mybatis中传入ids String类型参数异常

在使用mybatis时，写了一条sql语句，只有一个String类型的参数， 示例代码

<select id="getApplyNum" parameterType="java.lang.String" resultType="java.util.Map">  
         SELECT  
            t.activity_id AS "activityId",  
            COUNT(1) AS "count"  
        FROM  
            t_user t  
        WHERE  
            t.activity_id IN (${ids})  
        GROUP BY  
            t.activity_id  
    </select>

Mybatis查询sql传入一个字符串传参数，报There is no getter for property named 'ids' in 'class java.lang.String'。

后来改成如下写法，无论参数名是啥，都要改成"_parameter"。问题解决。

<select id="getApplyNum" parameterType="java.lang.String" resultType="java.util.Map">  
        SELECT  
           t.activity_id AS "activityId",  
           COUNT(1) AS "count"  
       FROM  
           t_user t  
       WHERE  
           t.activity_id IN (${_parameter})  
       GROUP BY  
           t.activity_id  
   </select>