Jsp org.apache.jasper.JasperException: Cannot forward after response has be

Question

what does this mean?
org.apache.jasper.JasperException: Cannot forward after response has been committed
I can't find an explanation of the error in general on the web? I am using dispatcher.,forward?
<%@ page import="java.io.*" %>
<%@ page import="javax.servlet.*" %>
<%@ page import="javax.servlet.http.*" %>
<%@ page import="java.util.*" %>
<%@ page import="java.text.*" %>
<%@ page import="javax.xml.parsers.DocumentBuilderFactory" %>
<%@ page import="javax.xml.parsers.*" %>
<%@ page import="org.w3c.dom.Document" %>
<%@ page import="org.w3c.dom.DOMException" %>
<%@ page import="org.w3c.dom.Element" %>
<%@ page language="java" import=".*" %>
<jsp:useBean id="manageAddressBook" scope="session" class=".ManageAddressBook" />
<%! .AddressBook ab; %>
<%! //String addJSP = "http://localhost:8080/examples/jsp//display.jsp"; %>
<%! String addJSP = "Add.jsp"; %>
<%! String displayJSP = "display.jsp"; %>
<%! boolean noPrev = false; %>
<% String displayPage = "display.jsp";
   String addPage     = "Add.jsp";
   String loginPage   = "login.jsp";
Enumeration enum = request.getParameterNames();
AddressBook ab;
String lastPageVar = request.getParameter("lastPageVar");
String button = (String)request.getParameter("submit");
if (lastPageVar == null)
   lastPageVar = "";
//Figure out what the dispatcher needs to do
String actionVar = request.getParameter("actionVar");
  if (actionVar == null)
     actionVar = "";
//check login status
Object loginObj = session.getAttribute("isLoggedIn");
boolean isLoggedIn;
if (loginObj != null)
   isLoggedIn = ((Boolean) loginObj).booleanValue();
else
   isLoggedIn = false;
if (lastPageVar.equals("")){
   session.setAttribute ("fromDispatcher", new Boolean(true));
   session.setAttribute("firstCall", new Boolean(false));
   callWebPage(request,response,session,loginPage);
else
   session.setAttribute("firstCall", new Boolean(false));
//if (!(isLoggedIn))
//  callWebPage(request,response,session,loginPage);
if (lastPageVar.equals("LOGIN_PAGE")){
   String username = (String)request.getParameter("username");
   String password = (String)request.getParameter("password");
   isLoggedIn = checkLogin(username,password);
   Boolean isLoggedInObj = new Boolean(isLoggedIn);
   session.setAttribute ("isLoggedIn", isLoggedInObj);
   if (isLoggedIn)
     callWebPage(request,response,session,displayPage);
   else
     callWebPage(request,response,session,loginPage);
if (lastPageVar.equals("DISPLAY_PAGE")){
  if (button.equals("logout"))
    callWebPage(request,response,session,loginPage);
  if (button.equals("next") || button.equals("previous"))
    callWebPage(request,response,session,displayPage);
  else
    callWebPage(request,response,session,addPage);
if (lastPageVar.equals("ADD_PAGE")){
  if (button.equals("logout"))
     callWebPage(request,response,session,loginPage);
  //if save or cancel, then just save or cancel and return to display screen.
  if (button.equals("save")){
    manageAddressBook.edit(request);
    callWebPage(request,response,session,displayPage);
  if (button.equals("Add"))
    callWebPage(request,response,session,displayPage);
  else
    ab = (AddressBook)session.getAttribute("currAddressBook");
  if (button.equals("Delete")) {
    ab = (AddressBook)session.getAttribute("currAddressBook");
    manageAddressBook.setDelete(ab);
    callWebPage(request,response,session,displayPage);
  if (button.equals("cancel"))
    callWebPage(request,response,session,displayPage);
%>
<%-- JSP FUNCTIONS  --%>
<%!
public boolean checkLogin(String username, String password) {
   if (username.equals("mypage") && password.equals("guesswho"))
     return true;
   else
     return false;
void callWebPage( HttpServletRequest request,HttpServletResponse response,HttpSession session,String webPage)
    throws ServletException, IOException {
  //verifies from dispather
  session.setAttribute ("fromDispatcher", new Boolean(true));
  RequestDispatcher dispatch = request.getRequestDispatcher( webPage );
  dispatch.forward(request,response);
%>

Answer 1

Its like this.. after you dispatch the request by the method call [callWeb...], the dispatcher would dispatch the request to that page and after the page is dispatched and response has been sent, the execution continues on this JSP, ie from the line after the call to the [callWeb..] function.. All you need to do to avoid this exception is,
a) put a return; statement after the call to the function.
Or
b) structure your code in such a way that after the call to the [callWeb..] function, the execution would not continue;
I would prefer the first condition though along with structuring the code in such a way that you need not have that many calls to the function. Instead have a String dest variable, which would change and make a call in the end
Cheers
-P