JPA学习---第十一节：JPA中的多对多双向关联实体定义与注解设置及操作

1、定义实体类，代码如下：

(1)、学生实体类：

package learn.jpa.entity;

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.JoinTable;
import javax.persistence.ManyToMany;
import javax.persistence.JoinColumn;

/**
 * @doc 多对多关系是对等的，认为决定谁是维护段，谁是被维护段
 *       通过中间表(中间表)来关联它们之间的关系
 */
@Entity
public class Student {

    private int id;
    private String name;
    private Set<Teacher> teacher = new HashSet<Teacher>();
    
    public Student() {
        super();
    }

    public Student(String name) {
        this.name = name;
    }
    
    @Id
    @GeneratedValue
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    @Column(length=10,nullable=false)
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    /**
     * inverseJoinColumns 与被维护端关联外键
     * joinColumns 与维护段关联外键
     */
    @ManyToMany(cascade=CascadeType.REFRESH)
    @JoinTable(name="student_teacher",inverseJoinColumns=@JoinColumn(name="teacher_id"),joinColumns=@JoinColumn(name="student_id"))
    public Set<Teacher> getTeacher() {
        return teacher;
    }
    public void setTeacher(Set<Teacher> teacher) {
        this.teacher = teacher;
    }
    
    // 建立老师和学生的关系
    public void addTeacher(Teacher teachers){
        this.teacher.add(teachers);
    }
    
    // 解除老师和学生的关系
    // 删除 teacher 是根据 id来删除的，所有在 Teacher 类中要重写 equals() 和 hashCode()
    public void removeTeacher(Teacher teachers){
        if(this.teacher.contains(teachers)){
            this.teacher.remove(teachers);
        }
    }
}

(2)、老师实体类：

package learn.jpa.entity;

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;

@Entity
public class Teacher {

    private int id;
    private String name;
    private Set<Student> student = new HashSet<Student>();
    
    // 如果声明了有参数的构造函数，那么一定要声明无参构造函数
    public Teacher(){}
    
    public Teacher(String name){
        this.name = name;
    }
    
    @Id
    @GeneratedValue
    public int getId() {
        return id;
    }
    public void setId(int 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,mappedBy="teacher")
    public Set<Student> getStudent() {
        return student;
    }
    public void setStudent(Set<Student> student) {
        this.student = student;
    }

    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + id;
        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 != other.id)
            return false;
        return true;
    }
    
}

2、相关操作类，代码如下：

package learn.jpa.test;

import static org.junit.Assert.*;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;

import learn.jpa.entity.Student;
import learn.jpa.entity.Teacher;

import org.junit.Test;

public class ManyToManyTest {

    /**
     * 测试数据库是否可以生成表
     */
    @Test
    public void test() {
        EntityManagerFactory factory = Persistence.createEntityManagerFactory("learn_jpa");
        factory.close();
    }

    @Test
    public void save(){
        EntityManagerFactory factory = Persistence.createEntityManagerFactory("learn_jpa");
        EntityManager em = factory.createEntityManager();
        em.getTransaction().begin();   // 开启事务
        em.persist(new Student("xx"));
        em.persist(new Teacher("xiuliu"));
        em.getTransaction().commit();
        em.close();
        factory.close();
    }
    
    /**
     * 建立老师和学生的关系（将关联的字段数据保存到中间表中）
     */
    @Test
    public void buildTS(){
        EntityManagerFactory factory = Persistence.createEntityManagerFactory("learn_jpa");
        EntityManager em = factory.createEntityManager();
        em.getTransaction().begin();   // 开启事务
        // 处于托管状态，可以对数据进行更新
        Student student = em.find(Student.class, 1);
        student.addTeacher(em.getReference(Teacher.class, 1));
        em.getTransaction().commit();
        em.close();
        factory.close();
    }
    
    // 解除老师和学生的关系
    @Test
    public void deleteTS(){
        EntityManagerFactory factory = Persistence.createEntityManagerFactory("learn_jpa");
        EntityManager em = factory.createEntityManager();
        em.getTransaction().begin();   // 开启事务
        // 处于托管状态，可以对数据进行更新
        Student student = em.find(Student.class, 1);
        student.removeTeacher(em.getReference(Teacher.class, 1));
        em.getTransaction().commit();
        em.close();
        factory.close();
    }
    
    // 删除老师
    @Test
    public void deleteTeacher(){
        EntityManagerFactory factory = Persistence.createEntityManagerFactory("learn_jpa");
        EntityManager em = factory.createEntityManager();
        em.getTransaction().begin();   // 开启事务
        // 处于托管状态，可以对数据进行更新
        // 先解除老师和学生的关系，在删除老师
        Student student = em.find(Student.class, 1);
        Teacher teacher = em.getReference(Teacher.class, 1);
        student.removeTeacher(teacher);
        em.remove(teacher);
        em.getTransaction().commit();
        em.close();
        factory.close();
    }
    
    // 删除学生
    @Test
    public void deleteStudent(){
        EntityManagerFactory factory = Persistence.createEntityManagerFactory("learn_jpa");
        EntityManager em = factory.createEntityManager();
        em.getTransaction().begin();   // 开启事务
        Student student = em.getReference(Student.class, 1);
        em.remove(student);
        em.getTransaction().commit();
        em.close();
        factory.close();
    }
}