jsp request.getAttribute内置对象问题

inputnumber

 <%@ page contentType="text/html;charset=utf-8" %>
<%@ taglib tagdir="/WEB-INF/tags" prefix="computer"%> 
<html>
    <body bgcolor="blanchedalmond">
        <font size="3">
            <form action="" method="post" name="form" >
            输入运算数，选择运算符号：
            <br><input type="text" name="numberone" size="6" />
            <select name="operator">
                <option value="+">+
                <option value="-">-
                <option value="*">*
                <option value="/">/
            </select>
            <input type="text" name="numbertwo" size="6" />
            <br><input type="submit" value="提交你的选择" name="submit" />
            </form>
            <%
                String a=request.getParameter("numberone");
                String b=request.getParameter("numbertwo");
                String operator=request.getParameter("operator");
                if(a==null||b==null){
                    a="";
                    b="";
                }
                if(a.length()>0&&b.length()>0){
            %><computer:computer numberA="<%=a%>" numberB="<%=b%>" operator="<%=operator%>" />
            计算结果：<%=a%><%=operator%><%=b%>=<%=request.getAttribute("result")%>
                <%}%>
        </font>
    </body>
</html>

computer.tag

 <%@ tag pageEncoding="utf-8" %>
<%@ attribute name="numberA" required="true" %>
<%@ attribute name="numberB" required="true" %>
<%@ attribute name="operator" required="true" %>
<%@ variable name-given="result" scope="AT-END" %>
<% try{
    double a=Double.parseDouble(numberA);
    double b=Double.parseDouble(numberB);
    double r=0;
    if(operator.equals("+"))
    r=a+b;
    else if(operator.equals("-"))
    r=a-b;
    else if(operator.equals("*"))
    r=a*b;
    else if(operator.equals("/"))
    r=a/b;
    jspContext.setAttribute("result",String.valueOf(r));
    }
    catch(Exception e){
    jspContext.setAttribute("result","发生异常："+e);
    }
%>

返回值总是空，不知啥问题，求大神解答下

JspContext 只能取本页面的值。超出他的作用域肯定取不到的

<%=request.getAttribute("result")%> 改为 <%=result%>