Thursday, March 24, 2011

Chapter 23: Invalidating Sessions

Invalidating sessions is very important because, the session is going to contain some data, data that you might not want someone else to use. For example, lets say you login to your banks website on your office computer and receive an urgent call from your wife. In a hurry you leave your desk unattended and also without logging off the banks website. Now, lets say after an hour someone else from your office walks by your desk and sees that you are logged on to your banks website and are nowhere to be seen. They can use the website to transfer funds to their account. Wouldn't that be an issue? Yes, it would be. That is exactly why all secure websites have a session timeout feature wherein, the users session will be terminated automatically if it senses that the session has been idle for a few mins (Usually 5 mins or 10). This way, you are safe.
So, after all this story, I guess you know where I am coming to. We are going to learn how to invalidate a session in this chapter.

So, lets get started!!!

Invalidating Sessions

Invalidating sessions is important as well as tricky. You need to be cautious when you encounter questions in the exam that asks whether the session would be invalidated under a particular scenario. They might lure you into thinking that the session might be invalidated where in reality the session would be very much active.

Exam Trivia
When is session invalid Surfing to another Web site does not invalidate a session, but quitting the browser does. The user can surf from your page to somewhere else and back again without losing the session. The session will remain intact unless the user was away longer than the timeout.

The six most commonly used methods to invalidate a session are

• Calling HttpSession.setMaxInactiveInterval(int secs) method, explicitly setting how many minutes the session will last.
• The session will automatically be invalid after a certain time of inactivity (Tomcat default is 30 minutes). You need to remember that this 30 minutes is not a hard and fast rule for all servers. It might vary from one server to another and is configurable. So you can have it configured to last 25 mins in your server and I can have it to last 20 mins.
• The user closes all browser windows. Note that, here the session will timeout rather than directly triggering a session invalidation.
• The session will expire when it is explicitly invalidated by a servlet by calling invalidate().
• The server is stopped or crashes. Note that this event might not trigger a session invalidation. A Web container that permits failover might persist the session and allow a backup Web container to take over when the original server fails.
• You can set the default timeout in the web.xml file ().

Don't worry about the web.xml file just yet. We shall be covering it in great detail in future so for now just remember that you can set the session timeout interval in the web.xml file and that is as much you need to know at this point of time.

Previous Chapter: Chapter 22 - Session Event Listeners

Next Chapter: Chapter 24 - Session Tracking Through URL Rewriting

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