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：

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。

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); 

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