EJB3.0，EJB查詢語言編寫自定義的查詢非常方便，不用擔心底層數據庫的詳細信息。這是很相似的HQL(Hibernate查詢語言)，通常被稱爲按名稱EJBQL。
要理解熟悉ejb的EJBQL，要做好以下幾項工作。
步驟 1. 在數據庫中創建表.
步驟 2. 創建一個無狀態EJB.
步驟 3. 更新無狀態的EJB。添加添加記錄並獲得通過實體管理器從數據庫中記錄的方法。
步驟 4. 一個基於控制檯應用程序客戶端將訪問無狀態EJB的持久化數據庫中的數據。
創建數據庫表
創建表 books 在默認數據庫 postgres.
CREATE TABLE books (
id integer PRIMARY KEY,
name varchar(50)
);
創建模型類
public class Book implements Serializable{
private int id;
private String name;
public Book(){
}
public int getId() {
return id;
}
...
}
創建無狀態EJB
@Stateless
public class LibraryPersistentBean implements LibraryPersistentBeanRemote {
public void addBook(Book book) {
//persist book using entity manager
}
public List getBooks() {
//get books using entity manager
}
...
}
構建EJB模塊後，我們需要一個無狀態的bean，我們將在下一節要創建客戶端來訪問。
示例應用程序
讓我們創建一個測試EJB應用程序來測試EJB的數據庫訪問機制。
StepDescription
1Create a project with a name EjbComponent under a package com.tutorialspoint.entity as explained in the EJB - Create Application chapter. You can also use the project created in EJB - Create Application chapter as such for this chapter to understand ejb data access concepts.
2Create Book.java under package com.tutorialspoint.entity and modify it as shown below.
3Create LibraryPersistentBean.java and LibraryPersistentBeanRemote as explained in the EJB - Create Application chapter and modify them as shown below.
4Clean and Build the application to make sure business logic is working as per the requirements.
5Finally, deploy the application in the form of jar file on JBoss Application Server. JBoss Application server will get started automatically if it is not started yet.
6Now create the ejb client, a console based application in the same way as explained in theEJB - Create Application chapter under topic Create Client to access EJB. Modify it as shown below.
EJBComponent (EJB Module)
Book.java
package com.tutorialspoint.entity;
import java.io.Serializable;
public class Book implements Serializable{
private int id;
private String name;
public Book(){
}
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;
}
}
LibraryPersistentBeanRemote.java
package com.tutorialspoint.stateless;
import com.tutorialspoint.entity.Book;
import java.util.List;
import javax.ejb.Remote;
@Remote
public interface LibraryPersistentBeanRemote {
void addBook(Book bookName);
List getBooks();
}
LibraryPersistentBean.java
package com.tutorialspoint.stateless;
import com.tutorialspoint.entity.Book;
import java.util.List;
import javax.ejb.Stateless;
import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;
import javax.persistence.Query;
@Stateless
public class LibraryPersistentBean implements LibraryPersistentBeanRemote {
public LibraryPersistentBean(){
}
@PersistenceContext(unitName="EntityEjbPU")
private EntityManager entityManager;
public void addBook(Book book) {
entityManager.persist(book);
}
public List getBooks() {
//create an ejbql expression
String ejbQL = "From Book b where b.name like ?1";
//create query
Query query = entityManager.createQuery(ejbQL);
//substitute parameter.
query.setParameter(1, "%test%");
//execute the query
return query.getResultList();
}
}
只要在JBoss部署 EjbComponent項目，會注意到jboss的日誌。
JBoss已經自動爲我們的會話bean創建一個JNDI條目 -LibraryPersistentBean/remote.
我們將使用這個查詢字符串來獲得遠程類型的業務對象 -com.tutorialspoint.stateless.LibraryPersistentBeanRemote
JBoss應用服務器的日誌輸出
...
16:30:01,401 INFO [JndiSessionRegistrarBase] Binding the following Entries in Global JNDI:
LibraryPersistentBean/remote - EJB3.x Default Remote Business Interface
LibraryPersistentBean/remote-com.tutorialspoint.stateless.LibraryPersistentBeanRemote - EJB3.x Remote Business Interface
16:30:02,723 INFO [SessionSpecContainer] Starting jboss.j2ee:jar=EjbComponent.jar,name=LibraryPersistentBeanRemote,service=EJB3
16:30:02,723 INFO [EJBContainer] STARTED EJB: com.tutorialspoint.stateless.LibraryPersistentBeanRemote ejbName: LibraryPersistentBean
16:30:02,731 INFO [JndiSessionRegistrarBase] Binding the following Entries in Global JNDI:
LibraryPersistentBean/remote - EJB3.x Default Remote Business Interface
LibraryPersistentBean/remote-com.tutorialspoint.stateless.LibraryPersistentBeanRemote - EJB3.x Remote Business Interface
...
EJBTester (EJB Client)
jndi.properties
java.naming.factory.initial=org.jnp.interfaces.NamingContextFactory
java.naming.factory.url.pkgs=org.jboss.naming:org.jnp.interfaces
java.naming.provider.url=localhost
這些屬性是用來初始化InitialContext對象的Java命名服務
InitialContext的對象將被用於查找無狀態會話bean
EJBTester.java
package com.tutorialspoint.test;
import com.tutorialspoint.stateless.LibraryPersistentBeanRemote;
import java.io.BufferedReader;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.List;
import java.util.Properties;
import javax.naming.InitialContext;
import javax.naming.NamingException;
public class EJBTester {
BufferedReader brConsoleReader = null;
Properties props;
InitialContext ctx;
{
props = new Properties();
try {
props.load(new FileInputStream("jndi.properties"));
} catch (IOException ex) {
ex.printStackTrace();
}
try {
ctx = new InitialContext(props);
} catch (NamingException ex) {
ex.printStackTrace();
}
brConsoleReader =
new BufferedReader(new InputStreamReader(System.in));
}
public static void main(String[] args) {
EJBTester ejbTester = new EJBTester();
ejbTester.testEntityEjb();
}
private void showGUI(){
System.out.println("**********************");
System.out.println("Welcome to Book Store");
System.out.println("**********************");
System.out.print("Options \n1. Add Book\n2. Exit \nEnter Choice: ");
}
private void testEntityEjb(){
try {
int choice = 1;
LibraryPersistentBeanRemote libraryBean =
LibraryPersistentBeanRemote)
ctx.lookup("LibraryPersistentBean/remote");
while (choice != 2) {
String bookName;
showGUI();
String strChoice = brConsoleReader.readLine();
choice = Integer.parseInt(strChoice);
if (choice == 1) {
System.out.print("Enter book name: ");
bookName = brConsoleReader.readLine();
Book book = new Book();
book.setName(bookName);
libraryBean.addBook(book);
} else if (choice == 2) {
break;
}
}
List booksList = libraryBean.getBooks();
System.out.println("Book(s) entered so far: " + booksList.size());
int i = 0;
for (Book book:booksList) {
System.out.println((i+1)+". " + book.getName());
i++;
}
} catch (Exception e) {
System.out.println(e.getMessage());
e.printStackTrace();
}finally {
try {
if(brConsoleReader !=null){
brConsoleReader.close();
}
} catch (IOException ex) {
System.out.println(ex.getMessage());
}
}
}
}
EJBTester做以下任務。
jndi.properties中加載和初始化的InitialContext對象。
在testStatefulEjb()方法，JNDI名稱進行查找 - "LibraryStatelessSessionBean/remote" 以獲得遠程業務對象(狀態EJB)。
然後用戶顯示一個庫存儲的用戶界面和他/她被要求輸入選擇。
如果用戶輸入1，系統要求輸入書籍名稱和節省了使用無狀態的會話bean addBook()方法。會話Bean堅持這本書中通過EntityManager的調用數據庫。
如果用戶輸入2，系統檢索書使用狀態會話Bean getBooks()方法和退出。
然後另一個JNDI名稱進行查找 - "LibraryStatelessSessionBean/remote"獲得遠程業務對象(狀態EJB)再次列出書籍。
運行客戶端訪問EJB
在項目資源管理器中找到EJBTester.java。右鍵點擊上EJBTester類，並選擇run file.
在Netbeans控制檯驗證以下輸出。
run:
**********************
Welcome to Book Store
**********************
Options
1. Add Book
2. Exit
Enter Choice: 1
Enter book name: Learn Testing
**********************
Welcome to Book Store
**********************
Options
1. Add Book
2. Exit
Enter Choice: 2
Book(s) entered so far: 1
1. learn Testing
BUILD SUCCESSFUL (total time: 15 seconds)
(編輯：雷林鵬 來源：網絡|侵刪)