的外鍵字段具有唯一性約束。

<class name="Person">
    <id name="id" column="personId">
        <generator class="native"/>
    </id>
    <many-to-one name="address" 
        column="addressId" 
        unique="true"
        not-null="true"/>
</class>

<class name="Address">
    <id name="id" column="addressId">
        <generator class="native"/>
    </id>
</class>

create table Person ( personId bigint not null primary key, addressId bigint not null unique )
create table Address ( addressId bigint not null primary key )
        

A unidirectional one-to-one association on a primary key usually uses a special id generator In this example, however, we have reversed the direction of the association:

<class name="Person">
    <id name="id" column="personId">
        <generator class="native"/>
    </id>
</class>

<class name="Address">
    <id name="id" column="personId">
        <generator class="foreign">
            <param name="property">person</param>
        </generator>
    </id>
    <one-to-one name="person" constrained="true"/>
</class>

create table Person ( personId bigint not null primary key )
create table Address ( personId bigint not null primary key )
        

7.2.3. One-to-many

A unidirectional one-to-many association on a foreign key is an unusual case, and is not recommended.

<class name="Person">
    <id name="id" column="personId">
        <generator class="native"/>
    </id>
    <set name="addresses">
        <key column="personId" 
            not-null="true"/>
        <one-to-many class="Address"/>
    </set>
</class>

<class name="Address">
    <id name="id" column="addressId">
        <generator class="native"/>
    </id>
</class>

create table Person ( personId bigint not null primary key )
create table Address ( addressId bigint not null primary key, personId bigint not null )
        

You should instead use a join table for this kind of association.

7.3. 使用連接表的單向關聯（Unidirectional associations with join tables）

7.3.1. One-to-many

A unidirectional one-to-many association on a join table is the preferred option. Specifyingunique="true", changes the multiplicity from many-to-many to one-to-many.

<class name="Person">
    <id name="id" column="personId">
        <generator class="native"/>
    </id>
    <set name="addresses" table="PersonAddress">
        <key column="personId"/>
        <many-to-many column="addressId"
            unique="true"
            class="Address"/>
    </set>
</class>

<class name="Address">
    <id name="id" column="addressId">
        <generator class="native"/>
    </id>
</class>

create table Person ( personId bigint not null primary key )
create table PersonAddress ( personId not null, addressId bigint not null primary key )
create table Address ( addressId bigint not null primary key )
        

7.3.2. Many-to-one

A unidirectional many-to-one association on a join table is common when the association is optional. For example:

<class name="Person">
    <id name="id" column="personId">
        <generator class="native"/>
    </id>
    <join table="PersonAddress" 
        optional="true">
        <key column="personId" unique="true"/>
        <many-to-one name="address"
            column="addressId" 
            not-null="true"/>
    </join>
</class>

<class name="Address">
    <id name="id" column="addressId">
        <generator class="native"/>
    </id>
</class>

create table Person ( personId bigint not null primary key )
create table PersonAddress ( personId bigint not null primary key, addressId bigint not null )
create table Address ( addressId bigint not null primary key )
        

7.3.3. One-to-one

A unidirectional one-to-one association on a join table is possible, but extremely unusual.

<class name="Person">
    <id name="id" column="personId">
        <generator class="native"/>
    </id>
    <join table="PersonAddress" 
        optional="true">
        <key column="personId" 
            unique="true"/>
        <many-to-one name="address"
            column="addressId" 
            not-null="true"
            unique="true"/>
    </join>
</class>

<class name="Address">
    <id name="id" column="addressId">
        <generator class="native"/>
    </id>
</class>

create table Person ( personId bigint not null primary key )
create table PersonAddress ( personId bigint not null primary key, addressId bigint not null unique )
create table Address ( addressId bigint not null primary key )
        

7.3.4. Many-to-many

Finally, here is an example of a unidirectional many-to-many association.

<class name="Person">
    <id name="id" column="personId">
        <generator class="native"/>
    </id>
    <set name="addresses" table="PersonAddress">
        <key column="personId"/>
        <many-to-many column="addressId"
            class="Address"/>
    </set>
</class>

<class name="Address">
    <id name="id" column="addressId">
        <generator class="native"/>
    </id>
</class>

create table Person ( personId bigint not null primary key )
create table PersonAddress ( personId bigint not null, addressId bigint not null, primary key (personId, addressId) )
create table Address ( addressId bigint not null primary key )
        

7.4. 雙向關聯（Bidirectional associations）

7.4.1. one-to-many / many-to-one

A bidirectional many-to-one association is the most common kind of association. The following example illustrates the standard parent/child relationship.

<class name="Person">
    <id name="id" column="personId">
        <generator class="native"/>
    </id>
    <many-to-one name="address" 
        column="addressId"
        not-null="true"/>
</class>

<class name="Address">
    <id name="id" column="addressId">
        <generator class="native"/>
    </id>
    <set name="people" inverse="true">
        <key column="addressId"/>
        <one-to-many class="Person"/>
    </set>
</class>

create table Person ( personId bigint not null primary key, addressId bigint not null )
create table Address ( addressId bigint not null primary key )
        

If you use a List, or other indexed collection, set thekey column of the foreign key to not null. Hibernate will manage the association from the collections side to maintain the index of each element, making the other side virtually inverse by settingupdate="false" and insert="false":

<class name="Person">
   <id name="id"/>
   ...
   <many-to-one name="address"
      column="addressId"
      not-null="true"
      insert="false"
      update="false"/>
</class>

<class name="Address">
   <id name="id"/>
   ...
   <list name="people">
      <key column="addressId" not-null="true"/>
      <list-index column="peopleIdx"/>
      <one-to-many class="Person"/>
   </list>
</class>

If the underlying foreign key column is NOT NULL, it is important that you definenot-null="true" on the <key> element of the collection mapping. Do not only declarenot-null="true" on a possible nested <column> element, but on the <key> element.

7.4.2. One-to-one

A bidirectional one-to-one association on a foreign key is common:

<class name="Person">
    <id name="id" column="personId">
        <generator class="native"/>
    </id>
    <many-to-one name="address" 
        column="addressId" 
        unique="true"
        not-null="true"/>
</class>

<class name="Address">
    <id name="id" column="addressId">
        <generator class="native"/>
    </id>
   <one-to-one name="person" 
        property-ref="address"/>
</class>

create table Person ( personId bigint not null primary key, addressId bigint not null unique )
create table Address ( addressId bigint not null primary key )
        

A bidirectional one-to-one association on a primary key uses the special id generator:

<class name="Person">
    <id name="id" column="personId">
        <generator class="native"/>
    </id>
    <one-to-one name="address"/>
</class>

<class name="Address">
    <id name="id" column="personId">
        <generator class="foreign">
            <param name="property">person</param>
        </generator>
    </id>
    <one-to-one name="person" 
        constrained="true"/>
</class>

create table Person ( personId bigint not null primary key )
create table Address ( personId bigint not null primary key )
        

7.5. 使用連接表的雙向關聯（Bidirectional associations with join tables）

7.5.1. one-to-many / many-to-one

The following is an example of a bidirectional one-to-many association on a join table. Theinverse="true" can go on either end of the association, on the collection, or on the join.

<class name="Person">
    <id name="id" column="personId">
        <generator class="native"/>
    </id>
    <set name="addresses" 
        table="PersonAddress">
        <key column="personId"/>
        <many-to-many column="addressId"
            unique="true"
            class="Address"/>
    </set>
</class>

<class name="Address">
    <id name="id" column="addressId">
        <generator class="native"/>
    </id>
    <join table="PersonAddress" 
        inverse="true" 
        optional="true">
        <key column="addressId"/>
        <many-to-one name="person"
            column="personId"
            not-null="true"/>
    </join>
</class>

create table Person ( personId bigint not null primary key )
create table PersonAddress ( personId bigint not null, addressId bigint not null primary key )
create table Address ( addressId bigint not null primary key )
        

7.5.2. 一對一（one to one）

A bidirectional one-to-one association on a join table is possible, but extremely unusual.

<class name="Person">
    <id name="id" column="personId">
        <generator class="native"/>
    </id>
    <join table="PersonAddress" 
        optional="true">
        <key column="personId" 
            unique="true"/>
        <many-to-one name="address"
            column="addressId" 
            not-null="true"
            unique="true"/>
    </join>
</class>

<class name="Address">
    <id name="id" column="addressId">
        <generator class="native"/>
    </id>
    <join table="PersonAddress" 
        optional="true"
        inverse="true">
        <key column="addressId" 
            unique="true"/>
        <many-to-one name="person"
            column="personId" 
            not-null="true"
            unique="true"/>
    </join>
</class>

create table Person ( personId bigint not null primary key )
create table PersonAddress ( personId bigint not null primary key, addressId bigint not null unique )
create table Address ( addressId bigint not null primary key )
        

7.5.3. Many-to-many

Here is an example of a bidirectional many-to-many association.

<class name="Person">
    <id name="id" column="personId">
        <generator class="native"/>
    </id>
    <set name="addresses" table="PersonAddress">
        <key column="personId"/>
        <many-to-many column="addressId"
            class="Address"/>
    </set>
</class>

<class name="Address">
    <id name="id" column="addressId">
        <generator class="native"/>
    </id>
    <set name="people" inverse="true" table="PersonAddress">
        <key column="addressId"/>
        <many-to-many column="personId"
            class="Person"/>
    </set>
</class>

create table Person ( personId bigint not null primary key )
create table PersonAddress ( personId bigint not null, addressId bigint not null, primary key (personId, addressId) )
create table Address ( addressId bigint not null primary key )
        

7.6. 更復雜的關聯映射

More complex association joins are extremely rare. Hibernate handles more complex situations by using SQL fragments embedded in the mapping document. For example, if a table with historical account information data defines accountNumber, effectiveEndDate andeffectiveStartDatecolumns, it would be mapped as follows:

<properties name="currentAccountKey">
    <property name="accountNumber" type="string" not-null="true"/>
    <property name="currentAccount" type="boolean">
        <formula>case when effectiveEndDate is null then 1 else 0 end</formula>
    </property>
</properties>
<property name="effectiveEndDate" type="date"/>
<property name="effectiveStateDate" type="date" not-null="true"/>

You can then map an association to the current instance, the one with nulleffectiveEndDate, by using:

<many-to-one name="currentAccountInfo" 
        property-ref="currentAccountKey"
        class="AccountInfo">
    <column name="accountNumber"/>
    <formula>'1'</formula>
</many-to-one>

In a more complex example, imagine that the association between Employee and Organization is maintained in anEmployment table full of historical employment data. An association to the employee'smost recent employer, the one with the most recentstartDate, could be mapped in the following way:

<join>
    <key column="employeeId"/>
    <subselect>
        select employeeId, orgId 
        from Employments 
        group by orgId 
        having startDate = max(startDate)
    </subselect>
    <many-to-one name="mostRecentEmployer" 
            class="Organization" 
            column="orgId"/>
</join>

This functionality allows a degree of creativity and flexibility, but it is more practical to handle these kinds of cases using HQL or a criteria query.

第 8 章 組件（Component）映射

The notion of a component is re-used in several different contexts and purposes throughout Hibernate.

8.1. 依賴對象（Dependent objects）

A component is a contained object that is persisted as a value type and not an entity reference. The term "component" refers to the object-oriented notion of composition and not to architecture-level components. For example, you can model a person like this:

public class Person {
    private java.util.Date birthday;
    private Name name;
    private String key;
    public String getKey() {
        return key;
    }
    private void setKey(String key) {
        this.key=key;
    }
    public java.util.Date getBirthday() {
        return birthday;
    }
    public void setBirthday(java.util.Date birthday) {
        this.birthday = birthday;
    }
    public Name getName() {
        return name;
    }
    public void setName(Name name) {
        this.name = name;
    }
    ......
    ......
}

public class Name {
    char initial;
    String first;
    String last;
    public String getFirst() {
        return first;
    }
    void setFirst(String first) {
        this.first = first;
    }
    public String getLast() {
        return last;
    }
    void setLast(String last) {
        this.last = last;
    }
    public char getInitial() {
        return initial;
    }
    void setInitial(char initial) {
        this.initial = initial;
    }
}

Now Name can be persisted as a component of Person. Name defines getter and setter methods for its persistent properties, but it does not need to declare any interfaces or identifier properties.

Our Hibernate mapping would look like this:

<class name="eg.Person" table="person">
    <id name="Key" column="pid" type="string">
        <generator class="uuid"/>
    </id>
    <property name="birthday" type="date"/>
    <component name="Name" class="eg.Name"> <!-- class attribute optional -->
        <property name="initial"/>
        <property name="first"/>
        <property name="last"/>
    </component>
</class>

人員(Person)表中將包括pid, birthday,initial, first和 last等字段。

Like value types, components do not support shared references. In other words, two persons could have the same name, but the two person objects would contain two independent name objects that were only "the same" by value. The null value semantics of a component are ad hoc. When reloading the containing object, Hibernate will assume that if all component columns are null, then the entire component is null. This is suitable for most purposes.

The properties of a component can be of any Hibernate type (collections, many-to-one associations, other components, etc). Nested components shouldnot be considered an exotic usage. Hibernate is intended to support a fine-grained object model.

<component> 元素還允許有 <parent>子元素，用來表明component類中的一個屬性是指向包含它的實體的引用。

<class name="eg.Person" table="person">
    <id name="Key" column="pid" type="string">
        <generator class="uuid"/>
    </id>
    <property name="birthday" type="date"/>
    <component name="Name" class="eg.Name" unique="true">
        <parent name="namedPerson"/> <!-- reference back to the Person -->
        <property name="initial"/>
        <property name="first"/>
        <property name="last"/>
    </component>
</class>

8.2. 在集合中出現的依賴對象 (Collections of dependent objects)

Collections of components are supported (e.g. an array of type Name). Declare your component collection by replacing the <element> tag with a <composite-element> tag:

<set name="someNames" table="some_names" lazy="true">
    <key column="id"/>
    <composite-element class="eg.Name"> <!-- class attribute required -->
        <property name="initial"/>
        <property name="first"/>
        <property name="last"/>
    </composite-element>
</set>

重要

If you define a Set of composite elements, it is important to implementequals() and hashCode() correctly.

Composite elements can contain components but not collections. If your composite element contains components, use the<nested-composite-element> tag. This case is a collection of components which themselves have components. You may want to consider if a one-to-many association is more appropriate. Remodel the composite element as an entity, but be aware that even though the Java model is the same, the relational model and persistence semantics are still slightly different.

A composite element mapping does not support null-able properties if you are using a<set>. There is no separate primary key column in the composite element table. Hibernate uses each column's value to identify a record when deleting objects, which is not possible with null values. You have to either use only not-null properties in a composite-element or choose a <list>,<map>, <bag> or <idbag>.

A special case of a composite element is a composite element with a nested <many-to-one> element. This mapping allows you to map extra columns of a many-to-many association table to the composite element class. The following is a many-to-many association fromOrder to Item, where purchaseDate, price and quantity are properties of the association:

<class name="eg.Order" .... >
    ....
    <set name="purchasedItems" table="purchase_items" lazy="true">
        <key column="order_id">
        <composite-element class="eg.Purchase">
            <property name="purchaseDate"/>
            <property name="price"/>
            <property name="quantity"/>
            <many-to-one name="item" class="eg.Item"/> <!-- class attribute is optional -->
        </composite-element>
    </set>
</class>

There cannot be a reference to the purchase on the other side for bidirectional association navigation. Components are value types and do not allow shared references. A singlePurchase can be in the set of an Order, but it cannot be referenced by the Item at the same time.

其實組合元素的這個用法可以擴展到三重或多重關聯:

<class name="eg.Order" .... >
    ....
    <set name="purchasedItems" table="purchase_items" lazy="true">
        <key column="order_id">
        <composite-element class="eg.OrderLine">
            <many-to-one name="purchaseDetails class="eg.Purchase"/>
            <many-to-one name="item" class="eg.Item"/>
        </composite-element>
    </set>
</class>

Composite elements can appear in queries using the same syntax as associations to other entities.

8.3. 組件作爲Map的索引（Components as Map indices ）

The <composite-map-key> element allows you to map a component class as the key of aMap. Ensure that you override hashCode() and equals() correctly on the component class.

8.4. 組件作爲聯合標識符(Components as composite identifiers)

You can use a component as an identifier of an entity class. Your component class must satisfy certain requirements:

• 它必須實現java.io.Serializable接口

• It must re-implement equals() and hashCode() consistently with the database's notion of composite key equality.

Note

In Hibernate3, although the second requirement is not an absolutely hard requirement of Hibernate, it is recommended.

You cannot use an IdentifierGenerator to generate composite keys. Instead the application must assign its own identifiers.

Use the <composite-id> tag, with nested <key-property> elements, in place of the usual <id> declaration. For example, theOrderLine class has a primary key that depends upon the (composite) primary key ofOrder.

<class name="OrderLine">
    
    <composite-id name="id" class="OrderLineId">
        <key-property name="lineId"/>
        <key-property name="orderId"/>
        <key-property name="customerId"/>
    </composite-id>
    
    <property name="name"/>
    
    <many-to-one name="order" class="Order"
            insert="false" update="false">
        <column name="orderId"/>
        <column name="customerId"/>
    </many-to-one>
    ....
    
</class>

Any foreign keys referencing the OrderLine table are now composite. Declare this in your mappings for other classes. An association toOrderLine is mapped like this:

<many-to-one name="orderLine" class="OrderLine">
<!-- the "class" attribute is optional, as usual -->
    <column name="lineId"/>
    <column name="orderId"/>
    <column name="customerId"/>
</many-to-one>

提示

The column element is an alternative to the column attribute everywhere. Using the column element just gives more declaration options, which are mostly useful when utilizinghbm2ddl

指向OrderLine的多對多關聯也使用聯合外鍵:

<set name="undeliveredOrderLines">
    <key column name="warehouseId"/>
    <many-to-many class="OrderLine">
        <column name="lineId"/>
        <column name="orderId"/>
        <column name="customerId"/>
    </many-to-many>
</set>

在Order中,OrderLine的集合則是這樣:

<set name="orderLines" inverse="true">
    <key>
        <column name="orderId"/>
        <column name="customerId"/>
    </key>
    <one-to-many class="OrderLine"/>
</set>

The <one-to-many> element declares no columns.

假若OrderLine本身擁有一個集合,它也具有組合外鍵。

<class name="OrderLine">
    ....
    ....
    <list name="deliveryAttempts">
        <key>   <!-- a collection inherits the composite key type -->
            <column name="lineId"/>
            <column name="orderId"/>
            <column name="customerId"/>
        </key>
        <list-index column="attemptId" base="1"/>
        <composite-element class="DeliveryAttempt">
            ...
        </composite-element>
    </set>
</class>

8.5. 動態組件 （Dynamic components）

You can also map a property of type Map:

<dynamic-component name="userAttributes">
    <property name="foo" column="FOO" type="string"/>
    <property name="bar" column="BAR" type="integer"/>
    <many-to-one name="baz" class="Baz" column="BAZ_ID"/>
</dynamic-component>

The semantics of a <dynamic-component> mapping are identical to<component>. The advantage of this kind of mapping is the ability to determine the actual properties of the bean at deployment time just by editing the mapping document. Runtime manipulation of the mapping document is also possible, using a DOM parser. You can also access, and change, Hibernate's configuration-time metamodel via theConfiguration object.

第 9 章 Inheritance mapping

9.1. The three strategies

Hibernate支持三種基本的繼承映射策略：

• 每個類分層結構一張表(table per class hierarchy)

• table per subclass

• 每個具體類一張表(table per concrete class)

此外，Hibernate還支持第四種稍有不同的多態映射策略：

• 隱式多態(implicit polymorphism)

It is possible to use different mapping strategies for different branches of the same inheritance hierarchy. You can then make use of implicit polymorphism to achieve polymorphism across the whole hierarchy. However, Hibernate does not support mixing<subclass>, <joined-subclass> and<union-subclass> mappings under the same root <class> element. It is possible to mix together the table per hierarchy and table per subclass strategies under the the same<class> element, by combining the <subclass> and <join> elements (see below for an example).

It is possible to define subclass, union-subclass, and joined-subclass mappings in separate mapping documents directly beneathhibernate-mapping. This allows you to extend a class hierarchy by adding a new mapping file. You must specify anextends attribute in the subclass mapping, naming a previously mapped superclass. Previously this feature made the ordering of the mapping documents important. Since Hibernate3, the ordering of mapping files is irrelevant when using the extends keyword. The ordering inside a single mapping file still needs to be defined as superclasses before subclasses.

 <hibernate-mapping>
     <subclass name="DomesticCat" extends="Cat" discriminator-value="D">
          <property name="name" type="string"/>
     </subclass>
 </hibernate-mapping>

9.1.1. 每個類分層結構一張表(Table per class hierarchy)

Suppose we have an interface Payment with the implementorsCreditCardPayment, CashPayment, andChequePayment. The table per hierarchy mapping would display in the following way:

<class name="Payment" table="PAYMENT">
    <id name="id" type="long" column="PAYMENT_ID">
        <generator class="native"/>
    </id>
    <discriminator column="PAYMENT_TYPE" type="string"/>
    <property name="amount" column="AMOUNT"/>
    ...
    <subclass name="CreditCardPayment" discriminator-value="CREDIT">
        <property name="creditCardType" column="CCTYPE"/>
        ...
    </subclass>
    <subclass name="CashPayment" discriminator-value="CASH">
        ...
    </subclass>
    <subclass name="ChequePayment" discriminator-value="CHEQUE">
        ...
    </subclass>
</class>

Exactly one table is required. There is a limitation of this mapping strategy: columns declared by the subclasses, such asCCTYPE, cannot have NOT NULL constraints.

9.1.2. 每個子類一張表(Table per subclass)

A table per subclass mapping looks like this:

<class name="Payment" table="PAYMENT">
    <id name="id" type="long" column="PAYMENT_ID">
        <generator class="native"/>
    </id>
    <property name="amount" column="AMOUNT"/>
    ...
    <joined-subclass name="CreditCardPayment" table="CREDIT_PAYMENT">
        <key column="PAYMENT_ID"/>
        <property name="creditCardType" column="CCTYPE"/>
        ...
    </joined-subclass>
    <joined-subclass name="CashPayment" table="CASH_PAYMENT">
        <key column="PAYMENT_ID"/>
        ...
    </joined-subclass>
    <joined-subclass name="ChequePayment" table="CHEQUE_PAYMENT">
        <key column="PAYMENT_ID"/>
        ...
    </joined-subclass>
</class>

Four tables are required. The three subclass tables have primary key associations to the superclass table so the relational model is actually a one-to-one association.

9.1.3. Table per subclass: using a discriminator

Hibernate's implementation of table per subclass does not require a discriminator column. Other object/relational mappers use a different implementation of table per subclass that requires a type discriminator column in the superclass table. The approach taken by Hibernate is much more difficult to implement, but arguably more correct from a relational point of view. If you want to use a discriminator column with the table per subclass strategy, you can combine the use of<subclass> and <join>, as follows:

<class name="Payment" table="PAYMENT">
    <id name="id" type="long" column="PAYMENT_ID">
        <generator class="native"/>
    </id>
    <discriminator column="PAYMENT_TYPE" type="string"/>
    <property name="amount" column="AMOUNT"/>
    ...
    <subclass name="CreditCardPayment" discriminator-value="CREDIT">
        <join table="CREDIT_PAYMENT">
            <key column="PAYMENT_ID"/>
            <property name="creditCardType" column="CCTYPE"/>
            ...
        </join>
    </subclass>
    <subclass name="CashPayment" discriminator-value="CASH">
        <join table="CASH_PAYMENT">
            <key column="PAYMENT_ID"/>
            ...
        </join>
    </subclass>
    <subclass name="ChequePayment" discriminator-value="CHEQUE">
        <join table="CHEQUE_PAYMENT" fetch="select">
            <key column="PAYMENT_ID"/>
            ...
        </join>
    </subclass>
</class>

可選的聲明fetch="select"，是用來告訴Hibernate，在查詢超類時， 不要使用外部連接(outer join)來抓取子類ChequePayment的數據。

9.1.4. 混合使用“每個類分層結構一張表”和“每個子類一張表”

You can even mix the table per hierarchy and table per subclass strategies using the following approach:

<class name="Payment" table="PAYMENT">
    <id name="id" type="long" column="PAYMENT_ID">
        <generator class="native"/>
    </id>
    <discriminator column="PAYMENT_TYPE" type="string"/>
    <property name="amount" column="AMOUNT"/>
    ...
    <subclass name="CreditCardPayment" discriminator-value="CREDIT">
        <join table="CREDIT_PAYMENT">
            <property name="creditCardType" column="CCTYPE"/>
            ...
        </join>
    </subclass>
    <subclass name="CashPayment" discriminator-value="CASH">
        ...
    </subclass>
    <subclass name="ChequePayment" discriminator-value="CHEQUE">
        ...
    </subclass>
</class>

對上述任何一種映射策略而言，指向根類Payment的 關聯是使用<many-to-one>進行映射的。

<many-to-one name="payment" column="PAYMENT_ID" class="Payment"/>

9.1.5. 每個具體類一張表(Table per concrete class)

There are two ways we can map the table per concrete class strategy. First, you can use<union-subclass>.

<class name="Payment">
    <id name="id" type="long" column="PAYMENT_ID">
        <generator class="sequence"/>
    </id>
    <property name="amount" column="AMOUNT"/>
    ...
    <union-subclass name="CreditCardPayment" table="CREDIT_PAYMENT">
        <property name="creditCardType" column="CCTYPE"/>
        ...
    </union-subclass>
    <union-subclass name="CashPayment" table="CASH_PAYMENT">
        ...
    </union-subclass>
    <union-subclass name="ChequePayment" table="CHEQUE_PAYMENT">
        ...
    </union-subclass>
</class>

這裏涉及三張與子類相關的表。每張表爲對應類的所有屬性（包括從超類繼承的屬性）定義相應字段。

The limitation of this approach is that if a property is mapped on the superclass, the column name must be the same on all subclass tables. The identity generator strategy is not allowed in union subclass inheritance. The primary key seed has to be shared across all unioned subclasses of a hierarchy.

If your superclass is abstract, map it with abstract="true". If it is not abstract, an additional table (it defaults toPAYMENT in the example above), is needed to hold instances of the superclass.

9.1.6. Table per concrete class using implicit polymorphism

另一種可供選擇的方法是採用隱式多態：

<class name="CreditCardPayment" table="CREDIT_PAYMENT">
    <id name="id" type="long" column="CREDIT_PAYMENT_ID">
        <generator class="native"/>
    </id>
    <property name="amount" column="CREDIT_AMOUNT"/>
    ...
</class>

<class name="CashPayment" table="CASH_PAYMENT">
    <id name="id" type="long" column="CASH_PAYMENT_ID">
        <generator class="native"/>
    </id>
    <property name="amount" column="CASH_AMOUNT"/>
    ...
</class>

<class name="ChequePayment" table="CHEQUE_PAYMENT">
    <id name="id" type="long" column="CHEQUE_PAYMENT_ID">
        <generator class="native"/>
    </id>
    <property name="amount" column="CHEQUE_AMOUNT"/>
    ...
</class>

Notice that the Payment interface is not mentioned explicitly. Also notice that properties ofPayment are mapped in each of the subclasses. If you want to avoid duplication, consider using XML entities (for example,[ <!ENTITY allproperties SYSTEM "allproperties.xml"> ] in theDOCTYPE declaration and &allproperties; in the mapping).

這種方法的缺陷在於，在Hibernate執行多態查詢時(polymorphic queries)無法生成帶 UNION的SQL語句。

對於這種映射策略而言，通常用<any>來實現到 Payment的多態關聯映射。

<any name="payment" meta-type="string" id-type="long">
    <meta-value value="CREDIT" class="CreditCardPayment"/>
    <meta-value value="CASH" class="CashPayment"/>
    <meta-value value="CHEQUE" class="ChequePayment"/>
    <column name="PAYMENT_CLASS"/>
    <column name="PAYMENT_ID"/>
</any>

9.1.7. 隱式多態和其他繼承映射混合使用

Since the subclasses are each mapped in their own <class> element, and sincePayment is just an interface), each of the subclasses could easily be part of another inheritance hierarchy. You can still use polymorphic queries against thePayment interface.

<class name="CreditCardPayment" table="CREDIT_PAYMENT">
    <id name="id" type="long" column="CREDIT_PAYMENT_ID">
        <generator class="native"/>
    </id>
    <discriminator column="CREDIT_CARD" type="string"/>
    <property name="amount" column="CREDIT_AMOUNT"/>
    ...
    <subclass name="MasterCardPayment" discriminator-value="MDC"/>
    <subclass name="VisaPayment" discriminator-value="VISA"/>
</class>

<class name="NonelectronicTransaction" table="NONELECTRONIC_TXN">
    <id name="id" type="long" column="TXN_ID">
        <generator class="native"/>
    </id>
    ...
    <joined-subclass name="CashPayment" table="CASH_PAYMENT">
        <key column="PAYMENT_ID"/>
        <property name="amount" column="CASH_AMOUNT"/>
        ...
    </joined-subclass>
    <joined-subclass name="ChequePayment" table="CHEQUE_PAYMENT">
        <key column="PAYMENT_ID"/>
        <property name="amount" column="CHEQUE_AMOUNT"/>
        ...
    </joined-subclass>
</class>

Once again, Payment is not mentioned explicitly. If we execute a query against thePayment interface, for example from Payment, Hibernate automatically returns instances of CreditCardPayment (and its subclasses, since they also implement Payment), CashPayment and ChequePayment, but not instances of NonelectronicTransaction.

9.2. 限制

There are limitations to the "implicit polymorphism" approach to the table per concrete-class mapping strategy. There are somewhat less restrictive limitations to<union-subclass> mappings.

下面表格中列出了在Hibernte中“每個具體類一張表”的策略和隱式多態的限制。

表 9.1. 繼承映射特性(Features of inheritance mappings)

繼承策略(Inheritance strategy)	多態多對一	多態一對一	多態一對多	多態多對多	Polymorphic load()/get()	多態查詢	多態連接(join)	外連接(Outer join)讀取
每個類分層結構一張表	<many-to-one>	<one-to-one>	<one-to-many>	<many-to-many>	s.get(Payment.class, id)	from Payment p	from Order o join o.payment p	支持
table per subclass	<many-to-one>	<one-to-one>	<one-to-many>	<many-to-many>	s.get(Payment.class, id)	from Payment p	from Order o join o.payment p	支持
每個具體類一張表(union-subclass)	<many-to-one>	<one-to-one>	<one-to-many> (for inverse="true" only)	<many-to-many>	s.get(Payment.class, id)	from Payment p	from Order o join o.payment p	支持
每個具體類一張表(隱式多態)	<any>	不支持	不支持	<many-to-any>	s.createCriteria(Payment.class).add( Restrictions.idEq(id) ).uniqueResult()	from Payment p	不支持	不支持

第 10 章 與對象共事

Hibernate is a full object/relational mapping solution that not only shields the developer from the details of the underlying database management system, but also offersstate management of objects. This is, contrary to the management of SQLstatements in common JDBC/SQL persistence layers, a natural object-oriented view of persistence in Java applications.

換句話說，使用Hibernate的開發者應該總是關注對象的狀態(state)，不必考慮SQL語句的執行。 這部分細節已經由Hibernate掌管妥當，只有開發者在進行系統性能調優的時候才需要進行了解。

10.1. Hibernate對象狀態(object states)

Hibernate定義並支持下列對象狀態(state):

• Transient - an object is transient if it has just been instantiated using thenew operator, and it is not associated with a HibernateSession. It has no persistent representation in the database and no identifier value has been assigned. Transient instances will be destroyed by the garbage collector if the application does not hold a reference anymore. Use the Hibernate Session to make an object persistent (and let Hibernate take care of the SQL statements that need to be executed for this transition).

• Persistent - a persistent instance has a representation in the database and an identifier value. It might just have been saved or loaded, however, it is by definition in the scope of aSession. Hibernate will detect any changes made to an object in persistent state and synchronize the state with the database when the unit of work completes. Developers do not execute manualUPDATE statements, or DELETE statements when an object should be made transient.

• Detached - a detached instance is an object that has been persistent, but itsSession has been closed. The reference to the object is still valid, of course, and the detached instance might even be modified in this state. A detached instance can be reattached to a newSession at a later point in time, making it (and all the modifications) persistent again. This feature enables a programming model for long running units of work that require user think-time. We call themapplication transactions, i.e., a unit of work from the point of view of the user.

We will now discuss the states and state transitions (and the Hibernate methods that trigger a transition) in more detail.

10.2. 使對象持久化

Hibernate認爲持久化類(persistent class)新實例化的對象是瞬時(Transient)的。 我們可通過將瞬時(Transient)對象與session關聯而把它變爲持久(Persistent)的。

DomesticCat fritz = new DomesticCat();
fritz.setColor(Color.GINGER);
fritz.setSex('M');
fritz.setName("Fritz");
Long generatedId = (Long) sess.save(fritz);

If Cat has a generated identifier, the identifier is generated and assigned to thecat when save() is called. IfCat has an assigned identifier, or a composite key, the identifier should be assigned to thecat instance before calling save(). You can also usepersist() instead of save(), with the semantics defined in the EJB3 early draft.

• persist() makes a transient instance persistent. However, it does not guarantee that the identifier value will be assigned to the persistent instance immediately, the assignment might happen at flush time.persist() also guarantees that it will not execute anINSERT statement if it is called outside of transaction boundaries. This is useful in long-running conversations with an extended Session/persistence context.

• save() does guarantee to return an identifier. If an INSERT has to be executed to get the identifier ( e.g. "identity" generator, not "sequence"), this INSERT happens immediately, no matter if you are inside or outside of a transaction. This is problematic in a long-running conversation with an extended Session/persistence context.

Alternatively, you can assign the identifier using an overloaded version of save().

DomesticCat pk = new DomesticCat();
pk.setColor(Color.TABBY);
pk.setSex('F');
pk.setName("PK");
pk.setKittens( new HashSet() );
pk.addKitten(fritz);
sess.save( pk, new Long(1234) );

If the object you make persistent has associated objects (e.g. the kittens collection in the previous example), these objects can be made persistent in any order you like unless you have aNOT NULL constraint upon a foreign key column. There is never a risk of violating foreign key constraints. However, you might violate aNOT NULL constraint if you save() the objects in the wrong order.

Usually you do not bother with this detail, as you will normally use Hibernate'stransitive persistence feature to save the associated objects automatically. Then, evenNOT NULL constraint violations do not occur - Hibernate will take care of everything. Transitive persistence is discussed later in this chapter.

10.3. 裝載對象

The load() methods of Session provide a way of retrieving a persistent instance if you know its identifier.load() takes a class object and loads the state into a newly instantiated instance of that class in a persistent state.

Cat fritz = (Cat) sess.load(Cat.class, generatedId);

// you need to wrap primitive identifiers
long id = 1234;
DomesticCat pk = (DomesticCat) sess.load( DomesticCat.class, new Long(id) );

此外, 你可以把數據(state)加載到指定的對象實例上（覆蓋掉該實例原來的數據）。

Cat cat = new DomesticCat();
// load pk's state into cat
sess.load( cat, new Long(pkId) );
Set kittens = cat.getKittens();

Be aware that load() will throw an unrecoverable exception if there is no matching database row. If the class is mapped with a proxy,load() just returns an uninitialized proxy and does not actually hit the database until you invoke a method of the proxy. This is useful if you wish to create an association to an object without actually loading it from the database. It also allows multiple instances to be loaded as a batch if batch-size is defined for the class mapping.

If you are not certain that a matching row exists, you should use the get() method which hits the database immediately and returns null if there is no matching row.

Cat cat = (Cat) sess.get(Cat.class, id);
if (cat==null) {
    cat = new Cat();
    sess.save(cat, id);
}
return cat;

You can even load an object using an SQL SELECT ... FOR UPDATE, using aLockMode. See the API documentation for more information.

Cat cat = (Cat) sess.get(Cat.class, id, LockMode.UPGRADE);

Any associated instances or contained collections will not be selected FOR UPDATE, unless you decide to specifylock or all as a cascade style for the association.

任何時候都可以使用refresh()方法強迫裝載對象和它的集合。如果你使用數據庫觸發器功能來處理對象的某些屬性，這個方法就很有用了。

sess.save(cat);
sess.flush(); //force the SQL INSERT
sess.refresh(cat); //re-read the state (after the trigger executes)

How much does Hibernate load from the database and how many SQL SELECTs will it use? This depends on the fetching strategy. This is explained in

第 19.1 節 “抓取策略(Fetching strategies)”.

10.4. 查詢

If you do not know the identifiers of the objects you are looking for, you need a query. Hibernate supports an easy-to-use but powerful object oriented query language (HQL). For programmatic query creation, Hibernate supports a sophisticated Criteria and Example query feature (QBC and QBE). You can also express your query in the native SQL of your database, with optional support from Hibernate for result set conversion into objects.

10.4.1. 執行查詢

HQL和原生SQL(native SQL)查詢要通過爲org.hibernate.Query的實例來表達。 這個接口提供了參數綁定、結果集處理以及運行實際查詢的方法。 你總是可以通過當前Session獲取一個Query對象：

List cats = session.createQuery(
    "from Cat as cat where cat.birthdate < ?")
    .setDate(0, date)
    .list();

List mothers = session.createQuery(
    "select mother from Cat as cat join cat.mother as mother where cat.name = ?")
    .setString(0, name)
    .list();

List kittens = session.createQuery(
    "from Cat as cat where cat.mother = ?")
    .setEntity(0, pk)
    .list();

Cat mother = (Cat) session.createQuery(
    "select cat.mother from Cat as cat where cat = ?")
    .setEntity(0, izi)
    .uniqueResult();]]

Query mothersWithKittens = (Cat) session.createQuery(
    "select mother from Cat as mother left join fetch mother.kittens");
Set uniqueMothers = new HashSet(mothersWithKittens.list());

A query is usually executed by invoking list(). The result of the query will be loaded completely into a collection in memory. Entity instances retrieved by a query are in a persistent state. TheuniqueResult() method offers a shortcut if you know your query will only return a single object. Queries that make use of eager fetching of collections usually return duplicates of the root objects, but with their collections initialized. You can filter these duplicates through a Set.

10.4.1.1. 迭代式獲取結果(Iterating results)

Occasionally, you might be able to achieve better performance by executing the query using theiterate() method. This will usually be the case if you expect that the actual entity instances returned by the query will already be in the session or second-level cache. If they are not already cached,iterate() will be slower than list() and might require many database hits for a simple query, usually 1 for the initial select which only returns identifiers, and n additional selects to initialize the actual instances.

// fetch ids
Iterator iter = sess.createQuery("from eg.Qux q order by q.likeliness").iterate();
while ( iter.hasNext() ) {
    Qux qux = (Qux) iter.next();  // fetch the object
    // something we couldnt express in the query
    if ( qux.calculateComplicatedAlgorithm() ) {
        // delete the current instance
        iter.remove();
        // dont need to process the rest
        break;
    }
}

10.4.1.2. 返回元組(tuples)的查詢

Hibernate queries sometimes return tuples of objects. Each tuple is returned as an array:

Iterator kittensAndMothers = sess.createQuery(
            "select kitten, mother from Cat kitten join kitten.mother mother")
            .list()
            .iterator();

while ( kittensAndMothers.hasNext() ) {
    Object[] tuple = (Object[]) kittensAndMothers.next();
    Cat kitten = (Cat) tuple[0];
    Cat mother = (Cat) tuple[1];
    ....
}

10.4.1.3. 標量(Scalar)結果

Queries can specify a property of a class in the select clause. They can even call SQL aggregate functions. Properties or aggregates are considered "scalar" results and not entities in persistent state.

Iterator results = sess.createQuery(
        "select cat.color, min(cat.birthdate), count(cat) from Cat cat " +
        "group by cat.color")
        .list()
        .iterator();

while ( results.hasNext() ) {
    Object[] row = (Object[]) results.next();
    Color type = (Color) row[0];
    Date oldest = (Date) row[1];
    Integer count = (Integer) row[2];
    .....
}

10.4.1.4. 綁定參數

Methods on Query are provided for binding values to named parameters or JDBC-style? parameters. Contrary to JDBC, Hibernate numbers parameters from zero. Named parameters are identifiers of the form:name in the query string. The advantages of named parameters are as follows:

• 命名參數(named parameters)與其在查詢串中出現的順序無關

• they can occur multiple times in the same query

• 它們本身是自我說明的

//named parameter (preferred)
Query q = sess.createQuery("from DomesticCat cat where cat.name = :name");
q.setString("name", "Fritz");
Iterator cats = q.iterate();

//positional parameter
Query q = sess.createQuery("from DomesticCat cat where cat.name = ?");
q.setString(0, "Izi");
Iterator cats = q.iterate();

//named parameter list
List names = new ArrayList();
names.add("Izi");
names.add("Fritz");
Query q = sess.createQuery("from DomesticCat cat where cat.name in (:namesList)");
q.setParameterList("namesList", names);
List cats = q.list();

10.4.1.5. 分頁

If you need to specify bounds upon your result set, that is, the maximum number of rows you want to retrieve and/or the first row you want to retrieve, you can use methods of theQuery interface:

Query q = sess.createQuery("from DomesticCat cat");
q.setFirstResult(20);
q.setMaxResults(10);
List cats = q.list();

Hibernate 知道如何將這個有限定條件的查詢轉換成你的數據庫的原生SQL(native SQL)。

10.4.1.6. 可滾動遍歷(Scrollable iteration)

If your JDBC driver supports scrollable ResultSets, theQuery interface can be used to obtain a ScrollableResults object that allows flexible navigation of the query results.

Query q = sess.createQuery("select cat.name, cat from DomesticCat cat " +
                            "order by cat.name");
ScrollableResults cats = q.scroll();
if ( cats.first() ) {

    // find the first name on each page of an alphabetical list of cats by name
    firstNamesOfPages = new ArrayList();
    do {
        String name = cats.getString(0);
        firstNamesOfPages.add(name);
    }
    while ( cats.scroll(PAGE_SIZE) );

    // Now get the first page of cats
    pageOfCats = new ArrayList();
    cats.beforeFirst();
    int i=0;
    while( ( PAGE_SIZE > i++ ) && cats.next() ) pageOfCats.add( cats.get(1) );

}
cats.close()

Note that an open database connection and cursor is required for this functionality. UsesetMaxResult()/setFirstResult() if you need offline pagination functionality.

10.4.1.7. 外置命名查詢(Externalizing named queries)

You can also define named queries in the mapping document. Remember to use a CDATA section if your query contains characters that could be interpreted as markup.

<query name="ByNameAndMaximumWeight"><![CDATA[
    from eg.DomesticCat as cat
        where cat.name = ?
        and cat.weight > ?
] ]></query>

參數綁定及執行以編程方式(programatically)完成：

Query q = sess.getNamedQuery("ByNameAndMaximumWeight");
q.setString(0, name);
q.setInt(1, minWeight);
List cats = q.list();

The actual program code is independent of the query language that is used. You can also define native SQL queries in metadata, or migrate existing queries to Hibernate by placing them in mapping files.

Also note that a query declaration inside a <hibernate-mapping> element requires a global unique name for the query, while a query declaration inside a<class> element is made unique automatically by prepending the fully qualified name of the class. For exampleeg.Cat.ByNameAndMaximumWeight.

10.4.2. 過濾集合

A collection filter is a special type of query that can be applied to a persistent collection or array. The query string can refer tothis, meaning the current collection element.

Collection blackKittens = session.createFilter(
    pk.getKittens(), 
    "where this.color = ?")
    .setParameter( Color.BLACK, Hibernate.custom(ColorUserType.class) )
    .list()
);

The returned collection is considered a bag that is a copy of the given collection. The original collection is not modified. This is contrary to the implication of the name "filter", but consistent with expected behavior.

Observe that filters do not require a from clause, although they can have one if required. Filters are not limited to returning the collection elements themselves.

Collection blackKittenMates = session.createFilter(
    pk.getKittens(), 
    "select this.mate where this.color = eg.Color.BLACK.intValue")
    .list();

Even an empty filter query is useful, e.g. to load a subset of elements in a large collection:

Collection tenKittens = session.createFilter(
    mother.getKittens(), "")
    .setFirstResult(0).setMaxResults(10)
    .list();

10.4.3. 條件查詢(Criteria queries)

HQL is extremely powerful, but some developers prefer to build queries dynamically using an object-oriented API, rather than building query strings. Hibernate provides an intuitiveCriteria query API for these cases:

Criteria crit = session.createCriteria(Cat.class);
crit.add( Restrictions.eq( "color", eg.Color.BLACK ) );
crit.setMaxResults(10);
List cats = crit.list();

Criteria以及相關的樣例(Example)API將會再

第 15 章條件查詢(Criteria Queries)中詳細討論。

10.4.4. 使用原生SQL的查詢

You can express a query in SQL, using createSQLQuery() and let Hibernate manage the mapping from result sets to objects. You can at any time callsession.connection() and use the JDBC Connection directly. If you choose to use the Hibernate API, you must enclose SQL aliases in braces:

List cats = session.createSQLQuery("SELECT {cat.*} FROM CAT {cat} WHERE ROWNUM<10")
    .addEntity("cat", Cat.class)
.list();

List cats = session.createSQLQuery(
    "SELECT {cat}.ID AS {cat.id}, {cat}.SEX AS {cat.sex}, " +
           "{cat}.MATE AS {cat.mate}, {cat}.SUBCLASS AS {cat.class}, ... " +
    "FROM CAT {cat} WHERE ROWNUM<10")
    .addEntity("cat", Cat.class)
.list()

SQL queries can contain named and positional parameters, just like Hibernate queries. More information about native SQL queries in Hibernate can be found in

第 16 章Native SQL查詢.

10.5. 修改持久對象

Transactional persistent instances (i.e. objects loaded, saved, created or queried by theSession) can be manipulated by the application, and any changes to persistent state will be persisted when theSession is flushed. This is discussed later in this chapter. There is no need to call a particular method (likeupdate(), which has a different purpose) to make your modifications persistent. The most straightforward way to update the state of an object is toload() it and then manipulate it directly while the Session is open:

DomesticCat cat = (DomesticCat) sess.load( Cat.class, new Long(69) );
cat.setName("PK");
sess.flush();  // changes to cat are automatically detected and persisted

Sometimes this programming model is inefficient, as it requires in the same session both an SQLSELECT to load an object and an SQL UPDATE to persist its updated state. Hibernate offers an alternate approach by using detached instances.

重要

Hibernate does not offer its own API for direct execution of UPDATE or DELETE statements. Hibernate is a state management service, you do not have to think in statements to use it. JDBC is a perfect API for executing SQL statements, you can get a JDBCConnection at any time by calling session.connection(). Furthermore, the notion of mass operations conflicts with object/relational mapping for online transaction processing-oriented applications. Future versions of Hibernate can, however, provide special mass operation functions. See

第 13 章批量处理（Batch processing） for some possible batch operation tricks.

10.6. 修改脫管(Detached)對象

很多程序需要在某個事務中獲取對象，然後將對象發送到界面層去操作，最後在一個新的事務保存所做的修改。 在高併發訪問的環境中使用這種方式，通常使用附帶版本信息的數據來保證這些“長“工作單元之間的隔離。

Hibernate通過提供Session.update()或Session.merge() 重新關聯脫管實例的辦法來支持這種模型。

// in the first session
Cat cat = (Cat) firstSession.load(Cat.class, catId);
Cat potentialMate = new Cat();
firstSession.save(potentialMate);

// in a higher layer of the application
cat.setMate(potentialMate);

// later, in a new session
secondSession.update(cat);  // update cat
secondSession.update(mate); // update mate

如果具有catId持久化標識的Cat之前已經被另一Session(secondSession)裝載了， 應用程序進行重關聯操作(reattach)的時候會拋出一個異常。

Use update() if you are certain that the session does not contain an already persistent instance with the same identifier. Usemerge() if you want to merge your modifications at any time without consideration of the state of the session. In other words,update() is usually the first method you would call in a fresh session, ensuring that the reattachment of your detached instances is the first operation that is executed.

The application should individually update() detached instances that are reachable from the given detached instanceonly if it wants their state to be updated. This can be automated usingtransitive persistence. See

第 10.11 節 “傳播性持久化(transitive persistence)” for more information.

The lock() method also allows an application to reassociate an object with a new session. However, the detached instance has to be unmodified.

//just reassociate:
sess.lock(fritz, LockMode.NONE);
//do a version check, then reassociate:
sess.lock(izi, LockMode.READ);
//do a version check, using SELECT ... FOR UPDATE, then reassociate:
sess.lock(pk, LockMode.UPGRADE);

Note that lock() can be used with various LockModes. See the API documentation and the chapter on transaction handling for more information. Reattachment is not the only usecase forlock().

其他用於長時間工作單元的模型會在第 11.3 節 “樂觀併發控制(Optimistic concurrency control)”中討論。

10.7. 自動狀態檢測

Hibernate的用戶曾要求一個既可自動分配新持久化標識(identifier)保存瞬時(transient)對象，又可更新/重新關聯脫管(detached)實例的通用方法。saveOrUpdate()方法實現了這個功能。

// in the first session
Cat cat = (Cat) firstSession.load(Cat.class, catID);

// in a higher tier of the application
Cat mate = new Cat();
cat.setMate(mate);

// later, in a new session
secondSession.saveOrUpdate(cat);   // update existing state (cat has a non-null id)
secondSession.saveOrUpdate(mate);  // save the new instance (mate has a null id)

saveOrUpdate()用途和語義可能會使新用戶感到迷惑。 首先，只要你沒有嘗試在某個session中使用來自另一session的實例，你就應該不需要使用update()，saveOrUpdate()，或merge()。有些程序從來不用這些方法。

通常下面的場景會使用update()或saveOrUpdate()：

• 程序在第一個session中加載對象

• 該對象被傳遞到表現層

• 對象發生了一些改動

• 該對象被返回到業務邏輯層

• 程序調用第二個session的update()方法持久這些改動

saveOrUpdate()做下面的事:

• 如果對象已經在本session中持久化了，不做任何事

• 如果另一個與本session關聯的對象擁有相同的持久化標識(identifier)，拋出一個異常

• 如果對象沒有持久化標識(identifier)屬性，對其調用save()

• 如果對象的持久標識(identifier)表明其是一個新實例化的對象，對其調用save()

• if the object is versioned by a <version> or <timestamp>, and the version property value is the same value assigned to a newly instantiated object,save() it

• 否則update() 這個對象

merge()可非常不同:

• 如果session中存在相同持久化標識(identifier)的實例，用用戶給出的對象的狀態覆蓋舊有的持久實例

• 如果session沒有相應的持久實例，則嘗試從數據庫中加載，或創建新的持久化實例

• 最後返回該持久實例

• 用戶給出的這個對象沒有被關聯到session上，它依舊是脫管的

10.8. 刪除持久對象

Session.delete() will remove an object's state from the database. Your application, however, can still hold a reference to a deleted object. It is best to think ofdelete() as making a persistent instance, transient.

sess.delete(cat);

You can delete objects in any order, without risk of foreign key constraint violations. It is still possible to violate aNOT NULL constraint on a foreign key column by deleting objects in the wrong order, e.g. if you delete the parent, but forget to delete the children.

10.9. 在兩個不同數據庫間複製對象

It is sometimes useful to be able to take a graph of persistent instances and make them persistent in a different datastore, without regenerating identifier values.

//retrieve a cat from one database
Session session1 = factory1.openSession();
Transaction tx1 = session1.beginTransaction();
Cat cat = session1.get(Cat.class, catId);
tx1.commit();
session1.close();

//reconcile with a second database
Session session2 = factory2.openSession();
Transaction tx2 = session2.beginTransaction();
session2.replicate(cat, ReplicationMode.LATEST_VERSION);
tx2.commit();
session2.close();

The ReplicationMode determines how replicate() will deal with conflicts with existing rows in the database:

• ReplicationMode.IGNORE: ignores the object when there is an existing database row with the same identifier

• ReplicationMode.OVERWRITE: overwrites any existing database row with the same identifier

• ReplicationMode.EXCEPTION: throws an exception if there is an existing database row with the same identifier

• ReplicationMode.LATEST_VERSION: overwrites the row if its version number is earlier than the version number of the object, or ignore the object otherwise

這個功能的用途包括使錄入的數據在不同數據庫中一致，產品升級時升級系統配置信息，回滾non-ACID事務中的修改等等。 （譯註，non-ACID，非ACID;ACID，Atomic，Consistent，Isolated and Durable的縮寫）

10.10. Session刷出(flush)

Sometimes the Session will execute the SQL statements needed to synchronize the JDBC connection's state with the state of objects held in memory. This process, calledflush, occurs by default at the following points:

• 在某些查詢執行之前

• 在調用org.hibernate.Transaction.commit()的時候

• 在調用Session.flush()的時候

The SQL statements are issued in the following order:

1. all entity insertions in the same order the corresponding objects were saved usingSession.save()

2. 所有對實體進行更新的語句

3. 所有進行集合刪除的語句

4. 所有對集合元素進行刪除，更新或者插入的語句

5. 所有進行集合插入的語句

6. all entity deletions in the same order the corresponding objects were deleted usingSession.delete()

An exception is that objects using native ID generation are inserted when they are saved.

Except when you explicitly flush(), there are absolutely no guarantees aboutwhen the Session executes the JDBC calls, only theorder in which they are executed. However, Hibernate does guarantee that theQuery.list(..) will never return stale or incorrect data.

It is possible to change the default behavior so that flush occurs less frequently. TheFlushMode class defines three different modes: only flush at commit time when the HibernateTransaction API is used, flush automatically using the explained routine, or never flush unlessflush() is called explicitly. The last mode is useful for long running units of work, where aSession is kept open and disconnected for a long time (see

第 11.3.2 節 “擴展週期的session和自動版本化”).

sess = sf.openSession();
Transaction tx = sess.beginTransaction();
sess.setFlushMode(FlushMode.COMMIT); // allow queries to return stale state

Cat izi = (Cat) sess.load(Cat.class, id);
izi.setName(iznizi);

// might return stale data
sess.find("from Cat as cat left outer join cat.kittens kitten");

// change to izi is not flushed!
...
tx.commit(); // flush occurs
sess.close();

刷出(flush)期間，可能會拋出異常。（例如一個DML操作違反了約束） 異常處理涉及到對Hibernate事務性行爲的理解，因此我們將在第 11 章 Transactions and Concurrency中討論。

10.11. 傳播性持久化(transitive persistence)

對每一個對象都要執行保存，刪除或重關聯操作讓人感覺有點麻煩，尤其是在處理許多彼此關聯的對象的時候。 一個常見的例子是父子關係。考慮下面的例子:

If the children in a parent/child relationship would be value typed (e.g. a collection of addresses or strings), their life cycle would depend on the parent and no further action would be required for convenient "cascading" of state changes. When the parent is saved, the value-typed child objects are saved and when the parent is deleted, the children will be deleted, etc. This works for operations such as the removal of a child from the collection. Since value-typed objects cannot have shared references, Hibernate will detect this and delete the child from the database.

Now consider the same scenario with parent and child objects being entities, not value-types (e.g. categories and items, or parent and child cats). Entities have their own life cycle and support shared references. Removing an entity from the collection does not mean it can be deleted), and there is by default no cascading of state from one entity to any other associated entities. Hibernate does not implementpersistence by reachability by default.

每個Hibernate session的基本操作 - 包括 persist(), merge(), saveOrUpdate(), delete(), lock(), refresh(), evict(), replicate() - 都有對應的級聯風格(cascade style)。 這些級聯風格(cascade style)風格分別命名爲create, merge, save-update, delete, lock, refresh, evict, replicate。 如果你希望一個操作被順着關聯關係級聯傳播，你必須在映射文件中指出這一點。例如：

<one-to-one name="person" cascade="persist"/>

級聯風格(cascade style)是可組合的:

<one-to-one name="person" cascade="persist,delete,lock"/>

You can even use cascade="all" to specify that all operations should be cascaded along the association. The defaultcascade="none" specifies that no operations are to be cascaded.

注意有一個特殊的級聯風格(cascade style) delete-orphan，只應用於one-to-many關聯，表明delete()操作 應該被應用於所有從關聯中刪除的對象。

建議:

• It does not usually make sense to enable cascade on a <many-to-one> or<many-to-many> association. Cascade is often useful for<one-to-one> and <one-to-many> associations.

• 如果子對象的壽命限定在父親對象的壽命之內，可通過指定cascade="all,delete-orphan"將其變爲自動生命週期管理的對象(life cycle object)。

• 其他情況，你可根本不需要級聯(cascade)。但是如果你認爲你會經常在某個事務中同時用到父對象與子對象，並且你希望少打點兒字，可以考慮使用cascade="persist,merge,save-update"。

可以使用cascade="all"將一個關聯關係（無論是對值對象的關聯，或者對一個集合的關聯）標記爲父/子關係的關聯。 這樣對父對象進行save/update/delete操作就會導致子對象也進行save/update/delete操作。

Furthermore, a mere reference to a child from a persistent parent will result in save/update of the child. This metaphor is incomplete, however. A child which becomes unreferenced by its parent isnot automatically deleted, except in the case of a<one-to-many> association mapped with cascade="delete-orphan". The precise semantics of cascading operations for a parent/child relationship are as follows:

• 如果父對象被persist()，那麼所有子對象也會被persist()

• 如果父對象被merge()，那麼所有子對象也會被merge()

• 如果父對象被save()，update()或saveOrUpdate()，那麼所有子對象則會被saveOrUpdate()

• 如果某個持久的父對象引用了瞬時(transient)或者脫管(detached)的子對象，那麼子對象將會被saveOrUpdate()

• 如果父對象被刪除，那麼所有子對象也會被delete()

• 除非被標記爲cascade="delete-orphan"（刪除“孤兒”模式，此時不被任何一個父對象引用的子對象會被刪除）， 否則子對象失掉父對象對其的引用時，什麼事也不會發生。 如果有特殊需要，應用程序可通過顯式調用delete()刪除子對象。

Finally, note that cascading of operations can be applied to an object graph atcall time or at flush time. All operations, if enabled, are cascaded to associated entities reachable when the operation is executed. However,save-update and delete-orphan are transitive for all associated entities reachable during flush of theSession.

10.12. 使用元數據

Hibernate requires a rich meta-level model of all entity and value types. This model can be useful to the application itself. For example, the application might use Hibernate's metadata to implement a "smart" deep-copy algorithm that understands which objects should be copied (eg. mutable value types) and which objects that should not (e.g. immutable value types and, possibly, associated entities).

Hibernate exposes metadata via the ClassMetadata andCollectionMetadata interfaces and the Type hierarchy. Instances of the metadata interfaces can be obtained from theSessionFactory.

Cat fritz = ......;
ClassMetadata catMeta = sessionfactory.getClassMetadata(Cat.class);

Object[] propertyValues = catMeta.getPropertyValues(fritz);
String[] propertyNames = catMeta.getPropertyNames();
Type[] propertyTypes = catMeta.getPropertyTypes();

// get a Map of all properties which are not collections or associations
Map namedValues = new HashMap();
for ( int i=0; i<propertyNames.length; i++ ) {
    if ( !propertyTypes[i].isEntityType() && !propertyTypes[i].isCollectionType() ) {
        namedValues.put( propertyNames[i], propertyValues[i] );
    }
}

第 11 章 Transactions and Concurrency

The most important point about Hibernate and concurrency control is that it is easy to understand. Hibernate directly uses JDBC connections and JTA resources without adding any additional locking behavior. It is recommended that you spend some time with the JDBC, ANSI, and transaction isolation specification of your database management system.

Hibernate does not lock objects in memory. Your application can expect the behavior as defined by the isolation level of your database transactions. ThroughSession, which is also a transaction-scoped cache, Hibernate provides repeatable reads for lookup by identifier and entity queries and not reporting queries that return scalar values.

In addition to versioning for automatic optimistic concurrency control, Hibernate also offers, using theSELECT FOR UPDATE syntax, a (minor) API for pessimistic locking of rows. Optimistic concurrency control and this API are discussed later in this chapter.

The discussion of concurrency control in Hibernate begins with the granularity ofConfiguration, SessionFactory, andSession, as well as database transactions and long conversations.

11.1. Session和事務範圍(transaction scope)

A SessionFactory is an expensive-to-create, threadsafe object, intended to be shared by all application threads. It is created once, usually on application startup, from aConfiguration instance.

A Session is an inexpensive, non-threadsafe object that should be used once and then discarded for: a single request, a conversation or a single unit of work. ASession will not obtain a JDBC Connection, or a Datasource, unless it is needed. It will not consume any resources until used.

In order to reduce lock contention in the database, a database transaction has to be as short as possible. Long database transactions will prevent your application from scaling to a highly concurrent load. It is not recommended that you hold a database transaction open during user think time until the unit of work is complete.

What is the scope of a unit of work? Can a single Hibernate Session span several database transactions, or is this a one-to-one relationship of scopes? When should you open and close aSession and how do you demarcate the database transaction boundaries? These questions are addressed in the following sections.

11.1.1. 操作單元(Unit of work)

First, let's define a unit of work. A unit of work is a design pattern described by Martin Fowler as “ [maintaining] a list of objects affected by a business transaction and coordinates the writing out of changes and the resolution of concurrency problems. ”[

PoEAA] In other words, its a series of operations we wish to carry out against the database together. Basically, it is a transaction, though fulfilling a unit of work will often span multiple physical database transactions (see第 11.1.2 節 “長對話”). So really we are talking about a more abstract notion of a transaction. The term "business transaction" is also sometimes used in lieu of unit of work.

Do not use the session-per-operation antipattern: do not open and close aSession for every simple database call in a single thread. The same is true for database transactions. Database calls in an application are made using a planned sequence; they are grouped into atomic units of work. This also means that auto-commit after every single SQL statement is useless in an application as this mode is intended for ad-hoc SQL console work. Hibernate disables, or expects the application server to disable, auto-commit mode immediately. Database transactions are never optional. All communication with a database has to occur inside a transaction. Auto-commit behavior for reading data should be avoided, as many small transactions are unlikely to perform better than one clearly defined unit of work. The latter is also more maintainable and extensible.

The most common pattern in a multi-user client/server application is session-per-request. In this model, a request from the client is sent to the server, where the Hibernate persistence layer runs. A new HibernateSession is opened, and all database operations are executed in this unit of work. On completion of the work, and once the response for the client has been prepared, the session is flushed and closed. Use a single database transaction to serve the clients request, starting and committing it when you open and close theSession. The relationship between the two is one-to-one and this model is a perfect fit for many applications.

The challenge lies in the implementation. Hibernate provides built-in management of the "current session" to simplify this pattern. Start a transaction when a server request has to be processed, and end the transaction before the response is sent to the client. Common solutions are ServletFilter, AOP interceptor with a pointcut on the service methods, or a proxy/interception container. An EJB container is a standardized way to implement cross-cutting aspects such as transaction demarcation on EJB session beans, declaratively with CMT. If you use programmatic transaction demarcation, for ease of use and code portability use the HibernateTransaction API shown later in this chapter.

Your application code can access a "current session" to process the request by callingsessionFactory.getCurrentSession(). You will always get aSession scoped to the current database transaction. This has to be configured for either resource-local or JTA environments, see第 2.5 節 “Contextual sessions”.

You can extend the scope of a Session and database transaction until the "view has been rendered". This is especially useful in servlet applications that utilize a separate rendering phase after the request has been processed. Extending the database transaction until view rendering, is achieved by implementing your own interceptor. However, this will be difficult if you rely on EJBs with container-managed transactions. A transaction will be completed when an EJB method returns, before rendering of any view can start. See the Hibernate website and forum for tips and examples relating to thisOpen Session in View pattern.

11.1.2. 長對話

The session-per-request pattern is not the only way of designing units of work. Many business processes require a whole series of interactions with the user that are interleaved with database accesses. In web and enterprise applications, it is not acceptable for a database transaction to span a user interaction. Consider the following example:

• The first screen of a dialog opens. The data seen by the user has been loaded in a particularSession and database transaction. The user is free to modify the objects.

• The user clicks "Save" after 5 minutes and expects their modifications to be made persistent. The user also expects that they were the only person editing this information and that no conflicting modification has occurred.

From the point of view of the user, we call this unit of work a long-running conversation or application transaction. There are many ways to implement this in your application.

A first naive implementation might keep the Session and database transaction open during user think time, with locks held in the database to prevent concurrent modification and to guarantee isolation and atomicity. This is an anti-pattern, since lock contention would not allow the application to scale with the number of concurrent users.

You have to use several database transactions to implement the conversation. In this case, maintaining isolation of business processes becomes the partial responsibility of the application tier. A single conversation usually spans several database transactions. It will be atomic if only one of these database transactions (the last one) stores the updated data. All others simply read data (for example, in a wizard-style dialog spanning several request/response cycles). This is easier to implement than it might sound, especially if you utilize some of Hibernate's features:

• Automatic Versioning: Hibernate can perform automatic optimistic concurrency control for you. It can automatically detect if a concurrent modification occurred during user think time. Check for this at the end of the conversation.

• Detached Objects: if you decide to use thesession-per-request pattern, all loaded instances will be in the detached state during user think time. Hibernate allows you to reattach the objects and persist the modifications. The pattern is calledsession-per-request-with-detached-objects. Automatic versioning is used to isolate concurrent modifications.

• Extended (or Long) Session: the HibernateSession can be disconnected from the underlying JDBC connection after the database transaction has been committed and reconnected when a new client request occurs. This pattern is known assession-per-conversation and makes even reattachment unnecessary. Automatic versioning is used to isolate concurrent modifications and theSession will not be allowed to be flushed automatically, but explicitly.

Both session-per-request-with-detached-objects andsession-per-conversation have advantages and disadvantages. These disadvantages are discussed later in this chapter in the context of optimistic concurrency control.

11.1.3. 關注對象標識(Considering object identity)

An application can concurrently access the same persistent state in two differentSessions. However, an instance of a persistent class is never shared between twoSession instances. It is for this reason that there are two different notions of identity:

數據庫標識

foo.getId().equals( bar.getId() )

JVM 標識

foo==bar

For objects attached to a particular Session (i.e., in the scope of a Session), the two notions are equivalent and JVM identity for database identity is guaranteed by Hibernate. While the application might concurrently access the "same" (persistent identity) business object in two different sessions, the two instances will actually be "different" (JVM identity). Conflicts are resolved using an optimistic approach and automatic versioning at flush/commit time.

This approach leaves Hibernate and the database to worry about concurrency. It also provides the best scalability, since guaranteeing identity in single-threaded units of work means that it does not need expensive locking or other means of synchronization. The application does not need to synchronize on any business object, as long as it maintains a single thread perSession. Within a Session the application can safely use== to compare objects.

However, an application that uses == outside of aSession might produce unexpected results. This might occur even in some unexpected places. For example, if you put two detached instances into the sameSet, both might have the same database identity (i.e., they represent the same row). JVM identity, however, is by definition not guaranteed for instances in a detached state. The developer has to override theequals() and hashCode() methods in persistent classes and implement their own notion of object equality. There is one caveat: never use the database identifier to implement equality. Use a business key that is a combination of unique, usually immutable, attributes. The database identifier will change if a transient object is made persistent. If the transient instance (usually together with detached instances) is held in aSet, changing the hashcode breaks the contract of theSet. Attributes for business keys do not have to be as stable as database primary keys; you only have to guarantee stability as long as the objects are in the sameSet. See the Hibernate website for a more thorough discussion of this issue. Please note that this is not a Hibernate issue, but simply how Java object identity and equality has to be implemented.

11.1.4. 常見問題

Do not use the anti-patterns session-per-user-session orsession-per-application (there are, however, rare exceptions to this rule). Some of the following issues might also arise within the recommended patterns, so ensure that you understand the implications before making a design decision:

• A Session is not thread-safe. Things that work concurrently, like HTTP requests, session beans, or Swing workers, will cause race conditions if aSession instance is shared. If you keep your HibernateSession in your HttpSession (this is discussed later in the chapter), you should consider synchronizing access to your Http session. Otherwise, a user that clicks reload fast enough can use the sameSession in two concurrently running threads.

• An exception thrown by Hibernate means you have to rollback your database transaction and close theSession immediately (this is discussed in more detail later in the chapter). If yourSession is bound to the application, you have to stop the application. Rolling back the database transaction does not put your business objects back into the state they were at the start of the transaction. This means that the database state and the business objects will be out of sync. Usually this is not a problem, because exceptions are not recoverable and you will have to start over after rollback anyway.

• The Session caches every object that is in a persistent state (watched and checked for dirty state by Hibernate). If you keep it open for a long time or simply load too much data, it will grow endlessly until you get an OutOfMemoryException. One solution is to call clear() and evict() to manage the Session cache, but you should consider a Stored Procedure if you need mass data operations. Some solutions are shown in

• 第 13 章批量处理（Batch processing）. Keeping a Session open for the duration of a user session also means a higher probability of stale data.

11.2. 數據庫事務聲明

Database, or system, transaction boundaries are always necessary. No communication with the database can occur outside of a database transaction (this seems to confuse many developers who are used to the auto-commit mode). Always use clear transaction boundaries, even for read-only operations. Depending on your isolation level and database capabilities this might not be required, but there is no downside if you always demarcate transactions explicitly. Certainly, a single database transaction is going to perform better than many small transactions, even for reading data.

A Hibernate application can run in non-managed (i.e., standalone, simple Web- or Swing applications) and managed J2EE environments. In a non-managed environment, Hibernate is usually responsible for its own database connection pool. The application developer has to manually set transaction boundaries (begin, commit, or rollback database transactions) themselves. A managed environment usually provides container-managed transactions (CMT), with the transaction assembly defined declaratively (in deployment descriptors of EJB session beans, for example). Programmatic transaction demarcation is then no longer necessary.

However, it is often desirable to keep your persistence layer portable between non-managed resource-local environments, and systems that can rely on JTA but use BMT instead of CMT. In both cases use programmatic transaction demarcation. Hibernate offers a wrapper API called Transaction that translates into the native transaction system of your deployment environment. This API is actually optional, but we strongly encourage its use unless you are in a CMT session bean.

Ending a Session usually involves four distinct phases:

• 同步session(flush,刷出到磁盤）

• 提交事務

• 關閉session

• 處理異常

We discussed Flushing the session earlier, so we will now have a closer look at transaction demarcation and exception handling in both managed and non-managed environments.

11.2.1. 非託管環境

If a Hibernate persistence layer runs in a non-managed environment, database connections are usually handled by simple (i.e., non-DataSource) connection pools from which Hibernate obtains connections as needed. The session/transaction handling idiom looks like this:

// Non-managed environment idiom
Session sess = factory.openSession();
Transaction tx = null;
try {
    tx = sess.beginTransaction();

    // do some work
    ...

    tx.commit();
}
catch (RuntimeException e) {
    if (tx != null) tx.rollback();
    throw e; // or display error message
}
finally {
    sess.close();
}

You do not have to flush() the Session explicitly: the call to commit() automatically triggers the synchronization depending on the

FlushMode for the session. A call toclose() marks the end of a session. The main implication ofclose() is that the JDBC connection will be relinquished by the session. This Java code is portable and runs in both non-managed and JTA environments.

As outlined earlier, a much more flexible solution is Hibernate's built-in "current session" context management:

// Non-managed environment idiom with getCurrentSession()
try {
    factory.getCurrentSession().beginTransaction();

    // do some work
    ...

    factory.getCurrentSession().getTransaction().commit();
}
catch (RuntimeException e) {
    factory.getCurrentSession().getTransaction().rollback();
    throw e; // or display error message
}

You will not see these code snippets in a regular application; fatal (system) exceptions should always be caught at the "top". In other words, the code that executes Hibernate calls in the persistence layer, and the code that handlesRuntimeException (and usually can only clean up and exit), are in different layers. The current context management by Hibernate can significantly simplify this design by accessing aSessionFactory. Exception handling is discussed later in this chapter.

You should select org.hibernate.transaction.JDBCTransactionFactory, which is the default, and for the second example select"thread" as your hibernate.current_session_context_class.

11.2.2. 使用JTA

If your persistence layer runs in an application server (for example, behind EJB session beans), every datasource connection obtained by Hibernate will automatically be part of the global JTA transaction. You can also install a standalone JTA implementation and use it without EJB. Hibernate offers two strategies for JTA integration.

If you use bean-managed transactions (BMT), Hibernate will tell the application server to start and end a BMT transaction if you use theTransaction API. The transaction management code is identical to the non-managed environment.

// BMT idiom
Session sess = factory.openSession();
Transaction tx = null;
try {
    tx = sess.beginTransaction();

    // do some work
    ...

    tx.commit();
}
catch (RuntimeException e) {
    if (tx != null) tx.rollback();
    throw e; // or display error message
}
finally {
    sess.close();
}

If you want to use a transaction-bound Session, that is, thegetCurrentSession() functionality for easy context propagation, use the JTAUserTransaction API directly:

// BMT idiom with getCurrentSession()
try {
    UserTransaction tx = (UserTransaction)new InitialContext()
                            .lookup("java:comp/UserTransaction");

    tx.begin();

    // Do some work on Session bound to transaction
    factory.getCurrentSession().load(...);
    factory.getCurrentSession().persist(...);

    tx.commit();
}
catch (RuntimeException e) {
    tx.rollback();
    throw e; // or display error message
}

With CMT, transaction demarcation is completed in session bean deployment descriptors, not programmatically. The code is reduced to:

// CMT idiom
 Session sess = factory.getCurrentSession();

 // do some work
 ...

In a CMT/EJB, even rollback happens automatically. An unhandled RuntimeException thrown by a session bean method tells the container to set the global transaction to rollback.You do not need to use the Hibernate Transaction API at all with BMT or CMT, and you get automatic propagation of the "current" Session bound to the transaction.

When configuring Hibernate's transaction factory, choose org.hibernate.transaction.JTATransactionFactory if you use JTA directly (BMT), andorg.hibernate.transaction.CMTTransactionFactory in a CMT session bean. Remember to also sethibernate.transaction.manager_lookup_class. Ensure that yourhibernate.current_session_context_class is either unset (backwards compatibility), or is set to"jta".

The getCurrentSession() operation has one downside in a JTA environment. There is one caveat to the use ofafter_statement connection release mode, which is then used by default. Due to a limitation of the JTA spec, it is not possible for Hibernate to automatically clean up any unclosedScrollableResults or Iterator instances returned byscroll() or iterate(). Youmust release the underlying database cursor by callingScrollableResults.close() or Hibernate.close(Iterator) explicitly from afinally block. Most applications can easily avoid usingscroll() or iterate() from the JTA or CMT code.)

11.2.3. 異常處理

If the Session throws an exception, including anySQLException, immediately rollback the database transaction, callSession.close() and discard the Session instance. Certain methods of Session willnot leave the session in a consistent state. No exception thrown by Hibernate can be treated as recoverable. Ensure that theSession will be closed by calling close() in a finally block.

The HibernateException, which wraps most of the errors that can occur in a Hibernate persistence layer, is an unchecked exception. It was not in older versions of Hibernate. In our opinion, we should not force the application developer to catch an unrecoverable exception at a low layer. In most systems, unchecked and fatal exceptions are handled in one of the first frames of the method call stack (i.e., in higher layers) and either an error message is presented to the application user or some other appropriate action is taken. Note that Hibernate might also throw other unchecked exceptions that are not aHibernateException. These are not recoverable and appropriate action should be taken.

Hibernate wraps SQLExceptions thrown while interacting with the database in aJDBCException. In fact, Hibernate will attempt to convert the exception into a more meaningful subclass ofJDBCException. The underlying SQLException is always available via JDBCException.getCause(). Hibernate converts theSQLException into an appropriate JDBCException subclass using the SQLExceptionConverter attached to theSessionFactory. By default, the SQLExceptionConverter is defined by the configured dialect. However, it is also possible to plug in a custom implementation. See the javadocs for theSQLExceptionConverterFactory class for details. The standardJDBCException subtypes are:

• JDBCConnectionException: indicates an error with the underlying JDBC communication.

• SQLGrammarException: indicates a grammar or syntax problem with the issued SQL.

• ConstraintViolationException: indicates some form of integrity constraint violation.

• LockAcquisitionException: indicates an error acquiring a lock level necessary to perform the requested operation.

• GenericJDBCException: a generic exception which did not fall into any of the other categories.

11.2.4. 事務超時

An important feature provided by a managed environment like EJB, that is never provided for non-managed code, is transaction timeout. Transaction timeouts ensure that no misbehaving transaction can indefinitely tie up resources while returning no response to the user. Outside a managed (JTA) environment, Hibernate cannot fully provide this functionality. However, Hibernate can at least control data access operations, ensuring that database level deadlocks and queries with huge result sets are limited by a defined timeout. In a managed environment, Hibernate can delegate transaction timeout to JTA. This functionality is abstracted by the HibernateTransaction object.

Session sess = factory.openSession();
try {
    //set transaction timeout to 3 seconds
    sess.getTransaction().setTimeout(3);
    sess.getTransaction().begin();

    // do some work
    ...

    sess.getTransaction().commit()
}
catch (RuntimeException e) {
    sess.getTransaction().rollback();
    throw e; // or display error message
}
finally {
    sess.close();
}

setTimeout() cannot be called in a CMT bean, where transaction timeouts must be defined declaratively.

11.3. 樂觀併發控制(Optimistic concurrency control)

The only approach that is consistent with high concurrency and high scalability, is optimistic concurrency control with versioning. Version checking uses version numbers, or timestamps, to detect conflicting updates and to prevent lost updates. Hibernate provides three possible approaches to writing application code that uses optimistic concurrency. The use cases we discuss are in the context of long conversations, but version checking also has the benefit of preventing lost updates in single database transactions.

11.3.1. 應用程序級別的版本檢查(Application version checking)

In an implementation without much help from Hibernate, each interaction with the database occurs in a newSession and the developer is responsible for reloading all persistent instances from the database before manipulating them. The application is forced to carry out its own version checking to ensure conversation transaction isolation. This approach is the least efficient in terms of database access. It is the approach most similar to entity EJBs.

// foo is an instance loaded by a previous Session
session = factory.openSession();
Transaction t = session.beginTransaction();

int oldVersion = foo.getVersion();
session.load( foo, foo.getKey() ); // load the current state
if ( oldVersion != foo.getVersion() ) throw new StaleObjectStateException();
foo.setProperty("bar");

t.commit();
session.close();

version 屬性使用 <version>來映射，如果對象 是髒數據，在同步的時候，Hibernate會自動增加版本號。

If you are operating in a low-data-concurrency environment, and do not require version checking, you can use this approach and skip the version check. In this case,last commit wins is the default strategy for long conversations. Be aware that this might confuse the users of the application, as they might experience lost updates without error messages or a chance to merge conflicting changes.

Manual version checking is only feasible in trivial circumstances and not practical for most applications. Often not only single instances, but complete graphs of modified objects, have to be checked. Hibernate offers automatic version checking with either an extended Session or detached instances as the design paradigm.

11.3.2. 擴展週期的session和自動版本化

A single Session instance and its persistent instances that are used for the whole conversation are known assession-per-conversation. Hibernate checks instance versions at flush time, throwing an exception if concurrent modification is detected. It is up to the developer to catch and handle this exception. Common options are the opportunity for the user to merge changes or to restart the business conversation with non-stale data.

The Session is disconnected from any underlying JDBC connection when waiting for user interaction. This approach is the most efficient in terms of database access. The application does not version check or reattach detached instances, nor does it have to reload instances in every database transaction.

// foo is an instance loaded earlier by the old session
Transaction t = session.beginTransaction(); // Obtain a new JDBC connection, start transaction

foo.setProperty("bar");

session.flush();    // Only for last transaction in conversation
t.commit();         // Also return JDBC connection
session.close();    // Only for last transaction in conversation

The foo object knows which Session it was loaded in. Beginning a new database transaction on an old session obtains a new connection and resumes the session. Committing a database transaction disconnects a session from the JDBC connection and returns the connection to the pool. After reconnection, to force a version check on data you are not updating, you can callSession.lock() with LockMode.READ on any objects that might have been updated by another transaction. You do not need to lock any data that youare updating. Usually you would set FlushMode.MANUAL on an extended Session, so that only the last database transaction cycle is allowed to actually persist all modifications made in this conversation. Only this last database transaction will include theflush() operation, and then close() the session to end the conversation.

This pattern is problematic if the Session is too big to be stored during user think time (for example, anHttpSession should be kept as small as possible). As theSession is also the first-level cache and contains all loaded objects, we can probably use this strategy only for a few request/response cycles. Use aSession only for a single conversation as it will soon have stale data.

Note

Earlier versions of Hibernate required explicit disconnection and reconnection of aSession. These methods are deprecated, as beginning and ending a transaction has the same effect.

Keep the disconnected Session close to the persistence layer. Use an EJB stateful session bean to hold theSession in a three-tier environment. Do not transfer it to the web layer, or even serialize it to a separate tier, to store it in theHttpSession.

The extended session pattern, or session-per-conversation, is more difficult to implement with automatic current session context management. You need to supply your own implementation of theCurrentSessionContext for this. See the Hibernate Wiki for examples.

11.3.3. 脫管對象(deatched object)和自動版本化

這種方式下，與持久化存儲的每次交互都發生在一個新的Session中。 然而，同一持久化對象實例可以在多次與數據庫的交互中重用。應用程序操縱脫管對象實例 的狀態，這個脫管對象實例最初是在另一個Session 中載入的，然後 調用Session.update()，Session.saveOrUpdate(), 或者Session.merge() 來重新關聯該對象實例。

// foo is an instance loaded by a previous Session
foo.setProperty("bar");
session = factory.openSession();
Transaction t = session.beginTransaction();
session.saveOrUpdate(foo); // Use merge() if "foo" might have been loaded already
t.commit();
session.close();

Again, Hibernate will check instance versions during flush, throwing an exception if conflicting updates occurred.

You can also call lock() instead of update(), and use LockMode.READ (performing a version check and bypassing all caches) if you are sure that the object has not been modified.

11.3.4. 定製自動版本化行爲

You can disable Hibernate's automatic version increment for particular properties and collections by setting theoptimistic-lock mapping attribute to false. Hibernate will then no longer increment versions if the property is dirty.

Legacy database schemas are often static and cannot be modified. Or, other applications might access the same database and will not know how to handle version numbers or even timestamps. In both cases, versioning cannot rely on a particular column in a table. To force a version check with a comparison of the state of all fields in a row but without a version or timestamp property mapping, turn onoptimistic-lock="all" in the <class> mapping. This conceptually only works if Hibernate can compare the old and the new state (i.e., if you use a single longSession and not session-per-request-with-detached-objects).

Concurrent modification can be permitted in instances where the changes that have been made do not overlap. If you setoptimistic-lock="dirty" when mapping the <class>, Hibernate will only compare dirty fields during flush.

In both cases, with dedicated version/timestamp columns or with a full/dirty field comparison, Hibernate uses a singleUPDATE statement, with an appropriate WHERE clause, per entity to execute the version check and update the information. If you use transitive persistence to cascade reattachment to associated entities, Hibernate may execute unnecessary updates. This is usually not a problem, buton update triggers in the database might be executed even when no changes have been made to detached instances. You can customize this behavior by settingselect-before-update="true" in the <class> mapping, forcing Hibernate to SELECT the instance to ensure that changes did occur before updating the row.

11.4. Pessimistic locking

It is not intended that users spend much time worrying about locking strategies. It is usually enough to specify an isolation level for the JDBC connections and then simply let the database do all the work. However, advanced users may wish to obtain exclusive pessimistic locks or re-obtain locks at the start of a new transaction.

Hibernate will always use the locking mechanism of the database; it never lock objects in memory.

The LockMode class defines the different lock levels that can be acquired by Hibernate. A lock is obtained by the following mechanisms:

• 當Hibernate更新或者插入一行記錄的時候，鎖定級別自動設置爲LockMode.WRITE。

• LockMode.UPGRADE can be acquired upon explicit user request usingSELECT ... FOR UPDATE on databases which support that syntax.

• LockMode.UPGRADE_NOWAIT can be acquired upon explicit user request using aSELECT ... FOR UPDATE NOWAIT under Oracle.

• LockMode.READ is acquired automatically when Hibernate reads data under Repeatable Read or Serializable isolation level. It can be re-acquired by explicit user request.

• LockMode.NONE 代表無需鎖定。在Transaction結束時， 所有的對象都切換到該模式上來。與session相關聯的對象通過調用update() 或者saveOrUpdate()脫離該模式。

"顯式的用戶指定"可以通過以下幾種方式之一來表示:

• 調用 Session.load()的時候指定鎖定模式(LockMode)。

• 調用Session.lock()。

• 調用Query.setLockMode()。

如果在UPGRADE或者UPGRADE_NOWAIT鎖定模式下調 用Session.load()，並且要讀取的對象尚未被session載入過，那麼對象 通過SELECT ... FOR UPDATE這樣的SQL語句被載入。如果爲一個對象調用load()方法時，該對象已經在另一個較少限制的鎖定模式下被載入了，那 麼Hibernate就對該對象調用lock() 方法。

Session.lock() performs a version number check if the specified lock mode isREAD, UPGRADE or UPGRADE_NOWAIT. In the case of UPGRADE or UPGRADE_NOWAIT, SELECT ... FOR UPDATE is used.

If the requested lock mode is not supported by the database, Hibernate uses an appropriate alternate mode instead of throwing an exception. This ensures that applications are portable.

11.5. Connection release modes

One of the legacies of Hibernate 2.x JDBC connection management meant that a Session would obtain a connection when it was first required and then maintain that connection until the session was closed. Hibernate 3.x introduced the notion of connection release modes that would instruct a session how to handle its JDBC connections. The following discussion is pertinent only to connections provided through a configuredConnectionProvider. User-supplied connections are outside the breadth of this discussion. The different release modes are identified by the enumerated values oforg.hibernate.ConnectionReleaseMode:

• ON_CLOSE: is the legacy behavior described above. The Hibernate session obtains a connection when it first needs to perform some JDBC access and maintains that connection until the session is closed.

• AFTER_TRANSACTION: releases connections after a org.hibernate.Transaction has been completed.

• AFTER_STATEMENT (also referred to as aggressive release): releases connections after every statement execution. This aggressive releasing is skipped if that statement leaves open resources associated with the given session. Currently the only situation where this occurs is through the use of org.hibernate.ScrollableResults.

The configuration parameter hibernate.connection.release_mode is used to specify which release mode to use. The possible values are as follows:

• auto (the default): this choice delegates to the release mode returned by theorg.hibernate.transaction.TransactionFactory.getDefaultReleaseMode() method. For JTATransactionFactory, this returns ConnectionReleaseMode.AFTER_STATEMENT; for JDBCTransactionFactory, this returns ConnectionReleaseMode.AFTER_TRANSACTION. Do not change this default behavior as failures due to the value of this setting tend to indicate bugs and/or invalid assumptions in user code.

• on_close: uses ConnectionReleaseMode.ON_CLOSE. This setting is left for backwards compatibility, but its use is discouraged.

• after_transaction: uses ConnectionReleaseMode.AFTER_TRANSACTION. This setting should not be used in JTA environments. Also note that with ConnectionReleaseMode.AFTER_TRANSACTION, if a session is considered to be in auto-commit mode, connections will be released as if the release mode were AFTER_STATEMENT.

• after_statement: uses ConnectionReleaseMode.AFTER_STATEMENT. Additionally, the configuredConnectionProvider is consulted to see if it supports this setting (supportsAggressiveRelease()). If not, the release mode is reset to ConnectionReleaseMode.AFTER_TRANSACTION. This setting is only safe in environments where we can either re-acquire the same underlying JDBC connection each time you make a call intoConnectionProvider.getConnection() or in auto-commit environments where it does not matter if we re-establish the same connection.

第 12 章 攔截器與事件(Interceptors and events)

It is useful for the application to react to certain events that occur inside Hibernate. This allows for the implementation of generic functionality and the extension of Hibernate functionality.

12.1. 攔截器(Interceptors)

The Interceptor interface provides callbacks from the session to the application, allowing the application to inspect and/or manipulate properties of a persistent object before it is saved, updated, deleted or loaded. One possible use for this is to track auditing information. For example, the following Interceptor automatically sets the createTimestamp when anAuditable is created and updates the lastUpdateTimestamp property when an Auditable is updated.

You can either implement Interceptor directly or extendEmptyInterceptor.

package org.hibernate.test;

import java.io.Serializable;
import java.util.Date;
import java.util.Iterator;

import org.hibernate.EmptyInterceptor;
import org.hibernate.Transaction;
import org.hibernate.type.Type;

public class AuditInterceptor extends EmptyInterceptor {

    private int updates;
    private int creates;
    private int loads;

    public void onDelete(Object entity,
                         Serializable id,
                         Object[] state,
                         String[] propertyNames,
                         Type[] types) {
        // do nothing
    }

    public boolean onFlushDirty(Object entity,
                                Serializable id,
                                Object[] currentState,
                                Object[] previousState,
                                String[] propertyNames,
                                Type[] types) {

        if ( entity instanceof Auditable ) {
            updates++;
            for ( int i=0; i < propertyNames.length; i++ ) {
                if ( "lastUpdateTimestamp".equals( propertyNames[i] ) ) {
                    currentState[i] = new Date();
                    return true;
                }
            }
        }
        return false;
    }

    public boolean onLoad(Object entity,
                          Serializable id,
                          Object[] state,
                          String[] propertyNames,
                          Type[] types) {
        if ( entity instanceof Auditable ) {
            loads++;
        }
        return false;
    }

    public boolean onSave(Object entity,
                          Serializable id,
                          Object[] state,
                          String[] propertyNames,
                          Type[] types) {

        if ( entity instanceof Auditable ) {
            creates++;
            for ( int i=0; i<propertyNames.length; i++ ) {
                if ( "createTimestamp".equals( propertyNames[i] ) ) {
                    state[i] = new Date();
                    return true;
                }
            }
        }
        return false;
    }

    public void afterTransactionCompletion(Transaction tx) {
        if ( tx.wasCommitted() ) {
            System.out.println("Creations: " + creates + ", Updates: " + updates, "Loads: " + loads);
        }
        updates=0;
        creates=0;
        loads=0;
    }

}

There are two kinds of inteceptors: Session-scoped andSessionFactory-scoped.

當使用某個重載的SessionFactory.openSession()使用Interceptor作爲參數調用打開一個session的時候，就指定了Session範圍內的攔截器。

Session session = sf.openSession( new AuditInterceptor() );

A SessionFactory-scoped interceptor is registered with theConfiguration object prior to building the SessionFactory. Unless a session is opened explicitly specifying the interceptor to use, the supplied interceptor will be applied to all sessions opened from thatSessionFactory. SessionFactory-scoped interceptors must be thread safe. Ensure that you do not store session-specific states, since multiple sessions will use this interceptor potentially concurrently.

new Configuration().setInterceptor( new AuditInterceptor() );

12.2. 事件系統(Event system)

If you have to react to particular events in your persistence layer, you can also use the Hibernate3event architecture. The event system can be used in addition, or as a replacement, for interceptors.

All the methods of the Session interface correlate to an event. You have aLoadEvent, a FlushEvent, etc. Consult the XML configuration-file DTD or theorg.hibernate.event package for the full list of defined event types. When a request is made of one of these methods, the HibernateSession generates an appropriate event and passes it to the configured event listeners for that type. Out-of-the-box, these listeners implement the same processing in which those methods always resulted. However, you are free to implement a customization of one of the listener interfaces (i.e., the LoadEvent is processed by the registered implementation of the LoadEventListener interface), in which case their implementation would be responsible for processing anyload() requests made of the Session.

The listeners should be considered singletons. This means they are shared between requests, and should not save any state as instance variables.

A custom listener implements the appropriate interface for the event it wants to process and/or extend one of the convenience base classes (or even the default event listeners used by Hibernate out-of-the-box as these are declared non-final for this purpose). Custom listeners can either be registered programmatically through the Configuration object, or specified in the Hibernate configuration XML. Declarative configuration through the properties file is not supported. Here is an example of a custom load event listener:

public class MyLoadListener implements LoadEventListener {
    // this is the single method defined by the LoadEventListener interface
    public void onLoad(LoadEvent event, LoadEventListener.LoadType loadType)
            throws HibernateException {
        if ( !MySecurity.isAuthorized( event.getEntityClassName(), event.getEntityId() ) ) {
            throw MySecurityException("Unauthorized access");
        }
    }
}

你還需要修改一處配置，來告訴Hibernate，除了默認的監聽器，還要附加選定的監聽器。

<hibernate-configuration>
    <session-factory>
        ...
        <event type="load">
            <listener class="com.eg.MyLoadListener"/>
            <listener class="org.hibernate.event.def.DefaultLoadEventListener"/>
        </event>
    </session-factory>
</hibernate-configuration>

Instead, you can register it programmatically:

Configuration cfg = new Configuration();
LoadEventListener[] stack = { new MyLoadListener(), new DefaultLoadEventListener() };
cfg.EventListeners().setLoadEventListeners(stack);

Listeners registered declaratively cannot share instances. If the same class name is used in multiple<listener/> elements, each reference will result in a separate instance of that class. If you need to share listener instances between listener types you must use the programmatic registration approach.

Why implement an interface and define the specific type during configuration? A listener implementation could implement multiple event listener interfaces. Having the type additionally defined during registration makes it easier to turn custom listeners on or off during configuration.

12.3. Hibernate的聲明式安全機制

Usually, declarative security in Hibernate applications is managed in a session facade layer. Hibernate3 allows certain actions to be permissioned via JACC, and authorized via JAAS. This is an optional functionality that is built on top of the event architecture.

首先，你必須要配置適當的事件監聽器（event listener），來激活使用JAAS管理授權的功能。

<listener type="pre-delete" class="org.hibernate.secure.JACCPreDeleteEventListener"/>
<listener type="pre-update" class="org.hibernate.secure.JACCPreUpdateEventListener"/>
<listener type="pre-insert" class="org.hibernate.secure.JACCPreInsertEventListener"/>
<listener type="pre-load" class="org.hibernate.secure.JACCPreLoadEventListener"/>

Note that <listener type="..." class="..."/> is shorthand for<event type="..."><listener class="..."/></event> when there is exactly one listener for a particular event type.

Next, while still in hibernate.cfg.xml, bind the permissions to roles:

<grant role="admin" entity-name="User" actions="insert,update,read"/>
<grant role="su" entity-name="User" actions="*"/>

這些角色的名字就是你的JACC provider所定義的角色的名字。

第 13 章 批量处理（Batch processing）

A naive approach to inserting 100,000 rows in the database using Hibernate might look like this:

Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();
for ( int i=0; i<100000; i++ ) {
    Customer customer = new Customer(.....);
    session.save(customer);
}
tx.commit();
session.close();

This would fall over with an OutOfMemoryException somewhere around the 50,000th row. That is because Hibernate caches all the newly insertedCustomer instances in the session-level cache. In this chapter we will show you how to avoid this problem.

If you are undertaking batch processing you will need to enable the use of JDBC batching. This is absolutely essential if you want to achieve optimal performance. Set the JDBC batch size to a reasonable number (10-50, for example):

hibernate.jdbc.batch_size 20

Hibernate disables insert batching at the JDBC level transparently if you use anidentity identifier generator.

You can also do this kind of work in a process where interaction with the second-level cache is completely disabled:

hibernate.cache.use_second_level_cache false

ä½†æ˜¯ï¼Œè¿™ä¸æ˜¯ç»å¯¹å¿…é¡»çš„ï¼Œå› ä¸ºæˆ‘ä»¬å¯ä»¥æ˜¾å¼è®¾ç½®CacheMode来关闭与二级缓存的交互。

13.1. 批量插入（Batch inserts）

When making new objects persistent flush() and thenclear() the session regularly in order to control the size of the first-level cache.

Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();
   
for ( int i=0; i<100000; i++ ) {
    Customer customer = new Customer(.....);
    session.save(customer);
    if ( i % 20 == 0 ) { //20, same as the JDBC batch size
        //flush a batch of inserts and release memory:
        session.flush();
        session.clear();
    }
}
   
tx.commit();
session.close();

13.2. 批量更新（Batch updates）

For retrieving and updating data, the same ideas apply. In addition, you need to usescroll() to take advantage of server-side cursors for queries that return many rows of data.

Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();
   
ScrollableResults customers = session.getNamedQuery("GetCustomers")
    .setCacheMode(CacheMode.IGNORE)
    .scroll(ScrollMode.FORWARD_ONLY);
int count=0;
while ( customers.next() ) {
    Customer customer = (Customer) customers.get(0);
    customer.updateStuff(...);
    if ( ++count % 20 == 0 ) {
        //flush a batch of updates and release memory:
        session.flush();
        session.clear();
    }
}
   
tx.commit();
session.close();

13.3. StatelessSession (æ— çŠ¶æ€session)接口

Alternatively, Hibernate provides a command-oriented API that can be used for streaming data to and from the database in the form of detached objects. AStatelessSession has no persistence context associated with it and does not provide many of the higher-level life cycle semantics. In particular, a stateless session does not implement a first-level cache nor interact with any second-level or query cache. It does not implement transactional write-behind or automatic dirty checking. Operations performed using a stateless session never cascade to associated instances. Collections are ignored by a stateless session. Operations performed via a stateless session bypass Hibernate's event model and interceptors. Due to the lack of a first-level cache, Stateless sessions are vulnerable to data aliasing effects. A stateless session is a lower-level abstraction that is much closer to the underlying JDBC.

StatelessSession session = sessionFactory.openStatelessSession();
Transaction tx = session.beginTransaction();
   
ScrollableResults customers = session.getNamedQuery("GetCustomers")
    .scroll(ScrollMode.FORWARD_ONLY);
while ( customers.next() ) {
    Customer customer = (Customer) customers.get(0);
    customer.updateStuff(...);
    session.update(customer);
}
   
tx.commit();
session.close();

In this code example, the Customer instances returned by the query are immediately detached. They are never associated with any persistence context.

The insert(), update() and delete() operations defined by the StatelessSession interface are considered to be direct database row-level operations. They result in the immediate execution of a SQLINSERT, UPDATE or DELETE respectively. They have different semantics to thesave(), saveOrUpdate() and delete() operations defined by theSession interface.

13.4. DML(数据操作语言)é£Žæ ¼çš„æ“ä½œ(DML-style operations)

As already discussed, automatic and transparent object/relational mapping is concerned with the management of the object state. The object state is available in memory. This means that manipulating data directly in the database (using the SQLData Manipulation Language (DML) the statements: INSERT, UPDATE, DELETE) will not affect in-memory state. However, Hibernate provides methods for bulk SQL-style DML statement execution that is performed through the Hibernate Query Language (

HQL).

The pseudo-syntax for UPDATE and DELETE statements is: ( UPDATE | DELETE ) FROM? EntityName (WHERE where_conditions)?.

Some points to note:

• 在FROM子句（from-clause）中，FROM关键字是可选的

• There can only be a single entity named in the from-clause. It can, however, be aliased. If the entity name is aliased, then any property references must be qualified using that alias. If the entity name is not aliased, then it is illegal for any property references to be qualified.

• No joins, either implicit or explicit, can be specified in a bulk HQL query. Sub-queries can be used in the where-clause, where the subqueries themselves may contain joins.

• 整个WHERE子句是可选的。

As an example, to execute an HQL UPDATE, use the Query.executeUpdate() method. The method is named for those familiar with JDBC'sPreparedStatement.executeUpdate():

Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();

String hqlUpdate = "update Customer c set c.name = :newName where c.name = :oldName";
// or String hqlUpdate = "update Customer set name = :newName where name = :oldName";
int updatedEntities = s.createQuery( hqlUpdate )
        .setString( "newName", newName )
        .setString( "oldName", oldName )
        .executeUpdate();
tx.commit();
session.close();

In keeping with the EJB3 specification, HQL UPDATE statements, by default, do not effect theversion or the timestamp property values for the affected entities. However, you can force Hibernate to reset theversion or timestamp property values through the use of aversioned update. This is achieved by adding the VERSIONED keyword after the UPDATE keyword.

Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();
String hqlVersionedUpdate = "update versioned Customer set name = :newName where name = :oldName";
int updatedEntities = s.createQuery( hqlUpdate )
        .setString( "newName", newName )
        .setString( "oldName", oldName )
        .executeUpdate();
tx.commit();
session.close();

Custom version types, org.hibernate.usertype.UserVersionType, are not allowed in conjunction with aupdate versioned statement.

执行一个HQL DELETEï¼ŒåŒæ ·ä½¿ç”¨ Query.executeUpdate() 方法:

Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();

String hqlDelete = "delete Customer c where c.name = :oldName";
// or String hqlDelete = "delete Customer where name = :oldName";
int deletedEntities = s.createQuery( hqlDelete )
        .setString( "oldName", oldName )
        .executeUpdate();
tx.commit();
session.close();

The int value returned by the Query.executeUpdate() method indicates the number of entities effected by the operation. This may or may not correlate to the number of rows effected in the database. An HQL bulk operation might result in multiple actual SQL statements being executed (for joined-subclass, for example). The returned number indicates the number of actual entities affected by the statement. Going back to the example of joined-subclass, a delete against one of the subclasses may actually result in deletes against not just the table to which that subclass is mapped, but also the "root" table and potentially joined-subclass tables further down the inheritance hierarchy.

INSERTè¯­å¥çš„ä¼ªç æ˜¯: INSERT INTO EntityName properties_list select_statement. 要注意的是:

• 只支持INSERT INTO ... SELECT ...形式,不支持INSERT INTO ... VALUES ...形式.

  The properties_list is analogous to the column specification in the SQLINSERT statement. For entities involved in mapped inheritance, only properties directly defined on that given class-level can be used in the properties_list. Superclass properties are not allowed and subclass properties do not make sense. In other words, INSERT statements are inherently non-polymorphic.

• select_statement can be any valid HQL select query, with the caveat that the return types must match the types expected by the insert. Currently, this is checked during query compilation rather than allowing the check to relegate to the database. This might, however, cause problems between Hibernate Types which areequivalent as opposed to equal. This might cause issues with mismatches between a property defined as aorg.hibernate.type.DateType and a property defined as aorg.hibernate.type.TimestampType, even though the database might not make a distinction or might be able to handle the conversion.

• For the id property, the insert statement gives you two options. You can either explicitly specify the id property in the properties_list, in which case its value is taken from the corresponding select expression, or omit it from the properties_list, in which case a generated value is used. This latter option is only available when using id generators that operate in the database; attempting to use this option with any "in memory" type generators will cause an exception during parsing. For the purposes of this discussion, in-database generators are considered to be org.hibernate.id.SequenceGenerator (and its subclasses) and any implementers oforg.hibernate.id.PostInsertIdentifierGenerator. The most notable exception here isorg.hibernate.id.TableHiLoGenerator, which cannot be used because it does not expose a selectable way to get its values.

• For properties mapped as either version or timestamp, the insert statement gives you two options. You can either specify the property in the properties_list, in which case its value is taken from the corresponding select expressions, or omit it from the properties_list, in which case theseed value defined by the org.hibernate.type.VersionType is used.

The following is an example of an HQL INSERT statement execution:

Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();

String hqlInsert = "insert into DelinquentAccount (id, name) select c.id, c.name from Customer c where ...";
int createdEntities = s.createQuery( hqlInsert )
        .executeUpdate();
tx.commit();
session.close();

第 14 章 HQL: Hibernate查詢語言

Hibernate uses a powerful query language (HQL) that is similar in appearance to SQL. Compared with SQL, however, HQL is fully object-oriented and understands notions like inheritance, polymorphism and association.

14.1. 大小寫敏感性問題

With the exception of names of Java classes and properties, queries are case-insensitive. SoSeLeCT is the same as sELEct is the same asSELECT, but org.hibernate.eg.FOO is notorg.hibernate.eg.Foo, and foo.barSet is notfoo.BARSET.

This manual uses lowercase HQL keywords. Some users find queries with uppercase keywords more readable, but this convention is unsuitable for queries embedded in Java code.

14.2. from子句

Hibernate中最簡單的查詢語句的形式如下：

from eg.Cat

This returns all instances of the class eg.Cat. You do not usually need to qualify the class name, sinceauto-import is the default. For example:

from Cat

In order to refer to the Cat in other parts of the query, you will need to assign analias. For example:

from Cat as cat

This query assigns the alias cat to Cat instances, so you can use that alias later in the query. The as keyword is optional. You could also write:

from Cat cat

Multiple classes can appear, resulting in a cartesian product or "cross" join.

from Formula, Parameter

from Formula as form, Parameter as param

It is good practice to name query aliases using an initial lowercase as this is consistent with Java naming standards for local variables (e.g.domesticCat).

14.3. 關聯(Association)與連接(Join)

You can also assign aliases to associated entities or to elements of a collection of values using ajoin. For example:

from Cat as cat
    inner join cat.mate as mate
    left outer join cat.kittens as kitten

from Cat as cat left join cat.mate.kittens as kittens

from Formula form full join form.parameter param

The supported join types are borrowed from ANSI SQL:

• inner join（內連接）

• left outer join（左外連接）

• right outer join（右外連接）

• full join (全連接，並不常用)

語句inner join, left outer join 以及right outer join 可以簡寫。

from Cat as cat
    join cat.mate as mate
    left join cat.kittens as kitten

通過HQL的with關鍵字，你可以提供額外的join條件。

from Cat as cat
    left join cat.kittens as kitten
        with kitten.bodyWeight > 10.0

A "fetch" join allows associations or collections of values to be initialized along with their parent objects using a single select. This is particularly useful in the case of a collection. It effectively overrides the outer join and lazy declarations of the mapping file for associations and collections. See

第 19.1 節 “抓取策略(Fetching strategies)” for more information.

from Cat as cat
    inner join fetch cat.mate
    left join fetch cat.kittens

A fetch join does not usually need to assign an alias, because the associated objects should not be used in thewhere clause (or any other clause). The associated objects are also not returned directly in the query results. Instead, they may be accessed via the parent object. The only reason you might need an alias is if you are recursively join fetching a further collection:

from Cat as cat
    inner join fetch cat.mate
    left join fetch cat.kittens child
    left join fetch child.kittens

The fetch construct cannot be used in queries called usingiterate() (though scroll() can be used).Fetch should be used together with setMaxResults() or setFirstResult(), as these operations are based on the result rows which usually contain duplicates for eager collection fetching, hence, the number of rows is not what you would expect.Fetch should also not be used together with impromptuwith condition. It is possible to create a cartesian product by join fetching more than one collection in a query, so take care in this case. Join fetching multiple collection roles can produce unexpected results for bag mappings, so user discretion is advised when formulating queries in this case. Finally, note thatfull join fetch and right join fetch are not meaningful.

If you are using property-level lazy fetching (with bytecode instrumentation), it is possible to force Hibernate to fetch the lazy properties in the first query immediately usingfetch all properties.

from Document fetch all properties order by name

from Document doc fetch all properties where lower(doc.name) like '%cats%'

14.4. join 語法的形式

HQL支持兩種關聯join的形式：implicit(隱式) 與explicit（顯式）。

The queries shown in the previous section all use the explicit form, that is, where the join keyword is explicitly used in the from clause. This is the recommended form.

implicit（隱式）形式不使用join關鍵字。關聯使用"點號"來進行“引用”。implicit join可以在任何HQL子句中出現.implicit join在最終的SQL語句中以inner join的方式出現。

from Cat as cat where cat.mate.name like '%s%'

14.5. Referring to identifier property

There are 2 ways to refer to an entity's identifier property:

• The special property (lowercase) id may be used to reference the identifier property of an entityprovided that the entity does not define a non-identifier property named id.

• If the entity defines a named identifier property, you can use that property name.

References to composite identifier properties follow the same naming rules. If the entity has a non-identifier property named id, the composite identifier property can only be referenced by its defined named. Otherwise, the specialid property can be used to reference the identifier property.

重要

Please note that, starting in version 3.2.2, this has changed significantly. In previous versions,id always referred to the identifier property regardless of its actual name. A ramification of that decision was that non-identifier properties namedid could never be referenced in Hibernate queries.

14.6. select子句

The select clause picks which objects and properties to return in the query result set. Consider the following:

select mate
from Cat as cat
    inner join cat.mate as mate

The query will select mates of other Cats. You can express this query more compactly as:

select cat.mate from Cat cat

Queries can return properties of any value type including properties of component type:

select cat.name from DomesticCat cat
where cat.name like 'fri%'

select cust.name.firstName from Customer as cust

Queries can return multiple objects and/or properties as an array of type Object[]:

select mother, offspr, mate.name
from DomesticCat as mother
    inner join mother.mate as mate
    left outer join mother.kittens as offspr

Or as a List:

select new list(mother, offspr, mate.name)
from DomesticCat as mother
    inner join mother.mate as mate
    left outer join mother.kittens as offspr

Or - assuming that the class Family has an appropriate constructor - as an actual typesafe Java object:

select new Family(mother, mate, offspr)
from DomesticCat as mother
    join mother.mate as mate
    left join mother.kittens as offspr

You can assign aliases to selected expressions using as:

select max(bodyWeight) as max, min(bodyWeight) as min, count(*) as n
from Cat cat

這種做法在與子句select new map一起使用時最有用:

select new map( max(bodyWeight) as max, min(bodyWeight) as min, count(*) as n )
from Cat cat

該查詢返回了一個Map的對象，內容是別名與被選擇的值組成的名-值映射。

14.7. 聚集函數

HQL queries can even return the results of aggregate functions on properties:

select avg(cat.weight), sum(cat.weight), max(cat.weight), count(cat)
from Cat cat

The supported aggregate functions are:

• avg(...), sum(...), min(...), max(...)

• count(*)

• count(...), count(distinct ...), count(all...)

You can use arithmetic operators, concatenation, and recognized SQL functions in the select clause:

select cat.weight + sum(kitten.weight)
from Cat cat
    join cat.kittens kitten
group by cat.id, cat.weight

select firstName||' '||initial||' '||upper(lastName) from Person

The distinct and all keywords can be used and have the same semantics as in SQL.

select distinct cat.name from Cat cat

select count(distinct cat.name), count(cat) from Cat cat

14.8. 多態查詢

一個如下的查詢語句:

from Cat as cat

returns instances not only of Cat, but also of subclasses likeDomesticCat. Hibernate queries can name any Java class or interface in the from clause. The query will return instances of all persistent classes that extend that class or implement the interface. The following query would return all persistent objects:

from java.lang.Object o

接口Named 可能被各種各樣的持久化類聲明：

from Named n, Named m where n.name = m.name

These last two queries will require more than one SQL SELECT. This means that theorder by clause does not correctly order the whole result set. It also means you cannot call these queries usingQuery.scroll().

14.9. where子句

The where clause allows you to refine the list of instances returned. If no alias exists, you can refer to properties by name:

from Cat where name='Fritz'

如果指派了別名，需要使用完整的屬性名:

from Cat as cat where cat.name='Fritz'

This returns instances of Cat named 'Fritz'.

The following query:

select foo
from Foo foo, Bar bar
where foo.startDate = bar.date

returns all instances of Foo with an instance of bar with a date property equal to the startDate property of the Foo. Compound path expressions make thewhere clause extremely powerful. Consider the following:

from Cat cat where cat.mate.name is not null

This query translates to an SQL query with a table (inner) join. For example:

from Foo foo
where foo.bar.baz.customer.address.city is not null

would result in a query that would require four table joins in SQL.

The = operator can be used to compare not only properties, but also instances:

from Cat cat, Cat rival where cat.mate = rival.mate

select cat, mate
from Cat cat, Cat mate
where cat.mate = mate

The special property (lowercase) id can be used to reference the unique identifier of an object. See

第 14.5 節 “Referring to identifier property” for more information.

from Cat as cat where cat.id = 123

from Cat as cat where cat.mate.id = 69

The second query is efficient and does not require a table join.

Properties of composite identifiers can also be used. Consider the following example wherePerson has composite identifiers consisting of country and medicareNumber:

from bank.Person person
where person.id.country = 'AU'
    and person.id.medicareNumber = 123456

from bank.Account account
where account.owner.id.country = 'AU'
    and account.owner.id.medicareNumber = 123456

Once again, the second query does not require a table join.

See 第 14.5 節 “Referring to identifier property” for more information regarding referencing identifier properties)

The special property class accesses the discriminator value of an instance in the case of polymorphic persistence. A Java class name embedded in the where clause will be translated to its discriminator value.

from Cat cat where cat.class = DomesticCat

You can also use components or composite user types, or properties of said component types. See第 14.17 節 “translator-credits” for more information.

An "any" type has the special properties id and class that allows you to express a join in the following way (where AuditLog.item is a property mapped with <any>):

from AuditLog log, Payment payment
where log.item.class = 'Payment' and log.item.id = payment.id

The log.item.class and payment.class would refer to the values of completely different database columns in the above query.

14.10. 表達式

Expressions used in the where clause include the following:

• mathematical operators: +, -, *, /

• binary comparison operators: =, >=, <=, <>, !=, like

• 邏輯運算符and, or, not

• Parentheses ( ) that indicates grouping

• in, not in, between, is null, is not null,is empty, is not empty,member of and not member of

• "簡單的" case, case ... when ... then ... else ... end,和 "搜索" case,case when ... then ... else ... end

• 字符串連接符...||... or concat(...,...)

• current_date(), current_time(), andcurrent_timestamp()

• second(...), minute(...),hour(...), day(...), month(...), and year(...)

• EJB-QL 3.0定義的任何函數或操作：substring(), trim(), lower(), upper(), length(), locate(), abs(), sqrt(), bit_length()， mod()

• coalesce() 和 nullif()

• str() 把數字或者時間值轉換爲可讀的字符串

• cast(... as ...), 其第二個參數是某Hibernate類型的名字，以及extract(... from ...)，只要ANSIcast() 和 extract() 被底層數據庫支持

• HQL index() 函數，作用於join的有序集合的別名。

• HQL functions that take collection-valued path expressions: size(), minelement(), maxelement(), minindex(), maxindex(), along with the specialelements() and indices functions that can be quantified usingsome, all, exists, any, in.

• Any database-supported SQL scalar function like sign(),trunc(), rtrim(), and sin()

• JDBC風格的參數傳入 ?

• named parameters :name, :start_date, and:x1

• SQL 直接常量 'foo', 69, 6.66E+2, '1970-01-01 10:00:01.0'

• Java public static final 類型的常量 eg.Color.TABBY

in and between can be used as follows:

from DomesticCat cat where cat.name between 'A' and 'B'

from DomesticCat cat where cat.name in ( 'Foo', 'Bar', 'Baz' )

The negated forms can be written as follows:

from DomesticCat cat where cat.name not between 'A' and 'B'

from DomesticCat cat where cat.name not in ( 'Foo', 'Bar', 'Baz' )

Similarly, is null and is not null can be used to test for null values.

Booleans can be easily used in expressions by declaring HQL query substitutions in Hibernate configuration:

<property name="hibernate.query.substitutions">true 1, false 0</property>

系統將該HQL轉換爲SQL語句時，該設置表明將用字符 1 和 0 來 取代關鍵字true 和 false:

from Cat cat where cat.alive = true

You can test the size of a collection with the special property size or the special size() function.

from Cat cat where cat.kittens.size > 0

from Cat cat where size(cat.kittens) > 0

For indexed collections, you can refer to the minimum and maximum indices usingminindex and maxindex functions. Similarly, you can refer to the minimum and maximum elements of a collection of basic type using theminelement and maxelement functions. For example:

from Calendar cal where maxelement(cal.holidays) > current_date

from Order order where maxindex(order.items) > 100

from Order order where minelement(order.items) > 10000

The SQL functions any, some, all, exists, in are supported when passed the element or index set of a collection (elements andindices functions) or the result of a subquery (see below):

select mother from Cat as mother, Cat as kit
where kit in elements(foo.kittens)

select p from NameList list, Person p
where p.name = some elements(list.names)

from Cat cat where exists elements(cat.kittens)

from Player p where 3 > all elements(p.scores)

from Show show where 'fizard' in indices(show.acts)

Note that these constructs - size, elements, indices, minindex,maxindex, minelement, maxelement - can only be used in the where clause in Hibernate3.

Elements of indexed collections (arrays, lists, and maps) can be referred to by index in a where clause only:

from Order order where order.items[0].id = 1234

select person from Person person, Calendar calendar
where calendar.holidays['national day'] = person.birthDay
    and person.nationality.calendar = calendar

select item from Item item, Order order
where order.items[ order.deliveredItemIndices[0] ] = item and order.id = 11

select item from Item item, Order order
where order.items[ maxindex(order.items) ] = item and order.id = 11

The expression inside [] can even be an arithmetic expression:

select item from Item item, Order order
where order.items[ size(order.items) - 1 ] = item

HQL also provides the built-in index() function for elements of a one-to-many association or collection of values.

select item, index(item) from Order order
    join order.items item
where index(item) < 5

Scalar SQL functions supported by the underlying database can be used:

from DomesticCat cat where upper(cat.name) like 'FRI%'

Consider how much longer and less readable the following query would be in SQL:

select cust
from Product prod,
    Store store
    inner join store.customers cust
where prod.name = 'widget'
    and store.location.name in ( 'Melbourne', 'Sydney' )
    and prod = all elements(cust.currentOrder.lineItems)

提示: 會像如下的語句

SELECT cust.name, cust.address, cust.phone, cust.id, cust.current_order
FROM customers cust,
    stores store,
    locations loc,
    store_customers sc,
    product prod
WHERE prod.name = 'widget'
    AND store.loc_id = loc.id
    AND loc.name IN ( 'Melbourne', 'Sydney' )
    AND sc.store_id = store.id
    AND sc.cust_id = cust.id
    AND prod.id = ALL(
        SELECT item.prod_id
        FROM line_items item, orders o
        WHERE item.order_id = o.id
            AND cust.current_order = o.id
    )

14.11. order by子句

The list returned by a query can be ordered by any property of a returned class or components:

from DomesticCat cat
order by cat.name asc, cat.weight desc, cat.birthdate

可選的asc或desc關鍵字指明瞭按照升序或降序進行排序.

14.12. group by子句

A query that returns aggregate values can be grouped by any property of a returned class or components:

select cat.color, sum(cat.weight), count(cat)
from Cat cat
group by cat.color

select foo.id, avg(name), max(name)
from Foo foo join foo.names name
group by foo.id

having子句在這裏也允許使用.

select cat.color, sum(cat.weight), count(cat)
from Cat cat
group by cat.color
having cat.color in (eg.Color.TABBY, eg.Color.BLACK)

SQL functions and aggregate functions are allowed in the having and order by clauses if they are supported by the underlying database (i.e., not in MySQL).

select cat
from Cat cat
    join cat.kittens kitten
group by cat.id, cat.name, cat.other, cat.properties
having avg(kitten.weight) > 100
order by count(kitten) asc, sum(kitten.weight) desc

Neither the group by clause nor the order by clause can contain arithmetic expressions. Hibernate also does not currently expand a grouped entity, so you cannot writegroup by cat if all properties of cat are non-aggregated. You have to list all non-aggregated properties explicitly.

14.13. 子查詢

對於支持子查詢的數據庫，Hibernate支持在查詢中使用子查詢。一個子查詢必須被圓括號包圍起來（經常是SQL聚集函數的圓括號）。 甚至相互關聯的子查詢（引用到外部查詢中的別名的子查詢）也是允許的。

from Cat as fatcat
where fatcat.weight > (
    select avg(cat.weight) from DomesticCat cat
)

from DomesticCat as cat
where cat.name = some (
    select name.nickName from Name as name
)

from Cat as cat
where not exists (
    from Cat as mate where mate.mate = cat
)

from DomesticCat as cat
where cat.name not in (
    select name.nickName from Name as name
)

select cat.id, (select max(kit.weight) from cat.kitten kit)
from Cat as cat

Note that HQL subqueries can occur only in the select or where clauses.

Note that subqueries can also utilize row value constructor syntax. See

第 14.18 節 “Row value constructor syntax” for more information.

14.14. HQL示例

Hibernate queries can be quite powerful and complex. In fact, the power of the query language is one of Hibernate's main strengths. The following example queries are similar to queries that have been used on recent projects. Please note that most queries you will write will be much simpler than the following examples.

The following query returns the order id, number of items, the given minimum total value and the total value of the order for all unpaid orders for a particular customer. The results are ordered by total value. In determining the prices, it uses the current catalog. The resulting SQL query, against the ORDER,ORDER_LINE, PRODUCT, CATALOG and PRICE tables has four inner joins and an (uncorrelated) subselect.

select order.id, sum(price.amount), count(item)
from Order as order
    join order.lineItems as item
    join item.product as product,
    Catalog as catalog
    join catalog.prices as price
where order.paid = false
    and order.customer = :customer
    and price.product = product
    and catalog.effectiveDate < sysdate
    and catalog.effectiveDate >= all (
        select cat.effectiveDate
        from Catalog as cat
        where cat.effectiveDate < sysdate
    )
group by order
having sum(price.amount) > :minAmount
order by sum(price.amount) desc

這簡直是一個怪物！實際上，在現實生活中，我並不熱衷於子查詢，所以我的查詢語句看起來更像這個：

select order.id, sum(price.amount), count(item)
from Order as order
    join order.lineItems as item
    join item.product as product,
    Catalog as catalog
    join catalog.prices as price
where order.paid = false
    and order.customer = :customer
    and price.product = product
    and catalog = :currentCatalog
group by order
having sum(price.amount) > :minAmount
order by sum(price.amount) desc

下面一個查詢計算每一種狀態下的支付的數目，除去所有處於AWAITING_APPROVAL狀態的支付，因爲在該狀態下 當前的用戶作出了狀態的最新改變。該查詢被轉換成含有兩個內連接以及一個相關聯的子選擇的SQL查詢，該查詢使用了表PAYMENT, PAYMENT_STATUS 以及PAYMENT_STATUS_CHANGE。

select count(payment), status.name
from Payment as payment
    join payment.currentStatus as status
    join payment.statusChanges as statusChange
where payment.status.name <> PaymentStatus.AWAITING_APPROVAL
    or (
        statusChange.timeStamp = (
            select max(change.timeStamp)
            from PaymentStatusChange change
            where change.payment = payment
        )
        and statusChange.user <> :currentUser
    )
group by status.name, status.sortOrder
order by status.sortOrder

If the statusChanges collection was mapped as a list, instead of a set, the query would have been much simpler to write.

select count(payment), status.name
from Payment as payment
    join payment.currentStatus as status
where payment.status.name <> PaymentStatus.AWAITING_APPROVAL
    or payment.statusChanges[ maxIndex(payment.statusChanges) ].user <> :currentUser
group by status.name, status.sortOrder
order by status.sortOrder

下面一個查詢使用了MS SQL Server的 isNull()函數用以返回當前用戶所屬組織的組織帳號及組織未支付的賬。 它被轉換成一個對錶ACCOUNT,PAYMENT, PAYMENT_STATUS,ACCOUNT_TYPE, ORGANIZATION 以及ORG_USER進行的三個內連接， 一個外連接和一個子選擇的SQL查詢。

select account, payment
from Account as account
    left outer join account.payments as payment
where :currentUser in elements(account.holder.users)
    and PaymentStatus.UNPAID = isNull(payment.currentStatus.name, PaymentStatus.UNPAID)
order by account.type.sortOrder, account.accountNumber, payment.dueDate

對於一些數據庫，我們需要棄用（相關的）子選擇。

select account, payment
from Account as account
    join account.holder.users as user
    left outer join account.payments as payment
where :currentUser = user
    and PaymentStatus.UNPAID = isNull(payment.currentStatus.name, PaymentStatus.UNPAID)
order by account.type.sortOrder, account.accountNumber, payment.dueDate

14.15. 批量的UPDATE和DELETE

HQL now supports update, delete andinsert ... select ... statements. See

第 13.4 節 “DML(数据操作语言)é£Žæ ¼çš„æ“ä½œ(DML-style operations)” for more information.

14.16. 小技巧 & 小竅門

You can count the number of query results without returning them:

( (Integer) session.createQuery("select count(*) from ....").iterate().next() ).intValue()

若想根據一個集合的大小來進行排序，可以使用如下的語句：

select usr.id, usr.name
from User as usr
    left join usr.messages as msg
group by usr.id, usr.name
order by count(msg)

如果你的數據庫支持子選擇，你可以在你的查詢的where子句中爲選擇的大小（selection size）指定一個條件:

from User usr where size(usr.messages) >= 1

If your database does not support subselects, use the following query:

select usr.id, usr.name
from User usr.name
    join usr.messages msg
group by usr.id, usr.name
having count(msg) >= 1

As this solution cannot return a User with zero messages because of the inner join, the following form is also useful:

select usr.id, usr.name
from User as usr
    left join usr.messages as msg
group by usr.id, usr.name
having count(msg) = 0

JavaBean的屬性可以被綁定到一個命名查詢（named query）的參數上：

Query q = s.createQuery("from foo Foo as foo where foo.name=:name and foo.size=:size");
q.setProperties(fooBean); // fooBean has getName() and getSize()
List foos = q.list();

通過將接口Query與一個過濾器（filter）一起使用，集合（Collections）是可以分頁的：

Query q = s.createFilter( collection, "" ); // the trivial filter
q.setMaxResults(PAGE_SIZE);
q.setFirstResult(PAGE_SIZE * pageNumber);
List page = q.list();

Collection elements can be ordered or grouped using a query filter:

Collection orderedCollection = s.filter( collection, "order by this.amount" );
Collection counts = s.filter( collection, "select this.type, count(this) group by this.type" );

不用通過初始化，你就可以知道一個集合（Collection）的大小：

( (Integer) session.createQuery("select count(*) from ....").iterate().next() ).intValue();

14.17. translator-credits

Components can be used similarly to the simple value types that are used in HQL queries. They can appear in theselect clause as follows:

select p.name from Person p

select p.name.first from Person p

where the Person's name property is a component. Components can also be used in thewhere clause:

from Person p where p.name = :name

from Person p where p.name.first = :firstName

Components can also be used in the order by clause:

from Person p order by p.name

from Person p order by p.name.first

Another common use of components is in

row value constructors.

14.18. Row value constructor syntax

HQL supports the use of ANSI SQL row value constructor syntax, sometimes referred to AStuple syntax, even though the underlying database may not support that notion. Here, we are generally referring to multi-valued comparisons, typically associated with components. Consider an entity Person which defines a name component:

from Person p where p.name.first='John' and p.name.last='Jingleheimer-Schmidt'

That is valid syntax although it is a little verbose. You can make this more concise by usingrow value constructor syntax:

from Person p where p.name=('John', 'Jingleheimer-Schmidt')

It can also be useful to specify this in the select clause:

select p.name from Person p

Using row value constructor syntax can also be beneficial when using subqueries that need to compare against multiple values:

from Cat as cat
where not ( cat.name, cat.color ) in (
    select cat.name, cat.color from DomesticCat cat
)

One thing to consider when deciding if you want to use this syntax, is that the query will be dependent upon the ordering of the component sub-properties in the metadata.

第 15 章 條件查詢(Criteria Queries)

具有一個直觀的、可擴展的條件查詢API是Hibernate的特色。

15.1. 創建一個Criteria 實例

org.hibernate.Criteria接口表示特定持久類的一個查詢。Session是Criteria實例的工廠。

Criteria crit = sess.createCriteria(Cat.class);
crit.setMaxResults(50);
List cats = crit.list();

15.2. 限制結果集內容

一個單獨的查詢條件是org.hibernate.criterion.Criterion 接口的一個實例。org.hibernate.criterion.Restrictions類 定義了獲得某些內置Criterion類型的工廠方法。

List cats = sess.createCriteria(Cat.class)
    .add( Restrictions.like("name", "Fritz%") )
    .add( Restrictions.between("weight", minWeight, maxWeight) )
    .list();

Restrictions can be grouped logically.

List cats = sess.createCriteria(Cat.class)
    .add( Restrictions.like("name", "Fritz%") )
    .add( Restrictions.or(
        Restrictions.eq( "age", new Integer(0) ),
        Restrictions.isNull("age")
    ) )
    .list();

List cats = sess.createCriteria(Cat.class)
    .add( Restrictions.in( "name", new String[] { "Fritz", "Izi", "Pk" } ) )
    .add( Restrictions.disjunction()
        .add( Restrictions.isNull("age") )
        .add( Restrictions.eq("age", new Integer(0) ) )
        .add( Restrictions.eq("age", new Integer(1) ) )
        .add( Restrictions.eq("age", new Integer(2) ) )
    ) )
    .list();

There are a range of built-in criterion types (Restrictions subclasses). One of the most useful allows you to specify SQL directly.

List cats = sess.createCriteria(Cat.class)
    .add( Restrictions.sqlRestriction("lower({alias}.name) like lower(?)", "Fritz%", Hibernate.STRING) )
    .list();

{alias}佔位符應當被替換爲被查詢實體的列別名。

You can also obtain a criterion from a Property instance. You can create aProperty by calling Property.forName():

Property age = Property.forName("age");
List cats = sess.createCriteria(Cat.class)
    .add( Restrictions.disjunction()
        .add( age.isNull() )
        .add( age.eq( new Integer(0) ) )
        .add( age.eq( new Integer(1) ) )
        .add( age.eq( new Integer(2) ) )
    ) )
    .add( Property.forName("name").in( new String[] { "Fritz", "Izi", "Pk" } ) )
    .list();

15.3. 結果集排序

You can order the results using org.hibernate.criterion.Order.

List cats = sess.createCriteria(Cat.class)
    .add( Restrictions.like("name", "F%")
    .addOrder( Order.asc("name") )
    .addOrder( Order.desc("age") )
    .setMaxResults(50)
    .list();

List cats = sess.createCriteria(Cat.class)
    .add( Property.forName("name").like("F%") )
    .addOrder( Property.forName("name").asc() )
    .addOrder( Property.forName("age").desc() )
    .setMaxResults(50)
    .list();

15.4. 關聯

By navigating associations using createCriteria() you can specify constraints upon related entities:

List cats = sess.createCriteria(Cat.class)
    .add( Restrictions.like("name", "F%") )
    .createCriteria("kittens")
        .add( Restrictions.like("name", "F%") )
    .list();

The second createCriteria() returns a new instance ofCriteria that refers to the elements of the kittens collection.

There is also an alternate form that is useful in certain circumstances:

List cats = sess.createCriteria(Cat.class)
    .createAlias("kittens", "kt")
    .createAlias("mate", "mt")
    .add( Restrictions.eqProperty("kt.name", "mt.name") )
    .list();

(createAlias()並不創建一個新的 Criteria實例。)

The kittens collections held by the Cat instances returned by the previous two queries arenot pre-filtered by the criteria. If you want to retrieve just the kittens that match the criteria, you must use aResultTransformer.

List cats = sess.createCriteria(Cat.class)
    .createCriteria("kittens", "kt")
        .add( Restrictions.eq("name", "F%") )
    .setResultTransformer(Criteria.ALIAS_TO_ENTITY_MAP)
    .list();
Iterator iter = cats.iterator();
while ( iter.hasNext() ) {
    Map map = (Map) iter.next();
    Cat cat = (Cat) map.get(Criteria.ROOT_ALIAS);
    Cat kitten = (Cat) map.get("kt");
}

15.5. 動態關聯抓取

You can specify association fetching semantics at runtime using setFetchMode().

List cats = sess.createCriteria(Cat.class)
    .add( Restrictions.like("name", "Fritz%") )
    .setFetchMode("mate", FetchMode.EAGER)
    .setFetchMode("kittens", FetchMode.EAGER)
    .list();

這個查詢可以通過外連接抓取mate和kittens。 查看

第 19.1 節 “抓取策略(Fetching strategies)”可以獲得更多信息。

15.6. 查詢示例

org.hibernate.criterion.Example類允許你通過一個給定實例 構建一個條件查詢。

Cat cat = new Cat();
cat.setSex('F');
cat.setColor(Color.BLACK);
List results = session.createCriteria(Cat.class)
    .add( Example.create(cat) )
    .list();

版本屬性、標識符和關聯被忽略。默認情況下值爲null的屬性將被排除。

你可以自行調整Example使之更實用。

Example example = Example.create(cat)
    .excludeZeroes()           //exclude zero valued properties
    .excludeProperty("color")  //exclude the property named "color"
    .ignoreCase()              //perform case insensitive string comparisons
    .enableLike();             //use like for string comparisons
List results = session.createCriteria(Cat.class)
    .add(example)
    .list();

你甚至可以使用examples在關聯對象上放置條件。

List results = session.createCriteria(Cat.class)
    .add( Example.create(cat) )
    .createCriteria("mate")
        .add( Example.create( cat.getMate() ) )
    .list();

15.7. 投影(Projections)、聚合（aggregation）和分組（grouping）

The class org.hibernate.criterion.Projections is a factory forProjection instances. You can apply a projection to a query by callingsetProjection().

List results = session.createCriteria(Cat.class)
    .setProjection( Projections.rowCount() )
    .add( Restrictions.eq("color", Color.BLACK) )
    .list();

List results = session.createCriteria(Cat.class)
    .setProjection( Projections.projectionList()
        .add( Projections.rowCount() )
        .add( Projections.avg("weight") )
        .add( Projections.max("weight") )
        .add( Projections.groupProperty("color") )
    )
    .list();

在一個條件查詢中沒有必要顯式的使用 "group by" 。某些投影類型就是被定義爲 分組投影，他們也出現在SQL的group by子句中。

An alias can be assigned to a projection so that the projected value can be referred to in restrictions or orderings. Here are two different ways to do this:

List results = session.createCriteria(Cat.class)
    .setProjection( Projections.alias( Projections.groupProperty("color"), "colr" ) )
    .addOrder( Order.asc("colr") )
    .list();

List results = session.createCriteria(Cat.class)
    .setProjection( Projections.groupProperty("color").as("colr") )
    .addOrder( Order.asc("colr") )
    .list();

alias()和as()方法簡便的將一個投影實例包裝到另外一個 別名的Projection實例中。簡而言之，當你添加一個投影到一個投影列表中時 你可以爲它指定一個別名：

List results = session.createCriteria(Cat.class)
    .setProjection( Projections.projectionList()
        .add( Projections.rowCount(), "catCountByColor" )
        .add( Projections.avg("weight"), "avgWeight" )
        .add( Projections.max("weight"), "maxWeight" )
        .add( Projections.groupProperty("color"), "color" )
    )
    .addOrder( Order.desc("catCountByColor") )
    .addOrder( Order.desc("avgWeight") )
    .list();

List results = session.createCriteria(Domestic.class, "cat")
    .createAlias("kittens", "kit")
    .setProjection( Projections.projectionList()
        .add( Projections.property("cat.name"), "catName" )
        .add( Projections.property("kit.name"), "kitName" )
    )
    .addOrder( Order.asc("catName") )
    .addOrder( Order.asc("kitName") )
    .list();

你也可以使用Property.forName()來表示投影：

List results = session.createCriteria(Cat.class)
    .setProjection( Property.forName("name") )
    .add( Property.forName("color").eq(Color.BLACK) )
    .list();

List results = session.createCriteria(Cat.class)
    .setProjection( Projections.projectionList()
        .add( Projections.rowCount().as("catCountByColor") )
        .add( Property.forName("weight").avg().as("avgWeight") )
        .add( Property.forName("weight").max().as("maxWeight") )
        .add( Property.forName("color").group().as("color" )
    )
    .addOrder( Order.desc("catCountByColor") )
    .addOrder( Order.desc("avgWeight") )
    .list();

15.8. 離線(detached)查詢和子查詢

The DetachedCriteria class allows you to create a query outside the scope of a session and then execute it using an arbitrarySession.

DetachedCriteria query = DetachedCriteria.forClass(Cat.class)
    .add( Property.forName("sex").eq('F') );
    
Session session = ....;
Transaction txn = session.beginTransaction();
List results = query.getExecutableCriteria(session).setMaxResults(100).list();
txn.commit();
session.close();

A DetachedCriteria can also be used to express a subquery. Criterion instances involving subqueries can be obtained viaSubqueries or Property.

DetachedCriteria avgWeight = DetachedCriteria.forClass(Cat.class)
    .setProjection( Property.forName("weight").avg() );
session.createCriteria(Cat.class)
    .add( Property.forName("weight").gt(avgWeight) )
    .list();

DetachedCriteria weights = DetachedCriteria.forClass(Cat.class)
    .setProjection( Property.forName("weight") );
session.createCriteria(Cat.class)
    .add( Subqueries.geAll("weight", weights) )
    .list();

Correlated subqueries are also possible:

DetachedCriteria avgWeightForSex = DetachedCriteria.forClass(Cat.class, "cat2")
    .setProjection( Property.forName("weight").avg() )
    .add( Property.forName("cat2.sex").eqProperty("cat.sex") );
session.createCriteria(Cat.class, "cat")
    .add( Property.forName("weight").gt(avgWeightForSex) )
    .list();

15.9. 根據自然標識查詢(Queries by natural identifier)

For most queries, including criteria queries, the query cache is not efficient because query cache invalidation occurs too frequently. However, there is a special kind of query where you can optimize the cache invalidation algorithm: lookups by a constant natural key. In some applications, this kind of query occurs frequently. The criteria API provides special provision for this use case.

First, map the natural key of your entity using <natural-id> and enable use of the second-level cache.

<class name="User">
    <cache usage="read-write"/>
    <id name="id">
        <generator class="increment"/>
    </id>
    <natural-id>
        <property name="name"/>
        <property name="org"/>
    </natural-id>
    <property name="password"/>
</class>

This functionality is not intended for use with entities with mutable natural keys.

Once you have enabled the Hibernate query cache, the Restrictions.naturalId() allows you to make use of the more efficient cache algorithm.

session.createCriteria(User.class)
    .add( Restrictions.naturalId()
        .set("name", "gavin")
        .set("org", "hb") 
    ).setCacheable(true)
    .uniqueResult();

第 16 章 Native SQL查詢

You can also express queries in the native SQL dialect of your database. This is useful if you want to utilize database-specific features such as query hints or theCONNECT keyword in Oracle. It also provides a clean migration path from a direct SQL/JDBC based application to Hibernate.

Hibernate3 allows you to specify handwritten SQL, including stored procedures, for all create, update, delete, and load operations.

16.1. 使用SQLQuery

Execution of native SQL queries is controlled via the SQLQuery interface, which is obtained by callingSession.createSQLQuery(). The following sections describe how to use this API for querying.

16.1.1. 標量查詢（Scalar queries）

最基本的SQL查詢就是獲得一個標量（數值）的列表。

sess.createSQLQuery("SELECT * FROM CATS").list();
sess.createSQLQuery("SELECT ID, NAME, BIRTHDATE FROM CATS").list();

These will return a List of Object arrays (Object[]) with scalar values for each column in the CATS table. Hibernate will use ResultSetMetadata to deduce the actual order and types of the returned scalar values.

To avoid the overhead of using ResultSetMetadata, or simply to be more explicit in what is returned, one can useaddScalar():

sess.createSQLQuery("SELECT * FROM CATS")
 .addScalar("ID", Hibernate.LONG)
 .addScalar("NAME", Hibernate.STRING)
 .addScalar("BIRTHDATE", Hibernate.DATE)

This query specified:

• SQL查詢字符串

• 要返回的字段和類型

This will return Object arrays, but now it will not use ResultSetMetadata but will instead explicitly get the ID, NAME and BIRTHDATE column as respectively a Long, String and a Short from the underlying resultset. This also means that only these three columns will be returned, even though the query is using* and could return more than the three listed columns.

對全部或者部分的標量值不設置類型信息也是可以的。

sess.createSQLQuery("SELECT * FROM CATS")
 .addScalar("ID", Hibernate.LONG)
 .addScalar("NAME")
 .addScalar("BIRTHDATE")

This is essentially the same query as before, but now ResultSetMetaData is used to determine the type of NAME and BIRTHDATE, where as the type of ID is explicitly specified.

How the java.sql.Types returned from ResultSetMetaData is mapped to Hibernate types is controlled by the Dialect. If a specific type is not mapped, or does not result in the expected type, it is possible to customize it via calls toregisterHibernateType in the Dialect.

16.1.2. 實體查詢(Entity queries)

上面的查詢都是返回標量值的，也就是從resultset中返回的“裸”數據。下面展示如何通過addEntity()讓原生查詢返回實體對象。

sess.createSQLQuery("SELECT * FROM CATS").addEntity(Cat.class);
sess.createSQLQuery("SELECT ID, NAME, BIRTHDATE FROM CATS").addEntity(Cat.class);

This query specified:

• SQL查詢字符串

• 要返回的實體

假設Cat被映射爲擁有ID,NAME和BIRTHDATE三個字段的類，以上的兩個查詢都返回一個List，每個元素都是一個Cat實體。

假若實體在映射時有一個many-to-one的關聯指向另外一個實體，在查詢時必須也返回那個實體，否則會導致發生一個"column not found"的數據庫錯誤。這些附加的字段可以使用*標註來自動返回，但我們希望還是明確指明，看下面這個具有指向Dog的many-to-one的例子：

sess.createSQLQuery("SELECT ID, NAME, BIRTHDATE, DOG_ID FROM CATS").addEntity(Cat.class);

這樣cat.getDog()就能正常運作。

16.1.3. 處理關聯和集合類(Handling associations and collections)

通過提前抓取將Dog連接獲得，而避免初始化proxy帶來的額外開銷也是可能的。這是通過addJoin()方法進行的，這個方法可以讓你將關聯或集合連接進來。

sess.createSQLQuery("SELECT c.ID, NAME, BIRTHDATE, DOG_ID, D_ID, D_NAME FROM CATS c, DOGS d WHERE c.DOG_ID = d.D_ID")
 .addEntity("cat", Cat.class)
 .addJoin("cat.dog");

In this example, the returned Cat's will have theirdog property fully initialized without any extra roundtrip to the database. Notice that you added an alias name ("cat") to be able to specify the target property path of the join. It is possible to do the same eager joining for collections, e.g. if the Cat had a one-to-many to Dog instead.

sess.createSQLQuery("SELECT ID, NAME, BIRTHDATE, D_ID, D_NAME, CAT_ID FROM CATS c, DOGS d WHERE c.ID = d.CAT_ID")
 .addEntity("cat", Cat.class)
 .addJoin("cat.dogs");

At this stage you are reaching the limits of what is possible with native queries, without starting to enhance the sql queries to make them usable in Hibernate. Problems can arise when returning multiple entities of the same type or when the default alias/column names are not enough.

16.1.4. 返回多個實體(Returning multiple entities)

Until now, the result set column names are assumed to be the same as the column names specified in the mapping document. This can be problematic for SQL queries that join multiple tables, since the same column names can appear in more than one table.

下面的查詢中需要使用字段別名注射（這個例子本身會失敗）：

sess.createSQLQuery("SELECT c.*, m.*  FROM CATS c, CATS m WHERE c.MOTHER_ID = c.ID")
 .addEntity("cat", Cat.class)
 .addEntity("mother", Cat.class)

The query was intended to return two Cat instances per row: a cat and its mother. The query will, however, fail because there is a conflict of names; the instances are mapped to the same column names. Also, on some databases the returned column aliases will most likely be on the form "c.ID", "c.NAME", etc. which are not equal to the columns specified in the mappings ("ID" and "NAME").

下面的形式可以解決字段名重複：

sess.createSQLQuery("SELECT {cat.*}, {mother.*}  FROM CATS c, CATS m WHERE c.MOTHER_ID = c.ID")
 .addEntity("cat", Cat.class)
 .addEntity("mother", Cat.class)

This query specified:

• SQL查詢語句，其中包含佔位附來讓Hibernate注射字段別名

• 查詢返回的實體

The {cat.*} and {mother.*} notation used above is a shorthand for "all properties". Alternatively, you can list the columns explicitly, but even in this case Hibernate injects the SQL column aliases for each property. The placeholder for a column alias is just the property name qualified by the table alias. In the following example, you retrieve Cats and their mothers from a different table (cat_log) to the one declared in the mapping metadata. You can even use the property aliases in the where clause.

String sql = "SELECT ID as {c.id}, NAME as {c.name}, " + 
         "BIRTHDATE as {c.birthDate}, MOTHER_ID as {c.mother}, {mother.*} " +
         "FROM CAT_LOG c, CAT_LOG m WHERE {c.mother} = c.ID";

List loggedCats = sess.createSQLQuery(sql)
        .addEntity("cat", Cat.class)
        .addEntity("mother", Cat.class).list()

16.1.4.1. 別名和屬性引用(Alias and property references)

In most cases the above alias injection is needed. For queries relating to more complex mappings, like composite properties, inheritance discriminators, collections etc., you can use specific aliases that allow Hibernate to inject the proper aliases.

The following table shows the different ways you can use the alias injection. Please note that the alias names in the result are simply examples; each alias will have a unique and probably different name when used.

表 16.1. 別名注射(alias injection names)

描述	語法	示例
簡單屬性	{[aliasname].[propertyname]	A_NAME as {item.name}
複合屬性	{[aliasname].[componentname].[propertyname]}	CURRENCY as {item.amount.currency}, VALUE as {item.amount.value}
實體辨別器(Discriminator of an entity)	{[aliasname].class}	DISC as {item.class}
實體的所有屬性	{[aliasname].*}	{item.*}
集合鍵(collection key)	{[aliasname].key}	ORGID as {coll.key}
集合id	{[aliasname].id}	EMPID as {coll.id}
集合元素	{[aliasname].element}	XID as {coll.element}
property of the element in the collection	{[aliasname].element.[propertyname]}	NAME as {coll.element.name}
集合元素的所有屬性	{[aliasname].element.*}	{coll.element.*}
集合的所有屬性	{[aliasname].*}	{coll.*}

16.1.5. 返回非受管實體(Returning non-managed entities)

It is possible to apply a ResultTransformer to native SQL queries, allowing it to return non-managed entities.

sess.createSQLQuery("SELECT NAME, BIRTHDATE FROM CATS")
        .setResultTransformer(Transformers.aliasToBean(CatDTO.class))

This query specified:

• SQL查詢字符串

• 結果轉換器(result transformer)

上面的查詢將會返回CatDTO的列表,它將被實例化並且將NAME和BIRTHDAY的值注射入對應的屬性或者字段。

16.1.6. 處理繼承（Handling inheritance）

Native SQL queries which query for entities that are mapped as part of an inheritance must include all properties for the baseclass and all its subclasses.

16.1.7. 參數（Parameters）

Native SQL queries support positional as well as named parameters:

Query query = sess.createSQLQuery("SELECT * FROM CATS WHERE NAME like ?").addEntity(Cat.class);
List pusList = query.setString(0, "Pus%").list();
     
query = sess.createSQLQuery("SELECT * FROM CATS WHERE NAME like :name").addEntity(Cat.class);
List pusList = query.setString("name", "Pus%").list();          

16.2. 命名SQL查詢

Named SQL queries can be defined in the mapping document and called in exactly the same way as a named HQL query. In this case, you donot need to call addEntity().

<sql-query name="persons">
    <return alias="person" class="eg.Person"/>
    SELECT person.NAME AS {person.name},
           person.AGE AS {person.age},
           person.SEX AS {person.sex}
    FROM PERSON person
    WHERE person.NAME LIKE :namePattern
</sql-query>

List people = sess.getNamedQuery("persons")
    .setString("namePattern", namePattern)
    .setMaxResults(50)
    .list();

The <return-join> element is use to join associations and the<load-collection> element is used to define queries which initialize collections,

<sql-query name="personsWith">
    <return alias="person" class="eg.Person"/>
    <return-join alias="address" property="person.mailingAddress"/>
    SELECT person.NAME AS {person.name},
           person.AGE AS {person.age},
           person.SEX AS {person.sex},
           address.STREET AS {address.street},
           address.CITY AS {address.city},
           address.STATE AS {address.state},
           address.ZIP AS {address.zip}
    FROM PERSON person
    JOIN ADDRESS address
        ON person.ID = address.PERSON_ID AND address.TYPE='MAILING'
    WHERE person.NAME LIKE :namePattern
</sql-query>

一個命名查詢可能會返回一個標量值.你必須使用<return-scalar>元素來指定字段的別名和 Hibernate類型

<sql-query name="mySqlQuery">
    <return-scalar column="name" type="string"/>
    <return-scalar column="age" type="long"/>
    SELECT p.NAME AS name,
           p.AGE AS age,
    FROM PERSON p WHERE p.NAME LIKE 'Hiber%'
</sql-query>

You can externalize the resultset mapping information in a <resultset> element which will allow you to either reuse them across several named queries or through thesetResultSetMapping() API.

<resultset name="personAddress">
    <return alias="person" class="eg.Person"/>
    <return-join alias="address" property="person.mailingAddress"/>
</resultset>

<sql-query name="personsWith" resultset-ref="personAddress">
    SELECT person.NAME AS {person.name},
           person.AGE AS {person.age},
           person.SEX AS {person.sex},
           address.STREET AS {address.street},
           address.CITY AS {address.city},
           address.STATE AS {address.state},
           address.ZIP AS {address.zip}
    FROM PERSON person
    JOIN ADDRESS address
        ON person.ID = address.PERSON_ID AND address.TYPE='MAILING'
    WHERE person.NAME LIKE :namePattern
</sql-query>

You can, alternatively, use the resultset mapping information in your hbm files directly in java code.

List cats = sess.createSQLQuery(
        "select {cat.*}, {kitten.*} from cats cat, cats kitten where kitten.mother = cat.id"
    )
    .setResultSetMapping("catAndKitten")
    .list();

16.2.1. 使用return-property來明確地指定字段/別名

You can explicitly tell Hibernate what column aliases to use with <return-property>, instead of using the {}-syntax to let Hibernate inject its own aliases.For example:

<sql-query name="mySqlQuery">
    <return alias="person" class="eg.Person">
        <return-property name="name" column="myName"/>
        <return-property name="age" column="myAge"/>
        <return-property name="sex" column="mySex"/>
    </return>
    SELECT person.NAME AS myName,
           person.AGE AS myAge,
           person.SEX AS mySex,
    FROM PERSON person WHERE person.NAME LIKE :name
</sql-query>

<return-property> also works with multiple columns. This solves a limitation with the{}-syntax which cannot allow fine grained control of multi-column properties.

<sql-query name="organizationCurrentEmployments">
    <return alias="emp" class="Employment">
        <return-property name="salary">
            <return-column name="VALUE"/>
            <return-column name="CURRENCY"/>
        </return-property>
        <return-property name="endDate" column="myEndDate"/>
    </return>
        SELECT EMPLOYEE AS {emp.employee}, EMPLOYER AS {emp.employer},
        STARTDATE AS {emp.startDate}, ENDDATE AS {emp.endDate},
        REGIONCODE as {emp.regionCode}, EID AS {emp.id}, VALUE, CURRENCY
        FROM EMPLOYMENT
        WHERE EMPLOYER = :id AND ENDDATE IS NULL
        ORDER BY STARTDATE ASC
</sql-query>

In this example <return-property> was used in combination with the{}-syntax for injection. This allows users to choose how they want to refer column and properties.

如果你映射一個識別器(discriminator),你必須使用<return-discriminator> 來指定識別器字段

16.2.2. 使用存儲過程來查詢

Hibernate3 provides support for queries via stored procedures and functions. Most of the following documentation is equivalent for both. The stored procedure/function must return a resultset as the first out-parameter to be able to work with Hibernate. An example of such a stored function in Oracle 9 and higher is as follows:

CREATE OR REPLACE FUNCTION selectAllEmployments
    RETURN SYS_REFCURSOR
AS
    st_cursor SYS_REFCURSOR;
BEGIN
    OPEN st_cursor FOR
 SELECT EMPLOYEE, EMPLOYER,
 STARTDATE, ENDDATE,
 REGIONCODE, EID, VALUE, CURRENCY
 FROM EMPLOYMENT;
      RETURN  st_cursor;
 END;

在Hibernate裏要要使用這個查詢,你需要通過命名查詢來映射它.

<sql-query name="selectAllEmployees_SP" callable="true">
    <return alias="emp" class="Employment">
        <return-property name="employee" column="EMPLOYEE"/>
        <return-property name="employer" column="EMPLOYER"/>
        <return-property name="startDate" column="STARTDATE"/>
        <return-property name="endDate" column="ENDDATE"/>
        <return-property name="regionCode" column="REGIONCODE"/>
        <return-property name="id" column="EID"/>
        <return-property name="salary">
            <return-column name="VALUE"/>
            <return-column name="CURRENCY"/>
        </return-property>
    </return>
    { ? = call selectAllEmployments() }
</sql-query>

Stored procedures currently only return scalars and entities. <return-join> and <load-collection> are not supported.

16.2.2.1. 使用存儲過程的規則和限制

You cannot use stored procedures with Hibernate unless you follow some procedure/function rules. If they do not follow those rules they are not usable with Hibernate. If you still want to use these procedures you have to execute them viasession.connection(). The rules are different for each database, since database vendors have different stored procedure semantics/syntax.

Stored procedure queries cannot be paged with setFirstResult()/setMaxResults().

The recommended call form is standard SQL92: { ? = call functionName(<parameters>) } or{ ? = call procedureName(<parameters>}. Native call syntax is not supported.

對於Oracle有如下規則:

• A function must return a result set. The first parameter of a procedure must be anOUT that returns a result set. This is done by using aSYS_REFCURSOR type in Oracle 9 or 10. In Oracle you need to define aREF CURSOR type. See Oracle literature for further information.

對於Sybase或者MS SQL server有如下規則:

• The procedure must return a result set. Note that since these servers can return multiple result sets and update counts, Hibernate will iterate the results and take the first result that is a result set as its return value. Everything else will be discarded.

• 如果你能夠在存儲過程裏設定SET NOCOUNT ON，這可能會效率更高，但這不是必需的。

16.3. 定製SQL用來create，update和delete

Hibernate3能夠使用定製的SQL語句來執行create,update和delete操作。在Hibernate中，持久化的類和集合已經 包含了一套配置期產生的語句(insertsql, deletesql, updatesql等等)，這些映射標記<sql-insert>, <sql-delete>, and<sql-update>重載了 這些語句。

<class name="Person">
    <id name="id">
        <generator class="increment"/>
    </id>
    <property name="name" not-null="true"/>
    <sql-insert>INSERT INTO PERSON (NAME, ID) VALUES ( UPPER(?), ? )</sql-insert>
    <sql-update>UPDATE PERSON SET NAME=UPPER(?) WHERE ID=?</sql-update>
    <sql-delete>DELETE FROM PERSON WHERE ID=?</sql-delete>
</class>

The SQL is directly executed in your database, so you can use any dialect you like. This will reduce the portability of your mapping if you use database specific SQL.

如果設定callable，則能夠支持存儲過程了。

<class name="Person">
    <id name="id">
        <generator class="increment"/>
    </id>
    <property name="name" not-null="true"/>
    <sql-insert callable="true">{call createPerson (?, ?)}</sql-insert>
    <sql-delete callable="true">{? = call deletePerson (?)}</sql-delete>
    <sql-update callable="true">{? = call updatePerson (?, ?)}</sql-update>
</class>

The order of the positional parameters is vital, as they must be in the same sequence as Hibernate expects them.

You can view the expected order by enabling debug logging for the org.hibernate.persister.entity level. With this level enabled, Hibernate will print out the static SQL that is used to create, update, delete etc. entities. To view the expected sequence, do not include your custom SQL in the mapping files, as this will override the Hibernate generated static SQL.

The stored procedures are in most cases required to return the number of rows inserted, updated and deleted, as Hibernate has some runtime checks for the success of the statement. Hibernate always registers the first statement parameter as a numeric output parameter for the CUD operations:

CREATE OR REPLACE FUNCTION updatePerson (uid IN NUMBER, uname IN VARCHAR2)
    RETURN NUMBER IS
BEGIN

    update PERSON
    set
        NAME = uname,
    where
        ID = uid;

    return SQL%ROWCOUNT;

END updatePerson;

16.4. 定製裝載SQL

You can also declare your own SQL (or HQL) queries for entity loading:

<sql-query name="person">
    <return alias="pers" class="Person" lock-mode="upgrade"/>
    SELECT NAME AS {pers.name}, ID AS {pers.id}
    FROM PERSON
    WHERE ID=?
    FOR UPDATE
</sql-query>

This is just a named query declaration, as discussed earlier. You can reference this named query in a class mapping:

<class name="Person">
    <id name="id">
        <generator class="increment"/>
    </id>
    <property name="name" not-null="true"/>
    <loader query-ref="person"/>
</class>

這也可以用於存儲過程

You can even define a query for collection loading:

<set name="employments" inverse="true">
    <key/>
    <one-to-many class="Employment"/>
    <loader query-ref="employments"/>
</set>

<sql-query name="employments">
    <load-collection alias="emp" role="Person.employments"/>
    SELECT {emp.*}
    FROM EMPLOYMENT emp
    WHERE EMPLOYER = :id
    ORDER BY STARTDATE ASC, EMPLOYEE ASC
</sql-query>

You can also define an entity loader that loads a collection by join fetching:

<sql-query name="person">
    <return alias="pers" class="Person"/>
    <return-join alias="emp" property="pers.employments"/>
    SELECT NAME AS {pers.*}, {emp.*}
    FROM PERSON pers
    LEFT OUTER JOIN EMPLOYMENT emp
        ON pers.ID = emp.PERSON_ID
    WHERE ID=?
</sql-query>

第 17 章 過濾數據

Hibernate3 provides an innovative new approach to handling data with "visibility" rules. AHibernate filter is a global, named, parameterized filter that can be enabled or disabled for a particular Hibernate session.

17.1. Hibernate 過濾器(filters)

Hibernate3 has the ability to pre-define filter criteria and attach those filters at both a class level and a collection level. A filter criteria allows you to define a restriction clause similar to the existing "where" attribute available on the class and various collection elements. These filter conditions, however, can be parameterized. The application can then decide at runtime whether certain filters should be enabled and what their parameter values should be. Filters can be used like database views, but they are parameterized inside the application.

要使用過濾器，必須首先在相應的映射節點中定義。而定義一個過濾器，要用到位於<hibernate-mapping/> 節點之內的<filter-def/>節點：

<filter-def name="myFilter">
    <filter-param name="myFilterParam" type="string"/>
</filter-def>

This filter can then be attached to a class:

<class name="myClass" ...>
    ...
    <filter name="myFilter" condition=":myFilterParam = MY_FILTERED_COLUMN"/>
</class>

Or, to a collection:

<set ...>
    <filter name="myFilter" condition=":myFilterParam = MY_FILTERED_COLUMN"/>
</set>

Or, to both or multiples of each at the same time.

The methods on Session are: enableFilter(String filterName), getEnabledFilter(String filterName), anddisableFilter(String filterName). By default, filters arenot enabled for a given session. Filters must be enabled through use of theSession.enableFilter() method, which returns an instance of theFilter interface. If you used the simple filter defined above, it would look like this:

session.enableFilter("myFilter").setParameter("myFilterParam", "some-value");

Methods on the org.hibernate.Filter interface do allow the method-chaining common to much of Hibernate.

The following is a full example, using temporal data with an effective record date pattern:

<filter-def name="effectiveDate">
    <filter-param name="asOfDate" type="date"/>
</filter-def>

<class name="Employee" ...>
...
    <many-to-one name="department" column="dept_id" class="Department"/>
    <property name="effectiveStartDate" type="date" column="eff_start_dt"/>
    <property name="effectiveEndDate" type="date" column="eff_end_dt"/>
...
    <!--
        Note that this assumes non-terminal records have an eff_end_dt set to
        a max db date for simplicity-sake
    -->
    <filter name="effectiveDate"
            condition=":asOfDate BETWEEN eff_start_dt and eff_end_dt"/>
</class>

<class name="Department" ...>
...
    <set name="employees" lazy="true">
        <key column="dept_id"/>
        <one-to-many class="Employee"/>
        <filter name="effectiveDate"
                condition=":asOfDate BETWEEN eff_start_dt and eff_end_dt"/>
    </set>
</class>

In order to ensure that you are provided with currently effective records, enable the filter on the session prior to retrieving employee data:

Session session = ...;
session.enableFilter("effectiveDate").setParameter("asOfDate", new Date());
List results = session.createQuery("from Employee as e where e.salary > :targetSalary")
         .setLong("targetSalary", new Long(1000000))
         .list();

Even though a salary constraint was mentioned explicitly on the results in the above HQL, because of the enabled filter, the query will return only currently active employees who have a salary greater than one million dollars.

If you want to use filters with outer joining, either through HQL or load fetching, be careful of the direction of the condition expression. It is safest to set this up for left outer joining. Place the parameter first followed by the column name(s) after the operator.

After being defined, a filter might be attached to multiple entities and/or collections each with its own condition. This can be problematic when the conditions are the same each time. Using<filter-def/> allows you to definine a default condition, either as an attribute or CDATA:

<filter-def name="myFilter" condition="abc > xyz">...</filter-def>
<filter-def name="myOtherFilter">abc=xyz</filter-def>

This default condition will be used whenever the filter is attached to something without specifying a condition. This means you can give a specific condition as part of the attachment of the filter that overrides the default condition in that particular case.

第 18 章 XML映射

XML Mapping is an experimental feature in Hibernate 3.0 and is currently under active development.

18.1. 用XML數據進行工作

Hibernate allows you to work with persistent XML data in much the same way you work with persistent POJOs. A parsed XML tree can be thought of as another way of representing the relational data at the object level, instead of POJOs.

Hibernate支持採用dom4j作爲操作XML樹的API。你可以寫一些查詢從數據庫中檢索出 dom4j樹，隨後你對這顆樹做的任何修改都將自動同步回數據庫。你甚至可以用dom4j解析 一篇XML文檔，然後使用Hibernate的任一基本操作將它寫入數據庫：persist(), saveOrUpdate(), merge(), delete(), replicate() (合併操作merge()目前還不支持)。

這一特性可以應用在很多場合，包括數據導入導出，通過JMS或SOAP具體化實體數據以及 基於XSLT的報表。

A single mapping can be used to simultaneously map properties of a class and nodes of an XML document to the database, or, if there is no class to map, it can be used to map just the XML.

18.1.1. 指定同時映射XML和類

這是一個同時映射POJO和XML的例子：

<class name="Account" 
        table="ACCOUNTS" 
        node="account">
        
    <id name="accountId" 
            column="ACCOUNT_ID" 
            node="@id"/>
            
    <many-to-one name="customer" 
            column="CUSTOMER_ID" 
            node="customer/@id" 
            embed-xml="false"/>
            
    <property name="balance" 
            column="BALANCE" 
            node="balance"/>
            
    ...
    
</class>

18.1.2. 只定義XML映射

這是一個不映射POJO的例子：

<class entity-name="Account" 
        table="ACCOUNTS" 
        node="account">
        
    <id name="id" 
            column="ACCOUNT_ID" 
            node="@id" 
            type="string"/>
            
    <many-to-one name="customerId" 
            column="CUSTOMER_ID" 
            node="customer/@id" 
            embed-xml="false" 
            entity-name="Customer"/>
            
    <property name="balance" 
            column="BALANCE" 
            node="balance" 
            type="big_decimal"/>
            
    ...
    
</class>

This mapping allows you to access the data as a dom4j tree, or as a graph of property name/value pairs or javaMaps. The property names are purely logical constructs that can be referred to in HQL queries.

18.2. XML映射元數據

A range of Hibernate mapping elements accept the node attribute. This lets you specify the name of an XML attribute or element that holds the property or entity data. The format of thenode attribute must be one of the following:

• "element-name": map to the named XML element

• "@attribute-name": map to the named XML attribute

• ".": map to the parent element

• "element-name/@attribute-name": map to the named attribute of the named element

For collections and single valued associations, there is an additional embed-xml attribute. If embed-xml="true", the default, the XML tree for the associated entity (or collection of value type) will be embedded directly in the XML tree for the entity that owns the association. Otherwise, ifembed-xml="false", then only the referenced identifier value will appear in the XML for single point associations and collections will not appear at all.

Do not leave embed-xml="true" for too many associations, since XML does not deal well with circularity.

<class name="Customer" 
        table="CUSTOMER" 
        node="customer">
        
    <id name="id" 
            column="CUST_ID" 
            node="@id"/>
            
    <map name="accounts" 
            node="." 
            embed-xml="true">
        <key column="CUSTOMER_ID" 
                not-null="true"/>
        <map-key column="SHORT_DESC" 
                node="@short-desc" 
                type="string"/>
        <one-to-many entity-name="Account"
                embed-xml="false" 
                node="account"/>
    </map>
    
    <component name="name" 
            node="name">
        <property name="firstName" 
                node="first-name"/>
        <property name="initial" 
                node="initial"/>
        <property name="lastName" 
                node="last-name"/>
    </component>
    
    ...
    
</class>

In this case, the collection of account ids is embedded, but not the actual account data. The following HQL query:

from Customer c left join fetch c.accounts where c.lastName like :lastName

would return datasets such as this:

<customer id="123456789">
    <account short-desc="Savings">987632567</account>
    <account short-desc="Credit Card">985612323</account>
    <name>
        <first-name>Gavin</first-name>
        <initial>A</initial>
        <last-name>King</last-name>
    </name>
    ...
</customer>

如果你把一對多映射<one-to-many>的embed-xml屬性置爲真(embed-xml="true")， 則數據看上去就像這樣：

<customer id="123456789">
    <account id="987632567" short-desc="Savings">
        <customer id="123456789"/>
        <balance>100.29</balance>
    </account>
    <account id="985612323" short-desc="Credit Card">
        <customer id="123456789"/>
        <balance>-2370.34</balance>
    </account>
    <name>
        <first-name>Gavin</first-name>
        <initial>A</initial>
        <last-name>King</last-name>
    </name>
    ...
</customer>

18.3. 操作XML數據

You can also re-read and update XML documents in the application. You can do this by obtaining a dom4j session:

Document doc = ....;
       
Session session = factory.openSession();
Session dom4jSession = session.getSession(EntityMode.DOM4J);
Transaction tx = session.beginTransaction();

List results = dom4jSession
    .createQuery("from Customer c left join fetch c.accounts where c.lastName like :lastName")
    .list();
for ( int i=0; i<results.size(); i++ ) {
    //add the customer data to the XML document
    Element customer = (Element) results.get(i);
    doc.add(customer);
}

tx.commit();
session.close();

Session session = factory.openSession();
Session dom4jSession = session.getSession(EntityMode.DOM4J);
Transaction tx = session.beginTransaction();

Element cust = (Element) dom4jSession.get("Customer", customerId);
for ( int i=0; i<results.size(); i++ ) {
    Element customer = (Element) results.get(i);
    //change the customer name in the XML and database
    Element name = customer.element("name");
    name.element("first-name").setText(firstName);
    name.element("initial").setText(initial);
    name.element("last-name").setText(lastName);
}

tx.commit();
session.close();

When implementing XML-based data import/export, it is useful to combine this feature with Hibernate'sreplicate() operation.

第 19 章 提升性能

19.1. 抓取策略(Fetching strategies)

Hibernate uses a fetching strategy to retrieve associated objects if the application needs to navigate the association. Fetch strategies can be declared in the O/R mapping metadata, or over-ridden by a particular HQL or Criteria query.

Hibernate3 定義瞭如下幾種抓取策略：

• Join fetching: Hibernate retrieves the associated instance or collection in the sameSELECT, using an OUTER JOIN.

• Select fetching: a second SELECT is used to retrieve the associated entity or collection. Unless you explicitly disable lazy fetching by specifyinglazy="false", this second select will only be executed when you access the association.

• Subselect fetching: a second SELECT is used to retrieve the associated collections for all entities retrieved in a previous query or fetch. Unless you explicitly disable lazy fetching by specifyinglazy="false", this second select will only be executed when you access the association.

• Batch fetching: an optimization strategy for select fetching. Hibernate retrieves a batch of entity instances or collections in a singleSELECT by specifying a list of primary or foreign keys.

Hibernate會區分下列各種情況：

• Immediate fetching: an association, collection or attribute is fetched immediately when the owner is loaded.

• Lazy collection fetching: a collection is fetched when the application invokes an operation upon that collection. This is the default for collections.

• "Extra-lazy" collection fetching: individual elements of the collection are accessed from the database as needed. Hibernate tries not to fetch the whole collection into memory unless absolutely needed. It is suitable for large collections.

• Proxy fetching: a single-valued association is fetched when a method other than the identifier getter is invoked upon the associated object.

• "No-proxy" fetching: a single-valued association is fetched when the instance variable is accessed. Compared to proxy fetching, this approach is less lazy; the association is fetched even when only the identifier is accessed. It is also more transparent, since no proxy is visible to the application. This approach requires buildtime bytecode instrumentation and is rarely necessary.

• Lazy attribute fetching: an attribute or single valued association is fetched when the instance variable is accessed. This approach requires buildtime bytecode instrumentation and is rarely necessary.

We have two orthogonal notions here: when is the association fetched andhow is it fetched. It is important that you do not confuse them. We usefetch to tune performance. We can use lazy to define a contract for what data is always available in any detached instance of a particular class.

19.1.1. 操作延遲加載的關聯

By default, Hibernate3 uses lazy select fetching for collections and lazy proxy fetching for single-valued associations. These defaults make sense for most associations in the majority of applications.

If you set hibernate.default_batch_fetch_size, Hibernate will use the batch fetch optimization for lazy fetching. This optimization can also be enabled at a more granular level.

Please be aware that access to a lazy association outside of the context of an open Hibernate session will result in an exception. For example:

s = sessions.openSession();
Transaction tx = s.beginTransaction();
            
User u = (User) s.createQuery("from User u where u.name=:userName")
    .setString("userName", userName).uniqueResult();
Map permissions = u.getPermissions();

tx.commit();
s.close();

Integer accessLevel = (Integer) permissions.get("accounts");  // Error!

Since the permissions collection was not initialized when the Session was closed, the collection will not be able to load its state. Hibernate does not support lazy initialization for detached objects. This can be fixed by moving the code that reads from the collection to just before the transaction is committed.

Alternatively, you can use a non-lazy collection or association, by specifyinglazy="false" for the association mapping. However, it is intended that lazy initialization be used for almost all collections and associations. If you define too many non-lazy associations in your object model, Hibernate will fetch the entire database into memory in every transaction.

On the other hand, you can use join fetching, which is non-lazy by nature, instead of select fetching in a particular transaction. We will now explain how to customize the fetching strategy. In Hibernate3, the mechanisms for choosing a fetch strategy are identical for single-valued associations and collections.

19.1.2. 調整抓取策略（Tuning fetch strategies）

查詢抓取（默認的）在N+1查詢的情況下是極其脆弱的，因此我們可能會要求在映射文檔中定義使用連接抓取：

<set name="permissions" 
            fetch="join">
    <key column="userId"/>
    <one-to-many class="Permission"/>
</set

<many-to-one name="mother" class="Cat" fetch="join"/>

在映射文檔中定義的抓取策略將會對以下列表條目產生影響：

• 通過get()或load()方法取得數據。

• 只有在關聯之間進行導航時，纔會隱式的取得數據。

• 條件查詢

• 使用了subselect抓取的HQL查詢

Irrespective of the fetching strategy you use, the defined non-lazy graph is guaranteed to be loaded into memory. This might, however, result in several immediate selects being used to execute a particular HQL query.

Usually, the mapping document is not used to customize fetching. Instead, we keep the default behavior, and override it for a particular transaction, usingleft join fetch in HQL. This tells Hibernate to fetch the association eagerly in the first select, using an outer join. In theCriteria query API, you would use setFetchMode(FetchMode.JOIN).

If you want to change the fetching strategy used by get() orload(), you can use a Criteria query. For example:

User user = (User) session.createCriteria(User.class)
                .setFetchMode("permissions", FetchMode.JOIN)
                .add( Restrictions.idEq(userId) )
                .uniqueResult();

This is Hibernate's equivalent of what some ORM solutions call a "fetch plan".

A completely different approach to problems with N+1 selects is to use the second-level cache.

19.1.3. 單端關聯代理（Single-ended association proxies）

Lazy fetching for collections is implemented using Hibernate's own implementation of persistent collections. However, a different mechanism is needed for lazy behavior in single-ended associations. The target entity of the association must be proxied. Hibernate implements lazy initializing proxies for persistent objects using runtime bytecode enhancement which is accessed via the CGLIB library.

At startup, Hibernate3 generates proxies by default for all persistent classes and uses them to enable lazy fetching ofmany-to-one and one-to-one associations.

The mapping file may declare an interface to use as the proxy interface for that class, with theproxy attribute. By default, Hibernate uses a subclass of the class.The proxied class must implement a default constructor with at least package visibility. This constructor is recommended for all persistent classes.

There are potential problems to note when extending this approach to polymorphic classes.For example:

<class name="Cat" proxy="Cat">
    ......
    <subclass name="DomesticCat">
        .....
    </subclass>
</class>

首先，Cat實例永遠不可以被強制轉換爲DomesticCat, 即使它本身就是DomesticCat實例。

Cat cat = (Cat) session.load(Cat.class, id);  // instantiate a proxy (does not hit the db)
if ( cat.isDomesticCat() ) {                  // hit the db to initialize the proxy
    DomesticCat dc = (DomesticCat) cat;       // Error!
    ....
}

Secondly, it is possible to break proxy ==:

Cat cat = (Cat) session.load(Cat.class, id);            // instantiate a Cat proxy
DomesticCat dc = 
        (DomesticCat) session.load(DomesticCat.class, id);  // acquire new DomesticCat proxy!
System.out.println(cat==dc);                            // false

雖然如此，但實際情況並沒有看上去那麼糟糕。雖然我們現在有兩個不同的引用，分別指向這兩個不同的代理對象， 但實際上，其底層應該是同一個實例對象：

cat.setWeight(11.0);  // hit the db to initialize the proxy
System.out.println( dc.getWeight() );  // 11.0

Third, you cannot use a CGLIB proxy for a final class or a class with anyfinal methods.

Finally, if your persistent object acquires any resources upon instantiation (e.g. in initializers or default constructor), then those resources will also be acquired by the proxy. The proxy class is an actual subclass of the persistent class.

These problems are all due to fundamental limitations in Java's single inheritance model. To avoid these problems your persistent classes must each implement an interface that declares its business methods. You should specify these interfaces in the mapping file where CatImpl implements the interface Cat and DomesticCatImpl implements the interfaceDomesticCat. For example:

<class name="CatImpl" proxy="Cat">
    ......
    <subclass name="DomesticCatImpl" proxy="DomesticCat">
        .....
    </subclass>
</class>

Then proxies for instances of Cat and DomesticCat can be returned by load() or iterate().

Cat cat = (Cat) session.load(CatImpl.class, catid);
Iterator iter = session.createQuery("from CatImpl as cat where cat.name='fritz'").iterate();
Cat fritz = (Cat) iter.next();

Note

list() does not usually return proxies.

這裏，對象之間的關係也將被延遲載入。這就意味着，你應該將屬性聲明爲Cat，而不是CatImpl。

Certain operations do not require proxy initialization:

• equals(): if the persistent class does not overrideequals()

• hashCode(): if the persistent class does not overridehashCode()

• 標誌符的getter方法。

Hibernate將會識別出那些重載了equals()、或hashCode()方法的持久化類。

By choosing lazy="no-proxy" instead of the defaultlazy="proxy", you can avoid problems associated with typecasting. However, buildtime bytecode instrumentation is required, and all operations will result in immediate proxy initialization.

19.1.4. 實例化集合和代理（Initializing collections and proxies）

A LazyInitializationException will be thrown by Hibernate if an uninitialized collection or proxy is accessed outside of the scope of theSession, i.e., when the entity owning the collection or having the reference to the proxy is in the detached state.

Sometimes a proxy or collection needs to be initialized before closing the Session. You can force initialization by calling cat.getSex() orcat.getKittens().size(), for example. However, this can be confusing to readers of the code and it is not convenient for generic code.

The static methods Hibernate.initialize() and Hibernate.isInitialized(), provide the application with a convenient way of working with lazily initialized collections or proxies.Hibernate.initialize(cat) will force the initialization of a proxy,cat, as long as its Session is still open.Hibernate.initialize( cat.getKittens() ) has a similar effect for the collection of kittens.

Another option is to keep the Session open until all required collections and proxies have been loaded. In some application architectures, particularly where the code that accesses data using Hibernate, and the code that uses it are in different application layers or different physical processes, it can be a problem to ensure that theSession is open when a collection is initialized. There are two basic ways to deal with this issue:

• In a web-based application, a servlet filter can be used to close the Session only at the end of a user request, once the rendering of the view is complete (theOpen Session in View pattern). Of course, this places heavy demands on the correctness of the exception handling of your application infrastructure. It is vitally important that theSession is closed and the transaction ended before returning to the user, even when an exception occurs during rendering of the view. See the Hibernate Wiki for examples of this "Open Session in View" pattern.

• In an application with a separate business tier, the business logic must "prepare" all collections that the web tier needs before returning. This means that the business tier should load all the data and return all the data already initialized to the presentation/web tier that is required for a particular use case. Usually, the application calls Hibernate.initialize() for each collection that will be needed in the web tier (this call must occur before the session is closed) or retrieves the collection eagerly using a Hibernate query with aFETCH clause or a FetchMode.JOIN inCriteria. This is usually easier if you adopt the Command pattern instead of a Session Facade.

• You can also attach a previously loaded object to a new Session with merge() or lock() before accessing uninitialized collections or other proxies. Hibernate does not, and certainlyshould not, do this automatically since it would introduce impromptu transaction semantics.

Sometimes you do not want to initialize a large collection, but still need some information about it, like its size, for example, or a subset of the data.

你可以使用集合過濾器得到其集合的大小，而不必實例化整個集合：

( (Integer) s.createFilter( collection, "select count(*)" ).list().get(0) ).intValue()

這裏的createFilter()方法也可以被用來有效的抓取集合的部分內容，而無需實例化整個集合：

s.createFilter( lazyCollection, "").setFirstResult(0).setMaxResults(10).list();

19.1.5. 使用批量抓取（Using batch fetching）

Using batch fetching, Hibernate can load several uninitialized proxies if one proxy is accessed. Batch fetching is an optimization of the lazy select fetching strategy. There are two ways you can configure batch fetching: on the class level and the collection level.

Batch fetching for classes/entities is easier to understand. Consider the following example: at runtime you have 25Cat instances loaded in a Session, and eachCat has a reference to its owner, aPerson. The Person class is mapped with a proxy,lazy="true". If you now iterate through all cats and callgetOwner() on each, Hibernate will, by default, execute 25SELECT statements to retrieve the proxied owners. You can tune this behavior by specifying abatch-size in the mapping of Person:

<class name="Person" batch-size="10">...</class>

Hibernate will now execute only three queries: the pattern is 10, 10, 5.

You can also enable batch fetching of collections. For example, if each Person has a lazy collection of Cats, and 10 persons are currently loaded in theSession, iterating through all persons will generate 10SELECTs, one for every call to getCats(). If you enable batch fetching for the cats collection in the mapping ofPerson, Hibernate can pre-fetch collections:

<class name="Person">
    <set name="cats" batch-size="3">
        ...
    </set>
</class>

如果整個的batch-size是3（筆誤？），那麼Hibernate將會分四次執行SELECT查詢， 按照3、3、3、1的大小分別載入數據。這裏的每次載入的數據量還具體依賴於當前Session中未實例化集合的個數。

Batch fetching of collections is particularly useful if you have a nested tree of items, i.e. the typical bill-of-materials pattern. However, anested set or a materialized path might be a better option for read-mostly trees.

19.1.6. 使用子查詢抓取（Using subselect fetching）

If one lazy collection or single-valued proxy has to be fetched, Hibernate will load all of them, re-running the original query in a subselect. This works in the same way as batch-fetching but without the piecemeal loading.

19.1.7. 使用延遲屬性抓取（Using lazy property fetching）

Hibernate3 supports the lazy fetching of individual properties. This optimization technique is also known asfetch groups. Please note that this is mostly a marketing feature; optimizing row reads is much more important than optimization of column reads. However, only loading some properties of a class could be useful in extreme cases. For example, when legacy tables have hundreds of columns and the data model cannot be improved.

可以在映射文件中對特定的屬性設置lazy，定義該屬性爲延遲載入。

<class name="Document">
       <id name="id">
        <generator class="native"/>
    </id>
    <property name="name" not-null="true" length="50"/>
    <property name="summary" not-null="true" length="200" lazy="true"/>
    <property name="text" not-null="true" length="2000" lazy="true"/>
</class>

Lazy property loading requires buildtime bytecode instrumentation. If your persistent classes are not enhanced, Hibernate will ignore lazy property settings and return to immediate fetching.

你可以在Ant的Task中，進行如下定義，對持久類代碼加入“二進制指令。”

<target name="instrument" depends="compile">
    <taskdef name="instrument" classname="org.hibernate.tool.instrument.InstrumentTask">
        <classpath path="${jar.path}"/>
        <classpath path="${classes.dir}"/>
        <classpath refid="lib.class.path"/>
    </taskdef>

    <instrument verbose="true">
        <fileset dir="${testclasses.dir}/org/hibernate/auction/model">
            <include name="*.class"/>
        </fileset>
    </instrument>
</target>

A different way of avoiding unnecessary column reads, at least for read-only transactions, is to use the projection features of HQL or Criteria queries. This avoids the need for buildtime bytecode processing and is certainly a preferred solution.

You can force the usual eager fetching of properties using fetch all properties in HQL.

19.2. 二級緩存（The Second Level Cache）

A Hibernate Session is a transaction-level cache of persistent data. It is possible to configure a cluster or JVM-level (SessionFactory-level) cache on a class-by-class and collection-by-collection basis. You can even plug in a clustered cache. Be aware that caches are not aware of changes made to the persistent store by another application. They can, however, be configured to regularly expire cached data.

You have the option to tell Hibernate which caching implementation to use by specifying the name of a class that implementsorg.hibernate.cache.CacheProvider using the propertyhibernate.cache.provider_class. Hibernate is bundled with a number of built-in integrations with the open-source cache providers that are listed below. You can also implement your own and plug it in as outlined above. Note that versions prior to 3.2 use EhCache as the default cache provider.

表 19.1. 緩存策略提供商（Cache Providers）

Cache	Provider class	Type	Cluster Safe	Query Cache Supported
Hashtable (not intended for production use)	org.hibernate.cache.HashtableCacheProvider	memory		yes
EHCache	org.hibernate.cache.EhCacheProvider	memory, disk		yes
OSCache	org.hibernate.cache.OSCacheProvider	memory, disk		yes
SwarmCache	org.hibernate.cache.SwarmCacheProvider	clustered (ip multicast)	yes (clustered invalidation)
JBoss Cache 1.x	org.hibernate.cache.TreeCacheProvider	clustered (ip multicast), transactional	yes (replication)	yes (clock sync req.)
JBoss Cache 2	org.hibernate.cache.jbc2.JBossCacheRegionFactory	clustered (ip multicast), transactional	yes (replication or invalidation)	yes (clock sync req.)

19.2.1. 緩存映射（Cache mappings）

類或者集合映射的“<cache>元素”可以有下列形式：

<cache 
    usage="transactional|read-write|nonstrict-read-write|read-only"  
    region="RegionName"                                              
    include="all|non-lazy"                                           
/>

	usage(必須)說明了緩存的策略: transactional、read-write、 nonstrict-read-write或read-only。
	region (optional: defaults to the class or collection role name): specifies the name of the second level cache region
	include (optional: defaults to all) non-lazy: specifies that properties of the entity mapped withlazy="true" cannot be cached when attribute-level lazy fetching is enabled

Alternatively, you can specify <class-cache> and <collection-cache> elements in hibernate.cfg.xml.

這裏的usage 屬性指明瞭緩存併發策略（cache concurrency strategy）。

19.2.2. 策略：只讀緩存（Strategy: read only）

If your application needs to read, but not modify, instances of a persistent class, aread-only cache can be used. This is the simplest and optimal performing strategy. It is even safe for use in a cluster.

<class name="eg.Immutable" mutable="false">
    <cache usage="read-only"/>
    ....
</class>

19.2.3. 策略:讀/寫緩存（Strategy: read/write）

If the application needs to update data, a read-write cache might be appropriate. This cache strategy should never be used if serializable transaction isolation level is required. If the cache is used in a JTA environment, you must specify the property hibernate.transaction.manager_lookup_class and naming a strategy for obtaining the JTATransactionManager. In other environments, you should ensure that the transaction is completed whenSession.close() or Session.disconnect() is called. If you want to use this strategy in a cluster, you should ensure that the underlying cache implementation supports locking. The built-in cache providersdo not support locking.

<class name="eg.Cat" .... >
    <cache usage="read-write"/>
    ....
    <set name="kittens" ... >
        <cache usage="read-write"/>
        ....
    </set>
</class>

19.2.4. 策略:非嚴格讀/寫緩存（Strategy: nonstrict read/write）

If the application only occasionally needs to update data (i.e. if it is extremely unlikely that two transactions would try to update the same item simultaneously), and strict transaction isolation is not required, anonstrict-read-write cache might be appropriate. If the cache is used in a JTA environment, you must specifyhibernate.transaction.manager_lookup_class. In other environments, you should ensure that the transaction is completed whenSession.close() or Session.disconnect() is called.

19.2.5. 策略:事務緩存（transactional）

The transactional cache strategy provides support for fully transactional cache providers such as JBoss TreeCache. Such a cache can only be used in a JTA environment and you must specifyhibernate.transaction.manager_lookup_class.

19.2.6. Cache-provider/concurrency-strategy compatibility

重要

None of the cache providers support all of the cache concurrency strategies.

The following table shows which providers are compatible with which concurrency strategies.

表 19.2. 各種緩存提供商對緩存併發策略的支持情況（Cache Concurrency Strategy Support）

Cache	read-only	nonstrict-read-write	read-write	transactional
Hashtable (not intended for production use)	yes	yes	yes
EHCache	yes	yes	yes
OSCache	yes	yes	yes
SwarmCache	yes	yes
JBoss Cache 1.x	yes			yes
JBoss Cache 2	yes			yes

19.3. 管理緩存（Managing the caches）

Whenever you pass an object to save(), update() or saveOrUpdate(), and whenever you retrieve an object usingload(), get(), list(), iterate() or scroll(), that object is added to the internal cache of theSession.

When flush() is subsequently called, the state of that object will be synchronized with the database. If you do not want this synchronization to occur, or if you are processing a huge number of objects and need to manage memory efficiently, the evict() method can be used to remove the object and its collections from the first-level cache.

ScrollableResult cats = sess.createQuery("from Cat as cat").scroll(); //a huge result set
while ( cats.next() ) {
    Cat cat = (Cat) cats.get(0);
    doSomethingWithACat(cat);
    sess.evict(cat);
}

Session還提供了一個contains()方法，用來判斷某個實例是否處於當前session的緩存中。

To evict all objects from the session cache, call Session.clear()

對於二級緩存來說，在SessionFactory中定義了許多方法， 清除緩存中實例、整個類、集合實例或者整個集合。

sessionFactory.evict(Cat.class, catId); //evict a particular Cat
sessionFactory.evict(Cat.class);  //evict all Cats
sessionFactory.evictCollection("Cat.kittens", catId); //evict a particular collection of kittens
sessionFactory.evictCollection("Cat.kittens"); //evict all kitten collections

The CacheMode controls how a particular session interacts with the second-level cache:

• CacheMode.NORMAL: will read items from and write items to the second-level cache

• CacheMode.GET: will read items from the second-level cache. Do not write to the second-level cache except when updating data

• CacheMode.PUT: will write items to the second-level cache. Do not read from the second-level cache

• CacheMode.REFRESH: will write items to the second-level cache. Do not read from the second-level cache. Bypass the effect ofhibernate.cache.use_minimal_puts forcing a refresh of the second-level cache for all items read from the database

如若需要查看二級緩存或查詢緩存區域的內容，你可以使用統計（Statistics） API。

Map cacheEntries = sessionFactory.getStatistics()
        .getSecondLevelCacheStatistics(regionName)
        .getEntries();

You will need to enable statistics and, optionally, force Hibernate to keep the cache entries in a more readable format:

hibernate.generate_statistics true
hibernate.cache.use_structured_entries true

19.4. 查詢緩存（The Query Cache）

Query result sets can also be cached. This is only useful for queries that are run frequently with the same parameters. You will first need to enable the query cache:

hibernate.cache.use_query_cache true

This setting creates two new cache regions: one holding cached query result sets (org.hibernate.cache.StandardQueryCache), the other holding timestamps of the most recent updates to queryable tables (org.hibernate.cache.UpdateTimestampsCache). Note that the query cache does not cache the state of the actual entities in the result set; it caches only identifier values and results of value type. The query cache should always be used in conjunction with the second-level cache.

Most queries do not benefit from caching, so by default, queries are not cached. To enable caching, callQuery.setCacheable(true). This call allows the query to look for existing cache results or add its results to the cache when it is executed.

If you require fine-grained control over query cache expiration policies, you can specify a named cache region for a particular query by callingQuery.setCacheRegion().

List blogs = sess.createQuery("from Blog blog where blog.blogger = :blogger")
    .setEntity("blogger", blogger)
    .setMaxResults(15)
    .setCacheable(true)
    .setCacheRegion("frontpages")
    .list();

如果查詢需要強行刷新其查詢緩存區域，那麼你應該調用Query.setCacheMode(CacheMode.REFRESH)方法。 這對在其他進程中修改底層數據（例如，不通過Hibernate修改數據），或對那些需要選擇性更新特定查詢結果集的情況特別有用。 這是對SessionFactory.evictQueries()的更爲有效的替代方案，同樣可以清除查詢緩存區域。

19.5. 理解集合性能（Understanding Collection performance）

In the previous sections we have covered collections and their applications. In this section we explore some more issues in relation to collections at runtime.

19.5.1. 分類（Taxonomy）

Hibernate定義了三種基本類型的集合：

• 值數據集合

• one-to-many associations

• many-to-many associations

這個分類是區分了不同的表和外鍵關係類型，但是它沒有告訴我們關係模型的所有內容。 要完全理解他們的關係結構和性能特點，我們必須同時考慮“用於Hibernate更新或刪除集合行數據的主鍵的結構”。 因此得到了如下的分類：

• 有序集合類

• 集合（sets）

• 包（bags)

All indexed collections (maps, lists, and arrays) have a primary key consisting of the<key> and <index> columns. In this case, collection updates are extremely efficient. The primary key can be efficiently indexed and a particular row can be efficiently located when Hibernate tries to update or delete it.

Sets have a primary key consisting of <key> and element columns. This can be less efficient for some types of collection element, particularly composite elements or large text or binary fields, as the database may not be able to index a complex primary key as efficiently. However, for one-to-many or many-to-many associations, particularly in the case of synthetic identifiers, it is likely to be just as efficient. If you wantSchemaExport to actually create the primary key of a<set>, you must declare all columns as not-null="true".

<idbag> mappings define a surrogate key, so they are efficient to update. In fact, they are the best case.

Bags are the worst case since they permit duplicate element values and, as they have no index column, no primary key can be defined. Hibernate has no way of distinguishing between duplicate rows. Hibernate resolves this problem by completely removing in a single DELETE and recreating the collection whenever it changes. This can be inefficient.

For a one-to-many association, the "primary key" may not be the physical primary key of the database table. Even in this case, the above classification is still useful. It reflects how Hibernate "locates" individual rows of the collection.

19.5.2. Lists, maps 和sets用於更新效率最高

From the discussion above, it should be clear that indexed collections and sets allow the most efficient operation in terms of adding, removing and updating elements.

There is, arguably, one more advantage that indexed collections have over sets for many-to-many associations or collections of values. Because of the structure of aSet, Hibernate does not UPDATE a row when an element is "changed". Changes to aSet always work via INSERT andDELETE of individual rows. Once again, this consideration does not apply to one-to-many associations.

After observing that arrays cannot be lazy, you can conclude that lists, maps and idbags are the most performant (non-inverse) collection types, with sets not far behind. You can expect sets to be the most common kind of collection in Hibernate applications. This is because the "set" semantics are most natural in the relational model.

However, in well-designed Hibernate domain models, most collections are in fact one-to-many associations withinverse="true". For these associations, the update is handled by the many-to-one end of the association, and so considerations of collection update performance simply do not apply.

19.5.3. Bag和list是反向集合類中效率最高的

There is a particular case, however, in which bags, and also lists, are much more performant than sets. For a collection withinverse="true", the standard bidirectional one-to-many relationship idiom, for example, we can add elements to a bag or list without needing to initialize (fetch) the bag elements. This is because, unlike aset, Collection.add() orCollection.addAll() must always return true for a bag orList. This can make the following common code much faster:

Parent p = (Parent) sess.load(Parent.class, id);
Child c = new Child();
c.setParent(p);
p.getChildren().add(c);  //no need to fetch the collection!
sess.flush();

19.5.4. 一次性刪除（One shot delete）

Deleting collection elements one by one can sometimes be extremely inefficient. Hibernate knows not to do that in the case of an newly-empty collection (if you calledlist.clear(), for example). In this case, Hibernate will issue a singleDELETE.

Suppose you added a single element to a collection of size twenty and then remove two elements. Hibernate will issue oneINSERT statement and two DELETE statements, unless the collection is a bag. This is certainly desirable.

但是，假設我們刪除了18個數據，只剩下2個，然後新增3個。則有兩種處理方式：

• 逐一的刪除這18個數據，再新增三個；

• remove the whole collection in one SQL DELETE and insert all five current elements one by one

Hibernate cannot know that the second option is probably quicker. It would probably be undesirable for Hibernate to be that intuitive as such behavior might confuse database triggers, etc.

Fortunately, you can force this behavior (i.e. the second strategy) at any time by discarding (i.e. dereferencing) the original collection and returning a newly instantiated collection with all the current elements.

One-shot-delete does not apply to collections mapped inverse="true".

19.6. 監測性能（Monitoring performance）

沒有監測和性能參數而進行優化是毫無意義的。Hibernate爲其內部操作提供了一系列的示意圖，因此可以從 每個SessionFactory抓取其統計數據。

19.6.1. 監測SessionFactory

你可以有兩種方式訪問SessionFactory的數據記錄，第一種就是自己直接調用 sessionFactory.getStatistics()方法讀取、顯示統計數據。

Hibernate can also use JMX to publish metrics if you enable the StatisticsService MBean. You can enable a single MBean for all your SessionFactory or one per factory. See the following code for minimalistic configuration examples:

// MBean service registration for a specific SessionFactory
Hashtable tb = new Hashtable();
tb.put("type", "statistics");
tb.put("sessionFactory", "myFinancialApp");
ObjectName on = new ObjectName("hibernate", tb); // MBean object name

StatisticsService stats = new StatisticsService(); // MBean implementation
stats.setSessionFactory(sessionFactory); // Bind the stats to a SessionFactory
server.registerMBean(stats, on); // Register the Mbean on the server

// MBean service registration for all SessionFactory's
Hashtable tb = new Hashtable();
tb.put("type", "statistics");
tb.put("sessionFactory", "all");
ObjectName on = new ObjectName("hibernate", tb); // MBean object name

StatisticsService stats = new StatisticsService(); // MBean implementation
server.registerMBean(stats, on); // Register the MBean on the server

You can activate and deactivate the monitoring for a SessionFactory:

• 在配置期間，將hibernate.generate_statistics設置爲true或false；

• 在運行期間，則可以可以通過sf.getStatistics().setStatisticsEnabled(true) 或hibernateStatsBean.setStatisticsEnabled(true)

Statistics can be reset programmatically using the clear() method. A summary can be sent to a logger (info level) using thelogSummary() method.

19.6.2. 數據記錄（Metrics）

Hibernate provides a number of metrics, from basic information to more specialized information that is only relevant in certain scenarios. All available counters are described in theStatistics interface API, in three categories:

• 使用Session的普通數據記錄，例如打開的Session的個數、取得的JDBC的連接數等；

• Metrics related to the entities, collections, queries, and caches as a whole (aka global metrics).

• 和具體實體、集合、查詢、緩存相關的詳細數據記錄

For example, you can check the cache hit, miss, and put ratio of entities, collections and queries, and the average time a query needs. Be aware that the number of milliseconds is subject to approximation in Java. Hibernate is tied to the JVM precision and on some platforms this might only be accurate to 10 seconds.

Simple getters are used to access the global metrics (i.e. not tied to a particular entity, collection, cache region, etc.). You can access the metrics of a particular entity, collection or cache region through its name, and through its HQL or SQL representation for queries. Please refer to the Statistics, EntityStatistics, CollectionStatistics, SecondLevelCacheStatistics, and QueryStatistics API Javadoc for more information. The following code is a simple example:

Statistics stats = HibernateUtil.sessionFactory.getStatistics();

double queryCacheHitCount  = stats.getQueryCacheHitCount();
double queryCacheMissCount = stats.getQueryCacheMissCount();
double queryCacheHitRatio =
  queryCacheHitCount / (queryCacheHitCount + queryCacheMissCount);

log.info("Query Hit ratio:" + queryCacheHitRatio);

EntityStatistics entityStats =
  stats.getEntityStatistics( Cat.class.getName() );
long changes =
        entityStats.getInsertCount()
        + entityStats.getUpdateCount()
        + entityStats.getDeleteCount();
log.info(Cat.class.getName() + " changed " + changes + "times"  );

You can work on all entities, collections, queries and region caches, by retrieving the list of names of entities, collections, queries and region caches using the following methods:getQueries(), getEntityNames(),getCollectionRoleNames(), and getSecondLevelCacheRegionNames().

第 20 章 工具箱指南

Roundtrip engineering with Hibernate is possible using a set of Eclipse plugins, commandline tools, and Ant tasks.

Hibernate Tools currently include plugins for the Eclipse IDE as well as Ant tasks for reverse engineering of existing databases:

• Mapping Editor: an editor for Hibernate XML mapping files that supports auto-completion and syntax highlighting. It also supports semantic auto-completion for class names and property/field names, making it more versatile than a normal XML editor.

• Console: the console is a new view in Eclipse. In addition to a tree overview of your console configurations, you are also provided with an interactive view of your persistent classes and their relationships. The console allows you to execute HQL queries against your database and browse the result directly in Eclipse.

• Development Wizards: several wizards are provided with the Hibernate Eclipse tools. You can use a wizard to quickly generate Hibernate configuration (cfg.xml) files, or to reverse engineer an existing database schema into POJO source files and Hibernate mapping files. The reverse engineering wizard supports customizable templates.

Please refer to the Hibernate Tools package documentation for more information.

However, the Hibernate main package comes bundled with an integrated tool : SchemaExport aka hbm2ddl.It can even be used from "inside" Hibernate.

20.1. Schema自動生成（Automatic schema generation）

DDL can be generated from your mapping files by a Hibernate utility. The generated schema includes referential integrity constraints, primary and foreign keys, for entity and collection tables. Tables and sequences are also created for mapped identifier generators.

You must specify a SQL Dialect via the hibernate.dialect property when using this tool, as DDL is highly vendor-specific.

First, you must customize your mapping files to improve the generated schema. The next section covers schema customization.

20.1.1. 對schema定製化(Customizing the schema)

Many Hibernate mapping elements define optional attributes named length, precision and scale. You can set the length, precision and scale of a column with this attribute.

<property name="zip" length="5"/>

<property name="balance" precision="12" scale="2"/>

Some tags also accept a not-null attribute for generating aNOT NULL constraint on table columns, and a unique attribute for generating UNIQUE constraint on table columns.

<many-to-one name="bar" column="barId" not-null="true"/>

<element column="serialNumber" type="long" not-null="true" unique="true"/>

A unique-key attribute can be used to group columns in a single, unique key constraint. Currently, the specified value of theunique-key attribute is not used to name the constraint in the generated DDL. It is only used to group the columns in the mapping file.

<many-to-one name="org" column="orgId" unique-key="OrgEmployeeId"/>
<property name="employeeId" unique-key="OrgEmployee"/>

An index attribute specifies the name of an index that will be created using the mapped column or columns. Multiple columns can be grouped into the same index by simply specifying the same index name.

<property name="lastName" index="CustName"/>
<property name="firstName" index="CustName"/>

A foreign-key attribute can be used to override the name of any generated foreign key constraint.

<many-to-one name="bar" column="barId" foreign-key="FKFooBar"/>

很多映射元素還接受<column>子元素。這在定義跨越多字段的類型時特別有用。

<property name="name" type="my.customtypes.Name"/>
    <column name="last" not-null="true" index="bar_idx" length="30"/>
    <column name="first" not-null="true" index="bar_idx" length="20"/>
    <column name="initial"/>
</property>

The default attribute allows you to specify a default value for a column.You should assign the same value to the mapped property before saving a new instance of the mapped class.

<property name="credits" type="integer" insert="false">
    <column name="credits" default="10"/>
</property>

<version name="version" type="integer" insert="false">
    <column name="version" default="0"/>
</property>

sql-type屬性允許用戶覆蓋默認的Hibernate類型到SQL數據類型的映射。

<property name="balance" type="float">
    <column name="balance" sql-type="decimal(13,3)"/>
</property>

check屬性允許用戶指定一個約束檢查。

<property name="foo" type="integer">
    <column name="foo" check="foo > 10"/>
</property>

<class name="Foo" table="foos" check="bar < 100.0">
    ...
    <property name="bar" type="float"/>
</class>

The following table summarizes these optional attributes.

表 20.1. Summary

屬性(Attribute)	值（Values）	解釋（Interpretation）
length	數字	字段長度
precision	數字	精度(decimal precision)
scale	數字	小數點位數(decimal scale)
not-null	true|false	specifies that the column should be non-nullable
unique	true|false	指明是否該字段具有惟一約束
index	index_name	指明一個（多字段）的索引(index)的名字
unique-key	unique_key_name	指明多字段惟一約束的名字（參見上面的說明）
foreign-key	foreign_key_name	specifies the name of the foreign key constraint generated for an association, for a<one-to-one>, <many-to-one>,<key>, or <many-to-many> mapping element. Note thatinverse="true" sides will not be considered by SchemaExport.
sql-type	SQL 字段類型	overrides the default column type (attribute of <column> element only)
default	SQL expression	爲字段指定默認值
check	SQL expression	對字段或表加入SQL約束檢查

<comment>元素可以讓你在生成的schema中加入註釋。

<class name="Customer" table="CurCust">
    <comment>Current customers only</comment>
    ...
</class>

<property name="balance">
    <column name="bal">
        <comment>Balance in USD</comment>
    </column>
</property>

This results in a comment on table or comment on column statement in the generated DDL where supported.

20.1.2. 運行該工具

SchemaExport工具把DDL腳本寫到標準輸出，同時/或者執行DDL語句。

The following table displays the SchemaExport command line options

java -cp hibernate_classpathsorg.hibernate.tool.hbm2ddl.SchemaExport options mapping_files

表 20.2. SchemaExport命令行選項

選項	Description
--quiet	do not output the script to stdout
--drop	只進行drop tables的步驟
--create	只創建表
--text	do not export to the database
--output=my_schema.ddl	把輸出的ddl腳本輸出到一個文件
--naming=eg.MyNamingStrategy	select a NamingStrategy
--config=hibernate.cfg.xml	從XML文件讀入Hibernate配置
--properties=hibernate.properties	read database properties from a file
--format	把腳本中的SQL語句對齊和美化
--delimiter=;	爲腳本設置行結束符

You can even embed SchemaExport in your application:

Configuration cfg = ....;
new SchemaExport(cfg).create(false, true);

20.1.3. 屬性(Properties)

Database properties can be specified:

• 通過-D<property>系統參數

• 在hibernate.properties文件中

• 位於一個其它名字的properties文件中,然後用 --properties參數指定

所需的參數包括:

表 20.3. SchemaExport 連接屬性

屬性名	Description
hibernate.connection.driver_class	jdbc driver class
hibernate.connection.url	jdbc url
hibernate.connection.username	database user
hibernate.connection.password	user password
hibernate.dialect	方言(dialect)

20.1.4. 使用Ant(Using Ant)

你可以在你的Ant build腳本中調用SchemaExport:

<target name="schemaexport">
    <taskdef name="schemaexport"
        classname="org.hibernate.tool.hbm2ddl.SchemaExportTask"
        classpathref="class.path"/>
    
    <schemaexport
        properties="hibernate.properties"
        quiet="no"
        text="no"
        drop="no"
        delimiter=";"
        output="schema-export.sql">
        <fileset dir="src">
            <include name="**/*.hbm.xml"/>
        </fileset>
    </schemaexport>
</target>

20.1.5. 對schema的增量更新(Incremental schema updates)

The SchemaUpdate tool will update an existing schema with "incremental" changes. TheSchemaUpdate depends upon the JDBC metadata API and, as such, will not work with all JDBC drivers.

java -cp hibernate_classpathsorg.hibernate.tool.hbm2ddl.SchemaUpdate options mapping_files

表 20.4. SchemaUpdate命令行選項

選項	Description
--quiet	do not output the script to stdout
--text	do not export the script to the database
--naming=eg.MyNamingStrategy	select a NamingStrategy
--properties=hibernate.properties	read database properties from a file
--config=hibernate.cfg.xml	specify a .cfg.xml file

You can embed SchemaUpdate in your application:

Configuration cfg = ....;
new SchemaUpdate(cfg).execute(false);

20.1.6. 用Ant來增量更新schema(Using Ant for incremental schema updates)

你可以在Ant腳本中調用SchemaUpdate：

<target name="schemaupdate">
    <taskdef name="schemaupdate"
        classname="org.hibernate.tool.hbm2ddl.SchemaUpdateTask"
        classpathref="class.path"/>
    
    <schemaupdate
        properties="hibernate.properties"
        quiet="no">
        <fileset dir="src">
            <include name="**/*.hbm.xml"/>
        </fileset>
    </schemaupdate>
</target>

20.1.7. Schema 校驗

The SchemaValidator tool will validate that the existing database schema "matches" your mapping documents. TheSchemaValidator depends heavily upon the JDBC metadata API and, as such, will not work with all JDBC drivers. This tool is extremely useful for testing.

java -cp hibernate_classpathsorg.hibernate.tool.hbm2ddl.SchemaValidator options mapping_files

表 20.5. SchemaValidator命令行參數

選項	Description
--naming=eg.MyNamingStrategy	select a NamingStrategy
--properties=hibernate.properties	read database properties from a file
--config=hibernate.cfg.xml	specify a .cfg.xml file

You can embed SchemaValidator in your application:

Configuration cfg = ....;
new SchemaValidator(cfg).validate();

20.1.8. 使用Ant進行schema校驗

你可以在Ant腳本中調用SchemaValidator:

<target name="schemavalidate">
    <taskdef name="schemavalidator"
        classname="org.hibernate.tool.hbm2ddl.SchemaValidatorTask"
        classpathref="class.path"/>
    
    <schemavalidator
        properties="hibernate.properties">
        <fileset dir="src">
            <include name="**/*.hbm.xml"/>
        </fileset>
    </schemavalidator>
</target>

第 21 章 示例：父子關係(Parent Child Relationships)

One of the first things that new users want to do with Hibernate is to model a parent/child type relationship. There are two different approaches to this. The most convenient approach, especially for new users, is to model bothParent and Child as entity classes with a<one-to-many> association from Parent to Child. The alternative approach is to declare theChild as a <composite-element>. The default semantics of a one-to-many association in Hibernate are much less close to the usual semantics of a parent/child relationship than those of a composite element mapping. We will explain how to use a bidirectional one-to-many association with cascades to model a parent/child relationship efficiently and elegantly.

21.1. 關於collections需要注意的一點

Hibernate collections are considered to be a logical part of their owning entity and not of the contained entities. Be aware that this is a critical distinction that has the following consequences:

• When you remove/add an object from/to a collection, the version number of the collection owner is incremented.

• If an object that was removed from a collection is an instance of a value type (e.g. a composite element), that object will cease to be persistent and its state will be completely removed from the database. Likewise, adding a value type instance to the collection will cause its state to be immediately persistent.

• Conversely, if an entity is removed from a collection (a one-to-many or many-to-many association), it will not be deleted by default. This behavior is completely consistent; a change to the internal state of another entity should not cause the associated entity to vanish. Likewise, adding an entity to a collection does not cause that entity to become persistent, by default.

Adding an entity to a collection, by default, merely creates a link between the two entities. Removing the entity will remove the link. This is appropriate for all sorts of cases. However, it is not appropriate in the case of a parent/child relationship. In this case, the life of the child is bound to the life cycle of the parent.

21.2. 雙向的一對多關係(Bidirectional one-to-many)

假設我們要實現一個簡單的從Parent到Child的<one-to-many>關聯。

<set name="children">
    <key column="parent_id"/>
    <one-to-many class="Child"/>
</set>

If we were to execute the following code:

Parent p = .....;
Child c = new Child();
p.getChildren().add(c);
session.save(c);
session.flush();

Hibernate會產生兩條SQL語句：

• 一條INSERT語句，爲c創建一條記錄

• 一條UPDATE語句，創建從p到c的連接

This is not only inefficient, but also violates any NOT NULL constraint on theparent_id column. You can fix the nullability constraint violation by specifyingnot-null="true" in the collection mapping:

<set name="children">
    <key column="parent_id" not-null="true"/>
    <one-to-many class="Child"/>
</set>

然而，這並非是推薦的解決方法。

The underlying cause of this behavior is that the link (the foreign key parent_id) from p to c is not considered part of the state of theChild object and is therefore not created in the INSERT. The solution is to make the link part of the Child mapping.

<many-to-one name="parent" column="parent_id" not-null="true"/>

You also need to add the parent property to the Child class.

Now that the Child entity is managing the state of the link, we tell the collection not to update the link. We use theinverse attribute to do this:

<set name="children" inverse="true">
    <key column="parent_id"/>
    <one-to-many class="Child"/>
</set>

The following code would be used to add a new Child:

Parent p = (Parent) session.load(Parent.class, pid);
Child c = new Child();
c.setParent(p);
p.getChildren().add(c);
session.save(c);
session.flush();

Only one SQL INSERT would now be issued.

You could also create an addChild() method of Parent.

public void addChild(Child c) {
    c.setParent(this);
    children.add(c);
}

The code to add a Child looks like this:

Parent p = (Parent) session.load(Parent.class, pid);
Child c = new Child();
p.addChild(c);
session.save(c);
session.flush();

21.3. 級聯生命週期（Cascading life cycle）

You can address the frustrations of the explicit call to save() by using cascades.

<set name="children" inverse="true" cascade="all">
    <key column="parent_id"/>
    <one-to-many class="Child"/>
</set>

This simplifies the code above to:

Parent p = (Parent) session.load(Parent.class, pid);
Child c = new Child();
p.addChild(c);
session.flush();

Similarly, we do not need to iterate over the children when saving or deleting aParent. The following removes p and all its children from the database.

Parent p = (Parent) session.load(Parent.class, pid);
session.delete(p);
session.flush();

However, the following code:

Parent p = (Parent) session.load(Parent.class, pid);
Child c = (Child) p.getChildren().iterator().next();
p.getChildren().remove(c);
c.setParent(null);
session.flush();

will not remove c from the database. In this case, it will only remove the link top and cause a NOT NULL constraint violation. You need to explicitlydelete() the Child.

Parent p = (Parent) session.load(Parent.class, pid);
Child c = (Child) p.getChildren().iterator().next();
p.getChildren().remove(c);
session.delete(c);
session.flush();

In our case, a Child cannot exist without its parent. So if we remove aChild from the collection, we do want it to be deleted. To do this, we must usecascade="all-delete-orphan".

<set name="children" inverse="true" cascade="all-delete-orphan">
    <key column="parent_id"/>
    <one-to-many class="Child"/>
</set>

Even though the collection mapping specifies inverse="true", cascades are still processed by iterating the collection elements. If you need an object be saved, deleted or updated by cascade, you must add it to the collection. It is not enough to simply call setParent().

21.4. 級聯與未保存值（Cascades andunsaved-value）

Suppose we loaded up a Parent in one Session, made some changes in a UI action and wanted to persist these changes in a new session by callingupdate(). The Parent will contain a collection of children and, since the cascading update is enabled, Hibernate needs to know which children are newly instantiated and which represent existing rows in the database. We will also assume that both Parent andChild have generated identifier properties of type Long. Hibernate will use the identifier and version/timestamp property value to determine which of the children are new. (See

第 10.7 節 “自動狀態檢測”.)In Hibernate3, it is no longer necessary to specify anunsaved-value explicitly.

The following code will update parent and child and insert newChild:

//parent and child were both loaded in a previous session
parent.addChild(child);
Child newChild = new Child();
parent.addChild(newChild);
session.update(parent);
session.flush();

This may be suitable for the case of a generated identifier, but what about assigned identifiers and composite identifiers? This is more difficult, since Hibernate cannot use the identifier property to distinguish between a newly instantiated object, with an identifier assigned by the user, and an object loaded in a previous session. In this case, Hibernate will either use the timestamp or version property, or will actually query the second-level cache or, worst case, the database, to see if the row exists.

21.5. 結論

The sections we have just covered can be a bit confusing. However, in practice, it all works out nicely. Most Hibernate applications use the parent/child pattern in many places.

We mentioned an alternative in the first paragraph. None of the above issues exist in the case of<composite-element> mappings, which have exactly the semantics of a parent/child relationship. Unfortunately, there are two big limitations with composite element classes: composite elements cannot own collections and they should not be the child of any entity other than the unique parent.

第 22 章 示例：Weblog 應用程序

22.1. 持久化類

The persistent classes here represent a weblog and an item posted in a weblog. They are to be modelled as a standard parent/child relationship, but we will use an ordered bag, instead of a set:

package eg;

import java.util.List;

public class Blog {
    private Long _id;
    private String _name;
    private List _items;

    public Long getId() {
        return _id;
    }
    public List getItems() {
        return _items;
    }
    public String getName() {
        return _name;
    }
    public void setId(Long long1) {
        _id = long1;
    }
    public void setItems(List list) {
        _items = list;
    }
    public void setName(String string) {
        _name = string;
    }
}

package eg;

import java.text.DateFormat;
import java.util.Calendar;

public class BlogItem {
    private Long _id;
    private Calendar _datetime;
    private String _text;
    private String _title;
    private Blog _blog;

    public Blog getBlog() {
        return _blog;
    }
    public Calendar getDatetime() {
        return _datetime;
    }
    public Long getId() {
        return _id;
    }
    public String getText() {
        return _text;
    }
    public String getTitle() {
        return _title;
    }
    public void setBlog(Blog blog) {
        _blog = blog;
    }
    public void setDatetime(Calendar calendar) {
        _datetime = calendar;
    }
    public void setId(Long long1) {
        _id = long1;
    }
    public void setText(String string) {
        _text = string;
    }
    public void setTitle(String string) {
        _title = string;
    }
}

22.2. Hibernate 映射

The XML mappings are now straightforward. For example:

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping package="eg">

    <class
        name="Blog"
        table="BLOGS">

        <id
            name="id"
            column="BLOG_ID">

            <generator class="native"/>

        </id>

        <property
            name="name"
            column="NAME"
            not-null="true"
            unique="true"/>

        <bag
            name="items"
            inverse="true"
            order-by="DATE_TIME"
            cascade="all">

            <key column="BLOG_ID"/>
            <one-to-many class="BlogItem"/>

        </bag>

    </class>

</hibernate-mapping>

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping package="eg">

    <class
        name="BlogItem"
        table="BLOG_ITEMS"
        dynamic-update="true">

        <id
            name="id"
            column="BLOG_ITEM_ID">

            <generator class="native"/>

        </id>

        <property
            name="title"
            column="TITLE"
            not-null="true"/>

        <property
            name="text"
            column="TEXT"
            not-null="true"/>

        <property
            name="datetime"
            column="DATE_TIME"
            not-null="true"/>

        <many-to-one
            name="blog"
            column="BLOG_ID"
            not-null="true"/>

    </class>

</hibernate-mapping>

22.3. Hibernate 代碼

The following class demonstrates some of the kinds of things we can do with these classes using Hibernate:

package eg;

import java.util.ArrayList;
import java.util.Calendar;
import java.util.Iterator;
import java.util.List;

import org.hibernate.HibernateException;
import org.hibernate.Query;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.cfg.Configuration;
import org.hibernate.tool.hbm2ddl.SchemaExport;

public class BlogMain {
    
    private SessionFactory _sessions;
    
    public void configure() throws HibernateException {
        _sessions = new Configuration()
            .addClass(Blog.class)
            .addClass(BlogItem.class)
            .buildSessionFactory();
    }
    
    public void exportTables() throws HibernateException {
        Configuration cfg = new Configuration()
            .addClass(Blog.class)
            .addClass(BlogItem.class);
        new SchemaExport(cfg).create(true, true);
    }
    
    public Blog createBlog(String name) throws HibernateException {
        
        Blog blog = new Blog();
        blog.setName(name);
        blog.setItems( new ArrayList() );
        
        Session session = _sessions.openSession();
        Transaction tx = null;
        try {
            tx = session.beginTransaction();
            session.persist(blog);
            tx.commit();
        }
        catch (HibernateException he) {
            if (tx!=null) tx.rollback();
            throw he;
        }
        finally {
            session.close();
        }
        return blog;
    }
    
    public BlogItem createBlogItem(Blog blog, String title, String text)
                        throws HibernateException {
        
        BlogItem item = new BlogItem();
        item.setTitle(title);
        item.setText(text);
        item.setBlog(blog);
        item.setDatetime( Calendar.getInstance() );
        blog.getItems().add(item);
        
        Session session = _sessions.openSession();
        Transaction tx = null;
        try {
            tx = session.beginTransaction();
            session.update(blog);
            tx.commit();
        }
        catch (HibernateException he) {
            if (tx!=null) tx.rollback();
            throw he;
        }
        finally {
            session.close();
        }
        return item;
    }
    
    public BlogItem createBlogItem(Long blogid, String title, String text)
                        throws HibernateException {
        
        BlogItem item = new BlogItem();
        item.setTitle(title);
        item.setText(text);
        item.setDatetime( Calendar.getInstance() );
        
        Session session = _sessions.openSession();
        Transaction tx = null;
        try {
            tx = session.beginTransaction();
            Blog blog = (Blog) session.load(Blog.class, blogid);
            item.setBlog(blog);
            blog.getItems().add(item);
            tx.commit();
        }
        catch (HibernateException he) {
            if (tx!=null) tx.rollback();
            throw he;
        }
        finally {
            session.close();
        }
        return item;
    }
    
    public void updateBlogItem(BlogItem item, String text)
                    throws HibernateException {
        
        item.setText(text);
        
        Session session = _sessions.openSession();
        Transaction tx = null;
        try {
            tx = session.beginTransaction();
            session.update(item);
            tx.commit();
        }
        catch (HibernateException he) {
            if (tx!=null) tx.rollback();
            throw he;
        }
        finally {
            session.close();
        }
    }
    
    public void updateBlogItem(Long itemid, String text)
                    throws HibernateException {
    
        Session session = _sessions.openSession();
        Transaction tx = null;
        try {
            tx = session.beginTransaction();
            BlogItem item = (BlogItem) session.load(BlogItem.class, itemid);
            item.setText(text);
            tx.commit();
        }
        catch (HibernateException he) {
            if (tx!=null) tx.rollback();
            throw he;
        }
        finally {
            session.close();
        }
    }
    
    public List listAllBlogNamesAndItemCounts(int max)
                    throws HibernateException {
        
        Session session = _sessions.openSession();
        Transaction tx = null;
        List result = null;
        try {
            tx = session.beginTransaction();
            Query q = session.createQuery(
                "select blog.id, blog.name, count(blogItem) " +
                "from Blog as blog " +
                "left outer join blog.items as blogItem " +
                "group by blog.name, blog.id " +
                "order by max(blogItem.datetime)"
            );
            q.setMaxResults(max);
            result = q.list();
            tx.commit();
        }
        catch (HibernateException he) {
            if (tx!=null) tx.rollback();
            throw he;
        }
        finally {
            session.close();
        }
        return result;
    }
    
    public Blog getBlogAndAllItems(Long blogid)
                    throws HibernateException {
        
        Session session = _sessions.openSession();
        Transaction tx = null;
        Blog blog = null;
        try {
            tx = session.beginTransaction();
            Query q = session.createQuery(
                "from Blog as blog " +
                "left outer join fetch blog.items " +
                "where blog.id = :blogid"
            );
            q.setParameter("blogid", blogid);
            blog  = (Blog) q.uniqueResult();
            tx.commit();
        }
        catch (HibernateException he) {
            if (tx!=null) tx.rollback();
            throw he;
        }
        finally {
            session.close();
        }
        return blog;
    }
    
    public List listBlogsAndRecentItems() throws HibernateException {
        
        Session session = _sessions.openSession();
        Transaction tx = null;
        List result = null;
        try {
            tx = session.beginTransaction();
            Query q = session.createQuery(
                "from Blog as blog " +
                "inner join blog.items as blogItem " +
                "where blogItem.datetime > :minDate"
            );

            Calendar cal = Calendar.getInstance();
            cal.roll(Calendar.MONTH, false);
            q.setCalendar("minDate", cal);
            
            result = q.list();
            tx.commit();
        }
        catch (HibernateException he) {
            if (tx!=null) tx.rollback();
            throw he;
        }
        finally {
            session.close();
        }
        return result;
    }
}

第 23 章 示例：複雜映射實例

This chapters explores some more complex association mappings.

23.1. Employer（僱主)/Employee(僱員)

The following model of the relationship between Employer andEmployee uses an entity class (Employment) to represent the association. You can do this when there might be more than one period of employment for the same two parties. Components are used to model monetary values and employee names.

Here is a possible mapping document:

<hibernate-mapping>
        
    <class name="Employer" table="employers">
        <id name="id">
            <generator class="sequence">
                <param name="sequence">employer_id_seq</param>
            </generator>
        </id>
        <property name="name"/>
    </class>

    <class name="Employment" table="employment_periods">

        <id name="id">
            <generator class="sequence">
                <param name="sequence">employment_id_seq</param>
            </generator>
        </id>
        <property name="startDate" column="start_date"/>
        <property name="endDate" column="end_date"/>

        <component name="hourlyRate" class="MonetaryAmount">
            <property name="amount">
                <column name="hourly_rate" sql-type="NUMERIC(12, 2)"/>
            </property>
            <property name="currency" length="12"/>
        </component>

        <many-to-one name="employer" column="employer_id" not-null="true"/>
        <many-to-one name="employee" column="employee_id" not-null="true"/>

    </class>

    <class name="Employee" table="employees">
        <id name="id">
            <generator class="sequence">
                <param name="sequence">employee_id_seq</param>
            </generator>
        </id>
        <property name="taxfileNumber"/>
        <component name="name" class="Name">
            <property name="firstName"/>
            <property name="initial"/>
            <property name="lastName"/>
        </component>
    </class>

</hibernate-mapping>

Here is the table schema generated by SchemaExport.

create table employers (
    id BIGINT not null, 
    name VARCHAR(255), 
    primary key (id)
)

create table employment_periods (
    id BIGINT not null,
    hourly_rate NUMERIC(12, 2),
    currency VARCHAR(12), 
    employee_id BIGINT not null, 
    employer_id BIGINT not null, 
    end_date TIMESTAMP, 
    start_date TIMESTAMP, 
    primary key (id)
)

create table employees (
    id BIGINT not null, 
    firstName VARCHAR(255), 
    initial CHAR(1), 
    lastName VARCHAR(255), 
    taxfileNumber VARCHAR(255), 
    primary key (id)
)

alter table employment_periods 
    add constraint employment_periodsFK0 foreign key (employer_id) references employers
alter table employment_periods 
    add constraint employment_periodsFK1 foreign key (employee_id) references employees
create sequence employee_id_seq
create sequence employment_id_seq
create sequence employer_id_seq

23.2. Author(作家)/Work(作品)

Consider the following model of the relationships between Work, Author and Person. In the example, the relationship betweenWork and Author is represented as a many-to-many association and the relationship betweenAuthor and Person is represented as one-to-one association. Another possibility would be to haveAuthor extend Person.

下面的映射文件正確的描述了這些關係：

<hibernate-mapping>

    <class name="Work" table="works" discriminator-value="W">

        <id name="id" column="id">
            <generator class="native"/>
        </id>
        <discriminator column="type" type="character"/>

        <property name="title"/>
        <set name="authors" table="author_work">
            <key column name="work_id"/>
            <many-to-many class="Author" column name="author_id"/>
        </set>

        <subclass name="Book" discriminator-value="B">
            <property name="text"/>
        </subclass>

        <subclass name="Song" discriminator-value="S">
            <property name="tempo"/>
            <property name="genre"/>
        </subclass>

    </class>

    <class name="Author" table="authors">

        <id name="id" column="id">
            <!-- The Author must have the same identifier as the Person -->
            <generator class="assigned"/> 
        </id>

        <property name="alias"/>
        <one-to-one name="person" constrained="true"/>

        <set name="works" table="author_work" inverse="true">
            <key column="author_id"/>
            <many-to-many class="Work" column="work_id"/>
        </set>

    </class>

    <class name="Person" table="persons">
        <id name="id" column="id">
            <generator class="native"/>
        </id>
        <property name="name"/>
    </class>

</hibernate-mapping>

There are four tables in this mapping: works, authors and persons hold work, author and person data respectively.author_work is an association table linking authors to works. Here is the table schema, as generated bySchemaExport:

create table works (
    id BIGINT not null generated by default as identity, 
    tempo FLOAT, 
    genre VARCHAR(255), 
    text INTEGER, 
    title VARCHAR(255), 
    type CHAR(1) not null, 
    primary key (id)
)

create table author_work (
    author_id BIGINT not null, 
    work_id BIGINT not null, 
    primary key (work_id, author_id)
)

create table authors (
    id BIGINT not null generated by default as identity, 
    alias VARCHAR(255), 
    primary key (id)
)

create table persons (
    id BIGINT not null generated by default as identity, 
    name VARCHAR(255), 
    primary key (id)
)

alter table authors 
    add constraint authorsFK0 foreign key (id) references persons
alter table author_work 
    add constraint author_workFK0 foreign key (author_id) references authors
alter table author_work
    add constraint author_workFK1 foreign key (work_id) references works

23.3. Customer(客戶)/Order(訂單)/Product(產品)

In this section we consider a model of the relationships between Customer, Order, Line Item andProduct. There is a one-to-many association between Customer and Order, but how can you representOrder / LineItem / Product? In the example, LineItem is mapped as an association class representing the many-to-many association betweenOrder and Product. In Hibernate this is called a composite element.

The mapping document will look like this:

<hibernate-mapping>

    <class name="Customer" table="customers">
        <id name="id">
            <generator class="native"/>
        </id>
        <property name="name"/>
        <set name="orders" inverse="true">
            <key column="customer_id"/>
            <one-to-many class="Order"/>
        </set>
    </class>

    <class name="Order" table="orders">
        <id name="id">
            <generator class="native"/>
        </id>
        <property name="date"/>
        <many-to-one name="customer" column="customer_id"/>
        <list name="lineItems" table="line_items">
            <key column="order_id"/>
            <list-index column="line_number"/>
            <composite-element class="LineItem">
                <property name="quantity"/>
                <many-to-one name="product" column="product_id"/>
            </composite-element>
        </list>
    </class>

    <class name="Product" table="products">
        <id name="id">
            <generator class="native"/>
        </id>
        <property name="serialNumber"/>
    </class>

</hibernate-mapping>

customers, orders, line_items 和 products 分別保存着customer, order, order line item 和 product的數據。line_items也作爲連接orders 和 products的關聯表。

create table customers (
    id BIGINT not null generated by default as identity, 
    name VARCHAR(255), 
    primary key (id)
)

create table orders (
    id BIGINT not null generated by default as identity, 
    customer_id BIGINT, 
    date TIMESTAMP, 
    primary key (id)
)

create table line_items (
    line_number INTEGER not null, 
    order_id BIGINT not null, 
    product_id BIGINT, 
    quantity INTEGER, 
    primary key (order_id, line_number)
)

create table products (
    id BIGINT not null generated by default as identity, 
    serialNumber VARCHAR(255), 
    primary key (id)
)

alter table orders 
    add constraint ordersFK0 foreign key (customer_id) references customers
alter table line_items
    add constraint line_itemsFK0 foreign key (product_id) references products
alter table line_items
    add constraint line_itemsFK1 foreign key (order_id) references orders

23.4. 雜例

These examples are available from the Hibernate test suite. You will find many other useful example mappings there by searching in thetest folder of the Hibernate distribution.

23.4.1. "Typed" one-to-one association

<class name="Person">
    <id name="name"/>
    <one-to-one name="address" 
            cascade="all">
        <formula>name</formula>
        <formula>'HOME'</formula>
    </one-to-one>
    <one-to-one name="mailingAddress" 
            cascade="all">
        <formula>name</formula>
        <formula>'MAILING'</formula>
    </one-to-one>
</class>

<class name="Address" batch-size="2" 
        check="addressType in ('MAILING', 'HOME', 'BUSINESS')">
    <composite-id>
        <key-many-to-one name="person" 
                column="personName"/>
        <key-property name="type" 
                column="addressType"/>
    </composite-id>
    <property name="street" type="text"/>
    <property name="state"/>
    <property name="zip"/>
</class>

23.4.2. Composite key example

<class name="Customer">

    <id name="customerId"
        length="10">
        <generator class="assigned"/>
    </id>

    <property name="name" not-null="true" length="100"/>
    <property name="address" not-null="true" length="200"/>

    <list name="orders"
            inverse="true"
            cascade="save-update">
        <key column="customerId"/>
        <index column="orderNumber"/>
        <one-to-many class="Order"/>
    </list>

</class>

<class name="Order" table="CustomerOrder" lazy="true">
    <synchronize table="LineItem"/>
    <synchronize table="Product"/>
    
    <composite-id name="id" 
            class="Order$Id">
        <key-property name="customerId" length="10"/>
        <key-property name="orderNumber"/>
    </composite-id>
    
    <property name="orderDate" 
            type="calendar_date"
            not-null="true"/>
    
    <property name="total">
        <formula>
            ( select sum(li.quantity*p.price) 
            from LineItem li, Product p 
            where li.productId = p.productId 
                and li.customerId = customerId 
                and li.orderNumber = orderNumber )
        </formula>
    </property>
    
    <many-to-one name="customer"
            column="customerId"
            insert="false"
            update="false" 
            not-null="true"/>
        
    <bag name="lineItems"
            fetch="join" 
            inverse="true"
            cascade="save-update">
        <key>
            <column name="customerId"/>
            <column name="orderNumber"/>
        </key>
        <one-to-many class="LineItem"/>
    </bag>
    
</class>
    
<class name="LineItem">
    
    <composite-id name="id" 
            class="LineItem$Id">
        <key-property name="customerId" length="10"/>
        <key-property name="orderNumber"/>
        <key-property name="productId" length="10"/>
    </composite-id>
    
    <property name="quantity"/>
    
    <many-to-one name="order"
            insert="false"
            update="false" 
            not-null="true">
        <column name="customerId"/>
        <column name="orderNumber"/>
    </many-to-one>
    
    <many-to-one name="product"
            insert="false"
            update="false" 
            not-null="true"
            column="productId"/>
        
</class>

<class name="Product">
    <synchronize table="LineItem"/>

    <id name="productId"
        length="10">
        <generator class="assigned"/>
    </id>
    
    <property name="description" 
        not-null="true" 
        length="200"/>
    <property name="price" length="3"/>
    <property name="numberAvailable"/>
    
    <property name="numberOrdered">
        <formula>
            ( select sum(li.quantity) 
            from LineItem li 
            where li.productId = productId )
        </formula>
    </property>
    
</class>

23.4.3. 共有組合鍵屬性的多對多(Many-to-many with shared composite key attribute)

<class name="User" table="`User`">
    <composite-id>
        <key-property name="name"/>
        <key-property name="org"/>
    </composite-id>
    <set name="groups" table="UserGroup">
        <key>
            <column name="userName"/>
            <column name="org"/>
        </key>
        <many-to-many class="Group">
            <column name="groupName"/>
            <formula>org</formula>
        </many-to-many>
    </set>
</class>
    
<class name="Group" table="`Group`">
    <composite-id>
        <key-property name="name"/>
        <key-property name="org"/>
    </composite-id>
    <property name="description"/>
    <set name="users" table="UserGroup" inverse="true">
        <key>
            <column name="groupName"/>
            <column name="org"/>
        </key>
        <many-to-many class="User">
            <column name="userName"/>
            <formula>org</formula>
        </many-to-many>
    </set>
</class>

23.4.4. Content based discrimination

<class name="Person"
    discriminator-value="P">
    
    <id name="id" 
        column="person_id" 
        unsaved-value="0">
        <generator class="native"/>
    </id>
    
            
    <discriminator 
        type="character">
        <formula>
            case 
                when title is not null then 'E' 
                when salesperson is not null then 'C' 
                else 'P' 
            end
        </formula>
    </discriminator>

    <property name="name" 
        not-null="true"
        length="80"/>
        
    <property name="sex" 
        not-null="true"
        update="false"/>
    
    <component name="address">
        <property name="address"/>
        <property name="zip"/>
        <property name="country"/>
    </component>
    
    <subclass name="Employee" 
        discriminator-value="E">
            <property name="title"
                length="20"/>
            <property name="salary"/>
            <many-to-one name="manager"/>
    </subclass>
    
    <subclass name="Customer" 
        discriminator-value="C">
            <property name="comments"/>
            <many-to-one name="salesperson"/>
    </subclass>
    
</class>

23.4.5. Associations on alternate keys

<class name="Person">
    
    <id name="id">
        <generator class="hilo"/>
    </id>
    
    <property name="name" length="100"/>
    
    <one-to-one name="address" 
        property-ref="person"
        cascade="all"
        fetch="join"/>
    
    <set name="accounts" 
        inverse="true">
        <key column="userId"
            property-ref="userId"/>
        <one-to-many class="Account"/>
    </set>
    
    <property name="userId" length="8"/>

</class>

<class name="Address">

    <id name="id">
        <generator class="hilo"/>
    </id>

    <property name="address" length="300"/>
    <property name="zip" length="5"/>
    <property name="country" length="25"/>
    <many-to-one name="person" unique="true" not-null="true"/>

</class>

<class name="Account">
    <id name="accountId" length="32">
        <generator class="uuid"/>
    </id>
    
    <many-to-one name="user"
        column="userId"
        property-ref="userId"/>
    
    <property name="type" not-null="true"/>
    
</class>

第 24 章 最佳實踐(Best Practices)

Write fine-grained classes and map them using <component>:

使用一個Address持久類來封裝 street,suburb, state, postcode. 這將有利於代碼重用和簡化代碼重構(refactoring)的工作。

Declare identifier properties on persistent classes:

Hibernate makes identifier properties optional. There are a range of reasons why you should use them. We recommend that identifiers be 'synthetic', that is, generated with no business meaning.

Identify natural keys:

對所有的實體都標識出自然鍵，用<natural-id>進行映射。實現equals()和hashCode()，在其中用組成自然鍵的屬性進行比較。

Place each class mapping in its own file:

Do not use a single monolithic mapping document. Map com.eg.Foo in the filecom/eg/Foo.hbm.xml. This makes sense, particularly in a team environment.

Load mappings as resources:

把映射文件和他們的映射類放在一起進行部署。

Consider externalizing query strings:

This is recommended if your queries call non-ANSI-standard SQL functions. Externalizing the query strings to mapping files will make the application more portable.

使用綁定變量

As in JDBC, always replace non-constant values by "?". Do not use string manipulation to bind a non-constant value in a query. You should also consider using named parameters in queries.

Do not manage your own JDBC connections:

Hibernate allows the application to manage JDBC connections, but his approach should be considered a last-resort. If you cannot use the built-in connection providers, consider providing your own implementation oforg.hibernate.connection.ConnectionProvider.

Consider using a custom type:

Suppose you have a Java type from a library that needs to be persisted but does not provide the accessors needed to map it as a component. You should consider implementingorg.hibernate.UserType. This approach frees the application code from implementing transformations to/from a Hibernate type.

Use hand-coded JDBC in bottlenecks:

In performance-critical areas of the system, some kinds of operations might benefit from direct JDBC. Do not assume, however, that JDBC is necessarily faster. Please wait until youknow something is a bottleneck. If you need to use direct JDBC, you can open a HibernateSession and usingfile:///usr/share/doc/HTML/en-US/index.html that JDBC connection. This way you can still use the same transaction strategy and underlying connection provider.

Understand Session flushing:

Sometimes the Session synchronizes its persistent state with the database. Performance will be affected if this process occurs too often. You can sometimes minimize unnecessary flushing by disabling automatic flushing, or even by changing the order of queries and other operations within a particular transaction.

In a three tiered architecture, consider using detached objects:

When using a servlet/session bean architecture, you can pass persistent objects loaded in the session bean to and from the servlet/JSP layer. Use a new session to service each request. UseSession.merge() or Session.saveOrUpdate() to synchronize objects with the database.

In a two tiered architecture, consider using long persistence contexts:

Database Transactions have to be as short as possible for best scalability. However, it is often necessary to implement long runningapplication transactions, a single unit-of-work from the point of view of a user. An application transaction might span several client request/response cycles. It is common to use detached objects to implement application transactions. An appropriate alternative in a two tiered architecture, is to maintain a single open persistence contact session for the whole life cycle of the application transaction. Then simply disconnect from the JDBC connection at the end of each request and reconnect at the beginning of the subsequent request. Never share a single session across more than one application transaction or you will be working with stale data.

Do not treat exceptions as recoverable:

This is more of a necessary practice than a "best" practice. When an exception occurs, roll back theTransaction and close the Session. If you do not do this, Hibernate cannot guarantee that in-memory state accurately represents the persistent state. For example, do not useSession.load() to determine if an instance with the given identifier exists on the database; useSession.get() or a query instead.

Prefer lazy fetching for associations:

Use eager fetching sparingly. Use proxies and lazy collections for most associations to classes that are not likely to be completely held in the second-level cache. For associations to cached classes, where there is an a extremely high probability of a cache hit, explicitly disable eager fetching using lazy="false". When join fetching is appropriate to a particular use case, use a query with aleft join fetch.

Use the open session in view pattern, or a disciplinedassembly phase to avoid problems with unfetched data:

Hibernate frees the developer from writing tedious Data Transfer Objects (DTO). In a traditional EJB architecture, DTOs serve dual purposes: first, they work around the problem that entity beans are not serializable; second, they implicitly define an assembly phase where all data to be used by the view is fetched and marshalled into the DTOs before returning control to the presentation tier. Hibernate eliminates the first purpose. Unless you are prepared to hold the persistence context (the session) open across the view rendering process, you will still need an assembly phase. Think of your business methods as having a strict contract with the presentation tier about what data is available in the detached objects. This is not a limitation of Hibernate. It is a fundamental requirement of safe transactional data access.

Consider abstracting your business logic from Hibernate:

Hide Hibernate data-access code behind an interface. Combine the DAO and Thread Local Session patterns. You can even have some classes persisted by handcoded JDBC associated to Hibernate via aUserType. This advice is, however, intended for "sufficiently large" applications. It is not appropriate for an application with five tables.

Do not use exotic association mappings:

Practical test cases for real many-to-many associations are rare. Most of the time you need additional information stored in the "link table". In this case, it is much better to use two one-to-many associations to an intermediate link class. In fact, most associations are one-to-many and many-to-one. For this reason, you should proceed cautiously when using any other association style.

Prefer bidirectional associations:

單向關聯更加難於查詢。在大型應用中，幾乎所有的關聯必須在查詢中可以雙向導航。

第 25 章 Database Portability Considerations

25.1. Portability Basics

One of the selling points of Hibernate (and really Object/Relational Mapping as a whole) is the notion of database portability. This could mean an internal IT user migrating from one database vendor to another, or it could mean a framework or deployable application consuming Hibernate to simultaneously target multiple database products by their users. Regardless of the exact scenario, the basic idea is that you want Hibernate to help you run against any number of databases without changes to your code, and ideally without any changes to the mapping metadata.

25.2. Dialect

The first line of portability for Hibernate is the dialect, which is a specialization of theorg.hibernate.dialect.Dialect contract. A dialect encapsulates all the differences in how Hibernate must communicate with a particular database to accomplish some task like getting a sequence value or structuring a SELECT query. Hibernate bundles a wide range of dialects for many of the most popular databases. If you find that your particular database is not among them, it is not terribly difficult to write your own.

25.3. Dialect resolution

Originally, Hibernate would always require that users specify which dialect to use. In the case of users looking to simultaneously target multiple databases with their build that was problematic. Generally this required their users to configure the Hibernate dialect or defining their own method of setting that value.

Starting with version 3.2, Hibernate introduced the notion of automatically detecting the dialect to use based on thejava.sql.DatabaseMetaData obtained from a java.sql.Connection to that database. This was much better, expect that this resolution was limited to databases Hibernate know about ahead of time and was in no way configurable or overrideable.

Starting with version 3.3, Hibernate has a fare more powerful way to automatically determine which dialect to should be used by relying on a series of delegates which implement theorg.hibernate.dialect.resolver.DialectResolver which defines only a single method:

public Dialect resolveDialect(DatabaseMetaData metaData) throws JDBCConnectionException

. The basic contract here is that if the resolver 'understands' the given database metadata then it returns the corresponding Dialect; if not it returns null and the process continues to the next resolver. The signature also identifiesorg.hibernate.exception.JDBCConnectionException as possibly being thrown. A JDBCConnectionException here is interpreted to imply a "non transient" (aka non-recoverable) connection problem and is used to indicate an immediate stop to resolution attempts. All other exceptions result in a warning and continuing on to the next resolver.

The cool part about these resolvers is that users can also register their own custom resolvers which will be processed ahead of the built-in Hibernate ones. This might be useful in a number of different situations: it allows easy integration for auto-detection of dialects beyond those shipped with HIbernate itself; it allows you to specify to use a custom dialect when a particular database is recognized; etc. To register one or more resolvers, simply specify them (seperated by commas, tabs or spaces) using the 'hibernate.dialect_resolvers' configuration setting (see the DIALECT_RESOLVERS constant onorg.hibernate.cfg.Environment).

25.4. Identifier generation

When considering portability between databases, another important decision is selecting the identifier generation stratagy you want to use. Originally Hibernate provided thenative generator for this purpose, which was intended to select between asequence, identity, ortable strategy depending on the capability of the underlying database. However, an insidious implication of this approach comes about when targtetting some databases which supportidentity generation and some which do not.identity generation relies on the SQL definition of an IDENTITY (or auto-increment) column to manage the identifier value; it is what is known as a post-insert generation strategy becauase the insert must actually happen before we can know the identifier value. Because Hibernate relies on this identifier value to uniquely reference entities within a persistence context it must then issue the insert immediately when the users requests the entitiy be associated with the session (like via save() e.g.) regardless of current transactional semantics.

注意

Hibernate was changed slightly once the implication of this was better understood so that the insert is delayed in cases where that is feasible.

The underlying issue is that the actual semanctics of the application itself changes in these cases.

Starting with version 3.2.3, Hibernate comes with a set of

enhanced identifier generators targetting portability in a much different way.

注意

There are specifically 2 bundled enhancedgenerators:

• org.hibernate.id.enhanced.SequenceStyleGenerator

• org.hibernate.id.enhanced.TableGenerator

The idea behind these generators is to port the actual semantics of the identifer value generation to the different databases. For example, theorg.hibernate.id.enhanced.SequenceStyleGenerator mimics the behavior of a sequence on databases which do not support sequences by using a table.

25.5. Database functions

警告

This is an area in Hibernate in need of improvement. In terms of portability concerns, this function handling currently works pretty well from HQL; however, it is quite lacking in all other aspects.

SQL functions can be referenced in many ways by users. However, not all databases support the same set of functions. Hibernate, provides a means of mapping alogical function name to a a delegate which knows how to render that particular function, perhaps even using a totally different physical function call.

重要

Technically this function registration is handled through the org.hibernate.dialect.function.SQLFunctionRegistry class which is intended to allow users to provide custom function definitions without having to provide a custom dialect. This specific behavior is not fully completed as of yet.

It is sort of implemented such that users can programatically register functions with theorg.hibernate.cfg.Configuration and those functions will be recognized for HQL.

25.6. Type mappings

This section scheduled for completion at a later date...

References

[PoEAA] Patterns of Enterprise Application Architecture.0-321-12742-0. 由 Martin Fowler. 版權 © 2003 Pearson Education, Inc..Addison-Wesley Publishing Company.

[JPwH] Java Persistence with Hibernate.Second Edition of Hibernate in Action. 1-932394-88-5.

http://www.manning.com/bauer2 . 由 Christian Bauer和GavinKing. 版權 © 2007 Manning Publications Co..Manning Publications Co..

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版權 © 2004 Red Hat Middleware, LLC.