The server encountered an internal error that prevented it from fulfilling this

help me！
各位大神

type Exception report

message javax.servlet.ServletException: javax.servlet.jsp.JspException: java.lang.NullPointerException

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

exception
org.apache.jasper.JasperException: javax.servlet.ServletException: javax.servlet.jsp.JspException: java.lang.NullPointerException
org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:560)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:461)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:396)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:340)
javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)


root cause
javax.servlet.ServletException: javax.servlet.jsp.JspException: java.lang.NullPointerException
org.apache.jasper.runtime.PageContextImpl.doHandlePageException(PageContextImpl.java:905)
org.apache.jasper.runtime.PageContextImpl.handlePageException(PageContextImpl.java:838)
org.apache.jsp.example4_005f2_jsp._jspService(example4_005f2_jsp.java:177)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:438)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:396)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:340)
javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)


root cause
java.lang.NullPointerException
org.apache.jsp.tag.web.example4_005f3_tag.doTag(example4_005f3_tag.java:115)
org.apache.jsp.example4_005f2_jsp._jspService(example4_005f2_jsp.java:136)
org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:438)
org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:396)
org.apache.jasper.servlet.JspServlet.service(JspServlet.java:340)
javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)


源代码：

<%@ page language="java" contentType="text/html; charset=GB2312"%>
<%@ taglib tagdir="/WEB-INF/tags" prefix="guessNumber" %>
<html>
<body>
<% String str=request.getParameter("guessNumber");
if(str==null){
str="*";
}
if(str.length()==0){
str="*";
}
%>
<guessNumber:example4_3 number="<%=str %>" />
当前的猜测结果：<%=message %>
<%if(message.startsWith("您猜对了"))
{
%> <br><a href="example4_1.jsp">重新获取随机数</a>
<% }
else{
%>
<br>输入您的猜测：
<form action="" method="post" name="form">
<input type="text" name="guessNumber">
<input type="submit" value="送出" name="submit">
</form>
<%}
%>
</body>
</html>

<%@ tag pageEncoding="GB2312"%>
<%@ attribute name="number" required="true" %>
<%@ variable name-given="message" scope="AT_END"%>
<%
String mess="";
Integer integer=(Integer)session.getAttribute("save");
int realnumber=integer.intValue();
int guessNumber=0;
boolean boo=true;
try{
guessNumber=Integer.parseInt(number);
}
catch(Exception e){boo=false;}
if(boo){
if(guessNumber==realnumber){
int n=((Integer)session.getAttribute("count")).intValue();
n=n+1;
session.setAttribute("count",new Integer(n));
mess="您猜对了，这是第"+n+"次猜测";
}
else if(guessNumber>realnumber){
int n=((Integer)session.getAttribute("count")).intValue();
n=n+1;
session.setAttribute("count",new Integer(n));
mess="您猜大了，这是第"+n+"次猜测";
}
else if(guessNumber<realnumber){
int n=((Integer)session.getAttribute("count")).intValue();
n=n+1;
session.setAttribute("count",new Integer(n));
mess="您猜小了，这是第"+n+"次猜测";
}
else if(number.equals("你还没开始猜测"))
{
mess="你还没开始猜测";
}
jspContext.setAttribute("message", mess);
}else{
jspContext.setAttribute("message","请输入您的猜测");
}
%>