jpa多对多

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维护端注解

@ManyToMany(cascade = CascadeType.REFRESH)

@JoinTable(//关联表

name = "student_teacher", //关联表名

inverseJoinColumns = @JoinColumn(name = "teacher_id"),//被维护端外键

joinColumns = @JoinColumn(name = "student_id"))//维护端外键

被维护端注解

@ManyToMany(

cascade = CascadeType.REFRESH,

mappedBy = "teachers",//通过维护端的属性关联

fetch = FetchType.LAZY)

关系维护端删除时，如果中间表存在些纪录的关联信息，则会删除该关联信息;

关系被维护端删除时，如果中间表存在些纪录的关联信息，则会删除失败.

以学生和老师的对应关系为例。一个学生可以拥有多个老师，一个老师也可以拥有多个学生。

学生实体类

Student.java代码

package com.taoistwar.jpa.entity.manytomany;
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.GenerationType;
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>();
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
@Column(nullable = false, length = 16)
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
@ManyToMany(cascade = CascadeType.REFRESH)
@JoinTable(name = "student_teacher", inverseJoinColumns = @JoinColumn(name = "teacher_id"), joinColumns = @JoinColumn(name = "student_id"))
public Set<Teacher> getTeachers() {
return teachers;
}
public void setTeachers(Set<Teacher> teachers) {
this.teachers = teachers;
}
public void addTeacher(Teacher teacher) {
this.teachers.add(teacher);
}
public void removeTeachers(Teacher teacher) {
this.teachers.remove(teacher);
}
}

重点在于：

Java代码

@ManyToMany(cascade = CascadeType.REFRESH)
@JoinTable(name = "student_teacher", inverseJoinColumns = @JoinColumn(name = "teacher_id"), joinColumns = @JoinColumn(name = "student_id"))
public Set<Teacher> getTeachers() {
return teachers;
}

老师实体类

Teacher.java代码

package com.taoistwar.jpa.entity.manytomany;
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.GenerationType;
import javax.persistence.Id;
import javax.persistence.ManyToMany;
@Entity
public class Teacher {
private Integer id;
private String name;
private Set<Student> students = new HashSet<Student>();
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
@Column(nullable = false, length = 16)
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
@ManyToMany(cascade = CascadeType.REFRESH, mappedBy = "teachers", fetch = FetchType.LAZY)
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;
}
}

重点在于：

Java代码

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

拥有mappedBy注解的实体类为关系被维护端，另外的实体类为关系维护端的。顾名思意，关系的维护端对关系（在多对多为中间关联表）的CRUD做操作。关系的被维护端没有该操作，不能维护关系。

测试类

Java代码

package com.taoistwar.jpa.entity.manytomany;
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import org.junit.Test;
public class ManyToMany {
@Test
public void save() {
EntityManagerFactory emf = Persistence
.createEntityManagerFactory("JPAPU");
EntityManager em = emf.createEntityManager();
em.getTransaction().begin();
Student student = new Student();
student.setName("小李");
Teacher teacher = new Teacher();
teacher.setName("大李");
em.persist(student);
em.persist(teacher);
em.getTransaction().commit();
emf.close();
}
@Test
public void bind() {
EntityManagerFactory emf = Persistence
.createEntityManagerFactory("JPAPU");
EntityManager em = emf.createEntityManager();
em.getTransaction().begin();
Student student = em.find(Student.class, 1);
Teacher teacher = em.find(Teacher.class, 1);
student.addTeacher(teacher);
em.persist(student);
em.getTransaction().commit();
emf.close();
}
@Test
public void unbind() {
EntityManagerFactory emf = Persistence
.createEntityManagerFactory("JPAPU");
EntityManager em = emf.createEntityManager();
em.getTransaction().begin();
Student student = em.find(Student.class, 1);
Teacher teacher = em.find(Teacher.class, 1);
student.removeTeachers(teacher);
em.persist(student);
em.getTransaction().commit();
emf.close();
}
@Test
public void removeTeacher() {
EntityManagerFactory emf = Persistence
.createEntityManagerFactory("JPAPU");
EntityManager em = emf.createEntityManager();
em.getTransaction().begin();
// 关系被维护端删除时，如果中间表存在些纪录的关联信息，则会删除失败
em.remove(em.getReference(Teacher.class, 1));
em.getTransaction().commit();
emf.close();
}
@Test
public void removeStudent() {
EntityManagerFactory emf = Persistence
.createEntityManagerFactory("JPAPU");
EntityManager em = emf.createEntityManager();
em.getTransaction().begin();
// 关系维护端删除时，如果中间表存在些纪录的关联信息，则会删除该关联信息
em.remove(em.getReference(Student.class, 1));
em.getTransaction().commit();
emf.close();
}
}