急：请高手帮我看看啊，这是什么问题呀，万分感谢！

我运行下面代码时：
<SCRIPT language="javascript">
function backward()
{
      history.back();
}
function teacher()
{
window.location="UntitledFrameset-5.htm";
}
</SCRIPT>
<%@ page contentType="text/html; charset=gb2312" language="java" import="java.sql.*" import="java.util.*" import="java.lang.*" %>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312">
<title>生成试卷</title>
<link href="../img/cssexamin.css" rel="stylesheet" type="text/css">
<style type="text/css">
<!--
.unnamed1 { font-size: 12px;
font-weight: lighter;
color: #FF0000;
text-decoration: none;
}
.style2 {font-size: 14px; font-weight: lighter; color: #FF0000; text-decoration: none; }
.style4 {font-size: 16px; font-weight: lighter; color: #FF0000; text-decoration: none; }
.style6 {font-size: 18px; font-weight: lighter; color: #FF0000; text-decoration: none; }
-->
</style>
</head><body>
<%!
void randomNum(int a[],int i,int j)
{
 a[i]=1+(int)(Math.random()*j);
for(int c=i-1;c>=0;c--)
    {
if(a[i]==a[c])
randomNum(a,i,j);
}
}
%>
<%
Class.forName("com.microsoft.jdbc.sqlserver.SQLServerDriver").newInstance();
String url="jdbc:microsoft:sqlserver://localhost:1433;DatabaseName=exam";
//mydb为数据库
String user="sa";
String password="";
Connection conn= DriverManager.getConnection(url,user,password);
Statement ps= conn.createStatement();
  //读取表单内容
  int shijuanbianhao=Integer.parseInt(request.getParameter("shijuanbianhao"));
  String sql="SELECT * FROM shijuan where sj_id="+shijuanbianhao;
  ResultSet rs=ps.executeQuery(sql);
  if(rs.next())
   {%>
   <p>本试卷编号已经存在，请重新选择填写编号。</p>
   <p><input type="button" name="fanhui" value="返回" onclick="backward()"></p>
   <%}
   else{
   int xuanzeshumu=Integer.parseInt(request.getParameter("xuanzeshumu"));
   int xuanzefenzhi=Integer.parseInt(request.getParameter("xuanzefenzhi"));
   int panduanshumu=Integer.parseInt(request.getParameter("panduanshumu"));
   int panduanfenzhi=Integer.parseInt(request.getParameter("panduanfenzhi"));
   int zhuguanshumu=Integer.parseInt(request.getParameter("zhuguanshumu"));
   int zhuguanfenzhi=0;
   int kaoshishijian=Integer.parseInt(request.getParameter("kaoshishijian"));
   int i=0;
   int j[]=new int [30];   for(i=0;i<xuanzeshumu;i++)
    {
      randomNum(j,i,30);
      sql="SELECT * from xuanzeti where id="+j[i];
      rs=ps.executeQuery(sql);
      //  while(rs.next()){
     rs.next();
     int id=Integer.parseInt(rs.getString("id"));
     String timu=rs.getString("question");
     String timuleixing="xuanzeti";
     String sql1="insert into shijuan values('"+shijuanbianhao+"','"+timu+"','"+xuanzefenzhi+"','"+timuleixing+"','"+id+"','"+kaoshishijian+"')";
     ps.executeUpdate(sql1);
     //}
   }   for(i=0;i<panduanshumu;i++)
    {
     randomNum(j,i,20);
     sql="SELECT * from panduanti where id="+j[i];
     rs=ps.executeQuery(sql);
     rs.next();
     int id=Integer.parseInt(rs.getString("id"));
     String timu=rs.getString("question");
     String timuleixing="panduanti";
     String sql1="insert into shijuan values('"+shijuanbianhao+"','"+timu+"','"+panduanfenzhi+"','"+timuleixing+"','"+id+"','"+kaoshishijian+"')";
     ps.executeUpdate(sql1);
    }    for(i=0;i<zhuguanshumu;i++)
    {
       randomNum(j,i,20);
       sql="SELECT * from panduanti where id="+j[i];
       rs=ps.executeQuery(sql);
      //  while(rs.next()){
      rs.next();
      int id=Integer.parseInt(rs.getString("id"));
      String timu=rs.getString("question");
      String timuleixing="zhuguanti";
      zhuguanfenzhi=(100-xuanzefenzhi*xuanzeshumu-panduanfenzhi*panduanshumu)/zhuguanshumu;
      String sql1="insert into shijuan values('"+shijuanbianhao+"','"+timu+"','"+zhuguanfenzhi+"','"+timuleixing+"','"+id+"','"+kaoshishijian+"')";
      ps.executeUpdate(sql1);
    }
 rs.close();
 ps.close();
 conn.close();
}
%>
<h1>试卷添加成功</h1>
<p></p>
</body>
</html>在JB里面运行时出现下面错误信息：
Apache Tomcat/4.0.6 - HTTP Status 500 - Internal Server Errortype Exception reportmessage Internal Server Errordescription The server encountered an internal error (Internal Server Error) that prevented it from fulfilling this request.exception java.lang.NumberFormatException: null
 at java.lang.Integer.parseInt(Integer.java:436)
 at java.lang.Integer.parseInt(Integer.java:518)
 at org.apache.jsp.chujuan1$jsp._jspService(chujuan1$jsp.java:90)
 at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:107)
 at javax.servlet.http.HttpServlet.service(HttpServlet.java:853)
 at org.apache.jasper.servlet.JspServlet$JspServletWrapper.service(JspServlet.java:201)
 at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:381)
 at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:473)
 at javax.servlet.http.HttpServlet.service(HttpServlet.java:853)
 at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:247)
 at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:193)
 at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:243)
 at org.apache.catalina.core.StandardPipeline.invokeNext(StandardPipeline.java:566)
 at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:472)
 at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:943)
 at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:190)
 at org.apache.catalina.core.StandardPipeline.invokeNext(StandardPipeline.java:566)
 at org.apache.catalina.valves.CertificatesValve.invoke(CertificatesValve.java:246)
 at org.apache.catalina.core.StandardPipeline.invokeNext(StandardPipeline.java:564)
 at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:472)
 at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:943)
 at org.apache.catalina.core.StandardContext.invoke(StandardContext.java:2347)
 at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:180)
 at org.apache.catalina.core.StandardPipeline.invokeNext(StandardPipeline.java:566)
 at org.apache.catalina.valves.ErrorDispatcherValve.invoke(ErrorDispatcherValve.java:170)
 at org.apache.catalina.core.StandardPipeline.invokeNext(StandardPipeline.java:564)
 at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:170)
 at org.apache.catalina.core.StandardPipeline.invokeNext(StandardPipeline.java:564)
 at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:472)
 at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:943)
 at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:174)
 at org.apache.catalina.core.StandardPipeline.invokeNext(StandardPipeline.java:566)
 at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:472)
 at org.apache.catalina.core.ContainerBase.invoke(ContainerBase.java:943)
 at org.apache.catalina.connector.http.HttpProcessor.process(HttpProcessor.java:1027)
 at org.apache.catalina.connector.http.HttpProcessor.run(HttpProcessor.java:1125)
 at java.lang.Thread.run(Thread.java:534)
哪位高手帮我看看是什么问题呀，谢谢！