JPA--联合主键

联合主键的一些知识：

使用@EmbeddedId标示联合主键；

在联合主键类中只是定义确定联合主键的字段即可；
* 联合主键类的规则
* 1.必须包含一个无参的构造函数
* 2.必须实现序列化接口
* 3.必须重写hashCode和equals方法，而且equals方法的参数必须包括确定联合主键的所有字段

联合主键类的定义：

 package com.yl.demo1.bean.JointPK;

 import java.io.Serializable;

 import javax.persistence.Column;
 import javax.persistence.Embeddable;

 /**
  * 在联合主键类中只是定义确定联合主键的字段即可
  * 联合主键类的规则
  *     1.必须包含一个无参的构造函数
  *     2.必须实现序列化接口
  *  3.必须重写hashCode和equals方法，而且equals方法的参数必须包括确定联合主键的所有字段
  * @author yul
  *
  *@Embeddable--告诉其它类，在使用AieLinePK时只是使用AieLinePK的属性作为实体的持久化属性
  */

 @Embeddable //嵌入
 public class AirLinePK implements Serializable {
     private String startCity;//航空中每个城市都有一个三位的字母标识符
     private String endCity;

     public AirLinePK(){}

     public AirLinePK(String startCity, String endCity) {
         this.startCity = startCity;
         this.endCity = endCity;
     }
     @Column(length=3)
     public String getStartCity() {
         return startCity;
     }
     public void setStartCity(String startCity) {
         this.startCity = startCity;
     }
     @Column(length=3)
     public String getEndCity() {
         return endCity;
     }
     public void setEndCity(String endCity) {
         this.endCity = endCity;
     }

     @Override
     public int hashCode() {
         final int prime = 31;
         int result = 1;
         result = prime * result + ((endCity == null) ? 0 : endCity.hashCode());
         result = prime * result
                 + ((startCity == null) ? 0 : startCity.hashCode());
         return result;
     }
     @Override
     public boolean equals(Object obj) {
         if (this == obj)
             return true;
         if (obj == null)
             return false;
         if (getClass() != obj.getClass())
             return false;
         AirLinePK other = (AirLinePK) obj;
         if (endCity == null) {
             if (other.endCity != null)
                 return false;
         } else if (!endCity.equals(other.endCity))
             return false;
         if (startCity == null) {
             if (other.startCity != null)
                 return false;
         } else if (!startCity.equals(other.startCity))
             return false;
         return true;
     }

 }

 package com.yl.demo1.bean.JointPK;

 import javax.persistence.Column;
 import javax.persistence.EmbeddedId;
 import javax.persistence.Entity;

 @Entity
 public class AirLine {
     private AirLinePK id;
     private String name;

     public AirLine(){}

     public AirLine(AirLinePK id) {
         this.id = id;
     }

     public AirLine(String startCity, String endCity, String name) {
         this.id = new AirLinePK(startCity, endCity);
         this.name = name;
     }

     @EmbeddedId
     public AirLinePK getId() {
         return id;
     }
     public void setId(AirLinePK id) {
         this.id = id;
     }
     @Column(length=20)
     public String getName() {
         return name;
     }
     public void setName(String name) {
         this.name = name;
     }

 }

常用操作:

 @Test
     public void save() {
         EntityManagerFactory factory = Persistence.createEntityManagerFactory("YL");
         EntityManager em = factory.createEntityManager();
         em.getTransaction().begin();//事务开始

         em.persist(new AirLine("PEK", "SHA", "北京飞上海"));

         em.getTransaction().commit();
         em.close();
         factory.close();
     }