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众所周知mybatis中标签< resultMap >是用来处理数据库库字段与java对象属性映射的。通常java对象属性（驼峰格式）与数据库表字段（下划线形式）是一 一对应的情况下，是不用定义< resultMap >标签的，是因为数据库表字段会自动映射到java对象的对应属性上的，比如数据库表一个字段为USER_NAME，java对象对应的属性为userName，默认情况下会自动映射的。

数据库表字段为什么会和java对象属性自动映射的？什么情况下会自动映射？什么情况下不会自动映射？实现这种自动映射行为就是通过< settings >标签下的autoMappingBehavior属性控制的。

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

autoMappingBehavior指定 MyBatis 应如何自动映射列到字段或属性。 该属性有3种取值NONE, PARTIAL, FULL, NONE 表示关闭自动映射；PARTIAL 只会自动映射没有定义嵌套结果映射的字段。 FULL 会自动映射任何复杂的结果集（无论是否嵌套）。如不配置，模式是PARTIAL部分映射的。下面分别看下三种情况是如何映射的

1. NONE关闭自动映射

关闭自动映射后，数据库表字段就不会自动映射到java对象属性中，此时就得自定义< resultMap >。
下面以查询用户信息为例，数据库表为USER，对应java类为User
package com.lzj.bean;

public class User  {private int id;private String name;private int age;//省略get/set/toString
}

对应数据库表USER为

mybatis配置文件中< settings >配置如下所示

    <settings><!--设置打印日志工具--><setting name="logImpl" value="STDOUT_LOGGING"/><--关闭自动映射--><setting name="autoMappingBehavior" value="NONE"/></settings>

mapper文件对应如下所示

<select id="selectOne" parameterType="int" resultType="com.lzj.bean.User">select * from user where id=#{id}
</select>

然后通过如下案例执行该mapper对应的SQL

public void sqlSessionTest(){SqlSessionFactory factory = mybatisUtil.getFactory();SqlSession sqlSession = factory.openSession(true);  //true表示自动提交List<Object> list = sqlSession.selectList("com.lzj.dao.UserDao.selectOne", 2);System.out.println("输出结果：" + list.get(0));
}

输出结果如下所示，发现该SQL命中了数据库中id=2的一条数据，但是没有映射到user对象上，导致打印出的user对象为null。

Logging initialized using 'class org.apache.ibatis.logging.stdout.StdOutImpl' adapter.
PooledDataSource forcefully closed/removed all connections.
PooledDataSource forcefully closed/removed all connections.
PooledDataSource forcefully closed/removed all connections.
Cache Hit Ratio [com.lzj.dao.UserDao]: 0.0
Opening JDBC Connection
Created connection 671187578.
==>  Preparing: select * from user where id=? 
==> Parameters: 2(Integer)
<==    Columns: id, name, age
<==        Row: 2, xiaoli, 26
<==      Total: 1
输出结果：null

那么在关闭自动映射的情况下如何把查到的数据库表数据映射到java对象user中呢？可以通过下面自定resultMap标签进行手动映射，把mapper文件改成如下方式

    <resultMap id="userResultMap" type="com.lzj.bean.User"><result property="id" column="ID"></result><result property="name" column="NAME"></result><result property="age" column="AGE"></result></resultMap><select id="selectOne" parameterType="int" resultMap="userResultMap">select * from user where id=#{id}</select>

然后再执行上述案例，输出结果如下所示，打印出的user对象已有值，说明数据库表中数据映射到了java对象user中。

Logging initialized using 'class org.apache.ibatis.logging.stdout.StdOutImpl' adapter.
PooledDataSource forcefully closed/removed all connections.
PooledDataSource forcefully closed/removed all connections.
PooledDataSource forcefully closed/removed all connections.
Cache Hit Ratio [com.lzj.dao.UserDao]: 0.0
Opening JDBC Connection
Created connection 953253248.
==>  Preparing: select * from user where id=? 
==> Parameters: 2(Integer)
<==    Columns: id, name, age
<==        Row: 2, xiaoli, 26
<==      Total: 1
输出结果：User{id=2, name='xiaoli', age=26}

2. PARTIAL非嵌套结果映射

PARTIAL 只会自动映射没有定义嵌套结果映射的字段。什么意思呢，不同于上面案例，一个库表对应一个简单的java对象，如果两个库表对应一个嵌套的java对象，如何进行映射的呢？
比如查询用户名下的车辆，对应的User类为

package com.lzj.bean;import java.util.List;public class User  {private int id;private String name;private int age;private List<Car> cars;//省略get/set/toString方法
}

对应的Car类为
package com.lzj.bean;

public class Car {private String name;private String brand;//省略get/set/toString方法
}

数据库表USER如下所示

数据库表CAR如下所示

mybatis的配置文件中setting标签改为如下配置

    <settings><setting name="logImpl" value="STDOUT_LOGGING"/><setting name="autoMappingBehavior" value="PARTIAL"/></settings>

mapper文件改为如下所示

<select id="selectOne" parameterType="int" resultMap="userResultMap">select * from user where id=#{id}
</select>

然后运行下面demo

public void sqlSessionTest2(){SqlSessionFactory factory = mybatisUtil.getFactory();SqlSession sqlSession = factory.openSession(true);  //true表示自动提交List<Object> list = sqlSession.selectList("com.lzj.dao.UserDao.select2", 2);System.out.println("输出结果：" + list.get(0));
}

输出结果如下所示，输出的user对象中id、name、age都有值，唯独cars没有值，说明mybatis只自动映射没有定义嵌套结果映射的字段，只映射了user对象中最外层的属性的值，没有映射嵌套的CAR数据库表中数据到cars对象中。

Logging initialized using 'class org.apache.ibatis.logging.stdout.StdOutImpl' adapter.
PooledDataSource forcefully closed/removed all connections.
PooledDataSource forcefully closed/removed all connections.
PooledDataSource forcefully closed/removed all connections.
Cache Hit Ratio [com.lzj.dao.UserDao]: 0.0
Opening JDBC Connection
Created connection 140778365.
==>  Preparing: select u.id,u.name,u.age,c.name,c.brand from user u, car c where u.id=? 
==> Parameters: 2(Integer)
<==    Columns: id, name, age, name, brand
<==        Row: 2, xiaoli, 26, xiaoli, BYD
<==      Total: 1
输出结果：User{id=2, name='xiaoli', age=26, cars=null}

那么这种情况下如何把CAR表中数据映射到嵌套的对象cars中呢，也是通过resultMap进行自定义映射，mapper改为如下方式

<resultMap id="userResultMap2" type="com.lzj.bean.User"><!--由于设置的是非嵌套自动映射，库表USER中数据会自动映射到user对象的id、name、age上，所以此处没有再自定义映射，只指定CAR表中字段数据是如何映射到cars上面的就可以了。--><collection property="cars" column="NAME" ofType="com.lzj.bean.Car"><result property="name" column="NAME"></result><result property="brand" column="BRAND"></result></collection>
</resultMap><select id="select2" resultMap="userResultMap2">select u.id,u.name,u.age,c.name,c.brand from user u, car c where u.id=#{id}
</select>

然后重新执行上面demo案例结果如下所示，发现user对象中嵌套的cars对象中属性也有值了，映射成功。

Logging initialized using 'class org.apache.ibatis.logging.stdout.StdOutImpl' adapter.
PooledDataSource forcefully closed/removed all connections.
PooledDataSource forcefully closed/removed all connections.
PooledDataSource forcefully closed/removed all connections.
Cache Hit Ratio [com.lzj.dao.UserDao]: 0.0
Opening JDBC Connection
Created connection 77215921.
==>  Preparing: select u.id,u.name,u.age,c.name,c.brand from user u, car c where u.id=? 
==> Parameters: 2(Integer)
<==    Columns: id, name, age, name, brand
<==        Row: 2, xiaoli, 26, xiaoli, BYD
<==      Total: 1
输出结果：User{id=0, name='null', age=0, cars=[Car{name='xiaoli', brand='BYD'}]}

3. FULL全自动映射

FULL 会自动映射任何复杂的结果集（无论是否嵌套）。
还是以上面嵌套映射的为例，把< settings 配置修改为如下所示>

    <settings><setting name="logImpl" value="STDOUT_LOGGING"/><setting name="autoMappingBehavior" value="FULL"/></settings>

mapper文件修改为如下所示，直接返回user对象，没有设置resultMap自定义映射

<select id="select3" resultType="com.lzj.bean.User">select u.id,u.name,u.age,c.name,c.brand from user u, car c where u.id=#{id}
</select>

执行上面案例结果输出显示，即使没有配置resultMap自定义映射，mybatis自动的进行了嵌套结果的映射

Logging initialized using 'class org.apache.ibatis.logging.stdout.StdOutImpl' adapter.
PooledDataSource forcefully closed/removed all connections.
PooledDataSource forcefully closed/removed all connections.
PooledDataSource forcefully closed/removed all connections.
Cache Hit Ratio [com.lzj.dao.UserDao]: 0.0
Opening JDBC Connection
Created connection 520082748.
==>  Preparing: select u.id,u.name,u.age,c.name,c.brand from user u, car c where u.id=? 
==> Parameters: 2(Integer)
<==    Columns: id, name, age, name, brand
<==        Row: 2, xiaoli, 26, xiaoli, BYD
<==      Total: 1
输出结果：User{id=2, name='null', age=26, cars=[Car{name='xiaoli', brand='BYD'}]}