Java Servlet : How to detect browser closing ?

Question

In my web app, when a user logs in, I add his Id to a vector of valid Ids in the servlet, when he logs out, I remove his Id from the vector, so I can see how many current users are active, if a user forgets to log out, my servelt generated html has :

<meta http-equiv="Refresh" content="30; url=My_Servlet?User_Action=logout&User_Id=1111">

in the tag to automatically log him out.

But I've noticed many users are there for ever, never logged out. I found out why, by closing their browsers, they never manually or automatically logged out, so their user Ids will never be removed from the valid user Ids vector.

So, my question is : how do I detect users closing their browsers, so my servlet can remove their Ids from the vector ?

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Edit :

Thanks for all the answers, I see some light at the end of the tunnel, but there is still a problem, my program has something like this :

Active User List :

User_1 : Machine_1 [ IP_1 address ]
User_2 : Machine_2 [ IP_2 address ]
User_3 : Machine_3 [ IP_3 address ]
...

How do I know, from the session listener, which user's session has ended and therefore remove him from my list ?

I was hoping when the session ends, the HttpServlet's destroy() method would be called and I can remove the user Id in there, but it never gets called when user closes his browser, why ? And is there any other method in the HttpServlet that gets called when a session closes ?

Frank

Answer 1

in JavaScript you can use the onbeforeclose event to pass a call back to the server when the user closes the browser.

I typically use a synchronous Ajax call to do this.

Answer 2

I suggest you remove the ID when the Servlet engine destroys the session. Register a HttpSessionListener that removes the user's ID when sessionDestroyed() is called.

Diodeus's idea will only help you detect that the session is over more immediately.

Answer 3

There is no foolproof way to do what you're trying to do, but both sblundy and Diodeus have plans that will cover most circumstances. There is nothing you can do about someone who turns off Javascript in their browser, or their internet connection goes down, or their power goes out. You should just cull sessions after a certain period of inactivity (which I think is what sblundy's suggestion of listening for session destruction will do).

Answer 4

There is no way to know on the server-side (unless you are using some JavaScript to send a message to the server) that the browser has closed. How could there be? Think of how HTTP works - everything is request and response.

However, the application server will track when Sessions are active and will even tell you when a Session has been destroyed (such as due to time-out). Take a look at this page to see how to configure a HttpSessionListener to receive these events. Then you can simply keep track of the number of active sessions.

The number of active sessions will lag behind the actual number of current users, since some period of (configurable) time has to elapse before a session is timed out; however, this should be somewhat close (you can lower the session-timeout to increase the accuracy) and it is a lot cleaner and easier than 1) tracking Sessions yourself or 2) sending some asynchronous JavaScript to the server when a browser is closed (which is not guaranteed to be sent).

Answer 5

I had to do that recently, and after some searches, I found some solutions on the Net... all of them non working universally!

onbeforeclose and onclose events are used for this task. But there are two catches: they are fired when the user reload the page or even just change the current page. There are tricks to see if the event is actually a window/page/tab closing (looking at some Dom properties going haywire on closing event), but:

• They are browser dependent
• The tricks are undocumented, thus brittle
• And actually they vary along the browser version/update...

And worst of all, these events are now ignored by most modern browsers, because they have been abused by rogue ads popping out windows when browser was closing. They are not fired in Safari, Opera, IE7, etc.

As pointed out, most Web applications with login destroy the user session after a while, eg. half an hour. I was asked to logout on browser closing to free faster a precious resource: licenses. Because users often forget to log out...

The solution I gave was to ping with an Ajax request (sending the user ID) the server on regular intervals (say 1 minute). If the server receives no ping for, say, 3 minutes, it disconnect the user.