jpa双向多对多关系

标签：jpa   多对多   

多对多关系相比其他其中关联关系，显得稍微复杂了一点点，这个复杂度主要体现在对这种关联关系的理解上。和其他关联关系不同的是这种关联多出来了一张中间表，操作上多了些许复杂，来随便看下吧

1  实体的定义

Student表：

package org.lxh.info;

import java.util.*;

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;

	@Id
	@GeneratedValue
	public Integer getId() {
		return id;
	}

	public void setId(Integer id) {
		this.id = id;
	}

	@Column(length = 50)
	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}
	@ManyToMany(cascade=CascadeType.REFRESH)
	@JoinTable(name="student_teacher",inverseJoinColumns=@JoinColumn(name="teacherId"),joinColumns=@JoinColumn(name="stuId"))
	public Set<Teacher> getTeachers() {
		return teachers;
	}

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

Teacher表：

package org.lxh.info;

import java.util.*;

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;
       @Id @GeneratedValue
	public Integer getId() {
		return id;
	}

	public void setId(Integer id) {
		this.id = id;
	}
        @Column(length=50)
	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;
	}
}

到这里实体就定义完了，要注意的地方就是只有关系维护的一方才能更新中间表里的外键记录。如果反过来则会报错

2  多对多数据保存

/**
	 * 双向多对多保存
	 */
	@org.junit.Test
	public void testMany2ManyInsert() {
		EntityManager em=null;
		EntityTransaction tx=null;
		try{
			em=JpaUtil.getEntityManager();
			tx=em.getTransaction();
			tx.begin();
			
			Student s=new Student();
			s.setName("张小华");
			Student s2=new Student();
			s2.setName("陈小明");
			Set<Student> students=new HashSet<Student>();
			students.add(s);
			students.add(s2);
			
			Teacher t=new Teacher();
			t.setName("李老师");
			t.setStudents(students);
			
			Teacher t2=new Teacher();
			t2.setName("刘老师");
			t2.setStudents(students);
			
			Set<Teacher> teachers=new HashSet<Teacher>();
			teachers.add(t);
			teachers.add(t2);
			
			s.setTeachers(teachers);
			s2.setTeachers(teachers);
			
			
                        em.persist(s);
                        em.persist(s2);
                        em.persist(t);
                        em.persist(t2);
			
			
		    
			tx.commit();
		}catch(Exception e){
			e.printStackTrace();
		}finally{
			if(em!=null){
				em.close();
			}
		}
		
	}

@org.junit.Test
	public void testMany2ManyDelete() {
		EntityManager em=null;
		EntityTransaction tx=null;
		try{
			em=JpaUtil.getEntityManager();
			tx=em.getTransaction();
			tx.begin();
			Student s=em.find(Student.class, 1);
			Student s2=em.find(Student.class, 2);
			Teacher t=em.find(Teacher.class, 2);
			//先解除关联关系
			s.getTeachers().remove(t);
			s2.getTeachers().remove(t);
			//解除关联关系后才能通过关系被维护端删除数据
		    em.remove(t);
			tx.commit();
		}catch(Exception e){
			e.printStackTrace();
		}finally{
			if(em!=null){
				em.close();
			}
		}
		
	}

jpa双向多对多关系

标签：jpa   多对多   

原文地址：http://blog.csdn.net/walkcode/article/details/39134963