专辑：MyBatis教程_自动映射，使用需谨慎-Java充电社【公众号：Java充电社】

### 案例代码

```java
链接：https://pan.baidu.com/s/1vt-MAX3oJOu9gyxZAhKkbg 
提取码：i8op
```

本文案例代码入口，配合源码看案例，效果更好。

```java
mybatis-series\chat05\src\test\java\com\javacode2018\chat05\demo7\Demo7Test.java
```

### 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：

```XML
<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：

```xml
<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：

```xml
<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：

```xml
<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：

```java
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

```xml
<settings>
    
    <setting name="autoMappingBehavior" value="NONE"/>
</settings>
```

##### OrderMapper.xml

```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

```java
OrderModel getById4(int id);
```

##### 测试用例

```java
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

```java
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

```xml
<settings>
    
    <setting name="autoMappingBehavior" value="PARTIAL"/>
</settings>
```

##### OrderMapper.xml

```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

```java
OrderModel getById5(int id);
```

##### 测试用例

```java
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

```java
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：

```xml
<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

```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

```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

```java
OrderModel getById6(int id);
```

测试用例

```java
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

```java
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。

```xml
<resultMap id="orderModelMap6" type="com.javacode2018.chat05.demo7.model.OrderModel">
    <association property="userModel">
    </association>
</resultMap>

修改一下autoMappingBehavior的值为`FULL`，看看效果。

##### mybatis配置

```xml
<settings>
    
    <setting name="autoMappingBehavior" value="FULL"/>
</settings>
```

##### 测试用例

```java
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：

```java
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：

```xml
<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

```java
com.javacode2018.chat05.demo7.Demo7Test#getById6_0
```

##### 输出

```java
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：

```xml
<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

```java
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

```java
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

```xml
<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

```xml
<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

```java
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

```java
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

```xml
<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：

```xml
<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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