回调（Callbacks）

回调是一种机制，通过该机制可以拦截企业bean的生命周期。 EJB 3.0规范已指定了为其创建回调处理程序方法的回调。 EJB容器调用这些回调。 我们可以在EJB类本身或单独的类中定义回调方法。 EJB 3.0为回调提供了许多注释。

以下是无状态bean的回调注释列表 -

以下是有状态bean的回调注释列表 -

以下是消息驱动bean的回调注释列表 -

以下是实体bean的回调注释列表 -

例子 Example Application

让我们创建一个测试EJB应用程序来测试EJB中的各种回调。

EJBComponent (EJB Module)

BookCallbackListener.java

package cn.xnip.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 cn.xnip.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 cn.xnip.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 cn.xnip.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.");
   }    
}

LibraryStatefulSessionBeanRemote.java

package cn.xnip.stateful;
import java.util.List;
import javax.ejb.Remote;
@Remote
public interface LibraryStatefulSessionBeanRemote {
   void addBook(String bookName);
   List getBooks();
}

LibraryPersistentBean.java

package cn.xnip.stateless;
import cn.xnip.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 cn.xnip.stateless;
import cn.xnip.entity.Book;
import java.util.List;
import javax.ejb.Remote;
@Remote
public interface LibraryPersistentBeanRemote {
   void addBook(Book bookName);
   List<Book> getBooks();
}

• 只要在JBOSS上部署EjbComponent项目，请注意jboss日志。

• JBoss已自动为会话bean创建了一个JNDI条目 - LibraryPersistentBean/remote 。

• 我们将使用此查找字符串来获取类型的远程业务对象 - cn.xnip.stateless.LibraryPersistentBeanRemote

JBoss应用服务器日志输出

...
16:30:01,401 INFO  [JndiSessionRegistrarBase] Binding the following Entries in Global JNDI:
   LibraryPersistentBean/remote - EJB3.x Default Remote Business Interface
   LibraryPersistentBean/remote-cn.xnip.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: cn.xnip.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

• 这些属性用于初始化java命名服务的InitialContext对象。

• InitialContext对象将用于查找无状态会话bean。

EJBTester.java

package cn.xnip.test;
import cn.xnip.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 \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<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执行以下任务 -

• 从jndi.properties加载属性并初始化InitialContext对象。

• 在testStatelessEjb（）方法中，使用名称“LibrarySessionBean/remote”完成jndi查找以获取远程业务对象（无状态EJB）。

• 然后向用户显示库存储用户界面，并要求他/她输入选项。

• 如果用户输入1，系统将询问书名并使用无状态会话bean addBook（）方法保存书籍。 会话Bean将书籍存储在数据库中。

• 如果用户输入2，系统将使用无状态会话bean getBooks（）方法检索书籍并退出。

运行客户端以访问EJB

在项目资源管理器中找到EJBTester.java。 右键单击EJBTester类并选择run file 。

在Netbeans控制台中验证以下输出。

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应用服务器日志输出

您可以在JBoss日志中找到以下回调条目

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
...