need help to create login machanism for web application

Hi,
I am making a web application in which login is must. I have made an html page in which i have made username and password fields and a submit button. I am using post method to send data to a servlet. On that servlet i am authenticating it and creating a session which get invalid after 30 seconds. But when i simply refresh the page, browser again ask for resend information and it get logged in again without entiring username and password again. I want it never happen but i am not getting any solution for this problem. Please tell me how can i solve this problem.

You should have log out option that does invalidate session.

ur servlet invalidates automatically. it should be invalidated only when u logout.
add that option(i.e. html button) to logout.