One approach can be to use a ServletFilter in front of your portal app that intercepts the request after successful login and re-directs to the index page of your web-app
You can also try using a custom Authenticator instead of the built-in tomcat Authenticator to accomplish this more cleanly, but it might be over kill for the problem you are trying to solve
Thanks, I got it.
Could you please share with the rest of us how you did this?
I am interested too...
First I created a portlet that show a button or link when click will call processAction (in doView mode) and sendRedirect to myWebapp in processAction method of this portlet.
Then, I created a module to check user information in my webapp (use getRemoteUser). In case the return value is null, I make a forward to login page of portal.