hibernate中关系映射

　在数据库中数据的对应关系有一对一,一对多和多对一.那么在hibernate中应该如何对应呢?如下:

　一对一:

　假设两个实体,一个实体类为husband,另外一个实体类为wife.一个丈夫只能对应一个妻子,反之亦然.那么在hibernate中使用annotation如下:(husband类)

 1 import javax.persistence.Entity;

 2 import javax.persistence.GeneratedValue;

 3 import javax.persistence.Id;

 4 import javax.persistence.JoinColumn;

 5 import javax.persistence.OneToOne;

 6 

 7 @Entity

 8 public class Husband {

 9     private int id;

10     private String name;

11     private Wife wife;

12     

13     @Id

14     @GeneratedValue

15     public int getId() {

16         return id;

17     }

18     public void setId(int id) {

19         this.id = id;

20     }

21     public String getName() {

22         return name;

23     }

24     public void setName(String name) {

25         this.name = name;

26     }

27     

28     @OneToOne　　　　//表明是一对一

29     @JoinColumn(name="wifeId")    //使用JoinColumn可以更改数据库中对应字段的名字

30     public Wife getWife() {

31         return wife;

32     }

33     public void setWife(Wife wife) {

34         this.wife = wife;

35     }

36 }

　wife类:

 1 import javax.persistence.Entity;

 2 import javax.persistence.GeneratedValue;

 3 import javax.persistence.Id;

 4 

 5 @Entity

 6 public class Wife {

 7     private int id;

 8     private String name;

 9     

10     @Id

11     @GeneratedValue

12     public int getId() {

13         return id;

14     }

15     public void setId(int id) {

16         this.id = id;

17     }

18     public String getName() {

19         return name;

20     }

21     public void setName(String name) {

22         this.name = name;

23     }

24 }

　一对多:

　设想一个小组有多个成员,而每一个成员只能加入一个小组.在设计数据的时候,按照常理,人们会把小组的Id设到每个成员中,作为外键;下面分两种情况:

　1.将小组Id设到成员中:

 1 import javax.persistence.Entity;

 2 import javax.persistence.GeneratedValue;

 3 import javax.persistence.Id;

 4 import javax.persistence.Table;

 5 

 6 @Entity

 7 @Table(name="t_Group")　　//这里注意:因为group是关键字,所以需要将表名更改

 8 public class Group {

 9     private int id;

10     private String name;

11     

12     public String getName() {

13         return name;

14     }

15     public void setName(String name) {

16         this.name = name;

17     }

18     @Id

19     @GeneratedValue

20     public int getId() {

21         return id;

22     }

23     public void setId(int id) {

24         this.id = id;

25     }

26 }

　Person类:

 1 import javax.persistence.Entity;

 2 import javax.persistence.GeneratedValue;

 3 import javax.persistence.Id;

 4 import javax.persistence.ManyToOne;

 5 

 6 @Entity

 7 public class Person {

 8     private Group group;

 9     private int id;

10     private String name;

11     

12     @ManyToOne　　　　　　　　//表明多个Person对应一个Group

13     public Group getGroup() {

14         return group;

15     }

16     

17     @Id

18     @GeneratedValue

19     public int getId() {

20         return id;

21     }

22     public String getName() {

23         return name;

24     }

25     public void setGroup(Group group) {

26         this.group = group;

27     }

28     public void setId(int id) {

29         this.id = id;

30     }

31     public void setName(String name) {

32         this.name = name;

33     }

34 }

　2.将person的Id作为外键设到group中.方法类似于上面.只需将person属性添加到group中并在get方法上添加注解@OneToMany和@JoinColumn(name="groupId"),另外:因为把person添加到group中去了,那么在group实体类中person的返回类型应该是一个数组,Map,Set或者List,个人觉得Set比较好,因为set中元素没有重复的,符合数据库中的规律.所以你的代码应该写成类似这样:

 1 private Set<Person> person = new HashSet<Person>();

 2     

 3     @OneToMany

 4     @JoinColumn(name="personId")

 5     public Set<Person> getPerson() {

 6         return person;

 7     }

 8     public void setPerson(Set<Person> person) {

 9         this.person = person;

10     }

　3.多对多就很简单了,分解为多对一即可.这里就不一一赘述了.