EJB持久性 - EJB

EJB2.0中使用的实体bean持久化机制在很大程度上被EJB3.0取代。现在实体bean是一个简单的POJO映射表。

以下是持久性API的关键角色

• Entity - 持久对象代表数据存储记录。这也是可序列化的。

• EntityManager - 持久性接口做数据操作，如添加/删除/更新/找到持久化对象（实体）。它还有助于执行查询使用query接口

• Persistence unit (persistence.xml) - 持久性单元介绍了持久性机制的属性。

• Data Source (*ds.xml) - 数据源描述了数据存储相关的属性，如连接URL。用户名，密码等。

为了证明EJB的持久化机制，我们要做好以下几项工作。

• Step 1. 在数据库中创建表.

• Step 2. 创建实体类对应的表.

• Step 3. 创建数据源和持久性单元

• Step 4. 创建一个无状态EJB EntityManager实例.

• Step 5. 更新无状态EJB。添加添加记录并获得通过实体管理器从数据库中记录的方法。

• Step 6. 一个基于控制台应用程序客户端将访问无状态EJB的持久化数据库中的数据。

创建表

创建一个表 books 在默认的数据库 postgres.

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

创建实体类

//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;
   }
   ...
}

创建数据源和持久性单元

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();
   }
   ...
}

构建EJB模块后，我们需要一个无状态的bean，我们将在下一节要创建客户端来访问。

示例应用程序

让我们创建一个测试EJB应用程序来测试EJB的持久化机制。

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应用服务器的日志输出

...
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.

运行客户端访问EJB

在项目资源管理器中找到EJBTester.java。右键点击上EJBTester类，并选择 run file.

在Netbeans控制台验证以下输出。

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)

再次运行客户端来访问EJB。

访问EJB之前重新启动JBoss。

在项目资源管理器中找到EJBTester.java。右键点击上EJBTester类，并选择 run file.

在Netbeans控制台验证以下输出。

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)

• 上面显示的输出状态书存储在持久性存储，并从数据库中检索。