JPA多对多关联

　　关于JPA多对多关系，这是使用学生与教师来表示。一个Student由多个Teacher教，同样一个Teacher也可以教多个学生。Student类如下：

 package com.yichun.bean;

 import java.util.HashSet;

 import java.util.Set;

 import javax.persistence.CascadeType;

 import javax.persistence.Column;

 import javax.persistence.Entity;

 import javax.persistence.GeneratedValue;

 import javax.persistence.Id;

 import javax.persistence.JoinColumn;

 import javax.persistence.JoinTable;

 import javax.persistence.ManyToMany;

 //关系的维护端

 @Entity

 public class Student {

     private Integer id;

     private String name;

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

     public Student() {

     }

     public Student(String name) {

         this.name = name;

     }

     @Id

     @GeneratedValue

     public Integer getId() {

         return id;

     }

     public void setId(Integer id) {

         this.id = id;

     }

     @Column(length = 10, nullable = false)

     public String getName() {

         return name;

     }

     public void setName(String name) {

         this.name = name;

     }

     @ManyToMany(cascade = CascadeType.REFRESH)

     // 关联表,inverseJoinColumns被维护端，

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

     public Set<Teacher> getTeachers() {

         return teachers;

     }

     public void setTeachers(Set<Teacher> teachers) {

         this.teachers = teachers;

     }

     // 添加teacher与student关系

     public void addTeacher(Teacher teacher) {

         this.teachers.add(teacher);

     }

     // 删除teacher与student关联

     public void removeTeacher(Teacher teacher) {

         if (teachers.contains(teacher))

             this.teachers.remove(teacher);

     }

 }

Teacher类如下：

 package com.yichun.bean;

 import java.util.HashSet;

 import java.util.Set;

 import javax.persistence.CascadeType;

 import javax.persistence.Column;

 import javax.persistence.Entity;

 import javax.persistence.GeneratedValue;

 import javax.persistence.Id;

 import javax.persistence.ManyToMany;

 //关系被维护端，中间表的记录，不能由teacher删除，

 @Entity

 public class Teacher {

     private Integer id;

     private String name;

     private Set<Student> students = new HashSet<Student>();

     public Teacher() {

     }

     public Teacher(String name) {

         this.name = name;

     }

     @Id

     @GeneratedValue

     public Integer getId() {

         return id;

     }

     public void setId(Integer id) {

         this.id = id;

     }

     @Column(length = 10, nullable = false)

     public String getName() {

         return name;

     }

     public void setName(String name) {

         this.name = name;

     }

     // 多对多基本不用级联删除

     @ManyToMany(cascade = CascadeType.REFRESH, mappedBy = "teachers")

     public Set<Student> getStudents() {

         return students;

     }

     public void setStudents(Set<Student> students) {

         this.students = students;

     }

     // id不为null返回id的hashCode，若没有id的时候，hashCode为0

     @Override

     public int hashCode() {

         final int prime = 31;

         int result = 1;

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

         return result;

     }

     @Override

     public boolean equals(Object obj) {

         if (this == obj)

             return true;

         if (obj == null)

             return false;

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

             return false;

         Teacher other = (Teacher) obj;

         if (id == null) {

             if (other.id != null)

                 return false;

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

             return false;

         return true;

     }

 }

　　这里Student是作为关系维护端，而Teacher是作为关系被维护端。所以当删除教师的时候，必须解除学生和教师的关联关系。代码如下：

     /**

      *<p>

      * 删除老师

      * </p>

      * 解除学生和老师的关系

      */

     @Test

     public void deleteTeacher() {

         EntityManagerFactory factory = Persistence

                 .createEntityManagerFactory("yichun");

         EntityManager manager = factory.createEntityManager();

         manager.getTransaction().begin();

         // 解除关系

         Student student = manager.find(Student.class, 1);

         Teacher teacher = manager.getReference(Teacher.class, 1);

         student.removeTeacher(teacher);

         // 删除教师

         manager.remove(teacher);

         manager.getTransaction().commit();

         manager.close();

         factory.close();

     }

　　而删除学生的时候，可以直接删除，jpa会自动解除学生与教师的关联关系，然后删除学生。

     /**

      *<p>

      * 删除学生

      * </p>

      * 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

      */

     @Test

     public void deleteStudent() {

         EntityManagerFactory factory = Persistence

                 .createEntityManagerFactory("yichun");

         EntityManager manager = factory.createEntityManager();

         manager.getTransaction().begin();

         // 删除学生

         Student student = manager.find(Student.class, 1);

         manager.remove(student);

         manager.getTransaction().commit();

         manager.close();

         factory.close();

     }