Resin session help

[Chad Zhou]

I am new to Resin. I have a web application which runs perfectly fine under Tomcat. Now I have to deploy it to the Resin server. The problem is that the session gets invalidated after user sign in. The following shows how I start a new session:

HttpSession session = req.getSession(true);
session.setAttribute("user", username);

I tried the following two ways and both of them lost the session info.
1. req.getRequestDispatcher(resp.encodeRedirectURL("/a.jsp")).forward(req,resp);
2. resp.sendRedirect(resp.encodeRedirectURL("a.jsp"));

My development environment is:
Windows 2000 Pro, Resin 2.1.14 (not work in Resin 3.0.8 either), MySQL
The Resin configuration looks like:

<session-config>
<session-max>4096</session-max>
<session-timeout>30</session-timeout>
<enable-cookies>true</enable-cookies>
<enable-url-rewriting>true</enable-url-rewriting>
<!--
- Store sessions in the filesystem, so they can persist across
- servlet and class changes.
-
- Uncomment this during development.
-->
<!--
- <file-store>WEB-INF/sessions</file-store>
-->
</session-config>

Thanks in advance for your help.