jsp表单中有多个按钮怎么调用Servlet实现不同的功能

麻烦了。。帮我看看！
jap：

<input type="submit" name="Submit" id="a" value="保存" />
<input type="submit" name="Submit"  id="b" value="修改" /> 
<input type="submit" name="Submit" id="c" value="删除" />


servlet：

String value=(String)request.getParameter("submit");
if(value.equals("保存"))
        {response.sendRedirect("a.jsp");}
      else if(value.equals("修改"))
        {response.sendRedirect("b.jsp");}
        else
        {response.sendRedirect("c.jsp");}

试了很久都是说错误：

type Exception report

message

description The server encountered an internal error () that prevented it from fulfilling this request.

exception

java.lang.NullPointerException
luru.luruxiugai.doPost(luruxiugai.java:82)
javax.servlet.http.HttpServlet.service(HttpServlet.java:710)
javax.servlet.http.HttpServlet.service(HttpServlet.java:803)


note The full stack trace of the root cause is available in the Apache Tomcat/6.0.13 logs.