JPA学习总结（四）--JPAManyToMany双向关联

最新推荐文章于 2025-06-06 15:01:27 发布

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#JPA #Hibernate #Bean #MySQL #junit

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本文介绍如何使用Hibernate实现Java项目的多对多关联映射，包括配置persistence.xml文件、编写实体Bean类（如Student和Teacher）、创建关联关系及进行相关操作。

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步骤1.新建java项目：JPAManyToMany，将hibernate包以及mysql驱动包导入

步骤2.在类路径下新建xml文件persistence.xml，将其放入META-INF文件夹下，其代码如下：

<?xml version="1.0" encoding="UTF-8"?>
<persistence xmlns="http://java.sun.com/xml/ns/persistence"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://java.sun.com/xml/ns/persistence http://java.sun.com/xml/ns/persistence/persistence_1_0.xsd"
	version="1.0">
	<persistence-unit name="itcast" transaction-type="RESOURCE_LOCAL">
		<properties>
		
			<property name="hibernate.connection.url" value="jdbc:mysql://localhost:3306/itcast?useUnicode=true&amp;characterEncoding=UTF-8" />
			<property name="hibernate.dialect" value="org.hibernate.dialect.MySQL5Dialect"/>	
			<property name="hibernate.connection.driver_class" value="org.gjt.mm.mysql.Driver" />
			<property name="hibernate.connection.username" value="root" />
			<property name="hibernate.connection.password" value="root" />
			<property name="hibernate.max_fetch_depth" value="3" />
			<property name="hibernate.hbm2ddl.auto" value="update" />
		</properties>
	</persistence-unit>
</persistence>

步骤3.编写实体Bean类

Student代码如下：

package cn.itcast.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;

@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;
	}
	
	/**
	 *Student为关系的维护端，有对外键增删改查的权利
	 *在关系的维护端创建关系表为student_teacher，并指定了关系被维护端的外键为teacher_id,
	 *关系维护端的外键为student_id
	 *	@JoinTable(name="student_teacher",inverseJoinColumns=@JoinColumn(name="teacher_id")
	 *		,joinColumns=@JoinColumn(name="student_id"))
	 */
	@ManyToMany(cascade=CascadeType.REFRESH)
	@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;
	}
	@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;
	}
	/**
	 * 增加teacher的方法
	 */
	public void addTeacher(Teacher teacher){
		this.teachers.add(teacher);
	}
	/**
	 * 删除teacher的方法
	 */
	public void removeTeacher(Teacher teacher){
		/*
		 * 判断该集合中是否包含该方法，包含就删除掉，
		 * 要在Teacher.java中重载hashCode()和equals(Object obj)方法，
		 * 作用是为了判断这个集合中的teacher是否与要删除的teacher对象相等，根据id判断
		 */
		if(this.teachers.contains(teacher))
			this.teachers.remove(teacher);
	}
}

Teacher的代码如下：

package cn.itcast.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;

@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;
	}
	/**
	 * mappedBy="teachers" 说明Teacher为关系被维护端，没有对外键增删改查的权利
	 */
	@ManyToMany(cascade=CascadeType.REFRESH,mappedBy="teachers")
	public Set<Student> getStudents() {
		return students;
	}
	public void setStudents(Set<Student> students) {
		this.students = students;
	}
	@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;
	}

	@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;
		final Teacher other = (Teacher) obj;
		if (id == null) {
			if (other.id != null)
				return false;
		} else if (!id.equals(other.id))
			return false;
		return true;
	}
	
}

步骤4.创建junit测试

package junit.test;


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

import org.junit.BeforeClass;
import org.junit.Test;

import cn.itcast.bean.Student;
import cn.itcast.bean.Teacher;

public class ManyToManyTest {

	@BeforeClass
	public static void setUpBeforeClass() throws Exception {
	}
	//分别保存老师和学生
	@Test public void save(){
		EntityManagerFactory factory = Persistence.createEntityManagerFactory("itcast");
		EntityManager em = factory.createEntityManager();
		em.getTransaction().begin();
		em.persist(new Student("小学生"));
		em.persist(new Teacher("大老师"));
		em.getTransaction().commit();
		em.close();
		factory.close();
	}
	//建立老师和学生的关系
	@Test public void buildTS(){
		EntityManagerFactory factory = Persistence.createEntityManagerFactory("itcast");
		EntityManager em = factory.createEntityManager();
		em.getTransaction().begin();
		Student student = em.getReference(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 removeTS(){
		EntityManagerFactory factory = Persistence.createEntityManagerFactory("itcast");
		EntityManager em = factory.createEntityManager();
		em.getTransaction().begin();
		Student student = em.getReference(Student.class, 1);
		student.removeTeacher(em.getReference(Teacher.class, 1));
		em.getTransaction().commit();
		em.close();
		factory.close();
	}
	/**
	 * 删除老师
	 * 在删除老师的时候，因为老师是关系被维护端
	 * 所以要首先解除老师跟学生的关系，然后才能删除老师
	 */
	@Test public void deleteTeacher(){
		EntityManagerFactory factory = Persistence.createEntityManagerFactory("itcast");
		EntityManager em = factory.createEntityManager();
		em.getTransaction().begin();
		Teacher teacher = em.getReference(Teacher.class, 1);
		Student student = em.getReference(Student.class, 1);
		student.removeTeacher(teacher);
		em.remove(teacher);
		em.getTransaction().commit();
		em.close();
		factory.close();
	}
	/**
	 * 删除学生
	 * 在删除学生的时候，因为学生是关系的维护端，
	 * 这样会直接解除老师跟学生的关系，并且可以删除掉学生
	 */
	@Test public void deleteStudent(){
		EntityManagerFactory factory = Persistence.createEntityManagerFactory("itcast");
		EntityManager em = factory.createEntityManager();
		em.getTransaction().begin();
		Student student = em.getReference(Student.class, 1);
		em.remove(student);
		em.getTransaction().commit();
		em.close();
		factory.close();
	}
}

看能将表创建出来，并插入数据即可。