如果要搭建JPA開發環境，請從JPA第一篇部落格看起。

下面的例子以老師和學生的例子進行多對多關係的編碼講解。

1、Student.java中的程式碼

package cn.sunft.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;

/**
 * 學生
 */
@Entity
public class Student {
	private Integer id;
	private String name;
	private Set<Teacher> teachers = new HashSet<>();

	public Student() {
		super();
	}

	public Student(String name) {
		super();
		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;
	}

	//維護關係的一方
	@ManyToMany(cascade=CascadeType.REFRESH)
	//如果中間表不寫inverseJoinColumns和joinColumns屬性,
	//Hibernate會自動生成對應的中間表的外來鍵
	@JoinTable(name="student_teacher", 
		
 
[email protected](name="teacher_id"),
		[email protected](name="student_id"))//指定關聯的中間表
	public Set<Teacher> getTeachers() {
		return teachers;
	}

	public void setTeachers(Set<Teacher> teachers) {
		this.teachers = teachers;
	}
	
	public void addTeacher(Teacher teacher){
		this.teachers.add(teacher);
	}

	public void removeTeacher(Teacher teacher){
		if(this.teachers.contains(teacher)){
			this.teachers.remove(teacher);
		}
	}

	//重寫hashCode方法,用於判斷兩個物件是否相等
	@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;
		Student other = (Student) obj;
		if (id == null) {
			if (other.id != null)
				return false;
		} else if (!id.equals(other.id))
			return false;
		return true;
	}

}
 

package cn.sunft.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;

/**
 * 老師
 */
@Entity
public class Teacher {
	private Integer id;
	private String name;
	private Set<Student> students = new HashSet<>();

	public Teacher() {
		super();
	}

	public Teacher(String name) {
		super();
		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;
	}

	//多對多關係中,級聯刪除基本用不上,這裡預設是懶載入
	//被維護關係的一方
	@ManyToMany(cascade=CascadeType.REFRESH, mappedBy="teachers")
	public Set<Student> getStudents() {
		return students;
	}

	public void setStudents(Set<Student> students) {
		this.students = students;
	}

}
 

package junit.test;

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

import org.junit.Test;

import cn.sunft.bean.Student;
import cn.sunft.bean.Teacher;

/**
 * 多對多的測試
 */
public class ManyToManyTest {
	
	/**
	 * 儲存學生和老師
	 */
	@Test
	public void save(){
		EntityManagerFactory factory 
			= Persistence.createEntityManagerFactory("sunft_first");
		EntityManager em = factory.createEntityManager();
		em.getTransaction().begin();
		em.persist(new Student("小張"));
		em.persist(new Teacher("李勇老師"));
		em.getTransaction().commit();
		em.close();
		factory.close();
	}
	
	/**
	 * 建立老師和學生的關係
	 */
	@Test
	public void buildTS(){
		EntityManagerFactory factory 
			= Persistence.createEntityManagerFactory("sunft_first");
		EntityManager em = factory.createEntityManager();
		em.getTransaction().begin();
		//這裡的配置是學生負責維護關係
		Student student = em.find(Student.class, 31);
		//這裡並不需要把老師及時載入進來,體現在中間表中新增一條資料
		student.addTeacher(em.getReference(Teacher.class, 32));
		em.getTransaction().commit();
		em.close();
		factory.close();
	}
	
	/**
	 * 解除老師和學生的關係
	 */
	@Test
	public void deleteTS(){
		EntityManagerFactory factory 
			= Persistence.createEntityManagerFactory("sunft_first");
		EntityManager em = factory.createEntityManager();
		em.getTransaction().begin();
		//這裡配置的是學生負責維護關係
		Student student = em.find(Student.class, 25);
		//這裡並不需要把老師及時載入進來,使用getReference增加效能,
		//體現在將中間表中的記錄刪除一條
		student.removeTeacher(em.getReference(Teacher.class, 26));
		em.getTransaction().commit();
		em.close();
		factory.close();
	}
	
	/**
	 * 刪除老師:只能先解除和學生的關係,再刪除老師,
	 * 老師是關係被維護端,沒有權利級聯刪除關係
	 */
	@Test
	public void deleteTeacher(){
		EntityManagerFactory factory 
			= Persistence.createEntityManagerFactory("sunft_first");
		EntityManager em = factory.createEntityManager();
		em.getTransaction().begin();
		Student student = em.find(Student.class, 25);
		Teacher teacher = em.getReference(Teacher.class, 26);
		//這裡並不需要把老師及時載入進來,體現在將中間表中的記錄刪除一條
		student.removeTeacher(teacher);//解除關係
		em.remove(em.getReference(Teacher.class, 26));//刪除老師
		em.getTransaction().commit();
		em.close();
		factory.close();
	}
	
	/**
	 * 刪除學生:因為由學生維護關係,因此刪除學生會刪除關係,
	 * 但是隻有當cascade=CascadeType.REMOVE時,才會連著
	 * 學生一起刪除
	 */
	@Test
	public void deleteStudent(){
		EntityManagerFactory factory 
			= Persistence.createEntityManagerFactory("sunft_first");
		EntityManager em = factory.createEntityManager();
		em.getTransaction().begin();
		Student student = em.getReference(Student.class, 31);
		//這裡會刪除關係
		em.remove(student);//刪除老師
		em.getTransaction().commit();
		em.close();
		factory.close();
	}
}