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eclipse + JBoss 5 + EJB3开发指南（7）：实现Entity Bean的一对一（one-to-one）映射
EJB3的一对多映射使用@OneToMany来设置，如果是双向的一对多映射，在many方需要使用@ManyToOne设置。在本书中给出两个表，其他一个表t_customers在上一篇文章中已给出了，另一个表的结构如图1所示。
图1 t_orders表
t_customers和t_orders表是一对多关系，一个Customer可能有多个Order，而一个Order只能有一个Customer。
在Customer类中需要定义一个集合类型的属性，用来保存多个Order对象，Customer类的代码如下：
<!--<br /> <br /> Code highlighting produced by Actipro CodeHighlighter (freeware)<br /> http://www.CodeHighlighter.com/<br /> <br /> -->
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
Stringname;
private
Refereereferee;
private
Collection
<
Order
>
orders;
@OneToMany(mappedBy
=
"
customer
"
,cascade
=
CascadeType.ALL)
public
Collection
<
Order
>
getOrders() {
return
orders; }
public
void
setOrders(Collection
<
Order
>
orders) {
this
.orders
=
orders; } @Id @GeneratedValue(strategy
=
GenerationType.IDENTITY)
public
int
getId() {
return
id; }
... ... }
其中@OneToMany的mappedBy属性指定了Order类中获得Customer对象的属性名。Order类的代码如下：
<!--<br /> <br /> Code highlighting produced by Actipro CodeHighlighter (freeware)<br /> http://www.CodeHighlighter.com/<br /> <br /> -->
package
entity;
import
javax.persistence.Column;
import
javax.persistence.Entity;
import
javax.persistence.GeneratedValue;
import
javax.persistence.GenerationType;
import
javax.persistence.Id;
import
javax.persistence.JoinColumn;
import
javax.persistence.JoinColumns;
import
javax.persistence.JoinTable;
import
javax.persistence.ManyToOne;
import
javax.persistence.Table; @Entity @Table(name
=
"
t_orders
"
)
public
class
Order {
private
int
id;
private
StringproductId;
private
int
count;
private
Customercustomer; @ManyToOne @JoinColumn(name
=
"
customer_id
"
)
public
CustomergetCustomer() {
return
customer; }
public
void
setCustomer(Customercustomer) {
this
.customer
=
customer; } @Id @GeneratedValue(strategy
=
GenerationType.IDENTITY)
public
int
getId() {
return
id; } ... ...
}
其中@JoinColumn注释的name属性指定t_orders表中用于连接t_customers表的外键名。
可以使用下面的代码进行测试：
<!--<br /> <br /> Code highlighting produced by Actipro CodeHighlighter (freeware)<br /> http://www.CodeHighlighter.com/<br /> <br /> -->
Customercustomer
=
new
Customer(); customer.setName(
"
微软
"
);
List
<
Order
>
orders
=
new
ArrayList
<
Order
>
(); Orderorder
=
new
Order(); order.setProductId(
"
1234
"
); order.setCount(
20
); order.setCustomer(customer);
orders.add(order); order
=
new
Order(); order.setProductId(
"
4321
"
); order.setCount(
12
); order.setCustomer(customer);
orders.add(order);
customer.setOrders(orders);
em.persist(customer);
除此之外，还可以使用@JoinTable指定连接表来映射one-to-many关系。连接表的结构如图2所示。
图2 t_customers_orders表
首选需要将Customer类的getOrders方法修改成下的形式：
<!--<br /> <br /> Code highlighting produced by Actipro CodeHighlighter (freeware)<br /> http://www.CodeHighlighter.com/<br /> <br /> -->
@OneToMany @JoinTable(name
=
"
t_customers_orders
"
,joinColumns
=
@JoinColumn(name
=
"
t_customers_id
"
, referencedColumnName
=
"
id
"
),inverseJoinColumns
=
@JoinColumn(name
=
"
orders_id
"
,referencedColumnName
=
"
id
"
))
public
Collection
<
Order
>
getOrders() {
return
orders; }
其中name属性指定图2所示的连接表的名称。joinColumns指定了t_customers表和t_customers_orders表中相连接的字段。inverseJoinColumns指定了t_orders表和t_customers_orders表中相连接的字段。这时t_orders表中的customer_id字段就不再需要了。因此，需要将Order表中的customer属性（getter和setter方法）去掉。
在持久化Customer和Order对象时，需要对Order对象进行持久化后才能对Customer对象进行持久化。
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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