EJB 3.0，EJB 2.0中使用的实体bean在很大程度上被持久性机制所取代.现在，实体bean是一个简单的POJO，它与表有映射.
以下是持久性API中的关键角色 :
实体 : 表示数据存储记录的持久对象.最好是可序列化的.
EntityManager : 持久性接口，用于对持久对象(实体)执行添加/删除/更新/查找等数据操作.它还有助于使用 Query 接口执行查询.
持久性单元(persistence.xml) : 持久性单元描述了持久性机制的属性.
数据源(* ds.xml) : 数据源描述了与数据存储相关的属性，如连接URL.用户名，密码等.
为了演示EJB持久性机制，我们需要执行以下任务 :
第1步 : 在数据库中创建表.
第2步 : 创建与表对应的实体类.
步骤3 : 创建数据源和持久性单元.
步骤4 : 创建具有EntityManager实例的无状态EJB.
步骤5 : 更新无状态EJB.添加方法以通过实体管理器添加记录并从数据库中获取记录.
步骤6 : 基于控制台的应用程序客户端将访问无状态EJB以将数据保存在数据库中.
在默认数据库 postgres 中创建一个表 books .
CREATE TABLE books ( id integer PRIMARY KEY, name varchar(50) );
//mark it entity using Entity annotation
//map table name using Table annotation
@Entity
@Table(name="books")
public class Book implements Serializable{
private int id;
private String name;
public Book() {
}
//mark id as primary key with autogenerated value
//map database column id with id field
@Id
@GeneratedValue(strategy= GenerationType.IDENTITY)
@Column(name="id")
public int getId() {
return id;
}
...
}<?xml version = "1.0" encoding = "UTF-8"?> <datasources> <local-tx-datasource> <jndi-name>PostgresDS</jndi-name> <connection-url>jdbc:postgresql://localhost:5432/postgres</connection-url> <driver-class>org.postgresql.driver</driver-class> <user-name>sa</user-name> <password>sa</password> <min-pool-size>5</min-pool-size> <max-pool-size>20</max-pool-size> <idle-timeout-minutes>5</idle-timeout-minutes> </local-tx-datasource> </datasources>
<persistence version = "1.0" xmlns = "http://java.sun.com/xml/ns/persistence" xmlns:xsi = "https://img01.yuandaxia.cn/Content/img/tutorials/ejb/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/persistence https://img01.yuandaxia.cn/Content/img/tutorials/ejb/index.html <persistence-unit name = "EjbComponentPU" transaction-type = "JTA"> <jta-data-source>java:/PostgresDS</jta-data-source> <exclude-unlisted-classes>false</exclude-unlisted-classes> <properties/> </persistence-unit> <persistence-unit name = "EjbComponentPU2" transaction-type = "JTA"> <provider>org.hibernate.ejb.HibernatePersistence</provider> <jta-data-source>java:/PostgresDS</jta-data-source> <exclude-unlisted-classes>false</exclude-unlisted-classes> <properties> <property name="hibernate.hbm2ddl.auto" value="update"/> </properties> </persistence-unit> </persistence>
@Stateless
public class LibraryPersistentBean implements LibraryPersistentBeanRemote {
//pass persistence unit to entityManager.
@PersistenceContext(unitName="EjbComponentPU")
private EntityManager entityManager;
public void addBook(Book book) {
entityManager.persist(book);
}
public List<Book> getBooks() {
return entityManager.createQuery("From Books").getResultList();
}
...
}构建EJB模块后，我们需要一个客户端来访问无状态bean ，我们将在下一节中创建.
让我们创建一个测试EJB应用程序来测试EJB持久性机制.
| Step | 描述 |
|---|---|
| 1 | 按照 EJB - 创建应用程序章节中的说明，在包 com.it1352.entity 下创建名为 EjbComponent 的项目.您还可以使用在本章中 EJB - 创建应用程序章节中创建的项目来了解EJB持久性概念. |
| 2 | 在包 com.IT下创建 Book.java 屋.entity 并按如下所示进行修改. |
| 3 | 按照 EJB - 创建应用程序章节中的说明创建 LibraryPersistentBean.java 和 LibraryPersistentBeanRemote 并修改它们，如下所示. |
| 4 | 在 EjbComponent>中创建 jboss-ds.xml EjbComponent>中的setup 文件夹和 persistence.xml src> conf 文件夹.这些文件夹可以在Netbeans的文件选项卡中看到.修改这些文件，如上所示. |
| 5 | 清理并构建应用程序以确保业务逻辑按照要求运行. |
| 6 | 最后，在JBoss Application Server上以jar文件的形式部署应用程序.如果JBoss应用服务器尚未启动，它将自动启动. |
| 7 | 现在创建EJB客户端，一个基于控制台的应用程序，其方式与主题创建 EJB - 创建应用程序章节中所述的相同客户端访问EJB .修改它，如下所示. |
package com.it1352.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;
}
}
package com.it1352.stateless;
import com.IT屋.entity.Book;
import java.util.List;
import javax.ejb.Remote;
@Remote
public interface LibraryPersistentBeanRemote {
void addBook(Book bookName);
List<Book> getBooks();
}
package com.it1352.stateless;
import com.it1352.entity.Book;
import java.util.List;
import javax.ejb.Stateless;
import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;
@Stateless
public class LibraryPersistentBean implements LibraryPersistentBeanRemote {
public LibraryPersistentBean() {
}
@PersistenceContext(unitName="EjbComponentPU")
private EntityManager entityManager;
public void addBook(Book book) {
entityManager.persist(book);
}
public List<Book> getBooks() {
return entityManager.createQuery("From Book").getResultList();
}
}一旦部署EjbComponent关于JBOSS的项目，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 - com.it1352.stateless.LibraryPersistentBeanRemote
... 16:30:01,401 INFO [JndiSessionRegistrarBase] Binding the following Entries in Global JNDI: LibraryPersistentBean/remote - EJB3.x Default Remote Business Interface LibraryPersistentBean/remote-com.IT屋.stateless.LibraryPersistentBeanRemote - EJB3.x Remote Business Interface 16:30:02,723 INFO [SessionSpecContainer] Starting jboss.j2ee:jar=EjbComponent.jar,name=LibraryPersistentBeanRemote,service=EJB3 16:30:02,723 INFO [EJBContainer] STARTED EJB: com.IT屋.stateless.LibraryPersistentBeanRemote ejbName: LibraryPersistentBean 16:30:02,731 INFO [JndiSessionRegistrarBase] Binding the following Entries in Global JNDI: LibraryPersistentBean/remote - EJB3.x Default Remote Business Interface LibraryPersistentBean/remote-com.IT屋.stateless.LibraryPersistentBeanRemote - EJB3.x Remote Business Interface ...
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.
package com.it1352.test;
import com.it1352.stateless.LibraryPersistentBeanRemote;
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testStatefulEjb()方法中，使用name - "LibraryStatefulSessionBean/remote"完成jndi查找以获取远程业务对象(有状态ejb).
然后用户是显示了一个库存储用户界面，并要求他/她输入选项.
如果用户输入1，系统会询问书名并使用无状态保存书籍session bean addBook()方法.会话Bean通过EntityManager调用将数据库保存在数据库中.
如果用户输入2，系统将使用有状态会话bean getBooks()方法检索书籍并退出./p>
然后用名称"LibraryStatelessSessionBean/remote"完成另一个jndi查找，以再次获取远程业务对象(无状态EJB)并完成书籍列表./p>
在项目资源管理器中找到EJBTester.java.右键单击EJBTester类并选择运行文件.
验证Netbeans控制台中的以下输出 :
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: 15 seconds)
在访问EJB之前重新启动JBoss.
在项目资源管理器中找到EJBTester.java.右键单击EJBTester类并选择运行文件.
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 Spring ********************** Welcome to Book Store ********************** Options 1. Add Book 2. Exit Enter Choice: 2 Book(s) entered so far: 2 1. learn java 2. Learn Spring BUILD SUCCESSFUL (total time: 15 seconds)
上面显示的输出表明书籍存储在持久存储中并从数据库中检索.
