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>

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