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.
时间： 2024-10-17 01:53:53