花非花

在前面已经读到过嵌入对象这一概念(@Embeddable),而复合主键只不过是嵌入对象的特例而已,简单例子:

Customer类的主键有id和name两个属性决定,我们写一个自定义的复合主键类CustomerPK:

@Embeddable
public class CustomerPK implements java.io.Serializable
{
   private long id;
   private String name;


   public CustomerPK()
   {
   }

   public CustomerPK(long id, String name)
   {
      this.id = id;
      this.name = name;
   }

   public long getId()
   {
      return id;
   }

   public void setId(long id)
   {
      this.id = id;
   }

   public String getName()
   {
      return name;
   }

   public void setName(String name)
   {
      this.name = name;
   }

   public int hashCode()
   {
      return (int) id + name.hashCode();
   }

   public boolean equals(Object obj)
   {
      if (obj == this) return true;
      if (!(obj instanceof CustomerPK)) return false;
      if (obj == null) return false;
      CustomerPK pk = (CustomerPK) obj;
      return pk.id == id && pk.name.equals(name);
   }
}
复合主键需要实现equals和hashCode方法，通过这两个方法来实现主键约束条件,而使用就更简单了:

@Entity
public class Customer implements java.io.Serializable
{
   CustomerPK pk;
    public Customer()
   {
   }

   @EmbeddedId
   public CustomerPK getPk()
   {
      return pk;
   }

   public void setPk(CustomerPK pk)
   {
      this.pk = pk;
   }
.........

现在的问题是,复合主键如何做关系映射?以many-to-many为例:

   @ManyToMany(cascade = {CascadeType.PERSIST, CascadeType.MERGE}, fetch = FetchType.EAGER, mappedBy="customers")
   @JoinTable(name="flight_customer_table",
              joinColumns = {@JoinColumn(name = "FLIGHT_ID")},
              inverseJoinColumns = {@JoinColumn(name = "CUSTOMER_ID"), @JoinColumn(name = "CUSTOMER_NAME")})
   public Set<Flight> getFlights()
   {
      return flights;
   }
Customer与Flight多对多关系,上面的代码是在Customer中配置,在Flight配置如下:

 @ManyToMany(cascade = {CascadeType.PERSIST, CascadeType.MERGE}, fetch = FetchType.EAGER, mappedBy="customers")
   @JoinTable(name="flight_customer_table",
              joinColumns={@JoinColumn(name = "FLIGHT_ID")},
              inverseJoinColumns={@JoinColumn(name = "CUSTOMER_ID"), @JoinColumn(name = "CUSTOMER_NAME")})
   public Set<Flight> getFlights()
   {
      return flights;
   }
设置@JoinTable中的inverseJoinColumns为复合主键即可