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：

 1 package com.yichun.bean;

 2 

 3 import java.io.Serializable;

 4 

 5 import javax.persistence.Column;

 6 import javax.persistence.Embeddable;

 7 

 8 /**

 9  * 联合主键。一般使用PK 只需要定义用作主键的字段

10  * <p>

11  * 联合主键类必须遵守的JPA规范：<br>

12  * 1、必须要提供一个public的无参数的构造方法<br>

13  * 2、必须要实现序列化接口<br>

14  * 3、必须重写hashCode（）与equals（）方法

15  */

16 // 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）

17 @Embeddable

18 public class AirLinePK implements Serializable {

19     private String startCity;// PEK，北京 CAN广州，SHA上海

20     private String endCity;

21 

22     public AirLinePK() {

23     }

24 

25     public AirLinePK(String startCity, String endCity) {

26         this.startCity = startCity;

27         this.endCity = endCity;

28     }

29 

30     @Column(length = 3, nullable = false)

31     public String getStartCity() {

32         return startCity;

33     }

34 

35     public void setStartCity(String startCity) {

36         this.startCity = startCity;

37     }

38 

39     @Column(length = 3, nullable = false)

40     public String getEndCity() {

41         return endCity;

42     }

43 

44     public void setEndCity(String endCity) {

45         this.endCity = endCity;

46     }

47 

48     // 以下两个方法判断是否相等

49     @Override

50     public int hashCode() {

51         final int prime = 31;

52         int result = 1;

53         result = prime * result + ((endCity == null) ? 0 : endCity.hashCode());

54         result = prime * result

55                 + ((startCity == null) ? 0 : startCity.hashCode());

56         return result;

57     }

58 

59     @Override

60     public boolean equals(Object obj) {

61         if (this == obj)

62             return true;

63         if (obj == null)

64             return false;

65         if (getClass() != obj.getClass())

66             return false;

67         AirLinePK other = (AirLinePK) obj;

68         if (endCity == null) {

69             if (other.endCity != null)

70                 return false;

71         } else if (!endCity.equals(other.endCity))

72             return false;

73         if (startCity == null) {

74             if (other.startCity != null)

75                 return false;

76         } else if (!startCity.equals(other.startCity))

77             return false;

78         return true;

79     }

80 

81 }

　　AirLine 代码如下：

 1 package com.yichun.bean;

 2 

 3 import javax.persistence.Column;

 4 import javax.persistence.EmbeddedId;

 5 import javax.persistence.Entity;

 6 

 7 

 8 @Entity

 9 public class AirLine {

10     private AirLinePK id;

11     private String name;

12 

13     public AirLine() {

14     }

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     // 用于标识该属性为实体的标识符，专门用于复合主键类

26     @EmbeddedId

27     public AirLinePK getId() {

28         return id;

29     }

30 

31     public void setId(AirLinePK id) {

32         this.id = id;

33     }

34 

35     @Column(length = 20)

36     public String getName() {

37         return name;

38     }

39 

40     public void setName(String name) {

41         this.name = name;

42     }

43 

44 }

保存数据如下：

 1     @Test

 2     public void save() {

 3         EntityManagerFactory factory = Persistence

 4                 .createEntityManagerFactory("yichun");

 5         EntityManager manager = factory.createEntityManager();

 6         manager.getTransaction().begin();

 7 

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

 9 

10         manager.getTransaction().commit();

11         manager.close();

12         factory.close();

13     }

查找数据如下：

 1 @Test

 2     public void find() {

 3         EntityManagerFactory factory = Persistence

 4                 .createEntityManagerFactory("yichun");

 5         EntityManager manager = factory.createEntityManager();

 6 

 7         AirLine airLine = manager.find(AirLine.class, new AirLinePK("PEK",

 8                 "SHA"));

 9         System.out.println(airLine.getName() + " : "

10                 + airLine.getId().getStartCity() + " --> "

11                 + airLine.getId().getEndCity());

12 

13         manager.close();

14         factory.close();

15     }