JSP Variable Scope

Question

Hello,
I'm fairly new to JSP, so I apologize if this is a newbie question. I'm having a problem where it appears as though some variables I defined in the declaration area of my JSP page are shared by multiple clients. Here is a code snippet...
<%@ page import="java.util.*" %>
<jsp:useBean id="postSearch" scope="request" class="healthweb.PostBean" />
<%!
String hridErr = "";
String emplHrid = "";
String supvHrid = "";
String yourHrid = "";
boolean youAreEmpl = true;
boolean youAreSupv = false;
Hashtable[] empl;
Hashtable[] you;
Hashtable[] supv;
%>
<SCRIPT LANGUAGE="JavaScript">
alert("<%= request.getParameter("yHrid") %>");
alert("<%= youAreEmpl %>");
</SCRIPT>
As you can see, I'm initializing the youAreEmpl variable to true. However, if a client executes this JSP and the variable gets set to false, it seems to be false when other clients access the JSP initially.
I'm very confused by this. Doesn't each client receive its own scope and its own instance of the object created by the JSP page?

Answer 1

First off, let's start by explaining what's going on here. A JSP page is converted to a servlet via the JSP parsing servlet that initially handles the request. When you use the declaration syntax <%!...%> you are declaring members (variables, methods, etc.) that are local to the underlying servlet. Since a servlet is a multi-threaded object, all clients share the same servlet object created from the JSP page. Thus, threading issues apply to all local members of the servlet object. In other words you have to make sure the members of the servlet are thread safe.
Many are confused by this because JSP shields you somewhat from these concerns if you stay within the confines of the predefined service() method. What I mean is this. everything you code within the <%...%> and <%=...%> delimiters is placed in the service method of the underlying servlet. This is the collection of code/objects that each client runs a unique copy of. These areas are not subject to threadding concerns/issues.
Now, There are a number of remedies to your situation. You can remove the exclamation point from the delimeters around your variable declarations (i.e. change all <%!...%> to <%...%>), or you could add the following implements clause to your JSP page via JSP directive: implements SingleThreadModel
Forgive me, I don't recall the exact syntax for this as I don't have any of my manuals at hand. I believe it's something like: <%@ page implements="SingleThreadModel" %> Either would solve the problem. The SingleThreadModel forces each servlet to execute in it's own thread.
Kind Regards,
Cliff