JPA多对多关联

 1 package com.yichun.bean;
 2 
 3 import java.util.HashSet;
 4 import java.util.Set;
 5 
 6 import javax.persistence.CascadeType;
 7 import javax.persistence.Column;
 8 import javax.persistence.Entity;
 9 import javax.persistence.GeneratedValue;
10 import javax.persistence.Id;
11 import javax.persistence.JoinColumn;
12 import javax.persistence.JoinTable;
13 import javax.persistence.ManyToMany;
14 
15 //关系的维护端
16 @Entity
17 public class Student {
18     private Integer id;
19     private String name;
20     private Set<Teacher> teachers = new HashSet<Teacher>();
21 
22     public Student() {
23     }
24 
25     public Student(String name) {
26         this.name = name;
27     }
28 
29     @Id
30     @GeneratedValue
31     public Integer getId() {
32         return id;
33     }
34 
35     public void setId(Integer id) {
36         this.id = id;
37     }
38 
39     @Column(length = 10, nullable = false)
40     public String getName() {
41         return name;
42     }
43 
44     public void setName(String name) {
45         this.name = name;
46     }
47 
48     @ManyToMany(cascade = CascadeType.REFRESH)
49     // 关联表,inverseJoinColumns被维护端，
50     @JoinTable(name = "student_teacher", inverseJoinColumns = @JoinColumn(name = "teacher_id"), joinColumns = @JoinColumn(name = "student_id"))
51     public Set<Teacher> getTeachers() {
52         return teachers;
53     }
54 
55     public void setTeachers(Set<Teacher> teachers) {
56         this.teachers = teachers;
57     }
58 
59     // 添加teacher与student关系
60     public void addTeacher(Teacher teacher) {
61         this.teachers.add(teacher);
62     }
63 
64     // 删除teacher与student关联
65     public void removeTeacher(Teacher teacher) {
66         if (teachers.contains(teacher))
67             this.teachers.remove(teacher);
68     }
69 
70 }

 1 package com.yichun.bean;
 2 
 3 import java.util.HashSet;
 4 import java.util.Set;
 5 
 6 import javax.persistence.CascadeType;
 7 import javax.persistence.Column;
 8 import javax.persistence.Entity;
 9 import javax.persistence.GeneratedValue;
10 import javax.persistence.Id;
11 import javax.persistence.ManyToMany;
12 
13 //关系被维护端，中间表的记录，不能由teacher删除，
14 @Entity
15 public class Teacher {
16     private Integer id;
17     private String name;
18     private Set<Student> students = new HashSet<Student>();
19 
20     public Teacher() {
21     }
22 
23     public Teacher(String name) {
24         this.name = name;
25     }
26 
27     @Id
28     @GeneratedValue
29     public Integer getId() {
30         return id;
31     }
32 
33     public void setId(Integer id) {
34         this.id = id;
35     }
36 
37     @Column(length = 10, nullable = false)
38     public String getName() {
39         return name;
40     }
41 
42     public void setName(String name) {
43         this.name = name;
44     }
45 
46     // 多对多基本不用级联删除
47     @ManyToMany(cascade = CascadeType.REFRESH, mappedBy = "teachers")
48     public Set<Student> getStudents() {
49         return students;
50     }
51 
52     public void setStudents(Set<Student> students) {
53         this.students = students;
54     }
55 
56     // id不为null返回id的hashCode，若没有id的时候，hashCode为0
57     @Override
58     public int hashCode() {
59         final int prime = 31;
60         int result = 1;
61         result = prime * result + ((id == null) ? 0 : id.hashCode());
62         return result;
63     }
64 
65     @Override
66     public boolean equals(Object obj) {
67         if (this == obj)
68             return true;
69         if (obj == null)
70             return false;
71         if (getClass() != obj.getClass())
72             return false;
73         Teacher other = (Teacher) obj;
74         if (id == null) {
75             if (other.id != null)
76                 return false;
77         } else if (!id.equals(other.id))
78             return false;
79         return true;
80     }
81 
82 }

 1     /**
 2      *<p>
 3      * 删除老师
 4      * </p>
 5      * 解除学生和老师的关系
 6      */
 7     @Test
 8     public void deleteTeacher() {
 9         EntityManagerFactory factory = Persistence
10                 .createEntityManagerFactory("yichun");
11         EntityManager manager = factory.createEntityManager();
12         manager.getTransaction().begin();
13 
14         // 解除关系
15         Student student = manager.find(Student.class, 1);
16         Teacher teacher = manager.getReference(Teacher.class, 1);
17         student.removeTeacher(teacher);
18 
19         // 删除教师
20         manager.remove(teacher);
21 
22         manager.getTransaction().commit();
23         manager.close();
24         factory.close();
25     }

 1     /**
 2      *<p>
 3      * 删除学生
 4      * </p>
 5      * 直接可以删除，hibernate会自动解除学生和老师的关系，之后删除学生
 6      */
 7     @Test
 8     public void deleteStudent() {
 9         EntityManagerFactory factory = Persistence
10                 .createEntityManagerFactory("yichun");
11         EntityManager manager = factory.createEntityManager();
12         manager.getTransaction().begin();
13 
14         // 删除学生
15         Student student = manager.find(Student.class, 1);
16         manager.remove(student);
17 
18         manager.getTransaction().commit();
19         manager.close();
20         factory.close();
21     }