JPA多对多双向关联

JPA多对多双向关联

SQL

create   table  Roles
(
   id                    int   not   null  auto_increment,
   name                  varchar ( 20 ),
    primary   key  (id)
);

create   table  Users
(
   id                    int   not   null  auto_increment,
   name                  varchar ( 20 ),
    primary   key  (id)
);

create   table  user_role
(
    user_id                int   not   null ,
   role_id               int   not   null ,
    primary   key  ( user_id , role_id)
);

alter   table  user_role  add   constraint  FK_Reference_1  foreign   key  ( user_id )
       references  Users (id)  on   delete   restrict   on   update   restrict ;

alter   table  user_role  add   constraint  FK_Reference_2  foreign   key  (role_id)
       references  Roles (id)  on   delete   restrict   on   update   restrict ;

Role.java

package  com.many2many.bean;

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;
import  javax.persistence.Table;

@Entity
@Table(name = " roles " ,catalog = " JPA_Many2Many " )
public   class  Role {
    
     private   int  id;
     private  String name;
     private  Set < User >  users;
    @Id
    @Column(name = " id " )
    @GeneratedValue(strategy = GenerationType.AUTO)
     public   int  getId() {
         return  id;
    }
     public   void  setId( int  id) {
         this .id  =  id;
    }
    @Column(name = " name " )
     public  String getName() {
         return  name;
    }
     public   void  setName(String name) {
         this .name  =  name;
    }
    @ManyToMany(cascade = CascadeType.PERSIST)
    @JoinTable(
            name = " user_role " ,
            joinColumns = @JoinColumn(name = " role_id " ),
            inverseJoinColumns = @JoinColumn(name = " user_id " )
    )
     public  Set < User >  getUsers() {
         return  users;
    }
     public   void  setUsers(Set < User >  users) {
         this .users  =  users;
    }
    
}

User.java

package  com.many2many.bean;

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.ManyToMany;
import  javax.persistence.Table;

@Entity
@Table(name = " users " ,catalog = " JPA_Many2Many " )
public   class  User {
    
     private   int  id;
     private  String name;
     private  Set < Role >  roles;
    @Id
    @Column(name = " id " )
    @GeneratedValue(strategy = GenerationType.AUTO)
     public   int  getId() {
         return  id;
    }
     public   void  setId( int  id) {
         this .id  =  id;
    }
    @Column(name = " name " )
     public  String getName() {
         return  name;
    }
     public   void  setName(String name) {
         this .name  =  name;
    }
    @ManyToMany(mappedBy = " users " ,cascade = CascadeType.REFRESH)
     public  Set < Role >  getRoles() {
         return  roles;
    }
     public   void  setRoles(Set < Role >  roles) {
         this .roles  =  roles;
    }
    
    
}

JPAUtil.java

package  com.many2many.util;

import  javax.persistence.EntityManager;
import  javax.persistence.EntityManagerFactory;
import  javax.persistence.Persistence;

public   class  JPAUtil {
    
     private   static  EntityManager entityManager;
     public   static  EntityManager getInstance(){
         if (entityManager != null ){
             return  entityManager;
        } else {
             return  makeInstance();
        }
    }
     private   static   synchronized  EntityManager makeInstance() {
         if (entityManager == null ){
            EntityManagerFactory entityManagerFactory = Persistence.createEntityManagerFactory( " JPA_Many2ManyPU " );
             return  entityManagerFactory.createEntityManager();
        }
         return   null ;
    }
}

测试代码

@Test
     public   void  insert(){
        EntityManager entityManager = JPAUtil.getInstance();
        EntityTransaction entityTransaction = entityManager.getTransaction();
         try  {
            entityTransaction.begin();
            
            User user = new  User();
            user.setName( " 张三 " );
            
            Role role = new  Role();
            role.setName( " 管理员 " );
            
            Set < User >  users = new  HashSet < User > ();
            users.add(user);
            
            role.setUsers(users);
            
            entityManager.persist(role);
            
            entityTransaction.commit();
        }  catch  (Exception e) {
            e.printStackTrace();
        }
    }
    
    @Test
     public   void  select(){
        EntityManager entityManager = JPAUtil.getInstance();
        Role role = entityManager.find(Role. class ,  1 );
        System.out.println(role.getName());
        Set < User >  set = role.getUsers();
         for  (Iterator < User >  iterator  =  set.iterator(); iterator.hasNext();) {
            User user  =  (User) iterator.next();
            System.out.println(user.getName());
        }
    }