回調是一種機制，可以截獲企業Bean的生命週期。 EJB 3.0規範指定的回調，回調處理方法是要創建。 EJB容器調用這些回調。 EJB類本身或在一個單獨的類，我們可以定義回調方法。 EJB3.0提供了許多回調的註解

以下是無狀態的bean回調的註解。

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.

示例應用程序

讓我們創建一個測試EJB應用程序來測試各種回調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 theEJB - 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項目，就會發現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=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

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.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.

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 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應用服務器的日誌輸出

你可以找到以下的回調在JBoss日誌項

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