junit测试类代码:(调用接口)

private SqlSessionFactory ssf = null;
    @Before
    public void before() throws Exception {
        InputStream is = Resources.getResourceAsStream("SqlMapConfig.xml");
        this.ssf = new SqlSessionFactoryBuilder().build(is);
    }

    @Test
    public void aaa() {
        SqlSession sqlSession = this.ssf.openSession();

        OrderMapper mapper = sqlSession.getMapper(OrderMapper.class);

        List<Orders> orders = mapper.selectAllOrdersWithUser();

        for (Orders order :
                orders) {
            System.out.println(order);
        }
    }

接口mapper类代码:

/**
 * Created by syk on 2018/2/16.
 */
public interface OrderMapper {

    public List<Orders> selectAllOrders();


    //多对一

    public List<Orders> selectAllOrdersWithUser();
}

接口对应的xml文件代码:

<?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">
<!-- namespace：命名空间，用于隔离sql，还有一个很重要的作用，后面会讲 -->




<mapper namespace="cn.itcast.mapper.OrderMapper">
    <resultMap id="ordersList" type="Orders">
        <result property="user_id" column="userId"></result>
    </resultMap>

    <select id="selectAllOrders" resultMap="ordersList">
        SELECT id,user_id,number,createtime,note FROM orders
    </select>

    <resultMap id="allOrdersWithUser" type="Orders">
        <result column="user_id" property="userId"/>
        <result column="id" property="id"/>
        <result column="number" property="number"/>
        <result column="createtime" property="createtime"/>
        <result column="note" property="note"/>
        <association property="user" javaType="User">
            <result column="user_id" property="id"/>
            <result column="username" property="username"/>
        </association>
    </resultMap>
    <select id="selectAllOrdersWithUser" resultMap="allOrdersWithUser">
        select
        o.*,
        u.*
        from orders o
        LEFT JOIN user u
        on o.user_id = u.id;
    </select>

需要注意的是:在一对一情况,对象中嵌套了另外一个对象,那么resultMap映射关系中正常可以忽略的属性都不能够忽略,否则无法成功映射