什么是自动映射？
介绍自动映射之前先看一下手动映射，如下：
<resultMap id="orderModelMap1" type="com.javacode2018.chat05.demo7.model.OrderModel">
<id column="id" property="id"/>
<result column="userId" property="userId" />
<result column="createTime" property="createTime" />
<result column="upTime" property="upTime" />
</resultMap>
<select id="getById1" resultMap="orderModelMap1">
<![CDATA[
SELECT
a.id,
a.user_id userId,
a.create_time createTime,
a.up_time upTime
FROM
t_order a
WHERE
a.id = #{value}
]]>
</select>
注意上面的resultMap元素中有4行配置，如下：
<id column="id" property="id"/> <result column="userId" property="userId" /> <result column="createTime" property="createTime" /> <result column="upTime" property="upTime" />
这4行代码用于配置sql结果的列和OrderModel对象中字段的映射关系。
大家有没有注意到，映射规则中column和property元素的值都是一样，mybatis中支持自动映射配置，当开启自动映射之后，当sql的列名和Model中的字段名称是一样的时候（不区分大小写），mybatis内部会进行自动映射，不需要我们手动去写上面的4行映射规则。
下面我们将上面的示例改成自动映射的方式，如下：
<resultMap id="orderModelMap2" type="com.javacode2018.chat05.demo7.model.OrderModel" autoMapping="true">
</resultMap>
<select id="getById2" resultMap="orderModelMap2">
<![CDATA[
SELECT
a.id,
a.user_id userId,
a.create_time createTime,
a.up_time upTime
FROM
t_order a
WHERE
a.id = #{value}
]]>
</select>
注意上面的resultMap中的autoMapping属性，是否开启自动映射，我们设置为true，这样mybatis会自动按照列名和Model中同名的字段进行映射赋值。
上面两个配置最后查询结果是一样的，都会将查询结果对应的4个字段的值自动赋值给OrderModel中同名的属性。
自动映射开关
mybatis中自动映射主要有2种配置，一种是全局的配置，对应用中所有的resultMap起效，这个是在mybatis配置文件中进行设置的；另外一种是通过resultMap的autoMapping属性进行配置。
mybatis判断某个resultMap是否开启自动映射配置的时候，会先查找自身的autoMapping属性，如果这个属性设置值了，就直接用这个属性的值，如果resultMap元素的autoMapping属性没有配置，则走全局配置的自动映射规则。
下面我们来详解介绍一下这款的内容。
mybatis自动映射全局配置
在mybatis全局配置文件中加入下面配置：
<settings> <setting name="autoMappingBehavior" value="自动映射规则"/> </settings>
autoMappingBehavior值来源于枚举：org.apache.ibatis.session.AutoMappingBehavior，源码：
public enum AutoMappingBehavior {
/**
* Disables auto-mapping.
*/
NONE,
/**
* Will only auto-map results with no nested result mappings defined inside.
*/
PARTIAL,
/**
* Will auto-map result mappings of any complexity (containing nested or otherwise).
*/
FULL
}
- NONE：关闭全局映射开关
- PARTIAL：对除在内部定义了嵌套结果映射（也就是连接的属性）以外的属性进行映射，这个也是默认值。
- FULL：自动映射所有属性。
小提示：settings元素中有很多配置，这些配置最后都会被解析成org.apache.ibatis.session.Configuration的属性，源码位于org.apache.ibatis.builder.xml.XMLConfigBuilder#settingsElement方法中。
下面我们来演示一下autoMappingBehavior每种配置的效果。
NONE
mybatis-config.xml加入配置
<settings> <!-- 关闭自动映射开关 --> <setting name="autoMappingBehavior" value="NONE"/> </settings>
OrderMapper.xml
<resultMap id="orderModelMap4" type="com.javacode2018.chat05.demo7.model.OrderModel">
</resultMap>
<select id="getById4" resultMap="orderModelMap4">
<![CDATA[
SELECT
a.id,
a.user_id userId,
a.create_time createTime,
a.up_time upTime
FROM
t_order a
WHERE
a.id = #{value}
]]>
</select>
OrderMapper.java加入
OrderModel getById4(int id);
测试用例
com.javacode2018.chat05.demo7.Demo7Test#getById4
@Test
public void getById4() throws IOException {
this.before("demo7/mybatis-config1.xml");
try (SqlSession sqlSession = this.sqlSessionFactory.openSession(true);) {
OrderMapper mapper = sqlSession.getMapper(OrderMapper.class);
OrderModel orderModel = mapper.getById4(2);
log.info("{}", orderModel);
}
}
运行结果
21:58.821 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById4 - ==> Preparing: SELECT a.id, a.user_id userId, a.create_time createTime, a.up_time upTime FROM t_order a WHERE a.id = ?
21:58.850 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById4 - ==> Parameters: 2(Integer)
21:58.868 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById4 - <== Total: 1
21:58.868 [main] INFO c.j.chat05.demo7.Demo7Test - null
从输出中可以看到最后一样输出结果为null，sql实际上返回的是有结果的，但是结果映射的时候返回的是空。
结果解释
由于mybatis全局配置中将autoMappingBehavior的值置为了NONE,表示全局自动映射被关闭了，而resultMapper中的orderModelMap4没有配置autoMapping属性，所以最终这个查询结果不会自动映射，所以最后查询结果为null。
PARTIAL
对除在内部定义了嵌套结果映射（也就是连接的属性）以外的属性进行映射，这个也是autoMappingBehavior的默认值。
mybatis-config.xml加入配置
<settings> <!-- 对除在内部定义了嵌套结果映射（也就是连接的属性）以外的属性进行映射，这个也是autoMappingBehavior的默认值。 --> <setting name="autoMappingBehavior" value="PARTIAL"/> </settings>
OrderMapper.xml
<resultMap id="orderModelMap5" type="com.javacode2018.chat05.demo7.model.OrderModel">
</resultMap>
<select id="getById5" resultMap="orderModelMap5">
<![CDATA[
SELECT
a.id,
a.user_id userId,
a.create_time createTime,
a.up_time upTime
FROM
t_order a
WHERE
a.id = #{value}
]]>
</select>
OrderMapper.java加入
OrderModel getById5(int id);
测试用例
com.javacode2018.chat05.demo7.Demo7Test#getById5
@Test
public void getById5() throws IOException {
this.before("demo7/mybatis-config2.xml");
try (SqlSession sqlSession = this.sqlSessionFactory.openSession(true);) {
OrderMapper mapper = sqlSession.getMapper(OrderMapper.class);
OrderModel orderModel = mapper.getById5(2);
log.info("{}", orderModel);
}
}
运行结果
28:32.612 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById5 - ==> Preparing: SELECT a.id, a.user_id userId, a.create_time createTime, a.up_time upTime FROM t_order a WHERE a.id = ?
28:32.648 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById5 - ==> Parameters: 2(Integer)
28:32.664 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById5 - <== Total: 1
28:32.665 [main] INFO c.j.chat05.demo7.Demo7Test - OrderModel(id=2, userId=1, createTime=1577947790, upTime=1577947790, userModel=null)
OrderModel中的4个属性被自动映射成功了。
结果解释
orderModelMap5中没有指定autoMapping属性，所以自动映射会走全局配置的规则，即PARTIAL，会进行自动映射。
我们再来看看PARTIAL的解释：对除在内部定义了嵌套结果映射（也就是连接的属性）以外的属性进行映射。这句话是什么意思？
有些复杂的查询映射会在resultMap中嵌套一些映射（如：association，collection），当使用PARTIAL的时候，如果有嵌套映射，则这个嵌套映射不会进行自动映射了。
通过订单id查询出订单以及订单用户的信息，sqlmap如下：
<resultMap id="orderModelMap6" type="com.javacode2018.chat05.demo7.model.OrderModel">
<association property="userModel">
</association>
</resultMap>
<select id="getById6" resultMap="orderModelMap6">
<![CDATA[
SELECT
a.id,
a.user_id userId,
a.create_time createTime,
a.up_time upTime
b.id as user_id,
b.name
FROM
t_order a,t_user b
WHERE
a.user_id = b.id
AND a.id = #{value}
]]>
</select>
OrderModel.java
package com.javacode2018.chat05.demo7.model;
import lombok.*;
import java.util.List;
@Getter
@Setter
@Builder
@ToString
@NoArgsConstructor
@AllArgsConstructor
public class OrderModel {
private Integer id;
private Integer userId;
private Long createTime;
private Long upTime;
private UserModel userModel;
}
内部有个userModel属性引用用户对象。
UserModel.java
package com.javacode2018.chat05.demo7.model;
import lombok.*;
@Getter
@Setter
@Builder
@ToString
@NoArgsConstructor
@AllArgsConstructor
public class UserModel {
private Integer id;
private String name;
}
OrderMapper.java中加入
OrderModel getById6(int id);
测试用例
com.javacode2018.chat05.demo7.Demo7Test#getById6
@Test
public void getById6() throws IOException {
this.before("demo7/mybatis-config2.xml");
try (SqlSession sqlSession = this.sqlSessionFactory.openSession(true);) {
OrderMapper mapper = sqlSession.getMapper(OrderMapper.class);
OrderModel orderModel = mapper.getById6(2);
log.info("{}", orderModel);
}
}
运行输出
52:49.037 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById6 - ==> Preparing: SELECT a.id, a.user_id userId, a.create_time createTime, a.up_time upTime, b.id as user_id, b.name FROM t_order a,t_user b WHERE a.user_id = b.id AND a.id = ?
52:49.066 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById6 - ==> Parameters: 2(Integer)
52:49.087 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById6 - <== Total: 1
52:49.088 [main] INFO c.j.chat05.demo7.Demo7Test - null
sql查询实际上是有一条记录的，但是最后返回的是null，说明没有进行自动映射。
FULL
自动映射所有属性。
这次Mapper我们不动，还是下面这样，没有手动指定映射规则。
<resultMap id="orderModelMap6" type="com.javacode2018.chat05.demo7.model.OrderModel">
<association property="userModel">
</association>
</resultMap>
<select id="getById6" resultMap="orderModelMap6">
<![CDATA[
SELECT
a.id,
a.user_id userId,
a.create_time createTime,
a.up_time upTime
b.id as user_id,
b.name
FROM
t_order a,t_user b
WHERE
a.user_id = b.id
AND a.id = #{value}
]]>
</select>
修改一下autoMappingBehavior的值为FULL，看看效果。
mybatis配置
<settings> <!-- 自动映射所有属性 --> <setting name="autoMappingBehavior" value="FULL"/> </settings>
测试用例
com.javacode2018.chat05.demo7.Demo7Test#getById6_0
@Test
public void getById6_0() throws IOException {
this.before("demo7/mybatis-config3.xml");
try (SqlSession sqlSession = this.sqlSessionFactory.openSession(true);) {
OrderMapper mapper = sqlSession.getMapper(OrderMapper.class);
OrderModel orderModel = mapper.getById6(2);
log.info("{}", orderModel);
}
}
运行输出
56:05.127 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById6 - ==> Preparing: SELECT a.id, a.user_id userId, a.create_time createTime, a.up_time upTime, b.id as user_id, b.name FROM t_order a,t_user b WHERE a.user_id = b.id AND a.id = ?
56:05.155 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById6 - ==> Parameters: 2(Integer)
56:05.186 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById6 - <== Total: 1
56:05.186 [main] INFO c.j.chat05.demo7.Demo7Test - OrderModel(id=2, userId=1, createTime=1577947790, upTime=1577947790, userModel=UserModel(id=2, name=张学友))
输出中可以看到OrderModel所有属性都是有值的，userModel的2个属性也有值，userModel.id是2，我们运行一下sql看看，用户id是多少，如下：
mysql> SELECT a.id, a.user_id userId, a.create_time createTime, a.up_time upTime, b.id as user_id, b.name FROM t_order a,t_user b WHERE a.user_id = b.id AND a.id = 2; +----+--------+------------+------------+---------+-----------+ | id | userId | createTime | upTime | user_id | name | +----+--------+------------+------------+---------+-----------+ | 2 | 1 | 1577947790 | 1577947790 | 1 | 张学友 | +----+--------+------------+------------+---------+-----------+ 1 row in set (0.00 sec)
user_id实际上是1，mybatis中按照sql字段和model结果字段同名进行自动映射，所以将订单的id赋值给userModel的id属性了。
此时需要我们orderModelMap6的配置，手动指定一下user_id和userModel.id的映射规则，如下：
<resultMap id="orderModelMap6" type="com.javacode2018.chat05.demo7.model.OrderModel">
<association property="userModel">
<id column="user_id" property="id"/>
</association>
</resultMap>
再次运行测试用例
com.javacode2018.chat05.demo7.Demo7Test#getById6_0
输出
15:02.751 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById6 - ==> Preparing: SELECT a.id, a.user_id userId, a.create_time createTime, a.up_time upTime, b.id as user_id, b.name FROM t_order a,t_user b WHERE a.user_id = b.id AND a.id = ?
15:02.783 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById6 - ==> Parameters: 2(Integer)
15:02.801 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById6 - <== Total: 1
15:02.801 [main] INFO c.j.chat05.demo7.Demo7Test - OrderModel(id=2, userId=1, createTime=1577947790, upTime=1577947790, userModel=UserModel(id=1, name=张学友))
这次userModel中的id正确了。
autoMapping使用
上面我们有说过，当在resultMap中指定了autoMapping属性之后，这个resultMap的自动映射就受autoMapping属性的控制，和mybatis中全局映射配置（autoMappingBehavior）行为无关了。
案例1
这个核心配置主要在sqlmap中，如下：
<resultMap id="orderModelMap7" type="com.javacode2018.chat05.demo7.model.OrderModel" autoMapping="true">
<association property="userModel" autoMapping="true">
<id column="user_id" property="id"/>
</association>
</resultMap>
<select id="getById7" resultMap="orderModelMap7">
<![CDATA[
SELECT
a.id,
a.user_id userId,
a.create_time createTime,
a.up_time upTime,
b.id as user_id,
b.name
FROM
t_order a,t_user b
WHERE
a.user_id = b.id
AND a.id = #{value}
]]>
</select>
对应测试用例
com.javacode2018.chat05.demo7.Demo7Test#getById7
@Test
public void getById7() throws IOException {
this.before("demo7/mybatis-config1.xml");
try (SqlSession sqlSession = this.sqlSessionFactory.openSession(true);) {
OrderMapper mapper = sqlSession.getMapper(OrderMapper.class);
OrderModel orderModel = mapper.getById7(2);
log.info("{}", orderModel);
}
}
运行输出
24:37.544 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById7 - ==> Preparing: SELECT a.id, a.user_id userId, a.create_time createTime, a.up_time upTime, b.id as user_id, b.name FROM t_order a,t_user b WHERE a.user_id = b.id AND a.id = ?
24:37.589 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById7 - ==> Parameters: 2(Integer)
24:37.610 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById7 - <== Total: 1
24:37.610 [main] INFO c.j.chat05.demo7.Demo7Test - OrderModel(id=2, userId=1, createTime=1577947790, upTime=1577947790, userModel=UserModel(id=1, name=张学友))
OrderModel中所有属性都自动映射成功。
自动装配并不是那么好玩，玩不转可能带来一些隐患，我们看一个案例，见下面的示例2。
示例2
根据订单编号，查询出订单信息，顺便查询出订单明细列表。这个我们使用mybatis中的一对多查询。
OrderDetaiMapper.xml加入
<select id="getListByOrderId1" resultType="com.javacode2018.chat05.demo7.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 = #{value}
]]>
</select>
这个可以根据订单的id，查询出订单关联的明细列表。
OrderMapper.xml加入
<resultMap id="orderModelMap8" type="com.javacode2018.chat05.demo7.model.OrderModel" autoMapping="true">
<collection property="orderDetailModelList" select="com.javacode2018.chat05.demo7.mapper.OrderDetailMapper.getListByOrderId1" column="id"/>
</resultMap>
<select id="getById8" resultMap="orderModelMap8">
<![CDATA[
SELECT
a.id,
a.user_id userId,
a.create_time createTime,
a.up_time upTime
FROM
t_order a
WHERE a.id = #{value}
]]>
</select>
测试用例
com.javacode2018.chat05.demo7.Demo7Test#getById8
@Test
public void getById8() throws IOException {
this.before("demo7/mybatis-config.xml");
try (SqlSession sqlSession = this.sqlSessionFactory.openSession(true);) {
OrderMapper mapper = sqlSession.getMapper(OrderMapper.class);
OrderModel orderModel = mapper.getById8(1);
log.info("{}", orderModel);
}
}
运行输出
11:06.193 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById8 - ==> Preparing: SELECT a.id, a.user_id userId, a.create_time createTime, a.up_time upTime FROM t_order a WHERE a.id = ?
11:06.229 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById8 - ==> Parameters: 1(Integer)
11:06.250 [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 = ?
11:06.251 [main] DEBUG c.j.c.d.m.O.getListByOrderId1 - ====> Parameters: 1(Integer)
11:06.255 [main] DEBUG c.j.c.d.m.O.getListByOrderId1 - <==== Total: 2
11:06.256 [main] DEBUG c.j.c.d.mapper.OrderMapper.getById8 - <== Total: 1
11:06.256 [main] INFO c.j.chat05.demo7.Demo7Test - OrderModel(id=null, userId=2, createTime=1577947790, upTime=1577947790, userModel=null, orderDetailModelList=[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OrderModel的id属性，怎么是null值？主要是下面这行配置导致的
<collection property="orderDetailModelList" select="com.javacode2018.chat05.demo7.mapper.OrderDetailMapper.getListByOrderId1" column="id"/>
上面这个配置中有个column属性，指定的是id，此时mybatis认为你对id字段手动指定了映射关系，就跳过了对id字段到OrderModel.id属性的自动映射，所以导致OrderModel对象的id属性没有赋值，此时需要我们在orderModelMap8手动指定id的映射规则，如下：
<resultMap id="orderModelMap8" type="com.javacode2018.chat05.demo7.model.OrderModel" autoMapping="true"> <id column="id" property="id" /> <collection property="orderDetailModelList" select="com.javacode2018.chat05.demo7.mapper.OrderDetailMapper.getListByOrderId1" column="id"/> </resultMap>
再去运行测试用例就正常了。
总结一下
对于咱们开发来说，自动映射确实可以帮助我们节省一些代码，不过也存在一些隐患，我们希望自己开发的系统是健壮的，建议大家写mapper xml的时候，还是花点时间将映射的配置都给写上去，这样能够杜绝一些隐患，使我们的系统更稳定。
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