专栏首页路人甲JavaMybatis系列第9篇:延迟加载、鉴别器、继承怎么玩?

Mybatis系列第9篇:延迟加载、鉴别器、继承怎么玩?

Mybatis系列目标:从入门开始开始掌握一个高级开发所需要的Mybatis技能。

这是mybatis系列第9篇,源码位于文章尾部!

建库建表

创建一个db:javacode2018

4张表:

t_user(用户表)

t_goods(商品表)

t_order(订单表)

t_order_detail(订单明细表)

表之间的关系:

t_order和t_user是一对一的关系,一条订单关联一个用户记录

t_order和t_order_detail是一对多关系,每个订单中可能包含多个子订单,每个子订单对应一个商品

DROP DATABASE IF EXISTS `javacode2018`;
CREATE DATABASE `javacode2018`;

USE `javacode2018`;

DROP TABLE IF EXISTS t_user;
CREATE TABLE t_user(
  id int AUTO_INCREMENT PRIMARY KEY COMMENT '用户id',
  name VARCHAR(32) NOT NULL DEFAULT '' COMMENT '用户名'
) COMMENT '用户表';
INSERT INTO t_user VALUES (1,'张学友'),(2,'路人甲Java');

DROP TABLE IF EXISTS t_goods;
CREATE TABLE t_goods(
  id int AUTO_INCREMENT PRIMARY KEY COMMENT '商品id',
  name VARCHAR(32) NOT NULL DEFAULT '' COMMENT '商品名称',
  price DECIMAL(10,2) NOT NULL DEFAULT 0 COMMENT '商品价格'
) COMMENT '商品信息表';
INSERT INTO t_goods VALUES (1,'Mybatis系列',8.88),(2,'maven高手系列',16.66);

DROP TABLE IF EXISTS t_order;
CREATE TABLE t_order(
  id int AUTO_INCREMENT PRIMARY KEY COMMENT '订单id',
  user_id INT NOT NULL DEFAULT 0 COMMENT '用户id,来源于t_user.id',
  create_time BIGINT NOT NULL DEFAULT 0 COMMENT '订单创建时间(时间戳,秒)',
  up_time BIGINT NOT NULL DEFAULT 0 COMMENT '订单最后修改时间(时间戳,秒)'
) COMMENT '订单表';
INSERT INTO t_order VALUES (1,2,unix_timestamp(now()),unix_timestamp(now())),(2,1,unix_timestamp(now()),unix_timestamp(now())),(3,1,unix_timestamp(now()),unix_timestamp(now()));

DROP TABLE IF EXISTS t_order_detail;
CREATE TABLE t_order_detail(
  id int AUTO_INCREMENT PRIMARY KEY COMMENT '订单明细id',
  order_id INT NOT NULL DEFAULT 0 COMMENT '订单id,来源于t_order.id',
  goods_id INT NOT NULL DEFAULT 0 COMMENT '商品id,来源于t_goods.id',
  num INT NOT NULL DEFAULT 0 COMMENT '商品数量',
  total_price DECIMAL(12,2) NOT NULL DEFAULT 0 COMMENT '商品总金额'
) COMMENT '订单表';
INSERT INTO t_order_detail VALUES (1,1,1,2,17.76),(2,1,1,1,16.66),(3,2,1,1,8.88),(4,3,1,1,8.88);

select * from t_user;
select * from t_goods;
select * from t_order;
select * from t_order_detail;

延迟加载

延迟加载其实就是将数据加载时机推迟,比如推迟嵌套查询的执行时机,在mybatis中经常用到关联查询,但是并不是任何时候都需要立即返回关联查询结果。比如查询订单信息,并不一定需要及时返回订单对应的用户信息或者订单详情信息等,这种情况需要一种机制,当需要查看关联的数据时,再去执行对应的查询,返回需要的结果,这种需求在mybatis中可以使用延迟加载机制来实现。

延迟加载2种设置方式

  1. 全局配置的方式
  2. sqlmap中配置的方式

方式1中会对所有关联查询起效,而方式2只会对相关设置的查询起效。

全局配置延迟加载

mybatis配置文件中通过下面两个属性来控制延迟加载:

<settings>
    <!--打开延迟加载的开关  -->
    <setting name="lazyLoadingEnabled" value="true"/>
    <!-- 当为true的时候,调用任意延迟属性,会去加载所有延迟属性,如果为false,则调用某个属性的时候,只会加载指定的属性 -->
    <setting name="aggressiveLazyLoading" value="false"/>
</settings>

lazyLoadingEnabled:这个属性比较好理解,是否开启延迟加载,默认为false,如果需要开启延迟加载,将其设置为true

aggressiveLazyLoading:当为true的时候,调用任意延迟属性,会去加载所有延迟属性,如果为false,则调用某个属性的时候,只会加载指定的属性

下面我们来个案例感受一下效果。

需求

通过订单id查询订单信息、订单用户信息、订单明细列表,而订单用户信息、订单明细列表采用延迟加载的方式获取。

mybatis配置

<settings>
    <!--打开延迟加载的开关  -->
    <setting name="lazyLoadingEnabled" value="true"/>
    <!-- 当为true的时候,调用任意延迟属性,会去加载所有延迟属性,如果为false,则调用某个属性的时候,只会加载指定的属性 -->
    <setting name="aggressiveLazyLoading" value="true"/>
</settings>

OrderMapper.xml

<resultMap id="orderModelMap1" type="com.javacode2018.chat05.demo5.model.OrderModel">
    <id column="id" property="id"/>
    <result column="user_id" property="userId"/>
    <result column="create_time" property="createTime"/>
    <result column="up_time" property="upTime"/>
    <!--  通过订单中user_id作为条件,查询下单人信息 -->
    <association property="userModel" select="com.javacode2018.chat05.demo5.mapper.UserMapper.getById1" column="user_Id"/>
    <!--  通过订单id作为条件,查询详情列表 -->
    <collection property="orderDetailModelList" select="com.javacode2018.chat05.demo5.mapper.OrderDetailMapper.getListByOrderId1" column="id"/>
</resultMap>

<select id="getById1" resultMap="orderModelMap1">
    <![CDATA[
    SELECT
        a.id ,
        a.user_id,
        a.create_time,
        a.up_time
    FROM
        t_order a
    WHERE
        a.id = #{value}
    ]]>
</select>

上面的orderModelMap1元素下面有两个关联查询,我们也写一下。

UserMapper.xml

<!-- 根据用户id查询用户信息 -->
<select id="getById1" resultType="com.javacode2018.chat05.demo5.model.UserModel">
    <![CDATA[
    SELECT id,name FROM t_user where id = #{user_id}
    ]]>
</select>

OrderDetailMapper.xml

<!-- 根据订单di查询订单明细列表 -->
<select id="getListByOrderId1" resultType="com.javacode2018.chat05.demo5.model.OrderDetailModel">
    <![CDATA[
    SELECT
        a.id,
        a.order_id AS orderId,
        a.goods_id AS goodsId,
        a.num,
        a.total_price AS totalPrice
    FROM
        t_order_detail a
    WHERE
        a.order_id = #{order_id}
    ]]>
</select>

对应的3个Model

@Getter
@Setter
@Builder
@ToString
@NoArgsConstructor
@AllArgsConstructor
public class OrderModel {
    private Integer id;
    private Integer userId;
    private Long createTime;
    private Long upTime;
    private UserModel userModel;
    //订单详情列表
    private List<OrderDetailModel> orderDetailModelList;
}

@Getter
@Setter
@Builder
@ToString
@NoArgsConstructor
@AllArgsConstructor
public class UserModel {
    private Integer id;
    private String name;
}

@Getter
@Setter
@Builder
@ToString
@NoArgsConstructor
@AllArgsConstructor
public class OrderDetailModel {
    private Integer id;
    private Integer orderId;
    private Integer goodsId;
    private Integer num;
    private Double totalPrice;
}

测试用例

com.javacode2018.chat05.demo5.Demo5Test#getById1

@Test
public void getById1() throws IOException {
    //指定mybatis全局配置文件
    mybatisConfig = "demo5/mybatis-config.xml";
    this.before();
    OrderModel orderModel = null;
    try (SqlSession sqlSession = this.sqlSessionFactory.openSession(true);) {
        OrderMapper mapper = sqlSession.getMapper(OrderMapper.class);
        orderModel = mapper.getById1(1);
    }
    log.info("-------分割线--------");
    log.info("{}", orderModel.getUserModel());
}

运行输出

01:55.343 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById1 - ==>  Preparing: SELECT a.id , a.user_id, a.create_time, a.up_time FROM t_order a WHERE a.id = ? 
01:55.372 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById1 - ==> Parameters: 1(Integer)
01:55.431 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById1 - <==      Total: 1
01:55.431 [main] INFO  c.j.chat05.demo5.Demo5Test - -------分割线--------
01:55.432 [main] DEBUG c.j.c.d.m.O.getListByOrderId1 - ==>  Preparing: SELECT a.id, a.order_id AS orderId, a.goods_id AS goodsId, a.num, a.total_price AS totalPrice FROM t_order_detail a WHERE a.order_id = ? 
01:55.432 [main] DEBUG c.j.c.d.m.O.getListByOrderId1 - ==> Parameters: 1(Integer)
01:55.435 [main] DEBUG c.j.c.d.m.O.getListByOrderId1 - <==      Total: 2
01:55.439 [main] DEBUG c.j.c.d.mapper.UserMapper.getById1 - ==>  Preparing: SELECT id,name FROM t_user where id = ? 
01:55.439 [main] DEBUG c.j.c.d.mapper.UserMapper.getById1 - ==> Parameters: 2(Integer)
01:55.441 [main] DEBUG c.j.c.d.mapper.UserMapper.getById1 - <==      Total: 1
01:55.441 [main] INFO  c.j.chat05.demo5.Demo5Test - UserModel(id=2, name=路人甲Java)

从日志中可以看出,总共有3次查询,后面2次查询在分割线之后出现的,说明是调用了orderModel.getUserModel()触发后面2次查询动作。

代码中我们调用的是获取用户信息,而订单列表信息也被加载了,这个主要是由于aggressiveLazyLoading被设置为true了,当使用到一个延迟加载的属性时,其他的延迟加载的属性也会被一起加载,所以触发了2个关联的查询。

下面我们看看将aggressiveLazyLoading设置为false的效果

<settings>
    <!--打开延迟加载的开关  -->
    <setting name="lazyLoadingEnabled" value="true"/>
    <!-- 当为true的时候,调用任意延迟属性,会去加载所有延迟属性,如果为false,则调用某个属性的时候,只会加载指定的属性 -->
    <setting name="aggressiveLazyLoading" value="false"/>
</settings>

再次运行测试用例输出

12:19.236 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById1 - ==>  Preparing: SELECT a.id , a.user_id, a.create_time, a.up_time FROM t_order a WHERE a.id = ? 
12:19.268 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById1 - ==> Parameters: 1(Integer)
12:19.336 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById1 - <==      Total: 1
12:19.337 [main] INFO  c.j.chat05.demo5.Demo5Test - -------分割线--------
12:19.338 [main] DEBUG c.j.c.d.mapper.UserMapper.getById1 - ==>  Preparing: SELECT id,name FROM t_user where id = ? 
12:19.338 [main] DEBUG c.j.c.d.mapper.UserMapper.getById1 - ==> Parameters: 2(Integer)
12:19.340 [main] DEBUG c.j.c.d.mapper.UserMapper.getById1 - <==      Total: 1
12:19.341 [main] INFO  c.j.chat05.demo5.Demo5Test - UserModel(id=2, name=路人甲Java)

sqlmap中设置延迟加载

全局的方式会对所有的关联查询起效,影响范围比较大,mybatis也提供了在关联查询中进行设置的方式,只会对当前设置的关联查询起效。

关联查询,一般我们使用association、collection,这两个元素都有个属性fetchType,通过这个属性可以指定关联查询的加载方式。

fetchType值有2种,eager:立即加载;lazy:延迟加载。

下面我们来实现一个需求:还是通过订单id查询订单信息,并获取关联的用户信息、订单详细列表,用户信息我们要求立即加载,而订单详情我们要求延迟加载。

mapper xml如下

<resultMap id="orderModelMap2" type="com.javacode2018.chat05.demo5.model.OrderModel">
    <id column="id" property="id"/>
    <result column="user_id" property="userId"/>
    <result column="create_time" property="createTime"/>
    <result column="up_time" property="upTime"/>
    <!--  通过订单中user_id作为条件,查询下单人信息 -->
    <association property="userModel" fetchType="eager" select="com.javacode2018.chat05.demo5.mapper.UserMapper.getById1" column="user_Id"/>
    <!--  通过订单id作为条件,查询详情列表 -->
    <collection property="orderDetailModelList" fetchType="lazy" select="com.javacode2018.chat05.demo5.mapper.OrderDetailMapper.getListByOrderId1" column="id"/>
</resultMap>

<select id="getById2" resultMap="orderModelMap2">
    <![CDATA[
    SELECT
        a.id ,
        a.user_id,
        a.create_time,
        a.up_time
    FROM
        t_order a
    WHERE
        a.id = #{value}
    ]]>
</select>

重点注意上面配置中association、collection这2个元素的fetchType属性,eager表示立即加载,lazy表示延迟加载。

测试用例

com.javacode2018.chat05.demo5.Demo5Test#getById2

@Test
public void getById2() throws IOException {
    //指定mybatis全局配置文件
    mybatisConfig = "demo5/mybatis-config2.xml";
    this.before();
    OrderModel orderModel = null;
    try (SqlSession sqlSession = this.sqlSessionFactory.openSession(true);) {
        OrderMapper mapper = sqlSession.getMapper(OrderMapper.class);
        orderModel = mapper.getById2(1);
    }
    log.info("-------分割线--------");
    log.info("{}", orderModel.getOrderDetailModelList());
}

运行输出

36:54.284 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById2 - ==>  Preparing: SELECT a.id , a.user_id, a.create_time, a.up_time FROM t_order a WHERE a.id = ? 
36:54.321 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById2 - ==> Parameters: 1(Integer)
36:54.385 [main] DEBUG c.j.c.d.mapper.UserMapper.getById1 - ====>  Preparing: SELECT id,name FROM t_user where id = ? 
36:54.385 [main] DEBUG c.j.c.d.mapper.UserMapper.getById1 - ====> Parameters: 2(Integer)
36:54.387 [main] DEBUG c.j.c.d.mapper.UserMapper.getById1 - <====      Total: 1
36:54.389 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById2 - <==      Total: 1
36:54.390 [main] INFO  c.j.chat05.demo5.Demo5Test - -------分割线--------
36:54.392 [main] DEBUG c.j.c.d.m.O.getListByOrderId1 - ==>  Preparing: SELECT a.id, a.order_id AS orderId, a.goods_id AS goodsId, a.num, a.total_price AS totalPrice FROM t_order_detail a WHERE a.order_id = ? 
36:54.392 [main] DEBUG c.j.c.d.m.O.getListByOrderId1 - ==> Parameters: 1(Integer)
36:54.397 [main] DEBUG c.j.c.d.m.O.getListByOrderId1 - <==      Total: 2
36:54.398 [main] INFO  c.j.chat05.demo5.Demo5Test - [OrderDetailModel(id=1, orderId=1, goodsId=1, num=2, totalPrice=17.76), OrderDetailModel(id=2, orderId=1, goodsId=1, num=1, totalPrice=16.66)]

注意输出中的分割线,可以分析得出,用户信息是和订单信息一起立即查出来的,而订单详情,是在我们调用orderModel.getOrderDetailModelList()获取订单列表的时候,采取懒加载的。

鉴别器(discriminator)

有时候,一个数据库查询可能会返回多个不同的结果集(但总体上还是有一定的联系的), 鉴别器(discriminator)元素就是被设计来应对这种情况的,鉴别器的概念很好理解——它很像 Java 语言中的 switch 语句。

discriminator标签常用的两个属性如下:

  • column:该属性用于设置要进行鉴别比较值的列。
  • javaType:该属性用于指定列的类型,保证使用相同的java类型来比较值。

discriminator标签可以有1个或多个case标签,case标签有一个比较重要的属性:

  • value:该值为discriminator指定column用来匹配的值,当匹配的时候,结果会走这个case关联的映射。

我们使用鉴别器实现一个功能:通过订单id查询订单信息,当传入的订单id为1的时候,获取订单信息及下单人信息;当传入的订单id为2的时候,获取订单信息、下单人信息、订单明细信息;其他情况默认只查询订单信息。

OrderMapper.xml

<resultMap id="orderModelMap1" type="com.javacode2018.chat05.demo6.model.OrderModel">
    <id column="id" property="id"/>
    <result column="user_id" property="userId"/>
    <result column="create_time" property="createTime"/>
    <result column="up_time" property="upTime"/>
    <!-- 鉴别器 -->
    <discriminator javaType="int" column="id">
        <case value="1">
            <!--通过用户id查询用户信息-->
            <association property="userModel" select="com.javacode2018.chat05.demo6.mapper.UserMapper.getById1" column="user_Id"/>
        </case>
        <case value="2">
            <!--通过用户id查询用户信息-->
            <association property="userModel" select="com.javacode2018.chat05.demo6.mapper.UserMapper.getById1" column="user_Id"/>
            <!--通过订单id查询订单列表-->
            <collection property="orderDetailModelList" select="com.javacode2018.chat05.demo6.mapper.OrderDetailMapper.getListByOrderId1" column="id"/>
        </case>
    </discriminator>
</resultMap>

<select id="getById1" resultMap="orderModelMap1">
    <![CDATA[
    SELECT
        a.id ,
        a.user_id,
        a.create_time,
        a.up_time
    FROM
        t_order a
    WHERE
        a.id = #{value}
    ]]>
</select>

注意上面的discriminator,这部分是关键,discriminator内部的case会和每行查询结果中的id字段进行匹配,匹配成功了case内部的关联查询会被执行,未匹配上的,只会走discriminator外部默认配置的映射映射规则。

UserMapper.xml

<!-- 通过用户id查询用户信息 -->
<select id="getById1" resultType="com.javacode2018.chat05.demo6.model.UserModel">
    <![CDATA[
    SELECT id,name FROM t_user where id = #{user_id}
    ]]>
</select>

OrderDetailMapper.xml

<!-- 通过订单id查询订单明细列表 -->
<select id="getListByOrderId1" resultType="com.javacode2018.chat05.demo6.model.OrderDetailModel">
    <![CDATA[
    SELECT
        a.id,
        a.order_id AS orderId,
        a.goods_id AS goodsId,
        a.num,
        a.total_price AS totalPrice
    FROM
        t_order_detail a
    WHERE
        a.order_id = #{order_id}
    ]]>
</select>

对应的3个Model类

@Getter
@Setter
@Builder
@ToString
@NoArgsConstructor
@AllArgsConstructor
public class OrderModel {
    private Integer id;
    private Integer userId;
    private Long createTime;
    private Long upTime;
    //用户信息
    private UserModel userModel;
    //订单详情列表
    private List<OrderDetailModel> orderDetailModelList;
}


@Getter
@Setter
@Builder
@ToString
@NoArgsConstructor
@AllArgsConstructor
public class UserModel {
    private Integer id;
    private String name;
}

@Getter
@Setter
@Builder
@ToString
@NoArgsConstructor
@AllArgsConstructor
public class OrderDetailModel {
    private Integer id;
    private Integer orderId;
    private Integer goodsId;
    private Integer num;
    private Double totalPrice;
}

测试用例

com.javacode2018.chat05.demo6.Demo6Test#getById1

@Test
public void getById1() throws IOException {
    try (SqlSession sqlSession = this.sqlSessionFactory.openSession(true);) {
        OrderMapper mapper = sqlSession.getMapper(OrderMapper.class);
        //查询订单为1的
        OrderModel orderModel = mapper.getById1(1);
        log.info("{}", orderModel);
        log.info("------------------------------------------------------------");
        //查询订单为2的
        orderModel = mapper.getById1(2);
        log.info("{}", orderModel);
        log.info("------------------------------------------------------------");
        //查询订单为3的
        orderModel = mapper.getById1(3);
        log.info("{}", orderModel);
    }
}

运行输出

58:16.413 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById1 - ==>  Preparing: SELECT a.id , a.user_id, a.create_time, a.up_time FROM t_order a WHERE a.id = ? 
58:16.457 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById1 - ==> Parameters: 1(Integer)
58:16.481 [main] DEBUG c.j.c.d.mapper.UserMapper.getById1 - ====>  Preparing: SELECT id,name FROM t_user where id = ? 
58:16.481 [main] DEBUG c.j.c.d.mapper.UserMapper.getById1 - ====> Parameters: 2(Integer)
58:16.488 [main] DEBUG c.j.c.d.mapper.UserMapper.getById1 - <====      Total: 1
58:16.489 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById1 - <==      Total: 1
58:16.489 [main] INFO  c.j.chat05.demo6.Demo6Test - OrderModel(id=1, userId=2, createTime=1578368161, upTime=1578368161, userModel=UserModel(id=2, name=路人甲Java), orderDetailModelList=null)
58:16.491 [main] INFO  c.j.chat05.demo6.Demo6Test - ------------------------------------------------------------
58:16.491 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById1 - ==>  Preparing: SELECT a.id , a.user_id, a.create_time, a.up_time FROM t_order a WHERE a.id = ? 
58:16.492 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById1 - ==> Parameters: 2(Integer)
58:16.493 [main] DEBUG c.j.c.d.mapper.UserMapper.getById1 - ====>  Preparing: SELECT id,name FROM t_user where id = ? 
58:16.493 [main] DEBUG c.j.c.d.mapper.UserMapper.getById1 - ====> Parameters: 1(Integer)
58:16.494 [main] DEBUG c.j.c.d.mapper.UserMapper.getById1 - <====      Total: 1
58:16.495 [main] DEBUG c.j.c.d.m.O.getListByOrderId1 - ====>  Preparing: SELECT a.id, a.order_id AS orderId, a.goods_id AS goodsId, a.num, a.total_price AS totalPrice FROM t_order_detail a WHERE a.order_id = ? 
58:16.495 [main] DEBUG c.j.c.d.m.O.getListByOrderId1 - ====> Parameters: 2(Integer)
58:16.505 [main] DEBUG c.j.c.d.m.O.getListByOrderId1 - <====      Total: 1
58:16.505 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById1 - <==      Total: 1
58:16.506 [main] INFO  c.j.chat05.demo6.Demo6Test - OrderModel(id=2, userId=1, createTime=1578368161, upTime=1578368161, userModel=UserModel(id=1, name=张学友), orderDetailModelList=[OrderDetailModel(id=3, orderId=2, goodsId=1, num=1, totalPrice=8.88)])
58:16.506 [main] INFO  c.j.chat05.demo6.Demo6Test - ------------------------------------------------------------
58:16.506 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById1 - ==>  Preparing: SELECT a.id , a.user_id, a.create_time, a.up_time FROM t_order a WHERE a.id = ? 
58:16.506 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById1 - ==> Parameters: 3(Integer)
58:16.508 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById1 - <==      Total: 1
58:16.509 [main] INFO  c.j.chat05.demo6.Demo6Test - OrderModel(id=3, userId=1, createTime=1578368161, upTime=1578368161, userModel=null, orderDetailModelList=null)

输出中可以看出,订单1查询了2次,订单2查询了3次,订单3查询了1次;鉴别器算是一个不错的功能。

继承(extends)

继承在java是三大特性之一,可以起到重用代码的作用,而mybatis也有继承的功能,和java中的继承的作用类似,主要在resultMap中使用,可以重用其他resultMap中配置的映射关系。

用法

<resultMap extends="被继承的resultMap的id"></resultMap>

案例

下面我们使用继承来对上面的鉴别器的案例改造一下,优化一下代码

OrderMapper.xml
<resultMap id="orderModelMap2" type="com.javacode2018.chat05.demo6.model.OrderModel">
    <id column="id" property="id"/>
    <result column="user_id" property="userId"/>
    <result column="create_time" property="createTime"/>
    <result column="up_time" property="upTime"/>
    <!-- 鉴别器 -->
    <discriminator javaType="int" column="id">
        <case value="1" resultMap="orderModelMap3" />
        <case value="2" resultMap="orderModelMap4" />
    </discriminator>
</resultMap>

<resultMap id="orderModelMap3" type="com.javacode2018.chat05.demo6.model.OrderModel" extends="orderModelMap2">
    <!--通过用户id查询用户信息-->
    <association property="userModel" select="com.javacode2018.chat05.demo6.mapper.UserMapper.getById1" column="user_Id"/>
</resultMap>

<resultMap id="orderModelMap4" type="com.javacode2018.chat05.demo6.model.OrderModel" extends="orderModelMap3">
    <!--通过订单id查询订单列表-->
    <collection property="orderDetailModelList" select="com.javacode2018.chat05.demo6.mapper.OrderDetailMapper.getListByOrderId1" column="id"/>
</resultMap>

<select id="getById2" resultMap="orderModelMap2">
    <![CDATA[
    SELECT
        a.id ,
        a.user_id,
        a.create_time,
        a.up_time
    FROM
        t_order a
    WHERE
        a.id = #{value}
    ]]>
</select>

重点在于上面两个extends属性,上面orderModelMap3继承了orderModelMap2中配置的映射关系(除鉴别器之外),自己又加入了一个association去查询用户信息;orderModelMap4继承了orderModelMap3,自己又加入了一个查询订单列表的collection元素。上面使用extends做到了代码重用,其实和下面这块代码写法效果一样:

<resultMap id="orderModelMap2" type="com.javacode2018.chat05.demo6.model.OrderModel">
    <id column="id" property="id"/>
    <result column="user_id" property="userId"/>
    <result column="create_time" property="createTime"/>
    <result column="up_time" property="upTime"/>
    <!-- 鉴别器 -->
    <discriminator javaType="int" column="id">
        <case value="1" resultMap="orderModelMap3" />
        <case value="2" resultMap="orderModelMap4" />
    </discriminator>
</resultMap>

<resultMap id="orderModelMap3" type="com.javacode2018.chat05.demo6.model.OrderModel">
    <id column="id" property="id"/>
    <result column="user_id" property="userId"/>
    <result column="create_time" property="createTime"/>
    <result column="up_time" property="upTime"/>
    <!--通过用户id查询用户信息-->
    <association property="userModel" select="com.javacode2018.chat05.demo6.mapper.UserMapper.getById1" column="user_Id"/>
</resultMap>

<resultMap id="orderModelMap4" type="com.javacode2018.chat05.demo6.model.OrderModel">
    <id column="id" property="id"/>
    <result column="user_id" property="userId"/>
    <result column="create_time" property="createTime"/>
    <result column="up_time" property="upTime"/>
    <!--通过用户id查询用户信息-->
    <association property="userModel" select="com.javacode2018.chat05.demo6.mapper.UserMapper.getById1" column="user_Id"/>
    <!--通过订单id查询订单列表-->
    <collection property="orderDetailModelList" select="com.javacode2018.chat05.demo6.mapper.OrderDetailMapper.getListByOrderId1" column="id"/>
</resultMap>

<select id="getById2" resultMap="orderModelMap2">
    <![CDATA[
    SELECT
        a.id ,
        a.user_id,
        a.create_time,
        a.up_time
    FROM
        t_order a
    WHERE
        a.id = #{value}
    ]]>
</select>
测试用例
com.javacode2018.chat05.demo6.Demo6Test#getById2

@Test
public void getById2() throws IOException {
    try (SqlSession sqlSession = this.sqlSessionFactory.openSession(true);) {
        OrderMapper mapper = sqlSession.getMapper(OrderMapper.class);
        //查询订单为1的
        OrderModel orderModel = mapper.getById2(1);
        log.info("{}", orderModel);
        log.info("------------------------------------------------------------");
        //查询订单为2的
        orderModel = mapper.getById2(2);
        log.info("{}", orderModel);
        log.info("------------------------------------------------------------");
        //查询订单为3的
        orderModel = mapper.getById2(3);
        log.info("{}", orderModel);
    }
}
运行输出
39:55.936 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById2 - ==>  Preparing: SELECT a.id , a.user_id, a.create_time, a.up_time FROM t_order a WHERE a.id = ? 
39:55.969 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById2 - ==> Parameters: 1(Integer)
39:55.986 [main] DEBUG c.j.c.d.mapper.UserMapper.getById1 - ====>  Preparing: SELECT id,name FROM t_user where id = ? 
39:55.987 [main] DEBUG c.j.c.d.mapper.UserMapper.getById1 - ====> Parameters: 2(Integer)
39:55.992 [main] DEBUG c.j.c.d.mapper.UserMapper.getById1 - <====      Total: 1
39:55.993 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById2 - <==      Total: 1
39:55.993 [main] INFO  c.j.chat05.demo6.Demo6Test - OrderModel(id=1, userId=2, createTime=1578368161, upTime=1578368161, userModel=UserModel(id=2, name=路人甲Java), orderDetailModelList=null)
39:55.994 [main] INFO  c.j.chat05.demo6.Demo6Test - ------------------------------------------------------------
39:55.994 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById2 - ==>  Preparing: SELECT a.id , a.user_id, a.create_time, a.up_time FROM t_order a WHERE a.id = ? 
39:55.995 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById2 - ==> Parameters: 2(Integer)
39:55.995 [main] DEBUG c.j.c.d.m.O.getListByOrderId1 - ====>  Preparing: SELECT a.id, a.order_id AS orderId, a.goods_id AS goodsId, a.num, a.total_price AS totalPrice FROM t_order_detail a WHERE a.order_id = ? 
39:55.996 [main] DEBUG c.j.c.d.m.O.getListByOrderId1 - ====> Parameters: 2(Integer)
39:56.000 [main] DEBUG c.j.c.d.m.O.getListByOrderId1 - <====      Total: 1
39:56.001 [main] DEBUG c.j.c.d.mapper.UserMapper.getById1 - ====>  Preparing: SELECT id,name FROM t_user where id = ? 
39:56.004 [main] DEBUG c.j.c.d.mapper.UserMapper.getById1 - ====> Parameters: 1(Integer)
39:56.005 [main] DEBUG c.j.c.d.mapper.UserMapper.getById1 - <====      Total: 1
39:56.005 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById2 - <==      Total: 1
39:56.005 [main] INFO  c.j.chat05.demo6.Demo6Test - OrderModel(id=2, userId=1, createTime=1578368161, upTime=1578368161, userModel=UserModel(id=1, name=张学友), orderDetailModelList=[OrderDetailModel(id=3, orderId=2, goodsId=1, num=1, totalPrice=8.88)])
39:56.005 [main] INFO  c.j.chat05.demo6.Demo6Test - ------------------------------------------------------------
39:56.005 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById2 - ==>  Preparing: SELECT a.id , a.user_id, a.create_time, a.up_time FROM t_order a WHERE a.id = ? 
39:56.006 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById2 - ==> Parameters: 3(Integer)
39:56.007 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById2 - <==      Total: 1
39:56.007 [main] INFO  c.j.chat05.demo6.Demo6Test - OrderModel(id=3, userId=1, createTime=1578368161, upTime=1578368161, userModel=null, orderDetailModelList=null)

案例代码

链接:https://pan.baidu.com/s/1vt-MAX3oJOu9gyxZAhKkbg 
提取码:i8op

本文分享自微信公众号 - 路人甲Java(javacode2018),作者:路人甲Java

原文出处及转载信息见文内详细说明,如有侵权,请联系 yunjia_community@tencent.com 删除。

原始发表时间:2020-01-07

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