Mybatis系列第9篇：延迟加载、鉴别器、继承怎么玩？
2022/4/23 6:13:11
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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
全局配置的方式
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@AllArgsConstructorpublic 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@AllArgsConstructorpublic class UserModel { private Integer id; private String name;} @Getter@Setter@Builder@ToString@NoArgsConstructor@AllArgsConstructorpublic 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 @Testpublic 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: 101:55.431 [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: 201:55.439 [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: 101:55.441 [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: 112:19.337 [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: 112:19.341 [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 @Testpublic 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: 136:54.389 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById2 - <== Total: 136:54.390 [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: 236:54.398 [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@AllArgsConstructorpublic 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@AllArgsConstructorpublic class UserModel { private Integer id; private String name;} @Getter@Setter@Builder@ToString@NoArgsConstructor@AllArgsConstructorpublic 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 @Testpublic 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: 158:16.489 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById1 - <== Total: 158:16.489 [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: 158:16.495 [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: 158:16.505 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById1 - <== Total: 158:16.506 [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: 158:16.509 [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 @Testpublic 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: 139:55.993 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById2 - <== Total: 139:55.993 [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: 139:56.001 [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: 139:56.005 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById2 - <== Total: 139:56.005 [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: 139:56.007 [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
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