这个异常是IDEA中漏加载mapper.xml文件，在classes中没找到，所以要在配置文件中加入：
!--如果不添加此节点mybatis的mapper.xml文件都会被漏掉。-->
<build>
<resources>
<resource>
<directory>src/main/java</directory>
<includes>
<include>**/*.properties</include>
<include>**/*.xml</include>
</includes>
<filtering>false</filtering>
</resource>
<resource>
<directory>src/main/resources</directory>
<includes>
<include>**/*.properties</include>
<include>**/*.xml</include>
</includes>
<filtering>false</filtering>
</resource>
</resources>
</build>
