JPA--多对多关系

JPA中，多对多关系用@ManyToMany标示。

关系维护端：

 1 package com.yl.demo1.bean.manyTomany;

 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 @Entity

16 public class Student {

17     private Integer id;

18     private String name;

19     private Set<Teacher> teachers = new HashSet<Teacher>();

20     

21     

22     public Student(){}

23     

24     public Student(String name) {

25         super();

26         this.name = name;

27     }

28     @Id @GeneratedValue

29     public Integer getId() {

30         return id;

31     }

32     public void setId(Integer id) {

33         this.id = id;

34     }

35     @Column(length=10, nullable=false)

36     public String getName() {

37         return name;

38     }

39     public void setName(String name) {

40         this.name = name;

41     }

42     /**

43      * Student是关系的维护端

44      * inverseJoinColumns和joinColumns是定义关联表中的字段，其中inverseJoinColumns定义关系被维护端的字段

45      * joinColumns定义关系维护端的字段

46      */

47     @ManyToMany(cascade=CascadeType.REFRESH)

48     @JoinTable(name="student_teacher", inverseJoinColumns=@JoinColumn(name="teacher_id"),

49     joinColumns=@JoinColumn(name="student_id"))

50     public Set<Teacher> getTeachers() {

51         return teachers;

52     }

53     public void setTeachers(Set<Teacher> teachers) {

54         this.teachers = teachers;

55     }

56     

57     public void addTeacher(Teacher teacher) {

58         this.teachers.add(teacher);

59     }

60     

61     public void removeTeacher(Teacher teacher) {

62         if (this.teachers.contains(teacher)) {

63             this.teachers.remove(teacher);

64         }

65     }

66 }

 

关系被维护端：

 1 package com.yl.demo1.bean.manyTomany;

 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.FetchType;

10 import javax.persistence.GeneratedValue;

11 import javax.persistence.Id;

12 import javax.persistence.ManyToMany;

13 

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     public Teacher(String name) {

23         super();

24         this.name = name;

25     }

26     @Id @GeneratedValue

27     public Integer getId() {

28         return id;

29     }

30     public void setId(Integer id) {

31         this.id = id;

32     }

33     @Column(length=10, nullable=false)

34     public String getName() {

35         return name;

36     }

37     public void setName(String name) {

38         this.name = name;

39     }

40     //teacher为关系的被维护端

41     @ManyToMany(cascade=CascadeType.REFRESH, mappedBy="teachers", fetch=FetchType.LAZY)

42     public Set<Student> getStudents() {

43         return students;

44     }

45     public void setStudents(Set<Student> students) {

46         this.students = students;

47     }

48 

49     @Override

50     public int hashCode() {

51         final int prime = 31;

52         int result = 1;

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

54         return result;

55     }

56 

57     @Override

58     public boolean equals(Object obj) {

59         if (this == obj)

60             return true;

61         if (obj == null)

62             return false;

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

64             return false;

65         Teacher other = (Teacher) obj;

66         if (id == null) {

67             if (other.id != null)

68                 return false;

69         } else if (!id.equals(other.id))

70             return false;

71         return true;

72     }

73     

74     

75     

76     

77 }

常见操作：

 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         Student student = new Student("yyyy");

 8         em.persist(student);

 9         

10         Teacher teacher = new Teacher("jiawenhui");

11         em.persist(teacher);

12         

13         em.getTransaction().commit();

14         em.close();

15         factory.close();

16     }

17 

18     /**

19      * 建立学生与老师之间的关系

20      */

21     @Test

22     public void buildConnection() {

23         EntityManagerFactory factory = Persistence.createEntityManagerFactory("YL");

24         EntityManager em = factory.createEntityManager();

25         em.getTransaction().begin();//事务开始

26         

27         Student student = em.find(Student.class, 1);

28         

29         Teacher teacher = em.getReference(Teacher.class, 1);

30         

31         student.addTeacher(teacher);

32         

33         em.getTransaction().commit();

34         em.close();

35         factory.close();

36     }

37 

38     /**

39      * 解除学生与老师之间的关系

40      */

41     @Test

42     public void deleteConnection() {

43         EntityManagerFactory factory = Persistence.createEntityManagerFactory("YL");

44         EntityManager em = factory.createEntityManager();

45         em.getTransaction().begin();//事务开始

46         

47         Student student = em.find(Student.class, 1);

48         

49         Teacher teacher = em.getReference(Teacher.class, 1);

50         

51         student.removeTeacher(teacher);

52         

53         em.getTransaction().commit();

54         em.close();

55         factory.close();

56     }

57     

58     /**

59      * 删除老师(删除关系被维护端)

60      */

61     @Test

62     public void deleteTeacher() {

63         EntityManagerFactory factory = Persistence.createEntityManagerFactory("YL");

64         EntityManager em = factory.createEntityManager();

65         em.getTransaction().begin();//事务开始

66         

67         Student student = em.find(Student.class, 1);

68         Teacher teacher = em.getReference(Teacher.class, 1);

69         //先解除Student和Teacher之间的关系

70         student.removeTeacher(teacher);

71         //在删除Teacher

72         em.remove(teacher);

73         

74         em.getTransaction().commit();

75         em.close();

76         factory.close();

77     }

78     

79     /**

80      * 删除学生(删除关系维护端)

81      */

82     @Test

83     public void deleteStudent() {

84         EntityManagerFactory factory = Persistence.createEntityManagerFactory("YL");

85         EntityManager em = factory.createEntityManager();

86         em.getTransaction().begin();//事务开始

87         

88         Student student = em.getReference(Student.class, 1);

89         em.remove(student);

90         

91         em.getTransaction().commit();

92         em.close();

93         factory.close();

94     }

95