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

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上一篇： eclipse + JBoss 5 + EJB3开发指南（8）：实现Entity Bean的一对多（one-to-many）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);

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