eclipse + JBoss 5 + EJB3开发指南（9）：实现Entity Bean的多对多映射

  在EJB3中需要使用@ManyToMany对封装多对多关系的字段或getter方法进行注释。先看看下面的表：

图1  t_addresses表

      t_addresses表和t_customers表是多对多的关系。需要使用一个关联表来描述这种关系，关联表的结构如下图所示。

图2  t_customers_addresses

    在Customer类中定义一个Collection<Address>类型的字段（addresses），用于保存与该Customer对象相对应的多个Address对象，代码如下：

package  entity;

import  java.util.Collection;

import  javax.persistence.CascadeType;
import  javax.persistence.Entity;
import  javax.persistence.FetchType;
import  javax.persistence.GeneratedValue;
import  javax.persistence.GenerationType;
import  javax.persistence.Id;
import  javax.persistence.JoinColumn;
import  javax.persistence.JoinTable;
import  javax.persistence.ManyToMany;
import  javax.persistence.OneToMany;
import  javax.persistence.OneToOne;
import  javax.persistence.PrimaryKeyJoinColumn;
import  javax.persistence.Table;

@Entity
@Table(name  =   " t_customers " )
public   class  Customer
{
     private   int  id;
     private  String name;
     private  Referee referee;
     private  Collection < Order >  orders;

     private  Collection < Address >  addresses;

    @ManyToMany(cascade  =  CascadeType.PERSIST, fetch  =  FetchType.LAZY)
    @JoinTable(name  =   " t_customers_addresses " , joinColumns  =  @JoinColumn(name  =   " customer_id " ,
     referencedColumnName  =   " id " ), inverseJoinColumns  =  @JoinColumn(name  =   " address_id " , referencedColumnName  =   " id " ))
     public  Collection < Address >  getAddresses()
    {
         return  addresses;
    }

    ... ...
}

    @JoinTable注释用于指定连接表和t_customers及t_addresses表的连接字段关系。
    Address类的代码如下：

package  entity;

import  java.util.Collection;

import  javax.persistence.CascadeType;
import  javax.persistence.Entity;
import  javax.persistence.FetchType;
import  javax.persistence.GeneratedValue;
import  javax.persistence.GenerationType;
import  javax.persistence.Id;
import  javax.persistence.ManyToMany;
import  javax.persistence.Table;
@Entity
@Table(name = " t_addresses " )
public   class  Address
{
     private   int  id;
     private  String addressLine;
     private  String country;
     private  String postCode;
     private  Collection < Customer >  customers;
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
     public   int  getId()
    {
         return  id;
    }
     public   void  setId( int  id)
    {
         this .id  =  id;
    }
     public  String getAddressLine()
    {
         return  addressLine;
    }
     public   void  setAddressLine(String addressLine)
    {
         this .addressLine  =  addressLine;
    }
     public  String getCountry()
    {
         return  country;
    }
     public   void  setCountry(String country)
    {
         this .country  =  country;
    }
     public  String getPostCode()
    {
         return  postCode;
    }
     public   void  setPostCode(String postCode)
    {
         this .postCode  =  postCode;
    }
    @ManyToMany(mappedBy ="addresses" )
     public  Collection < Customer >  getCustomers()
    {
         return  customers;
    }
     public   void  setCustomers(Collection < Customer >  customers)
    {
         this .customers  =  customers;
    }    
}

    由于是多对多的关系，因此，在Address类中需要定义一个Collection<Customer>类型的字段(customers）用来保存与该Address对象相对应的Customer对象。getCustomers方法也需要使用@ManyToMany进行注释。可以使用下面代码进行测试：

Customer customer  =   new  Customer();
customer.setName( " 微软11 " );
List < Address >  addresses  =   new  ArrayList < Address > ();
Address address  =   new  entity.Address();
address.setAddressLine( " address1 " );
address.setCountry( " 中国 " ); 
address.setPostCode( " 12345678 " );
addresses.add(address);
address  =   new  entity.Address();
address.setAddressLine( " address2 " );
address.setCountry( " 美国 " ); 
address.setPostCode( " 4321 " );
addresses.add(address);
customer.setAddresses(addresses);
em.persist(customer);