Mybatis 自动映射(使用需谨慎)
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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