MyBatis 关联查询延迟加载实战案例

// 用户实体（一的一方）
@Data
public class User {
    private Integer id;
    private String username;
    // 一对多：存放当前用户的所有订单
    private List<Order> orderList;
}
// 订单实体（多的一方）
@Data
public class Order {
    private Integer id;
    private String orderNo;
    // 多对一：存放订单所属的用户
    private User user;
}

<!-- 单表查询：resultType 直接映射，无需配置resultMap -->
<select id="selectById" resultType="com.itheima.pojo.User">
    select * from tb_user where id = #{id}
</select>

<?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.itheima.mapper.OrderMapper">
    <!-- 一对一结果映射 -->
    <resultMap id="orderResultMap" type="order" autoMapping="true">
        <id property="id" column="id"/>
        <result property="orderNo" column="order_no"/>
        <!-- 一对一关联配置：延迟加载用户信息 -->
        <association
            property="user"
            javaType="user"
            select="com.itheima.mapper.UserMapper.selectById"
            column="user_id"
            fetchType="lazy">
        </association>
    </resultMap>
    <!-- 根据id查询订单 -->
    <select id="selectById" resultMap="orderResultMap">
        select * from tb_order where id = #{id}
    </select>
</mapper>

<?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.itheima.mapper.UserMapper">
    <!-- 1. 定义复杂映射规则：id唯一标识，type映射实体类，autoMapping自动匹配字段 -->
    <resultMap id="userResultMap" type="user" autoMapping="true">
        <!-- 2. 主键映射（规范写法，可省略） -->
        <id property="id" column="id"/>
        <!-- 普通字段映射（autoMapping=true可自动匹配，可省略） -->
        <result property="username" column="username"/>
        <!-- 3. 核心：一对多关联映射，延迟加载配置 -->
        <collection
            property="orderList"
            ofType="order"
            select="com.itheima.mapper.OrderMapper.findByUid"
            column="id"
            fetchType="lazy">
        </collection>
    </resultMap>
    <!-- 4. 查询用户，绑定上面的映射规则 -->
    <select id="selectById" resultMap="userResultMap">
        select * from tb_user where id = #{id}
    </select>
</mapper>

<settings>
    <!-- 开启延迟加载总开关 -->
    <setting name="lazyLoadingEnabled" value="true"/>
    <!-- 关闭积极加载（3.4.1后默认false，配置更保险） -->
    <setting name="aggressiveLazyLoading" value="false"/>
</settings>

<mapper namespace="com.itheima.mapper.OrderMapper">
    <!-- 根据用户id查询订单：供一对多延迟加载调用 -->
    <select id="findByUid" resultType="com.itheima.pojo.Order">
        select * from tb_order where user_id = #{userId}
    </select>
</mapper>

// UserMapper
public interface UserMapper {
    User selectById(Integer id);
}
// OrderMapper
public interface OrderMapper {
    List<Order> findByUid(Integer userId);
    Order selectById(Integer id);
}

@Test
public void testOneToOne(){
    // 1. 仅查询订单：只执行1条SQL → select * from tb_order where id=1
    Order order = orderMapper.selectById(1);
    System.out.println("订单编号：" + order.getOrderNo());
    // 2. 调用user：触发一对一延迟加载，查询用户信息
    System.out.println("订单所属用户：" + order.getUser());
}

@Test
public void testLazyLoad(){
    // 1. 仅查询用户：只执行1条SQL → select * from tb_user where id=1
    User user = userMapper.selectById(1);
    System.out.println("查询到用户：" + user.getUsername());
    // 2. 未使用订单数据：不执行订单查询SQL
    // 3. 调用orderList：触发延迟加载，执行订单SQL → select * from tb_order where user_id=1
    System.out.println("用户订单：" + user.getOrderList());
}