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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