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();
    }