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();
     }