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);