autoMappingUnknownColumnBehavior是< settings >配置下的属性，该属性是指定发现自动映射目标未知列（或未知属性类型）的行为。就是说当数据库中的字段找不到映射java对象的属性或者与java对象对应属性的类型不匹配时会产生什么样的行为。

autoMappingUnknownColumnBehavior属性可以指定当映射失败时会产生三种行为，默认情况配置的是NONE

• NONE: 不做任何反应；
• WARNING: 输出警告日志（‘org.apache.ibatis.session.AutoMappingUnknownColumnBehavior’ 的日志等级必须设置为 WARN）；
• FAILING: 映射失败 (抛出 SqlSessionException)。

下面分别看下三种情况

1. NONE

假设java类User中只有id和name两个属性，而对应数据库表user有三个字段，分别为ID、NAME、AGE，库表和java对象不能完全匹配，看下会发生什么

public class User  {
    private int id;
    private String name;
 } 

user表

在mybatis配置文件配置autoMappingUnknownColumnBehavior的值为NONE

<settings>
    <setting name="logImpl" value="STDOUT_LOGGING"/>
    <setting name="autoMappingUnknownColumnBehavior" value="NONE"/>
</settings>

下面为mapper文件中对应的SQL

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

执行这个SQL，输出结果如下所示，输出的id=2，name=‘xiaoli’，库表中的AGE字段没有对应的映射属性，丢弃掉了，程序也不会报错。

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 1713847712.
==>  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'}

2. WARNING

下面看下第二种情况，当autoMappingUnknownColumnBehavior取值WARNING时，修改mybatis的 < settings >配置如下所示

<settings>
    <setting name="logImpl" value="STDOUT_LOGGING"/>
    <setting name="autoMappingUnknownColumnBehavior" value="WARNING"/>
</settings>

重新执行上面的SQL，输出结果如下所示，发现也能正常输出User{id=2, name=‘xiaoli’}，库表user的AGE列没有找到对应的java对象user的属性，丢弃掉了，程序也没有报错，但在自动映射后打印出了下面一条警告“Unknown column is detected on ‘com.lzj.dao.UserDao.selectOne’ auto-mapping. Mapping parameters are [columnName=age,propertyName=age,propertyType=null]”。所以当autoMappingUnknownColumnBehavior取值WARNING时，也不会报错但会打印出警告（设置的日志级别一定要小于等于warn）。

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 2070438137.
==>  Preparing: select * from user where id=? 
==> Parameters: 2(Integer)
<==    Columns: id, name, age
<==        Row: 2, xiaoli, 26
Unknown column is detected on 'com.lzj.dao.UserDao.selectOne' auto-mapping. Mapping parameters are [columnName=age,propertyName=age,propertyType=null]
<==      Total: 1
输出结果：User{id=2, name='xiaoli'}

3. FAILING

下面看下最后一种，当autoMappingUnknownColumnBehavior取值FAILING时，看下如果没有完全映射上会发生情况
修改mybatis配置文件如下所示

<settings>
    <setting name="logImpl" value="STDOUT_LOGGING"/>
    <setting name="autoMappingUnknownColumnBehavior" value="FAILING"/>
</settings>

然后重新执行上面的SQL，输出结果如下所示，程序直接抛异常了，说明当库表中列不能映射到java对象中属性上，程序抛异常处理。

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 1617550648.
==>  Preparing: select * from user where id=? 
==> Parameters: 2(Integer)
<==    Columns: id, name, age
<==        Row: 2, xiaoli, 26
Exception in thread "main" org.apache.ibatis.exceptions.PersistenceException: 
### Error querying database.  Cause: org.apache.ibatis.session.SqlSessionException: Unknown column is detected on 'com.lzj.dao.UserDao.selectOne' auto-mapping. Mapping parameters are [columnName=age,propertyName=age,propertyType=null]
### The error may exist in mapper/UserMapper.xml
### The error may involve com.lzj.dao.UserDao.selectOne
### The error occurred while handling results