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求教各位高手，我编择JSP程序时出现了这样的问题：Exception reportmessage description The server encountered an internal error () that prevented it from fulfilling this request.exception org.apache.jasper.JasperException: Exception in JSP: /SetContentType.jsp:2421:     </form>
22:         </center>
23:         <%String docType=request.getParameter("format");
24:         if(docType.equals("text")){
25:         docType="text/html";
26:         }else if(docType.equals("word")){
27:         docType="application/msword";
Stacktrace:
org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:451)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:373)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:329)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:265)
javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
root cause java.lang.NullPointerException
org.apache.jsp.SetContentType_jsp._jspService(SetContentType_jsp.java:66)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:98)
javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:331)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:329)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:265)
javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
note The full stack trace of the root cause is available in the Apache Tomcat/5.5.28 logs.我的代码：
<%@ page contentType="text/html; charset=gb2312" language="java" %>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
  <head>
     <title>动态响应contentType属性  </title>
   </head>
    <body>
    <center>
    <p>动态响应contentType属性的案例</p>
    <hr><br>
    请选择你的保存的格式：
    <form action="SetContentType.jsp" method="post" name="MyForm">
    <select name="format" id="format">
    <option value="text">文本文件</option>
    <option value="word">Word文件</option>
    <option value="excel">Excel文件</option>
    </select>
    <br><br>
    <input type="submit" name="Submit" value="提交">
    </form>
        </center>
        <%String docType=request.getParameter("format");
        if(docType.equals("text")){
        docType="text/html";
        }else if(docType.equals("word")){
        docType="application/msword";
        }else if(docType.equals("excel")){
         docType="application/x-msexcel";
         }
         response.setContentType(docType); %> 
        
        
  </body>
</html>