JSP: check if user is logged on

[Vivek Kr Agrawal]

I am creating a small project (CRUD) for my college in JSP. The problem is that a user can create,read,update and delete items only after he gets logged in. I have created all things and its working all fine. But how can I check that this is a logged user or not in my every jsp page.
If it is not a logged one then it should be redirected to index.jsp.
I know I have to session variables but don't know how to use it? Please also provide me the best coding practice to write this code once and include in all subsequent files.

Thanks in advance!!

[Bear Bibeault]

Servlet filter.