📌  相关文章
📜  EJB-回调

📅  最后修改于: 2020-11-16 06:18:52             🧑  作者: Mango

回调是一种可以拦截企业Bean生命周期的机制。 EJB 3.0规范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的回调注释的列表-

Annotation Description
@PostConstruct Invoked when a bean is created for the first time.
@PreDestroy Invoked when a bean is removed from the bean pool or is destroyed.

以下是有状态bean的回调注释的列表-

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

以下是消息驱动的bean的回调注释的列表-

Annotation Description
@PostConstruct Invoked when a bean is created for the first time.
@PreDestroy Invoked when a bean is removed from the bean pool or is destroyed.

以下是实体bean的回调注释的列表-

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

应用范例

让我们创建一个测试EJB应用程序以测试EJB中的各种回调。

Step Description
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 EJB.
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 separation 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模块)

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 bookShelf;    

   public LibraryStatefulSessionBean() {
      bookShelf = new ArrayList();
   }

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

   public List 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 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 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 getBooks();
    
}

  • 在JBOSS上部署EjbComponent项目后，请注意jboss日志。

  • JBoss已经为我们的会话bean- LibraryPersistentBean / remote自动创建了一个JNDI条目。

  • 我们将使用此查找字符串获取类型为com.tutorialspoint.stateless.LibraryPersistentBeanRemote的远程业务对象。

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-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客户端)

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 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 \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 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，以获得远程业务对象(无状态EJB)。

  • 然后，向用户显示一个库存储用户界面，并要求他/她输入选择。

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

  • 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，然后选择运行文件。

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
...