JPA ManyToMany映射

package com.jvwl.model;

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;

/**
 * 学生实体类，关系的维护端
 * 
 * @author Jerval
 * 
 */
@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)
	@JoinTable(name = "student_teacher", inverseJoinColumns = @JoinColumn(name = "tid"), joinColumns = @JoinColumn(name = "sid"))
	public Set<Teacher> getTeachers() {
		return teachers;
	}

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

	public void addTeacher(Teacher teacher) {
		teachers.add(teacher);
	}

	public void removeTeacher(Teacher teacher) {
		// 要能判断出是否包含在Set中，必须重写Teacher中的hashCode方法和equals方法
		if (teachers.contains(teacher)) {
			teachers.remove(teacher);
		}
	}
}

 

package com.jvwl.model;

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;
/**
 * 教师实体类
 * @author Jerval
 *
 */
@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;
	}

	@ManyToMany(cascade = CascadeType.REFRESH, mappedBy = "teachers")
	public Set<Student> getStudents() {
		return students;
	}

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

	@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;
	}

}

 

package junit.test;

import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import org.junit.Test;
import com.jvwl.model.Student;
import com.jvwl.model.Teacher;

public class JPATest {
	/**
	 * 可以通过这种方式生成表
	 */
	@Test
	public void makeTable() {
		EntityManagerFactory factory = Persistence
				.createEntityManagerFactory("jvwl");
		factory.close();
	}

	/*
	 * 向数据库中放入学生和老师的信息
	 */
	@Test
	public void addInfo() {
		EntityManagerFactory factory = Persistence
				.createEntityManagerFactory("jvwl");
		EntityManager em = factory.createEntityManager();
		em.getTransaction().begin();
		em.persist(new Teacher("李老师"));
		em.persist(new Student("蹇伟"));
		em.persist(new Teacher("王老师"));
		em.persist(new Student("蹇洁"));
		em.getTransaction().commit();
		em.close();
		factory.close();
	}

	/*
	 * 建立老师和学生的关系
	 */
	@Test
	public void buidTS() {
		EntityManagerFactory factory = Persistence
				.createEntityManagerFactory("jvwl");
		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();
	}

	/*
	 * 解除老师和学生的关系
	 */
	@Test
	public void removeTS() {
		EntityManagerFactory factory = Persistence
				.createEntityManagerFactory("jvwl");
		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 注：此方法不能删除老师和学生的关系
	 */
	@Test
	public void removeTeacher() {
		EntityManagerFactory factory = Persistence
				.createEntityManagerFactory("jvwl");
		EntityManager em = factory.createEntityManager();
		em.getTransaction().begin();
		em.remove(em.getReference(Teacher.class, 1));
		em.getTransaction().commit();
		em.close();
		factory.close();
	}

	/*
	 * 删除学生 注：此方法能删除学生和老师的关系
	 */
	@Test
	public void removeStudent() {
		EntityManagerFactory factory = Persistence
				.createEntityManagerFactory("jvwl");
		EntityManager em = factory.createEntityManager();
		em.getTransaction().begin();
		em.remove(em.getReference(Student.class, 1));
		em.getTransaction().commit();
		em.close();
		factory.close();
	}
}