EJB持久性

EJB2.0中使用的實體bean持久化機製在很大程度上被EJB3.0取代。現在實體bean是一個簡單的POJO映射表。

以下是持久性API的關鍵角色

Entity - 持久對象代表數據存儲記錄。這也是可序列化的。
EntityManager - 持久性接口做數據操作，如添加/刪除/更新/找到持久化對象（實體）。它還有助於執行查詢使用query接口
Persistence unit (persistence.xml) - 持久性單元介紹了持久性機製的屬性。
Data Source (*ds.xml) - 數據源描述了數據存儲相關的屬性，如連接URL。用戶名，密碼等。

為了證明EJB的持久化機製，我們要做好以下幾項工作。

Step 1. 在數據庫中創建表.
Step 2. 創建實體類對應的表.
Step 3. 創建數據源和持久性單元
Step 4. 創建一個無狀態EJB EntityManager實例.
Step 5. 更新無狀態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。
Step 6. 一個基於控製台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。

創建表

創建一個表 books 在默認的數據庫 postgres.

CREATE TABLE books (
   id     integer PRIMARY KEY,
   name   varchar(50)
);

創建實體類

//mark it entity using Entity annotation 
//map table name using Table annoation
@Entity
@Table(name="books")
public class Book implements Serializable{
    
   private int id;
   private String name;

   public Book(){        
   }

   //mark id as primary key with autogenerated value
   //map database column id with id field
   @Id
   @GeneratedValue(strategy= GenerationType.IDENTITY)
   @Column(name="id")
   public int getId() {
      return id;
   }
   ...
}

創建數據源和持久性單元

DataSource (jboss-ds.xml)

<?xml version="1.0" encoding="UTF-8"?>
<datasources>
   <local-tx-datasource>
      <jndi-name>PostgresDS</jndi-name>
      <connection-url>jdbc:postgresql://localhost:5432/postgres</connection-url>
      <driver-class>org.postgresql.driver</driver-class>
      <user-name>sa</user-name>
      <password>sa</password>
      <min-pool-size>5</min-pool-size>
      <max-pool-size>20</max-pool-size>
      <idle-timeout-minutes>5</idle-timeout-minutes>
   </local-tx-datasource>
</datasources>

Persistence Unit (persistence.xml)

<persistence version="1.0" xmlns="http://java.sun.com/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_1_0.xsd">
   <persistence-unit name="EjbComponentPU" transaction-type="JTA">
      <jta-data-source>java:/PostgresDS</jta-data-source>
      <exclude-unlisted-classes>false</exclude-unlisted-classes>
      <properties/>
   </persistence-unit>
   <persistence-unit name="EjbComponentPU2" transaction-type="JTA">
      <provider>org.hibernate.ejb.HibernatePersistence</provider>
      <jta-data-source>java:/PostgresDS</jta-data-source>
      <exclude-unlisted-classes>false</exclude-unlisted-classes>
      <properties>
         <property name="hibernate.hbm2ddl.auto" value="update"/>
      </properties>
   </persistence-unit>
</persistence>

Create Stateless EJB having EntityManager instance

@Stateless
public class LibraryPersistentBean implements LibraryPersistentBeanRemote {
	
   //pass persistence unit to entityManager.
   @PersistenceContext(unitName="EjbComponentPU")
   private EntityManager entityManager;         

   public void addBook(Book book) {
      entityManager.persist(book);
   }    

   public List<Book> getBooks() {        
      return entityManager.createQuery("From Books").getResultList();
   }
   ...
}

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的持久化機製。

Step	描述
1	Create 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 persistence concepts.
2	Create Book.java under package com.tutorialspoint.entity and modify it as shown below.
3	Create LibraryPersistentBean.java and LibraryPersistentBeanRemote as explained in the EJB - Create Application chapter and modify them as shown below.
4	Create jboss-ds.xml in EjbComponent > setup folder and persistence.xml in EjbComponent > src > conf folder. These folder can be seen in files tab in Netbeans. Modify these files as shown above.
5	清理並生成應用程序以確保業務邏輯是按要求工作。
6	最後，將應用程序部署在JBoss應用服務器上的jar文件的形式。 JBoss應用服務器將自動開始瀏覽網頁，如果它尚未啟動。
7	Now 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;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.EntityListeners;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.Table;

@Entity
@Table(name="books")
public class Book implements Serializable{
    
   private int id;
   private String name;

   public Book(){        
   }

   @Id
   @GeneratedValue(strategy= GenerationType.IDENTITY)
   @Column(name="id")
   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<Book> 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;

@Stateless
public class LibraryPersistentBean implements LibraryPersistentBeanRemote {
    
   public LibraryPersistentBean(){
   }

   @PersistenceContext(unitName="EjbComponentPU")
   private EntityManager entityManager;         

   public void addBook(Book book) {
      entityManager.persist(book);
   }    

   public List<Book> getBooks() {
      return entityManager.createQuery("From Book").getResultList();
   }
}

As soon as you deploy the EjbComponent project on JBOSS, notice the jboss log.
JBoss has automatically created a JNDI entry for our session bean -LibraryPersistentBean/remote.
We'll using this lookup string to get remote business object of type -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

These properties are used to initialize the InitialContext object of java naming service
InitialContext object will be used to lookup stateless session 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 
1. Add Book
2. Exit 
Enter 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<Book> 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 is doing the following tasks.

Load properties from jndi.properties and initialize the InitialContext object.
In testStatefulEjb() method, jndi lookup is done with name - "LibraryStatefulSessionBean/remote" to obtain the remote business object (stateful ejb).
Then user is shown a library store User Interface and he/she is asked to enter choice.
If user enters 1, system asks for book name and saves the book using stateless session bean addBook() method. Session Bean is persisting the book in database via EntityManager call.
If user enters 2, system retrieves books using stateful session bean getBooks() method and exits.
Then another jndi lookup is done with name - "LibraryStatelessSessionBean/remote" to obtain the remote business object (stateless ejb) again and listing of books is done.

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控製台驗證以下輸出。

run:
**********************
Welcome to Book Store
**********************
Options 
1. Add Book
2. Exit 
Enter Choice: 1
Enter book name: Learn Java
**********************
Welcome to Book Store
**********************
Options 
1. Add Book
2. Exit 
Enter Choice: 2
Book(s) entered so far: 1
1. learn java
BUILD SUCCESSFUL (total time: 15 seconds)

再次運行客戶端來訪問EJB。

訪問EJB之前重新啟動JBoss。

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控製台驗證以下輸出。

run:
**********************
Welcome to Book Store
**********************
Options 
1. Add Book
2. Exit 
Enter Choice: 1
Enter book name: Learn Spring
**********************
Welcome to Book Store
**********************
Options 
1. Add Book
2. Exit 
Enter Choice: 2
Book(s) entered so far: 2
1. learn java
2. Learn Spring
BUILD SUCCESSFUL (total time: 15 seconds)

上麵顯示的輸出狀態書存儲在持久性存儲，並從數據庫中檢索。