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EJB回调 - EJB

回调是一种机制，可以截获企业Bean的生命周期。 EJB 3.0规范指定的回调，回调处理方法是要创建。 EJB容器调用这些回调。 EJB类本身或在一个单独的类，我们可以定义回调方法。 EJB3.0提供了许多回调的注解

以下是无状态的bean回调的注解。

Annotation 描述
@PostConstruct method is invoked when a bean is created for the first time
--- ---
@PreDestroy method is invoked when a bean is removed from the bean pool or is destroyed.
--- ---

Following is the list of callback annotations for stateful bean.

Annotation 描述
@PostConstruct method is invoked when a bean is created for the first time
--- ---
@PreDestroy method is invoked when a bean is removed from the bean pool or is destroyed.
--- ---
@PostActivate method is invoked when a bean is loaded to be used.
--- ---
@PrePassivate method is invoked when a bean is put back to bean pool.
--- ---

Following is the list of callback annotations for message driven bean.

Annotation 描述
@PostConstruct method is invoked when a bean is created for the first time
--- ---
@PreDestroy method is invoked when a bean is removed from the bean pool or is destroyed.
--- ---

Following is the list of callback annotations for entity bean.

Annotation Description
@PrePersist method is invoked when an entity is created in database.
--- ---
@PostPersist method is invoked after an entity is created in database.
--- ---
@PreRemove method is invoked when an entity is deleted from the database.
--- ---
@PostRemove method is invoked after an entity is deleted from the database.
--- ---
@PreUpdate method is invoked before an entity is to be updated in the database.
--- ---
@PostLoad method is invoked when a record is fetched from database and loaded into the entity.
--- ---

示例应用程序

让我们创建一个测试EJB应用程序来测试各种回调EJB。

Step 描述
1 Create a project with a name EjbComponent under a package com.tutorialspoint.stateless as explained in the EJB - Create Application chapter. You can also use the project created in EJB - Persistence chapter as such for this chapter to add various callbacks to ejbs.
2 Create LibrarySessionBean.java and LibrarySessionBeanRemote as explained in the EJB - Create Application chapter. Keep rest of the files unchanged.
3 Use Beans created in the EJB - Persistence chapter. Add callback methods as shown below. Keep rest of the files unchanged.
4 Create a java class BookCallbackListener under package com.tutorialspoint.callback. This class will demonstrates the seperation of callback methods.
5 Clean and Build the application to make sure business logic is working as per the requirements.
6 Finally, 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.
7 Now create the ejb client, a console based application in the same way as explained in the_EJB - Create Application_ chapter under topic Create Client to access EJB.

EJBComponent (EJB Module)

BookCallbackListener.java

package com.tutorialspoint.callback;

import javax.persistence.PrePersist;
import javax.persistence.PostLoad;
import javax.persistence.PostPersist;
import javax.persistence.PostRemove;
import javax.persistence.PostUpdate;
import javax.persistence.PreRemove;
import javax.persistence.PreUpdate;
import com.tutorialspoint.entity.Book;

public class BookCallbackListener {

   @PrePersist
   public void prePersist(Book book){
      System.out.println("BookCallbackListener.prePersist:" 
         + "Book to be created with book id: "+book.getId());
   }

   @PostPersist
   public void postPersist(Object book){
      System.out.println("BookCallbackListener.postPersist::"
         + "Book created with book id: "+((Book)book).getId());
   }

   @PreRemove
   public void preRemove(Book book)
   {
      System.out.println("BookCallbackListener.preRemove:"
         + " About to delete Book: " + book.getId());
   }

   @PostRemove
   public void postRemove(Book book)
   {
      System.out.println("BookCallbackListener.postRemove::"
         + " Deleted Book: " + book.getId());
   }

   @PreUpdate
   public void preUpdate(Book book)
   {
      System.out.println("BookCallbackListener.preUpdate::"
         + " About to update Book: " + book.getId());
   }

   @PostUpdate
   public void postUpdate(Book book)
   {
      System.out.println("BookCallbackListener.postUpdate::"
         + " Updated Book: " + book.getId());
   }

   @PostLoad
   public void postLoad(Book book)
   {
      System.out.println("BookCallbackListener.postLoad::"
         + " Loaded Book: " + book.getId());
   }
}

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;
   }    
}

LibraryStatefulSessionBean.java

package com.tutorialspoint.stateful;

import java.util.ArrayList;
import java.util.List;
import javax.annotation.PostConstruct;
import javax.annotation.PreDestroy;
import javax.ejb.PostActivate;
import javax.ejb.PrePassivate;
import javax.ejb.Stateful;

@Stateful
public class LibraryStatefulSessionBean 
   implements LibraryStatefulSessionBeanRemote {
   List<String> bookShelf;    

   public LibraryStatefulSessionBean(){
      bookShelf = new ArrayList<String>();
   }

   public void addBook(String bookName) {
      bookShelf.add(bookName);
   }    

   public List<String> getBooks() {
      return bookShelf;
   }

   @PostConstruct
   public void postConstruct(){
      System.out.println("LibraryStatefulSessionBean.postConstruct::"
         + " bean created.");
   }

   @PreDestroy
   public void preDestroy(){
      System.out.println("LibraryStatefulSessionBean.preDestroy:"
         + " bean removed.");
   }

   @PostActivate
   public void postActivate(){
      System.out.println("LibraryStatefulSessionBean.postActivate:"
         + " bean activated.");
   }

   @PrePassivate
   public void prePassivate(){
      System.out.println("LibraryStatefulSessionBean.prePassivate:"
         + " bean passivated.");
   }    
}

LibraryStatefulSessionBeanRempote.java

package com.tutorialspoint.stateful;

import java.util.List;
import javax.ejb.Remote;

@Remote
public interface LibraryStatefulSessionBeanRemote {
   void addBook(String bookName);
   List getBooks();
}

LibraryPersistentBean.java

package com.tutorialspoint.stateless;

import com.tutorialspoint.entity.Book;
import java.util.List;
import javax.annotation.PostConstruct;
import javax.annotation.PreDestroy;
import javax.ejb.Stateless;
import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;

@Stateless
public class LibraryPersistentBean 
   implements LibraryPersistentBeanRemote {

   public LibraryPersistentBean(){}

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

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

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

   @PostConstruct
   public void postConstruct(){
      System.out.println("postConstruct:: LibraryPersistentBean session bean"
         + " created with entity Manager object: ");
   }

   @PreDestroy
   public void preDestroy(){
      System.out.println("preDestroy: LibraryPersistentBean session"
      + " bean is removed ");
   }
}

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();

}

  • 只要你部署在JBoss EjbComponent项目，就会发现jboss的日志。

  • JBoss已经自动为我们的会话bean创建一个JNDI条目 -LibraryPersistentBean/remote.

  • 我们将使用这个查询字符串来获得远程类型的业务对象 -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=LibraryPersistentBean,service=EJB3
16:30:02,723 INFO  [EJBContainer] STARTED EJB: com.tutorialspoint.stateless.LibrarySessionBeanRemote ejbName: LibraryPersistentBean
...

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

  • 这些属性是用来初始化InitialContext对象的Java命名服务

  • InitialContext的对象将被用于查找无状态会话bean

EJBTester.java

package com.tutorialspoint.test;

import com.tutorialspoint.stateful.LibrarySessionBeanRemote;
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 testStatelessEjb() method, jndi lookup is done with name - "LibrarySessionBean/remote" to obtain the remote business object (stateless 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 storing the book in the database.

  • If user enters 2, system retrieves books using stateless session bean getBooks() method and exits.

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: 13 seconds)

JBoss应用服务器的日志输出

你可以找到以下的回调在JBoss日志项

14:08:34,293 INFO  [STDOUT] postConstruct:: LibraryPersistentBean session bean created with entity Manager object
...
16:39:09,484 INFO  [STDOUT] BookCallbackListener.prePersist:: Book to be created with book id: 0
16:39:09,531 INFO  [STDOUT] BookCallbackListener.postPersist:: Book created with book id: 1
16:39:09,900 INFO  [STDOUT] BookCallbackListener.postLoad:: Loaded Book: 1