Session and Refresh problem, Urgent, please help

Question

          Hi, Dear Everyone:
          I have two questions to ask. (Environment: OS: Nt4.0, Server: Weblogic5.1)
          (1). When I use session to store an object in servlet (processed results
          from database) then dispatch to a jsp page to display the results, I got the following
          error message.
          Wed May 16 15:54:31 CDT 2001:<I> <ServletContext-General> Generated java file:
          C:\weblogic\myserver\classfiles\jsp_servlet\_ccproject\_subdisplay.java
          java.lang.NullPointerException
          at jsp_servlet._ccproject._subdisplay._jspService(_subdisplay.java, Comp
          iled Code)
          at weblogic.servlet.jsp.JspBase.service(JspBase.java:27)
          at weblogic.servlet.internal.ServletStubImpl.invokeServlet(ServletStubIm
          pl.java:106)
          at weblogic.servlet.internal.ServletStubImpl.invokeServlet(ServletStubIm
          pl.java:124)
          at weblogic.servlet.internal.RequestDispatcherImpl.forward(RequestDispat
          cherImpl.java:154)
          at com.voy.CCPro.DataDumper.doPost(DataDumper.java:69)
          at com.voy.CCPro.DataDumper.doGet(DataDumper.java:85)
          at javax.servlet.http.HttpServlet.service(HttpServlet.java:740)
          at javax.servlet.http.HttpServlet.service(HttpServlet.java:865)
          at weblogic.servlet.internal.ServletStubImpl.invokeServlet(ServletStubIm
          pl.java:106)
          at weblogic.servlet.internal.ServletContextImpl.invokeServlet(ServletCon
          textImpl.java:907)
          at weblogic.servlet.internal.ServletContextImpl.invokeServlet(ServletCon
          textImpl.java:851)
          at weblogic.servlet.internal.ServletContextManager.invokeServlet(Servlet
          ContextManager.java:252)
          at weblogic.socket.MuxableSocketHTTP.invokeServlet(MuxableSocketHTTP.jav
          a:364)
          at weblogic.socket.MuxableSocketHTTP.execute(MuxableSocketHTTP.java:252)
          at weblogic.kernel.ExecuteThread.run(ExecuteThread.java, Compiled Code)
          Here is my servlet code:
          HttpSession session = req.getSession(true);
          Vector temp = buildData.BuildForDisplay(rst);
          session.setAttribute("PostTitlePool", temp);
          String url = "/ccproject/subdisplay.jsp";
          RequestDispatcher dis = getServletContext().getRequestDispatcher(url);
          dis.forward(req, res);
          Here is my jsp
          <%@ page import="com.voy.CCPro.*" %>
          <%@ page import="java.util.*" %>
          <%
          Vector v = new Vector();
          v = (Vector)session.getAttribute("PostTitlePool");
          Enumeration enum = v.elements();
          SequenceData sq = new SequenceData();
          while(enum.hasMoreElements())
               sq = (SequenceData)enum.nextElement();
               if(sq.getIndicator()==0)
                    out.println(sq.getTitle());
               else
               out.print(sq.getTitle());
          %>
          Please tell me what I did wrong or What more do I need to do?
          (2). If I changed to use request.setAttribute in servlet, it works. But I got another
          strange
          result from browser. That is, if I keep on clicking refresh (IE5.5), the same result
          appending
          the previous one on the same page.
          Here is my code in servlet:
          req.setAttribute("PostTitlePool",temp);
          String url = "/ccproject/subdisplay.jsp";
          RequestDispatcher dis = getServletContext().getRequestDispatcher(url);
          dis.forward(req, res);
          Here is my jsp:
          <%@ page import="com.onvoy.CCPro.*" %>
          <%@ page import="java.util.*" %>
          <%
          Vector v = new Vector();
          v = (Vector)request.getAttribute("PostTitlePool");
          Enumeration enum = v.elements();
          SequenceData sq = new SequenceData();
          while(enum.hasMoreElements())
               sq = (SequenceData)enum.nextElement();
               if(sq.getIndicator()==0)
                    out.println(sq.getTitle());
               else
               out.print(sq.getTitle());
          %>
          Please explain it to me why that and how to fix it. I appreciate it very much.
          

Answer 1

I control this as follows
--> all JSP's don't create session (<@page session="false">)
--> my FrontServlet checks
session=request.getSession(false);
if(session==null)
//invoke method from LoginHandler (Helper Class)
//for check logindata and create session
else
//invoke method from ProtectedResource (Helper Class)
//for check, if user logged on
--> LoginHandler
//method UserAllowed returns true or false
if(!UserAllowed())
//invoke ServiceDispatcher method who displays
//the 'AccessDenied'Page
else
//set loggedIn into the session
session.setAttribute("loggedIn","OK");
//invoke ServiceDispatcher method to display the requested Page
--> ProtectedResource
//check if user logged in
Object o=session.getAttribute("loggedIn");
if(o==null)
//User not logged in
//invoke ServiceDispatcher method to display 'Login' Page
else
//User logged in
//invoke ServiceDispatcher method to display the requested Page
--> URL of the Application
Welcome: http://localhost:8000/APP/login.jsp
submitted FORM sent to FrontServlet
(FORM ACTION="servlet/FrontServlet")
When everything is alright (username/password)
new URL http://localhost:8000/APP/servlet/FrontServlet
and the 'Main' Page is shown
when session is timed out and i refresh page
data is sent again (without asking for username and password)and new
session is created and the 'Main' Page is shown again.
How can i fix this problem
thank you in advance :-)