TCP ServerSocket stops listening

[teletubo]

Hello guys . I am having a really strange problem with my ServerSockets.
first, this is my code of starting a serverSocket:

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ServerSocket serverSocket = new ServerSocket(P.PORT); while (true)  {  ...  Socket connectionSocket= serverSocket .accept(); //launch thread to listen to data on this connectionSocket and go to accept new connections }

basically that's it . It works quite fine , for a long while . It accepts the connections sends and receives a few data, then closes the connection .
 
But something happens that it just does not accept connections anymore . In the client, I don't get any errors, the thread just hangs waiting for the connection (if a timeout is not set). I dont get a "Server not found", it seems that it can see the server, but the connection is never stabilished.

Sometimes  my serverSocket lasts for a week, sometimes a few days . That's with 200 connections/day .

Anybody has an idea ?
Thanks in advance .

[Riven]

I have had ServerSockets listening for many months, with hundreds of connections per second, not a single problem.

Probably out of file descriptors?

For a more robust serversocket listener:
1. ensure all connections are properly closed, no matter what
2. prevent a flood-attack from bringing down your server (lack of file descriptors will crash other processes too)

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final Semaphore preventFlooding = new Semaphore(maxConcurrentConnections); while(true) {    preventFlooding.acquireUninterruptibly();    final Socket socket;    try {        socket = server.accept();    } catch(IOException exc) {        preventFlooding.release();        continue; // maybe next time!   }    Runnable task = new Runnable() {       public void run() {            InputStream input = null;            OutputStream output = null;            try {                socket.setSoTimeout(2000); // we must receive the first byte within 2 sec, change your timeout later to something larger               input = socket.getInputStream();                output = socket.getOutputStream();                DO_A_LOT_OF_WORK(socket, input, output);            }            finally {                preventFlooding.release();                try { if(input!=null) input.close(); } catch(IOException exc) { /* ignore */ }                try { if(output!=null) output.close(); } catch(IOException exc) { /* ignore */ }                try { socket.close(); } catch(IOException exc) { /* ignore */ }            }       }    }    // better: a thread-pool, with a few connections per second   // creating threads really starts to drain performance   new Thread(task).start(); }

Regarding the flood attack: I got hit by this so many times that it's silly. Thousands of connections in a few seconds, and if you're not careful, something breaks. Either VM heap, RAM (swapping), threads, running out of file descriptors... that is a serious issue for a server, as every process relies on them. Avoid IOException("Too many files open") at all cost, and my Semaphore will cap your file-descriptor count pretty well.

[teletubo]

Riven, that way I would only accept connections  after I DO_A_LOT_OF_WORK.
This is not actually desirable, since this can take several seconds, and I would be out of listeners in that interval, right ?
Of course I could start like 5 Threads of listeners but then I think it just breaks the anti-flooding strategy that the semaphore is there for .

I think I'll stick to 1. ensure all connections are properly closed, no matter what 
Come to think of it, I wrote my networking code a long time ago, so there is probably some error handling missing .

Thank you for now .

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[Riven]

Riven, that way I would only accept connections  after I DO_A_LOT_OF_WORK.

No.

You can have N (=maxConcurrentConnections) threads doing A_LOT_OF_WORK(...)

See: new Thread(task).start()

If you set maxConcurrentConnections to 100, you will at most have 100 thread handling 100 connections.

[teletubo]

ah okay I just skipped the Semaphore(maxConcurrentConnections); and thought of a mutex .

Thanks again