1+N問題的描述：舉例，多個主題（Topic）屬於一個帖子（Category），一個帖子含有多個主題。當只需要查詢Topic時不要查詢Category時，如果@ManyToOne的屬性fetch=FetchType.EAGER,這時查詢所有Topic時，每查詢一個Topic就會多產生一個SQL語句查詢相關的Category表的數據，這樣要是有N條Topic數據，就會產生1+N條SQL語句。同樣的在@OneToMany的情況下，要是在Many方設置fetch=FetchType.EAGER，同樣也會產生1+N的問題。

解決方案有三種：

fetch=FetchType.LAZY，設爲懶加載
@BatchSize（size=5）代表一次取5條數據，這樣取5條數據只要發出一條SQL語句，注意是用在被關聯類上的（不建議用）
迫切左外連接檢索 join fetch(Criteria 查詢默認就是join fetch)

下面請看代碼：

Category類

Java代碼

package com.lbx.model;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import org.hibernate.annotations.BatchSize;
@Entity
@BatchSize(size=2)
public class Category {
private int id;
private String name;
@Id
@GeneratedValue
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}

package com.lbx.model;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;

import org.hibernate.annotations.BatchSize;

@Entity
@BatchSize(size=2)
public class Category {
	
	private int id;
	private String name;
	
	@Id
	@GeneratedValue
	public int getId() {
		return id;
	}
	public void setId(int id) {
		this.id = id;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	
}

Topic類

Java代碼

package com.lbx.model;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.ManyToOne;
@Entity
public class Topic {
private int id;
private String title;
private Category category;
@Id
@GeneratedValue
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getTitle() {
return title;
}
public void setTitle(String title) {
this.title = title;
}
//@ManyToOne(fetch=FetchType.LAZY)
@ManyToOne
public Category getCategory() {
return category;
}
public void setCategory(Category category) {
this.category = category;
}
}

package com.lbx.model;

import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.ManyToOne;

@Entity
public class Topic {

	private int id;
	private String title;
	private Category category;
	
	@Id
	@GeneratedValue
	public int getId() {
		return id;
	}
	public void setId(int id) {
		this.id = id;
	}
	public String getTitle() {
		return title;
	}
	public void setTitle(String title) {
		this.title = title;
	}
	
	//@ManyToOne(fetch=FetchType.LAZY)
	@ManyToOne
	public Category getCategory() {
		return category;
	}
	public void setCategory(Category category) {
		this.category = category;
	}
	
}

hibernate.cfg.xml文件配置

Xml代碼

<?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>
<property name="connection.driver_class">com.mysql.jdbc.Driver</property>
<property name="connection.url">jdbc:mysql://localhost:3306/testhib</property>
<property name="connection.username">root</property>
<property name="connection.password">root</property>
<property name="dialect">org.hibernate.dialect.MySQLDialect</property>
<property name="show_sql">true</property>
<property name="hibernate.format_sql">true</property>
<property name="hbm2ddl.auto">update</property>
<mapping class="com.lbx.model.Category"/>
<mapping class="com.lbx.model.Topic"/>
<mapping class="com.lbx.model.Msg"/>
</session-factory>
</hibernate-configuration>

<?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">com.mysql.jdbc.Driver</property>
        <property name="connection.url">jdbc:mysql://localhost:3306/testhib</property>
        <property name="connection.username">root</property>
        <property name="connection.password">root</property>

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

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

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

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

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

		<mapping class="com.lbx.model.Category"/>
		<mapping class="com.lbx.model.Topic"/>
		<mapping class="com.lbx.model.Msg"/>
		

    </session-factory>

</hibernate-configuration>

測試代碼

Java代碼

package com.lbx.model.test;
import java.util.List;
import javax.persistence.FetchType;
import javax.persistence.ManyToOne;
import junit.framework.TestCase;
import org.hibernate.Query;
import org.hibernate.Session;
import org.hibernate.lucene.Text;
import com.lbx.hibernate.Util.HibUtil;
import com.lbx.model.Category;
import com.lbx.model.Msg;
import com.lbx.model.Topic;
public class Test extends TestCase {
@Text
public void save(){
Session session = HibUtil.getSession();
session.beginTransaction();
for (int i = 0; i < 3; i++) {
Category c = new Category();
c.setName("c" + i);
Topic t = new Topic();
t.setTitle("t" + i);
t.setCategory(c);
session.save(c);
session.save(t);
}
session.beginTransaction().commit();
session.close();
}
//1+N問題(這裏我只要取出Topic就可以了)
@Text
public void testHQL_01(){
Session session = HibUtil.getSession();
session.beginTransaction();
//List<Topic> topics = (List<Topic>)session.createCriteria(Topic.class).list();
/**
* 這裏要是不把Topic類中不設 @ManyToOne(fetch=FetchType.LAZY)，這裏就要發很多SQL語句，關聯的表都會查
* 但是設了@ManyToOne(fetch=FetchType.LAZY) 之後就不會發出查詢相關表的查詢語句，用到的時候才發出
*/
Query q = session.createQuery("from Topic");
List<Topic> topics = (List<Topic>)q.list();
System.out.println(topics.size());
for (int i = 0; i < topics.size(); i++) {
System.out.println(topics.get(i).getId() + " " + topics.get(i).getTitle());
}
session.beginTransaction().commit();
session.close();
}
//用到被關聯表的信息
@Text
public void testHQL_02(){
Session session = HibUtil.getSession();
session.beginTransaction();
//List<Topic> topics = (List<Topic>)session.createCriteria(Topic.class).list();
/**
* 這裏要是不把Topic類中不設 @ManyToOne(fetch=FetchType.LAZY)，這裏就要發很多SQL語句，關聯的表都會查
* 但是設了@ManyToOne(fetch=FetchType.LAZY) 之後就不會發出查詢相關表的查詢語句，用到的時候才發出
*/
Query q = session.createQuery("from Topic");
List<Topic> topics = (List<Topic>)q.list();
System.out.println(topics.size());
for (int i = 0; i < topics.size(); i++) {
System.out.println(topics.get(i).getId() + " " + topics.get(i).getTitle());
/**
* 注意，在這裏要用到Category類的信息,所以就會發出相關的查詢信息
*/
System.out.println(topics.get(i).getCategory().getId() + " " +
topics.get(i).getCategory().getName());
}
session.beginTransaction().commit();
session.close();
}
//@BatchSize的使用，其屬性size=5就代表一次取5個
@Text
public void testHQL_03(){
Session session = HibUtil.getSession();
session.beginTransaction();
//List<Topic> topics = (List<Topic>)session.createCriteria(Topic.class).list();
/**
* 這裏要是不把Topic類中不設 @ManyToOne(fetch=FetchType.LAZY)，這裏就要發很多SQL語句，關聯的表都會查
* 但是設了@ManyToOne(fetch=FetchType.LAZY) 之後就不會發出查詢相關表的查詢語句，用到的時候才發出
*/
Query q = session.createQuery("from Topic");
List<Topic> topics = (List<Topic>)q.list();
System.out.println(topics.size());
for (int i = 0; i < topics.size(); i++) {
System.out.println(topics.get(i).getId() + " " + topics.get(i).getTitle());
/**
* 注意，在這裏要用到Category類的信息,所以就會發出相關的查詢信息
*/
System.out.println(topics.get(i).getCategory().getId() + " " +
topics.get(i).getCategory().getName());
}
session.beginTransaction().commit();
session.close();
}
// join fetch,迫切左外連接檢索
@Text
public void testHQL_04(){
Session session = HibUtil.getSession();
session.beginTransaction();
//Criteria 查詢默認就是join fetch
//List<Topic> topics = (List<Topic>)session.createCriteria(Topic.class).list();
/**
* 這裏要是不把Topic類中不設 @ManyToOne(fetch=FetchType.LAZY)，這裏就要發很多SQL語句，關聯的表都會查
* 但是設了@ManyToOne(fetch=FetchType.LAZY) 之後就不會發出查詢相關表的查詢語句，用到的時候才發出
*/
Query q = session.createQuery("from Topic t left join fetch t.category c");
List<Topic> topics = (List<Topic>)q.list();
System.out.println(topics.size());
for (int i = 0; i < topics.size(); i++) {
System.out.println(topics.get(i).getId() + " " + topics.get(i).getTitle());
}
session.beginTransaction().commit();
session.close();
}
}

Hibernate 1+N 問題

解決方案有三種：

fetch=FetchType.LAZY，設爲懶加載

@BatchSize（size=5）代表一次取5條數據，這樣取5條數據只要發出一條SQL語句，注意是用在被關聯類上的（不建議用）

迫切左外連接檢索 join fetch(Criteria 查詢默認就是join fetch)

下面請看代碼：

Category類

Topic類

hibernate.cfg.xml文件配置

測試代碼

工作中用到的腳本合集

24-5-18 X

Ext驗證彙總

來自官方的開始時間和結束時間及密碼和重複密碼驗證

分組表格和表格響應表單實例

ExtJs錯誤信息提示的四種方式

PropertyUtils MethodUtils用法

https://yachay.unat.edu.pe/blog/index.php?comment_area=format_blog&comment_component=blog&comment_co

linux以太網驅動總結