JPA之ManyToMany的处理

  在处理关系中，最为麻烦的可能是多对多的关系处理，因为多对多的关系要要一个中间表来进行维护。但是实际上多对多的关系也是我们比较常见的一种关系。

业务需求：在学校的选课系统中老师和学生的关系则是一种多对多的关系
首先还是将老师和学生的实体类编写出来。

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>();//用集合类来存放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;
	}

	@ManyToMany(cascade = CascadeType.REFRESH)
	@JoinTable(name = "student_teacher", inverseJoinColumns = @JoinColumn(name = "teacher_id"), joinColumns = @JoinColumn(name = "student_id")) //JoinTable就是定义中间表的名字以及关联字段名
	public Set<Teacher> getTeacher() {
		return teachers;
	}
	public void setTeacher(Set<Teacher> teacher) {
		this.teachers = teacher;
	}

	public void setName(String name) {
		this.name = name;
	}
	
         
	public void addTeacher(Teacher teacher){
		this.teachers.add(teacher);
	}
	
	public void removeTeacher(Teacher teacher){
		 if(this.teachers.contains(teacher)){//判断该学生是否包含了这个老师，如果存在就删除。这里要重新Teacher类中的hashCode和equals方法
			 this.teachers.remove(teacher);
		 }
	}
}

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> student = 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;
	}
	
    @ManyToMany(cascade=CascadeType.REFRESH,mappedBy="teacher")//这里说明了关系维护端是student，teacher是关系被维护端
	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 == 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;
	}
}

测试类

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 {

	}

	@Test
	public void save() {
		EntityManagerFactory factory = Persistence
				.createEntityManagerFactory("test");
		EntityManager em = factory.createEntityManager();
		em.getTransaction().begin();
		em.persist(new Student("Twe"));
		em.persist(new Teacher("蒋老师"));
		em.getTransaction().commit();
		em.close();
		factory.close();
	}

	/**
	 * 建立学生和老师的关系
	 */
	@Test
	public void buildTS() {
		EntityManagerFactory factory = Persistence
				.createEntityManagerFactory("test");
		EntityManager em = factory.createEntityManager();
		em.getTransaction().begin();
		Student student = em.find(Student.class, 1);
		student.addTeacher(em.getReference(Teacher.class, 1));

		em.getTransaction().commit();
		em.close();
		factory.close();
	}

	/**
	 * 解除学生和老师的关系
	 */
	@Test
	public void deleteTS() {
		EntityManagerFactory factory = Persistence
				.createEntityManagerFactory("test");
		EntityManager em = factory.createEntityManager();
		em.getTransaction().begin();
		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("test");
		EntityManager em = factory.createEntityManager();
		em.getTransaction().begin();
		Student student = em.find(Student.class, 1);
		Teacher teacher = em.getReference(Teacher.class, 1);
		student.removeTeacher(teacher);
		em.remove(em.getReference(Teacher.class, 1));
		em.getTransaction().commit();
		em.close();
		factory.close();
	}

	/**
	 * 删除学生
	 */
	@Test
	public void deleteStudent() {
		EntityManagerFactory factory = Persistence
				.createEntityManagerFactory("test");
		EntityManager em = factory.createEntityManager();
		em.getTransaction().begin();
		Student student = em.getReference(Student.class, 1);
		em.remove(student);
		em.getTransaction().commit();
		em.close();
		factory.close();
	}
}

总结：这里更要注意关系维护端和关系被维护端。