Testing for a User in a Session (JSP/Servlet)

Hi all,

I'm really struggling trying to figure out how best to approach a
problem I have.

My project is a simple web application that requires user
authentication. To accomplish this, the first page is a login JSP
that forwards a user's credentials to a loginCheck servlet. Upon
approval, it forwards to the main JSP of the app.

If a user has already logged in (User bean object created and in the
Session), I want it to redirect all requests to the main JSP rather
then the login JSP.

I created a tag to handle this but I ran into an endless loop. Basic
logic for checking:
UserBean user = (UserBean)
pageContext.getSession().getAttribute("user");

        try
        {
            if (user != null)
            {
                RequestDispatcher dis =
pageContext.getRequest().getRequestDispatcher("welcome.jsp");
                dis.forward(pageContext.getRequest(),
pageContext.getResponse());
            }
        } catch (Exception e)
        {
            e.printStackTrace();
        }

        return (SKIP_BODY);

Can anyone help me with the logic here? I want this tag to be usable
on all pages of the system, but I want to avoid the endless
redirecting that occurs when you get logged in and load "welcome.jsp".

Thanks for your help!