mybatis java.lang.UnsupportedOperationException

mybatis拋出下面的異常：

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

對應的語句如下：

  <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>

錯誤原因是 resultType="java.util.Set" ，這里應該改成： resultType="java.lang.String"

這里 resultType 指的是 select 返回的每一條記錄 的類型，而不是所有記錄組成的類型。