mybatis抛出下面的异常：
org.mybatis.spring.MyBatisSystemException:
nested exception is org.apache.ibatis.exceptions.PersistenceException:
### Error querying database. Cause: java.lang.UnsupportedOperationException
### The error may exist in file
[C:\apache-tomcat-8.0.21\webapps\ems\WEB-INF\classes\config\mappers\UserMapper.xml]
### The error may involve com.ems.mapper.UserMapper.findPermissionsByUserId
### The error occurred while handling results
### SQL: select distinct permission from privilege p inner join user_privilege up
on p.id=up.privilege_id inner join user u on u.id=up.user_id where u.id=?
### Cause: java.lang.UnsupportedOperationException
对应的语句如下：
<select id="findPermissionsByUserId" parameterType="java.lang.Integer" resultType="java.util.Set">
select distinct permission from privilege p
inner join user_privilege up on p.id=up.privilege_id
inner join user u on u.id=up.user_id
where u.id=#{userId, jdbcType=INTEGER}
</select>
错误原因是 resultType="java.util.Set" ，这里应该改成： resultType="java.lang.String"
这里 resultType 指的是 select 返回的每一条记录 的类型，而不是所有记录组成的类型。