可嵌入对象（Embeddable Objects）

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2023-12-01

EJB 3.0提供了将JAVA POJO（Plain Old Java Object）嵌入到实体bean中的选项，并允许使用嵌入式POJO类的方法映射列名。要嵌入的Java POJO必须注释为@Embeddable。

@Embeddable
public class Publisher implements Serializable{
   private String name;
   private String address;
   ...
}

可以使用@Embedded批注嵌入上述类。

@Entity
public class Book implements Serializable{
   private int id;
   private String name;
   private Publisher publisher;
   ...
   @Embedded
   @AttributeOverrides({
      @AttributeOverride(name = "name", 
                      column = @Column(name = "PUBLISHER")),
      @AttributeOverride(name = "address", 
                      column = @Column(name = "PUBLISHER_ADDRESS"))
   })
   public Publisher getPublisher() {
      return publisher;
   }
   ...
}

例子 Example Application

让我们创建一个测试EJB应用程序来测试EJB 3.0中的嵌入对象。

步	描述
1	在EJB - Create Application章节中解释，在cn.xnip.entity包下创建一个名为EjbComponent的项目。请使用在EJB - Persistence章节中创建的项目本章来理解EJB概念中的嵌入对象。
2	按照EJB - Create Application章节中的说明，在cn.xnip.entity包下创建Publisher.java 。保持其余文件不变。
3	在cn.xnip.entity包下创建cn.xnip.entity 。使用EJB - Persistence章节作为参考。保持其余文件不变。
4	清理并构建应用程序以确保业务逻辑按照要求运行。
5	最后，在JBoss Application Server上以jar文件的形式部署应用程序。如果JBoss Application服务器尚未启动，它将自动启动。
6	现在创建EJB客户端，一个基于控制台的应用程序，其方式与EJB - Create Application章节中主题Create Client to access EJB 。

创建/更改书籍表

CREATE TABLE book (
   id     integer PRIMARY KEY,
   name   varchar(50)
);
Alter table book add publisher varchar(100);
Alter table book add publisher_address varchar(200);

EJBComponent (EJB Module)

Publisher.java

package cn.xnip.entity;
import java.io.Serializable;
import javax.persistence.Embeddable;
@Embeddable
public class Publisher implements Serializable{
   private String name;
   private String address;
   public Publisher(){}
   public Publisher(String name, String address){
      this.name = name;
      this.address = address;
   }
   public String getName() {
      return name;
   }
   public void setName(String name) {
      this.name = name;
   }
   public String getAddress() {
      return address;
   }
   public void setAddress(String address) {
      this.address = address;
   }
   public String toString(){
      return name + "," + address;
   }    
}

Book.java


package cn.xnip.entity;
import cn.xnip.callback.BookCallbackListener;
import java.io.Serializable;
import javax.persistence.AttributeOverride;
import javax.persistence.AttributeOverrides;
import javax.persistence.Column;
import javax.persistence.Embedded;
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="book")
public class Book implements Serializable{
   private int id;
   private String name;
   private Publisher publisher;
   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;
   }
   @Embedded
   @AttributeOverrides({
      @AttributeOverride(name = "name", 
         column = @Column(name = "PUBLISHER")),
      @AttributeOverride(name = "address", 
         column = @Column(name = "PUBLISHER_ADDRESS"))
   })
   public Publisher getPublisher() {
      return publisher;
   }
   public void setPublisher(Publisher publisher) {
      this.publisher = publisher;
   }    
}

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();
}

LibraryPersistentBean.java

package cn.xnip.stateless;
import cn.xnip.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();
   }
}

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.interceptor.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.interceptor.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.interceptor.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-cn.xnip.interceptor.LibraryPersistentBeanRemote - EJB3.x Remote Business Interface
...

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.LibraryBeanRemote;
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.testEmbeddedObjects();
   }
   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 testEmbeddedObjects(){
      try {
         int choice = 1; 
         LibraryPersistentBeanRemote libraryBean = 
        (LibraryPersistentBeanRemote)
         ctx.lookup("LibraryPersistentBean/remote");
         while (choice != 2) {
            String bookName;
            String publisherName;
            String publisherAddress;
            showGUI();
            String strChoice = brConsoleReader.readLine();
            choice = Integer.parseInt(strChoice);
            if (choice == 1) {
               System.out.print("Enter book name: ");
               bookName = brConsoleReader.readLine();
               System.out.print("Enter publisher name: ");
               publisherName = brConsoleReader.readLine();
               System.out.print("Enter publisher address: ");
               publisherAddress = brConsoleReader.readLine();
               Book book = new Book();
               book.setName(bookName);
               book.setPublisher
              (new Publisher(publisherName,publisherAddress));
               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());
            System.out.println("Publication: "+book.getPublisher());
            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testInterceptedEjb（）方法中，使用名称“LibraryPersistenceBean/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 html5
Enter publisher name: SAMS
Enter publisher address: DELHI
**********************
Welcome to Book Store
**********************
Options 
1. Add Book
2. Exit 
Enter Choice: 2
Book(s) entered so far: 1
1. learn html5
Publication: SAMS,DELHI
BUILD SUCCESSFUL (total time: 21 seconds)