Eclipse+JBoss+EJB3實現Entity Bean的一對多映射

EJB3的一對多映射使用@OneToMany來設置，如果是雙向的一對多映射，在many方需要使用@ManyToOne設置。在本書中給出兩個表，其他一個表t_customers在上一篇文章中已給出了，另一個表的結構如圖1所示。

t_customers和t_orders表是一對多關系，一個Customer可能有多個Order，而一個Order只能有一個Customer。

在Customer類中需要定義一個集合類型的屬性，用來保存多個Order對象，Customer類的代碼如下：

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 orders;  
 
    @OneToMany(mappedBy = "customer", cascade = CascadeType.ALL)  
    public Collection getOrders()  
    {  
        return orders;  
    }  
    public void setOrders(Collection orders)  
    {  
        this.orders = orders;  
    }  
    @Id 
    @GeneratedValue(strategy = GenerationType.IDENTITY)  
    public int getId()  
    {  
        return id;  
    }  
    ... ...  
}  

其中@OneToMany的mappedBy屬性指定了Order類中獲得Customer對象的屬性名。Order類的代碼如下：

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注釋的name屬性指定t_orders表中用於連接t_customers表的外鍵名。

可以使用下面的代碼進行測試：

Customer customer = new Customer();  
customer.setName("微軟");  
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，還可以使用@JoinTable指定連接表來映射one-to-many關系。連接表的結構如圖2所示。

首選需要將Customer類的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 getOrders()  
{  
return orders;  
} 

其中name屬性指定圖2所示的連接表的名稱。joinColumns指定了t_customers表和t_customers_orders表中相連接的字段。inverseJoinColumns指定了t_orders表和t_customers_orders表中相連接的字段。這時t_orders表中的customer_id字段就不再需要了。因此，需要將Order表中的customer屬性（getter和setter方法）去掉。

在持久化Customer和Order對象時，需要對Order對象進行持久化後才能對Customer對象進行持久化。