本文章主要介绍了jpa多对多映射案例，具有不错的的参考价值，希望对您有所帮助，如解说有误或未考虑完全的地方，请您留言指出，谢谢！
学生和老师就是多对多的关系。一个学生有多个老师，一个老师教多个学生。多对多映射采取中间表连接的映射策略，建立的中间表将分别引入两边的主键作为外键。jpa 对于中间表的元数据提供了可配置的方式，用户可以自定义中间表的表名，列名。
下面就以学生和老师为例介绍多对多映射关系的实例开发
Student实体类
package com.ljq.entity;
import java.util.HashSet;
import java.util.Set;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.ManyToMany;
@SuppressWarnings( " serial " )
@Entity
public class Student implements java.io.Serializable {
/** 学生ID * */
private Integer studentid;
/** 学生姓名 * */
private String name;
private Set < Teacher > teachers = new HashSet < Teacher > ();
public Student() {
super ();
}
public Student(String name) {
super ();
this .name = name;
}
@Id
@GeneratedValue
public Integer getStudentid() {
return studentid;
}
public void setStudentid(Integer studentid) {
this .studentid = studentid;
}
@Column(nullable = false ,length = 32 )
public String getName() {
return name;
}
public void setName(String name) {
this .name = name;
}
// @ManyToMany注释表示Student是多对多关系的一边，mappedBy属性定义了Student为双向关系的维护端
// Teacher表是关系的维护者，owner side，有主导权，它有个外键指向Student表。
@ManyToMany(mappedBy = " students " )
public Set < Teacher > getTeachers() {
return teachers;
}
public void setTeachers(Set < Teacher > teachers) {
this .teachers = teachers;
}
}
Teacher实体类
package com.ljq.entity;
import java.util.HashSet;
import java.util.Set;
import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.JoinTable;
import javax.persistence.ManyToMany;
@SuppressWarnings( " serial " )
@Entity
public class Teacher implements java.io.Serializable {
/** 教师ID * */
private Integer teacherid;
/** 教师姓名 * */
private String name;
private Set < Student > students = new HashSet < Student > ();
public Teacher() {
super ();
}
public Teacher(String name) {
super ();
this .name = name;
}
@Id
@GeneratedValue
public Integer getTeacherid() {
return teacherid;
}
public void setTeacherid(Integer teacherid) {
this .teacherid = teacherid;
}
@Column(nullable = false ,length = 32 )
public String getName() {
return name;
}
public void setName(String name) {
this .name = name;
}
// @ManyToMany注释表示Teacher是多对多关系的一端。
// @JoinTable描述了多对多关系的数据表关系。name属性指定中间表名称，joinColumns定义中间表与Teacher表的外键关系。
// 中间表Teacher_Student的Teacher_ID列是Teacher表的主键列对应的外键列，inverseJoinColumns属性定义了中间表与另外一端(Student)的外键关系。
@ManyToMany(cascade = CascadeType.PERSIST, fetch = FetchType.LAZY)
@JoinTable(name = " Teacher_Student " ,
joinColumns = { @JoinColumn(name = " Teacher_ID " , referencedColumnName = " teacherid " ) },
inverseJoinColumns = { @JoinColumn(name = " Student_ID " , referencedColumnName = " studentid " ) })
public Set < Student > getStudents() {
return students;
}
public void setStudents(Set < Student > students) {
this .students = students;
}
/**
* 添加学生
*
* @param student
*/
public void addStudent(Student student) {
if ( ! this .students.contains(student)) {
this .students.add(student);student.setTeacher(this);
}
}
/**
* 删除学生
*
* @param student
*/
public void removeStudent(Student student) {
if ( this .students.contains(student)){student.setTeacher(null);
this .students.remove(student);
}
}
}
ManyToManyTest测试类
package com.ljq.test;
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import org.junit.Test;
import com.ljq.entity.Student;
import com.ljq.entity.Teacher;
public class ManyToManyTest {
@Test
public void save() {
EntityManagerFactory factory = Persistence.createEntityManagerFactory( " ljq " );
EntityManager em = factory.createEntityManager();
em.getTransaction().begin();
em.persist( new Teacher( " 张老师 " ));
em.persist( new Student( " 小张 " ));
em.getTransaction().commit();
em.close();
factory.close();
}
/**
* 为老师添加一个学生
*
*/
@Test
public void build() {
EntityManagerFactory factory = Persistence.createEntityManagerFactory( " ljq " );
EntityManager em = factory.createEntityManager();
em.getTransaction().begin();
Teacher teacher = em.find(Teacher. class , 2 );
teacher.addStudent(em.getReference(Student. class , 2 ));
em.getTransaction().commit();
em.close();
factory.close();
}
/**
* 解除学生跟老师的关系
*
*/
@Test
public void remove() {
EntityManagerFactory factory = Persistence.createEntityManagerFactory( " ljq " );
EntityManager em = factory.createEntityManager();
em.getTransaction().begin();
Teacher teacher = em.find(Teacher. class , 2 );
teacher.removeStudent(em.getReference(Student. class , 2 ));
em.getTransaction().commit();
em.close();
factory.close();
}
/**
* 删除学生，因为学生不是关系维护者，所以需要先手工解除老师与学生的关联，然后再删除学生
*
*/
@Test
public void deleteStudent() {
EntityManagerFactory factory = Persistence.createEntityManagerFactory( " ljq " );
EntityManager em = factory.createEntityManager();
em.getTransaction().begin();
Teacher teacher = em.find(Teacher. class , 2 );
Student student = em.getReference(Student. class , 2 );
teacher.removeStudent(student); // 手工解除老师与学生的关系
em.remove(student); // 删除学生
em.getTransaction().commit();
em.close();
factory.close();
}
/**
* 删除老师，因为老师是关系维护者，所以可以直接解除老师与学生的关系，不用我们手工解除
*
*/
@Test
public void deleteTeacher() {
EntityManagerFactory factory = Persistence.createEntityManagerFactory( " ljq " );
EntityManager em = factory.createEntityManager();
em.getTransaction().begin();
em.remove(em.getReference(Teacher. class , 3 ));
em.getTransaction().commit();
em.close();
factory.close();
}
/**
* 用来判断映射是否成功
*
*/
@Test
public void test() {
EntityManagerFactory factory = Persistence.createEntityManagerFactory( " ljq " );
factory.close();
}
}
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