JPA联合主键

　　联合主键也就是说需要多个字段才能确定数据库记录中的唯一一行。这样就需要多个字段一起，组成主键，也叫联合主键。例如飞机航线，我们需要知道飞机起飞的地点以及飞机降落的地点。所以需要飞机起飞的地点和降落的地方才能确定一条航线。AirLine表示航线，AirLinePK表示主键类。AirLinePK代码如下：

 package com.yichun.bean;

 import java.io.Serializable;

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

 /**
  * 联合主键。一般使用PK 只需要定义用作主键的字段
  * <p>
  * 联合主键类必须遵守的JPA规范：<br>
  * 1、必须要提供一个public的无参数的构造方法<br>
  * 2、必须要实现序列化接口<br>
  * 3、必须重写hashCode（）与equals（）方法
  */
 // 用在实体里面,只是使用该类中的属性。（该类中的属性用在持久化的类中的字段）
 @Embeddable
 public class AirLinePK implements Serializable {
     private String startCity;// PEK，北京 CAN广州，SHA上海
     private String endCity;

     public AirLinePK() {
     }

     public AirLinePK(String startCity, String endCity) {
         this.startCity = startCity;
         this.endCity = endCity;
     }

     @Column(length = 3, nullable = false)
     public String getStartCity() {
         return startCity;
     }

     public void setStartCity(String startCity) {
         this.startCity = startCity;
     }

     @Column(length = 3, nullable = false)
     public String getEndCity() {
         return endCity;
     }

     public void setEndCity(String endCity) {
         this.endCity = endCity;
     }

     // 以下两个方法判断是否相等
     @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;
     }

 }

　　AirLine 代码如下：

 package com.yichun.bean;

 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;
     }

     // 用于标识该属性为实体的标识符，专门用于复合主键类
     @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;
     }

 }

保存数据如下：

     @Test
     public void save() {
         EntityManagerFactory factory = Persistence
                 .createEntityManagerFactory("yichun");
         EntityManager manager = factory.createEntityManager();
         manager.getTransaction().begin();

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

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

查找数据如下：

 @Test
     public void find() {
         EntityManagerFactory factory = Persistence
                 .createEntityManagerFactory("yichun");
         EntityManager manager = factory.createEntityManager();

         AirLine airLine = manager.find(AirLine.class, new AirLinePK("PEK",
                 "SHA"));
         System.out.println(airLine.getName() + " : "
                 + airLine.getId().getStartCity() + " --> "
                 + airLine.getId().getEndCity());

         manager.close();
         factory.close();
     }