【MyBatis】Mybatis 多表查询（二）多对多

• 需求：
  实现查询所有对象并且加载它所分配的用户信息。
• 分析：
  查询角色我们需要用到Role表，但角色分配的用户的信息我们并不能直接找到用户信息，而是要通过中间表(USER_ROLE 表)才能关联到用户信息。

案例前提数据库中已经存在user,role表，以及中间表user_role
那么我们的sql语句应该是：

SELECT
r.*,u.id uid,u.username username,u.birthday birthday,u.sex sex,u.address address
FROM ROLE r
INNER JOIN USER_ROLE ur ON ( r.id = ur.rid)
INNER JOIN USER u ON (ur.uid = u.id);

角色实体类：

package com.siyi.domain;

import java.io.Serializable;
import java.util.List;

public class Role implements Serializable {

    private Integer roleId;
    private String roleName;
    private String roleDesc;

    //多对多的关系映射,一个角色可以赋予多个用户
    private List<User> users;

    public List<User> getUsers() {
        return users;
    }

    public void setUsers(List<User> users) {
        this.users = users;
    }

    public Integer getRoleId() {
        return roleId;
    }

    public void setRoleId(Integer roleId) {
        this.roleId = roleId;
    }

    public String getRoleName() {
        return roleName;
    }

    public void setRoleName(String roleName) {
        this.roleName = roleName;
    }

    public String getRoleDesc() {
        return roleDesc;
    }

    public void setRoleDesc(String roleDesc) {
        this.roleDesc = roleDesc;
    }

    @Override
    public String toString() {
        return "Role{" +
                "roleId=" + roleId +
                ", roleName='" + roleName + '\'' +
                ", roleDesc='" + roleDesc + '\'' +
                '}';
    }
}

IRole 持久层接口

package com.siyi.dao;

import com.siyi.domain.Role;

import java.util.List;

public interface IRoleDao {

    /**
     * 查询所有角色
     * @return
     */
    public List<Role> findAll();
}

映射文件IRoleDao.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">

<mapper namespace="com.siyi.dao.IRoleDao">

    <!-- 定义role表的ResultMap -->
    <resultMap id="roleMap" type="role">
        <id property="roleId" column="rid"/>
        <result property="roleName" column="role_name"/>
        <result property="roleDesc" column="role_desc"/>
        <collection property="users" ofType="user">
            <id property="id" column="id"/>
            <result property="username" column="username"/>
            <result property="address" column="address"/>
            <result property="sex" column="sex"/>
            <result property="birthday" column="birthday"/>
        </collection>
    </resultMap>

    <!-- 查询所有 -->
    <select id="findAll" resultMap="roleMap">
        select u.*,r.id as rid,r.role_name,r.role_desc from role r
        left outer join user_role ur on r.id = ur.rid
        left outer join user u on u.id = ur.uid
    </select>
</mapper>

测试类

package com.siyi.test;

import com.siyi.dao.IRoleDao;
import com.siyi.dao.IUSerDao;
import com.siyi.domain.Role;
import com.siyi.domain.User;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;

import java.io.IOException;
import java.io.InputStream;
import java.util.List;

public class RoleTest {

    private InputStream in;
    private SqlSession session;
    private IRoleDao iRoleDao;

    @Before//用于在测试方法执行之前执行
    public void init() throws IOException {
        //1.读取配置文件生成字节输入流
        in = Resources.getResourceAsStream("SqlMapConfig.xml");
        //2.获取SqlSessionFactory对象
        SqlSessionFactoryBuilder builder = new SqlSessionFactoryBuilder();
        SqlSessionFactory factory = builder.build(in);
        //3.获取SqlSession对象
        session = factory.openSession(true);
        //4.获取DAO的代理对象
        iRoleDao = session.getMapper(IRoleDao.class);
    }

    @After//用于在测试方法之后执行
    public void destroy() throws IOException {
        //事务提交
        //session.commit();
        //释放资源
        session.close();
        in.close();
    }

    /**
     * 测试查询所有
     * @throws IOException
     */
    @Test
    public void testFindAll() throws IOException {
        //5.执行查询所有方法
        List<Role> roles = iRoleDao.findAll();
        for (Role role : roles) {
            System.out.println("------------------------");
            System.out.println(role);
            System.out.println(role.getUsers());
        }
    }
}