Session sharing between BlazeDS Servlets and Http Servlets

I'm developing a Flex application that employs BlazeDS with Tomcat on the backend. The Flex client authenticates through the ChannelSet which in turn creates an authenticated session on Tomcat. From then on, all of the client calls are authenticated transparently. However, I have a regular HttpServlet for uploading files, which does not get the session information from the BlazeDS servlets.

Is there a way that I can get the Principal object in the HttpServlet? I know that I开发者_StackOverflow社区 can get the session ID from the BlazeDS servlet, pass it to the client, and then send that along with the upload request. However, I can't figure out how to get the principal from the session ID. Any ideas?

There is a method getUserPrincipal() in the interface HttpServletRequest.