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
@PostConstructmethod is invoked when a bean is created for the first time
@PreDestroymethod 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
@PostConstructmethod is invoked when a bean is created for the first time
@PreDestroymethod is invoked when a bean is removed from the bean pool or is destroyed.
@PostActivatemethod is invoked when a bean is loaded to be used.
@PrePassivatemethod 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
@PostConstructmethod is invoked when a bean is created for the first time
@PreDestroymethod is invoked when a bean is removed from the bean pool or is destroyed.
Following is the list of callback annotations for entity bean.
AnnotationDescription
@PrePersistmethod is invoked when an entity is created in database.
@PostPersistmethod is invoked after an entity is created in database.
@PreRemovemethod is invoked when an entity is deleted from the database.
@PostRemovemethod is invoked after an entity is deleted from the database.
@PreUpdatemethod is invoked before an entity is to be updated in the database.
@PostLoadmethod 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
1Create 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.
2Create LibrarySessionBean.java and LibrarySessionBeanRemote as explained in the EJB - Create Application chapter. Keep rest of the files unchanged.
3Use Beans created in the EJB - Persistence chapter. Add callback methods as shown below. Keep rest of the files unchanged.
4Create a java class BookCallbackListener under package com.tutorialspoint.callback. This class will demonstrates the seperation of callback methods.
5Clean and Build the application to make sure business logic is working as per the requirements.
6Finally, 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.
7Now 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 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.");
}
}
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 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();
}
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 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是做了以下工作。
从jndi.properties负荷特性和初始化InitialContext对象。
在testStatelessEjb()方法，JNDI查找完成与名字 - “LibrarySessionBean /远程”，以获得远程业务对象(无状态EJB)。
然后用户显示一个图书馆商店用户界面和他/她被要求输入选择。
如果用户输入1，系统要求书的名称和保存的书，用无状态会话bean addBook()方法。会话bean存储该书在数据库中。
如果用户输入2，系统检索使用无状态会话bean getBooks()方法，并退出书籍。
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
(编辑：雷林鹏 来源：网络|侵删)