（九） MyBatis从入门到入土——延迟加载、鉴别器以及继承
这是mybatis系列第9篇，没看前文的建议先去【Java冢狐】公众号中查看前文，方便理解和掌握。在上一篇中咱们介绍了关于MyBatis的自动映射是如何开启以及使用的，想必你们对于这方面的知识有所了解了。java
今天要给你们带来的主要是MyBatis延迟加载以及鉴别器相关方面的知识以及内容。sql
延迟加载
延迟加载介绍
所谓的延迟加载就是将数据加载时机推迟，其中比较典型的应用就是推迟嵌套查询的执行时机。数据库
由于在mybatis中常常用到关联查询，可是并非任什么时候候都须要当即返回关联查询结果。就好比查询订单信息，并不必定须要及时返回订单对应的用户信息或者订单详情信息等，因此当咱们遇到这种状况时就须要一种机制，当须要查看关联的数据时，再去执行对应的查询，返回须要的结果。编程
这种需求在mybatis中可使用延迟加载机制来实现。mybatis
延迟加载2种设置方式
MyBatis对于延迟加载提供了两种设置方式，分别是：app
- 全局配置的方式
- sqlmap中配置的方式
从这两种方式的名称来看就能发现这两种延迟加载的区别，第一种方法会对全部的关联查询有效，而第二种方法只会对相关设置的查询有效。测试
下面咱们就分别看一下这两种延迟加载的使用方式。fetch
全局配置延迟加载
要想实现全局配置延迟加载就要经过mybatis的配置文件。优化
mybatis配置文件中经过下面两个属性来控制延迟加载：ui
<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，会去加载全部延迟属性，若是为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，若是须要开启延迟加载，将其设置为true
- aggressiveLazyLoading：当为true的时候，调用任意延迟属性，会去加载全部延迟属性，若是为false，则调用某个属性的时候，只会加载指定的属性
关于全局配置延迟加载的说明就是这些，下面咱们来用一个具体的例子来讲明下全局配置延迟加载是如何使用的。
需求
此次咱们要使用MyBatis实现的需求就是经过订单id查询订单的各类信息，诸如：订单用户信息、订单明细列表。其中订单用户信息和订单明细信息采用延迟加载的方式来进行获取。
mybatis配置
按照前面的介绍第一步就是经过mybatis的配置文件来进行设置。以下所示：
<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，会去加载全部延迟属性，若是为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
进行全局配置的设置完毕之后，咱们就进行咱们正常开发的途径，首先就是编写xml文件，在xml文件中编写咱们的sql语句。
<resultMap id="orderModelMap1" type="com.zhonghu.chat09.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"/>
<!-- 经过订单中user_id做为条件，查询下单人信息 -->
<association property="userModel" select="com.zhonghu.chat09.demo5.mapper.UserMapper.getById1" column="user_Id"/>
<!-- 经过订单id做为条件，查询详情列表 -->
<collection property="orderDetailModelList" select="com.zhonghu.chat09.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
orders a
WHERE
a.id = #{value}
]]>
</select>
上面的orderModelMap1元素下面有两个关联查询，咱们也写一下。
UserMapper.xml
<!-- 根据用户id查询用户信息 -->
<select id="getById1" resultType="com.zhonghu.chat09.demo5.model.UserModel">
<![CDATA[
SELECT id,name FROM 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.zhonghu.chat09.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
order_detail a
WHERE
a.order_id = #{order_id}
]]>
</select>
对应的3个Model
上面咱们写了三个xml，那么接来下咱们编写一下xml对应的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
写完了Model，咱们的代码基本就大功告成了，接下来咱们就来看一看延迟加载的使用效果。
com.zhonghu.chat09.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 orders 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 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 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=Java冢狐)
从日志中能够看出，总共有3次查询，后面2次查询在分割线以后出现的，说明是调用了orderModel.getUserModel()触发后面2次查询动做。
代码中咱们调用的是获取用户信息，而订单列表信息也被加载了，这个主要是因为aggressiveLazyLoading被设置为true了，当使用到一个延迟加载的属性时，其余的延迟加载的属性也会被一块儿加载，因此触发了2个关联的查询。
下面咱们看看将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，会去加载全部延迟属性，若是为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 orders 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 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=Java冢狐)
经过这两次对比，咱们能够很轻易的看出延迟加载开启与否的效果区别。
sqlmap中设置延迟加载
上面的篇幅咱们介绍了全局的延迟加载是如何起做用以及如何使用的。全局的方式会对全部的关联查询起效，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，通常咱们使用association、collection，这两个元素都有个属性fetchType，经过这个属性能够指定关联查询的加载方式。
fetchType有两个值
- eager：当即加载
- lazy：延迟加载
下面咱们来实现一个需求：仍是经过订单id查询订单信息，并获取关联的用户信息、订单详细列表，用户信息咱们要求当即加载，而订单详情咱们要求延迟加载。
mapper xml以下
<resultMap id="orderModelMap2" type="com.zhonghu.chat09.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"/>
<!-- 经过订单中user_id做为条件，查询下单人信息 -->
<association property="userModel" fetchType="eager" select="com.zhonghu.chat09.demo5.mapper.UserMapper.getById1" column="user_Id"/>
<!-- 经过订单id做为条件，查询详情列表 -->
<collection property="orderDetailModelList" fetchType="lazy" select="com.zhonghu.chat09.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
orders 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表示当即加载，lazy表示延迟加载。
测试用例
com.zhonghu.chat09.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 orders 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 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 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=16.00), OrderDetailModel(id=2, orderId=1, goodsId=1, num=1, totalPrice=16.00)]
注意输出中的分割线，能够分析得出，用户信息是和订单信息一块儿当即查出来的，而订单详情，是在咱们调用orderModel.getOrderDetailModelList()获取订单列表的时候，采起懒加载的。
鉴别器(discriminator)
有时候，一个数据库查询可能会返回多个不一样的结果集（但整体上仍是有必定的联系的）， 鉴别器（discriminator）元素就是被设计来应对这种状况的，鉴别器的概念很好理解——它很像 Java 语言中的 switch 语句。
discriminator标签经常使用的两个属性以下：
- 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标签能够有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。
咱们使用鉴别器实现一个功能：经过订单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；其余状况默认只查询订单信息。
OrderMapper.xml
<resultMap id="orderModelMap1" type="com.zhonghu.chat09.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">
<!--经过用户id查询用户信息-->
<association property="userModel" select="com.zhonghu.chat09.demo6.mapper.UserMapper.getById1" column="user_Id"/>
</case>
<case value="2">
<!--经过用户id查询用户信息-->
<association property="userModel" select="com.zhonghu.chat09.demo6.mapper.UserMapper.getById1" column="user_Id"/>
<!--经过订单id查询订单列表-->
<collection property="orderDetailModelList" select="com.zhonghu.chat09.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
orders 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
<!-- 经过用户id查询用户信息 -->
<select id="getById1" resultType="com.zhonghu.chat09.demo6.model.UserModel">
<![CDATA[
SELECT id,name FROM user where id = #{user_id}
]]>
</select>
OrderDetailMapper.xml
<!-- 经过订单id查询订单明细列表 -->
<select id="getListByOrderId1" resultType="com.zhonghu.chat09.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
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.zhonghu.chat09.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 orders 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 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=1610803573, upTime=1610803573, userModel=UserModel(id=2, name=Java冢狐), 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 orders 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 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 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=1610803573, upTime=1610803573, userModel=UserModel(id=1, name=冢狐), orderDetailModelList=[OrderDetailModel(id=3, orderId=2, goodsId=1, num=1, totalPrice=8.00)]) 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 orders 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=1610803573, upTime=1610803573, userModel=null, orderDetailModelList=null)
输出中能够看出，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，能够起到重用代码的做用，而mybatis也有继承的功能，和java中的继承的做用相似，主要在resultMap中使用，能够重用其余resultMap中配置的映射关系。
用法
<resultMap extends="被继承的resultMap的id"></resultMap>
案例
下面咱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.zhonghu.chat09.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.zhonghu.chat09.demo6.model.OrderModel" extends="orderModelMap2">
<!--经过用户id查询用户信息-->
<association property="userModel" select="com.zhonghu.chat09.demo6.mapper.UserMapper.getById1" column="user_Id"/>
</resultMap>
<resultMap id="orderModelMap4" type="com.zhonghu.chat09.demo6.model.OrderModel" extends="orderModelMap3">
<!--经过订单id查询订单列表-->
<collection property="orderDetailModelList" select="com.zhonghu.chat09.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
orders 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（除鉴别器以外），本身又加入了一个association去查询用户信息；orderModelMap4继承了orderModelMap3，本身又加入了一个查询订单列表的collection元素。上面使用extends作到了代码重用，其实和下面这块代码写法效果同样：
<resultMap id="orderModelMap2" type="com.zhonghu.chat09.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.zhonghu.chat09.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"/>
<!--经过用户id查询用户信息-->
<association property="userModel" select="com.zhonghu.chat09.demo6.mapper.UserMapper.getById1" column="user_Id"/>
</resultMap>
<resultMap id="orderModelMap4" type="com.zhonghu.chat09.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"/>
<!--经过用户id查询用户信息-->
<association property="userModel" select="com.zhonghu.chat09.demo6.mapper.UserMapper.getById1" column="user_Id"/>
<!--经过订单id查询订单列表-->
<collection property="orderDetailModelList" select="com.zhonghu.chat09.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
orders a
WHERE
a.id = #{value}
]]>
</select>
测试用例
com.zhonghu.chat09.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 orders 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 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=1610803573, upTime=1610803573, userModel=UserModel(id=2, name=Java冢狐), 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 orders 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 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 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=1610803573, upTime=1610803573, userModel=UserModel(id=1, name=冢狐), orderDetailModelList=[OrderDetailModel(id=3, orderId=2, goodsId=1, num=1, totalPrice=8.00)]) 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 orders 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=1610803573, upTime=1610803573, userModel=null, orderDetailModelList=null)
总结
本篇开篇重点介绍了有关MyBatis的延迟加载的相关的内容，对其中的全局延迟加载和部分延迟加载进行了注重的介绍，介绍完延迟加载后，又介绍了一些与之相关的鉴别器和继承的相关内容。
最后
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