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。