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How does the servlet know the user just press STOP button?

Sam Zheng
Ranch Hand

Joined: Nov 29, 2000
Posts: 61
Hi, everybody,
I have a servlet that is doing heavy data crunching. The
structure of my web application is like this:
(Request Dispatch) JSP2
The JSP1 will invoke the Servlet, which does all number
crunching and then dispatch to the JSP2 for presentation
to the web browser. My problem is that if a user clicks
the STOP button, or simply closes the browser, my Servlet
will continue to run because it has no way of telling
whether the user is still waiting for output. This gives
me two headaches:
1. it consumes computing resources because the user doesn't
need the result anymore.
2. it causes exception to be thrown out since there is no
place for the output data.
What I need is a way to alert the servlet when the user
clicks the STOP button in the brower or completely exits
the browser. Any help, please?
William Brogden
Author and all-around good cowpoke

Joined: Mar 22, 2000
Posts: 13037
Seems to me you should get an IOException because the socket is closed, but I don't think it will happen until you actually write some output. Maybe your servlet should be writing (and flushing) small chunks of HTML to check that the user is still there.

author of:
Sam Zheng
Ranch Hand

Joined: Nov 29, 2000
Posts: 61
Hi Bill,
You are exactly right. I would get an IOException
when the servlet/JSP try to write the things out.
Your suggestion is also useful if I don't use
the so-called servlet centric design, in which my
servlet will not output anything, all the output
(presentation phase) will be handled by the
JSP dispatched by the servlet.
However, my problem is to detect that the
user is not interested in the work of the servlet
thus to stop the running of the servlet. Keep in
mind that in my servlet I cannot output anything
information, otherwise I will not be able to dispatch.
My intention is to detect that the user has pressed
the STOP button (does it mean that the session is
also invalidated at the same time) and then to
Any other way to detect such an event as pressing
STOP button?

Mike Nightsky
Ranch Hand

Joined: Aug 18, 2001
Posts: 48
I think there is no way to detect if the user presses the stop button, exept, the browser will send this event as a request to the server and you have to identify him to stop the servlet.
Http is a stateless protocoll so the session will not be invalidated until the user resubmit his data, or you send an answer.

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Tim Holloway
Saloon Keeper

Joined: Jun 25, 2001
Posts: 17421

HTTP is not only stateless, it's message-based. There are, however, no messages for "button-press" events and the like as in a regular GUI / client-server app. This keeps overhead low and browser independence high.
What the "STOP" button REALLY means is "STOP the output" - it breaks the pipeline connection that the server was using to send the response back to the client. If by some wierd quantum mechanical effect the client disappeared while the server was calculating, the server wouldn't even be capable of noticing - it's only when it comes time for output that the client has to be there.
There's the implication here that whatever you are calculating takes just long enough to be annoying, but not so long as to cause the browser to time out. That, obviously, could change if you started to scale up requests.
I'd like to be able to suggest a solution, but about the best that come to mind are safeguards. Make it hard for the client to trigger calculations frivilously (that is, do things like ask "Are you sure?"). Consider offloading the calculations to another server where the wasted horsepower isn't such an issue.
Or better yet, see if you can offload all or part of the heavy-duty stuff to run on the client's machine. It will be easier to attach a "Cancel" button there, and if not, at least they won't batter your server!

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