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：

 1 package com.yl.demo1.bean.JointPK;
 2
 3 import java.io.Serializable;
 4
 5 import javax.persistence.Column;
 6 import javax.persistence.Embeddable;
 7
 8 /**
 9  * 在联合主键类中只是定义确定联合主键的字段即可
10  * 联合主键类的规则
11  *     1.必须包含一个无参的构造函数
12  *     2.必须实现序列化接口
13  *  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
14  * @author yul
15  *
16  *@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
17  */
18
19 @Embeddable //嵌入
20 public class AirLinePK implements Serializable {
21     private String startCity;//航空中每个城市都有一个三位的字母标识符
22     private String endCity;
23
24     public AirLinePK(){}
25
26
27     public AirLinePK(String startCity, String endCity) {
28         this.startCity = startCity;
29         this.endCity = endCity;
30     }
31     @Column(length=3)
32     public String getStartCity() {
33         return startCity;
34     }
35     public void setStartCity(String startCity) {
36         this.startCity = startCity;
37     }
38     @Column(length=3)
39     public String getEndCity() {
40         return endCity;
41     }
42     public void setEndCity(String endCity) {
43         this.endCity = endCity;
44     }
45
46     @Override
47     public int hashCode() {
48         final int prime = 31;
49         int result = 1;
50         result = prime * result + ((endCity == null) ? 0 : endCity.hashCode());
51         result = prime * result
52                 + ((startCity == null) ? 0 : startCity.hashCode());
53         return result;
54     }
55     @Override
56     public boolean equals(Object obj) {
57         if (this == obj)
58             return true;
59         if (obj == null)
60             return false;
61         if (getClass() != obj.getClass())
62             return false;
63         AirLinePK other = (AirLinePK) obj;
64         if (endCity == null) {
65             if (other.endCity != null)
66                 return false;
67         } else if (!endCity.equals(other.endCity))
68             return false;
69         if (startCity == null) {
70             if (other.startCity != null)
71                 return false;
72         } else if (!startCity.equals(other.startCity))
73             return false;
74         return true;
75     }
76
77
78 }

 1 package com.yl.demo1.bean.JointPK;
 2
 3 import javax.persistence.Column;
 4 import javax.persistence.EmbeddedId;
 5 import javax.persistence.Entity;
 6
 7 @Entity
 8 public class AirLine {
 9     private AirLinePK id;
10     private String name;
11
12
13
14     public AirLine(){}
15
16     public AirLine(AirLinePK id) {
17         this.id = id;
18     }
19
20     public AirLine(String startCity, String endCity, String name) {
21         this.id = new AirLinePK(startCity, endCity);
22         this.name = name;
23     }
24
25     @EmbeddedId
26     public AirLinePK getId() {
27         return id;
28     }
29     public void setId(AirLinePK id) {
30         this.id = id;
31     }
32     @Column(length=20)
33     public String getName() {
34         return name;
35     }
36     public void setName(String name) {
37         this.name = name;
38     }
39
40 }

常用操作:

 1 @Test
 2     public void save() {
 3         EntityManagerFactory factory = Persistence.createEntityManagerFactory("YL");
 4         EntityManager em = factory.createEntityManager();
 5         em.getTransaction().begin();//事务开始
 6
 7         em.persist(new AirLine("PEK", "SHA", "北京飞上海"));
 8
 9         em.getTransaction().commit();
10         em.close();
11         factory.close();
12     }