java ssm框架 mapper文件里的#符号和$符号的区别

Java SSM框架里面，Mapper.xml文件

（一）#符号生成的sql语句是作为传参的

<!-- 获得数据列表（包括课程相关信息） -->
  <select id="GetListByUidAndType" resultMap="BaseResultMap" parameterType="Map">  
    select
    <include refid="Base_Column_List" />
    from my_favorites
    where uid=#{uid} and type=#{type}
    <if test="strWhere !=''">
        and #{strWhere}
    </if>    
    <if test="filedOrder!=''">
        order by #{filedOrder}
    </if>        
    limit #{page.startPos},#{page.pageSize}
  </select>

DEBUG - ==>  Preparing: select nid, uid, title,brief, key_value, type, addtime from my_favorites where uid=? and type=? order by ? limit ?,? 
DEBUG - ==> Parameters: 1(String), 1(Integer), addtime desc(String), 0(Integer), 10(Integer)

（二）而$符号生成的sql语句是作为占位符显示的

 <!-- 获得数据列表（包括课程相关信息） -->
  <select id="GetListByUidAndType" resultMap="BaseResultMap" parameterType="Map">  
    select
    <include refid="Base_Column_List" />
    from my_favorites
    where uid=${uid} and type=${type}
    <if test="strWhere !=''">
        and ${strWhere}
    </if>    
    <if test="filedOrder!=''">
        order by ${filedOrder}
    </if>        
    limit ${page.startPos},${page.pageSize}
  </select>

DEBUG - ==>  Preparing: select nid, uid, title,brief, key_value, type, addtime from my_favorites where uid=1 and type=1 order by addtime desc limit 0,10 
DEBUG - ==> Parameters:

 （三）需要注意的是：当strWhere为拼接sql语句字符串时，如status=1，这时候使用#号会查不出结果，必须使用$号作为变量

<select id="GetRecordCount" resultType="java.lang.Integer" parameterType="Map">
        select count(*) from course_books
        <if test="strWhere !=''">
            where ${strWhere}
        </if>
        <if test="filedOrder!=''">
            order by #{filedOrder}
        </if>
</select>