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上一篇： 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。
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：
可以使用下面的代码进行测试：
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。
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上一篇： 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：
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;
@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：
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;
@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;
}
... ...
}
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 String productId;
private int count;
private Customer customer;
@ManyToOne
@JoinColumn(name = " customer_id " )
public Customer getCustomer()
{
return customer;
}
public void setCustomer(Customer customer)
{
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。
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 String productId;
private int count;
private Customer customer;
@ManyToOne
@JoinColumn(name = " customer_id " )
public Customer getCustomer()
{
return customer;
}
public void setCustomer(Customer customer)
{
this .customer = customer;
}
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
public int getId()
{
return id;
}
... ...
}
可以使用下面的代码进行测试：
Customer customer
=
new
Customer();
customer.setName( " 微软 " );
List < Order > orders = new ArrayList < Order > ();
Order order = 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);
customer.setName( " 微软 " );
List < Order > orders = new ArrayList < Order > ();
Order order = 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：
@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;
}
@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
国内最棒的Google Android技术社区（eoeandroid），欢迎访问！
《银河系列原创教程》发布
《Java Web开发速学宝典》出版，欢迎定购
