EJB回調

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
  3. Add Book
  4. Exit Enter Choice: 2 Book(s) entered so far: 1
  5. 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