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
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
- @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
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