jpa多对多映射案例

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();
}

}