Java mybatis 参数

一 、返回List<String>

List<String> getExistStudents(Map<String ,Object> params);

<select id="getExistStudents" parameterType="java.util.Map" resultType="java.lang.String">

  select username
  from  a
  where class_id=#{classcode}
  and userid in (#{ids})

</select>

二、