What's the best way to check whether a user is logged in or not?

Question

I have a question about basic session handling.
I'm running Tomcat 5.0.30 and have a web application where users can register with a username and password, and then log into a "member site".
What is the best way of making sure that a user actually has logged in or not?
What I've done in previous applications I've made is that I've just put a simple variable into the users session after he has successfully entered his password (i.e. Boolean loggedIn=true). Then I just test if this flag is true to grant him access to the member site. Is that a smart way of doing it?
Are there any libraries I can use which handles sessions for me in a secure way?
All comments and suggestions are appreciated!

Answer 1

Maintain a flag using session attributes it to one when the user is logs in set it to value.... and change set it zero or invalidate that session... when the user is logged out...
use something like this while user had logged in
session.setAttribute("flag",<unique_number>);
for checking whether the user had logged in or not...
if(Integer.parseInt(session.getAttribute("flag").toString())=!<unique_number>){
out.println("The session had expired");
out.close();
}