JPA多对多@manytomany注解配置实例

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 teachers = new HashSet(); 

@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 getTeachers() {
return teachers;
} 

public void setTeachers(Set 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：

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

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 students = new HashSet(); 

@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 getStudents() {
return students;
} 

public void setStudents(Set 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：

@ManyToMany(cascade = CascadeType.REFRESH, mappedBy = "teachers", fetch = FetchType.LAZY)
public Set 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。