Mybatis：延迟加载

拼搏现实的明天。 2024-03-23 13:27 58阅读 0赞

#### 局部延迟加载 ####

<resultMap id="userMap" type="com.wuxian.pojo.User">
        <result property="id" column="uid"></result>
        <result property="username" column="username"></result>
        
        <collection property="orderList" ofType="com.wuxian.pojo.Order" 
                    select="com.wuxian.mapper.IOrderMapper.findOrderByUid" column="uid" fetchType="lazy">
            <result property="id" column="id"></result>
            <result property="orderTime" column="orderTime"></result>
            <result property="total" column="total"></result>
        </collection>
    </resultMap>

#### 全局延迟加载 ####

<settings>
        
        <setting name="lazyLoadingEnabled" value="true"/>
    </settings>

#### **注意** ####

fetchType = "eager"

#### 基本使用 ####

**sql**

-- 延迟加载是基于嵌套查询来实现的
    -- 联合查询
    select u,*,o.id oid,o.ordertime,o,total,o,uid from user u left join orders 0 on o,uid = u.id
    
    
    -- 嵌套查询:将多表操作拆分成多个单表操作
    -- 1.先查询用户信息
    select * from user where id = 1
    -- 2再根据tom用户信息查询关联的订单信息
    select * from orders where uid = l

**Mapper.xml**

<mapper namespace="com.wuxian.mapper.IUserMapper">
        <resultMap id="userMap" type="com.wuxian.pojo.User">
            <result property="id" column="uid"></result>
            <result property="username" column="username"></result>
            <collection property="orderList" ofType="com.wuxian.pojo.Order" 
                        select="com.wuxian.mapper.IOrderMapper.findOrderByUid" column="uid">
                <result property="id" column="id"></result>
                <result property="orderTime" column="orderTime"></result>
                <result property="total" column="total"></result>
            </collection>
        </resultMap>
    
        <select id="findById" resultMap="userMap">
            select * from user where id = #{id}
        </select>
    </mapper>
    
    
    <mapper namespace="com.wuxian.mapper.IOrderMapper">
    
        <select id="findOrderByUid" resultType="com.wuxian.pojo.Order">
            select * from orders where uid = #{uid}
        </select>
    </mapper>