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：

<!--<br /> <br /> Code highlighting produced by Actipro CodeHighlighter (freeware)<br /> http://www.CodeHighlighter.com/<br /> <br /> -->packageentity;

importjava.util.Collection;

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

@Entity
@Table(name="t_customers")
publicclassCustomer
{
privateintid;
privateStringname;
privateRefereereferee;
privateCollection<Order>orders;

privateCollection<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"))
publicCollection<Address>getAddresses()
{
returnaddresses;
}

... ...
}

@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：

<!--<br /> <br /> Code highlighting produced by Actipro CodeHighlighter (freeware)<br /> http://www.CodeHighlighter.com/<br /> <br /> -->packageentity;

importjava.util.Collection;

importjavax.persistence.CascadeType;
importjavax.persistence.Entity;
importjavax.persistence.FetchType;
importjavax.persistence.GeneratedValue;
importjavax.persistence.GenerationType;
importjavax.persistence.Id;
importjavax.persistence.ManyToMany;
importjavax.persistence.Table;
@Entity
@Table(name="t_addresses")
publicclassAddress
{
privateintid;
privateStringaddressLine;
privateStringcountry;
privateStringpostCode;
privateCollection<Customer>customers;
@Id
@GeneratedValue(strategy=GenerationType.IDENTITY)
publicintgetId()
{
returnid;
}
publicvoidsetId(intid)
{
this.id=id;
}
publicStringgetAddressLine()
{
returnaddressLine;
}
publicvoidsetAddressLine(StringaddressLine)
{
this.addressLine=addressLine;
}
publicStringgetCountry()
{
returncountry;
}
publicvoidsetCountry(Stringcountry)
{
this.country=country;
}
publicStringgetPostCode()
{
returnpostCode;
}
publicvoidsetPostCode(StringpostCode)
{
this.postCode=postCode;
}
@ManyToMany(mappedBy="addresses")
publicCollection<Customer>getCustomers()
{
returncustomers;
}
publicvoidsetCustomers(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：

<!--<br /> <br /> Code highlighting produced by Actipro CodeHighlighter (freeware)<br /> http://www.CodeHighlighter.com/<br /> <br /> -->Customercustomer=newCustomer();
customer.setName("微软11");
List<Address>addresses=newArrayList<Address>();
Addressaddress=newentity.Address();
address.setAddressLine("address1");
address.setCountry("中国");
address.setPostCode("12345678");
addresses.add(address);
address=newentity.Address();
address.setAddressLine("address2");
address.setCountry("美国");
address.setPostCode("4321");
addresses.add(address);
customer.setAddresses(addresses);
em.persist(customer);

下一篇：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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