How can I timeout a function that blocks in Java?

Basically, I'm calling BufferedReader.ReadLine(); However I'm in a multithreaded server, where I'm synchronizing a node in a tree. So when this ReadLine function is called, if someone else reaches the node, they're locked out. I can't figure out how to make a timelimit on the amount of time ReadLine waits for a response before quitting the thread. The closest I got was to create a new thread that would sleep for 1ms then check to see if the variable that I set ReadLine to is changed. So something like this:

synchronized (pointer) {
    String answer = "";
    Thread d = new Thread(new Runnable() {
        public void run() {
            try {
                int i = 0;
                while (answer.equals("")) {
                    if (i == 10000) {
                        System.out.println("Timeout Occured");
                        System.exit(0);
                    }
                    try {
                        Thread.sleep(1);
                        i++;
                    }
                    catch(Exception e) {
                        System.out.println("sleep problem occured");
                    }
                }
            }
            catch (IOException ex) {
            }
        }    
    });

    d.start();
    answer = socketIn.readLine();
}

This did what I wanted it to, but I couldn't figure out how to stop the current thread to unlock the node so other users could continue instead of killing the whole server. Finally, I thought maybe I could do this:

开发者_StackOverflow社区    Thread d = new Thread(new Runnable() {
        public void run() {
            try {
                answer = socketIn.readLine(); 
            } catch (IOException ex) {
            }
        }    
    });

    d.join(10000);
    catch (InterruptedException e){
    socketOut.println("Timeout Occured. Returning you to the beginning...");
    socketOut.flush();
    return;
}

But this still seems to block and not be able to continue. Could someone help me out with this? I can't understand what I'm doing wrong?

I also tried to get the ExecutorService to work, but couldn't. Is this my answer? How would I implement it?

[EDIT] socketIn is a BufferedReader, should have said that explicitly sorry. Also, the client is connecting via telnet, though I don't think that matters.

What I'm doing here is a "celebrity guessing game" where users can add celebrities to the tree. So I need to lock the node that the person is editing for thread-safety

Is this homework? It's suspiciously close to a question asked by someone else yesterday. If so it should have the homework tag.

You only need to lock on something when a thread will modify data that other threads may read/modify.

If you're locking something waiting on input, the scope of your lock is far too broad.

Your flow should be:

• read input from client (blocking readLine())
• lock shared resource
• modify
• unlock

(This is assuming you've got one thread per connection/client, and are blocking on the read from the client)

That being said ... if you're reading from a socket and want it to time out, you need to use clientSocket.setSoTimeout(1000); when you first accept the connection. If your BufferedReader is waiting for that amount of time (in milliseconds) and doesn't get input, it will throw a java.net.SocketTimeoutException

String inputLine = null;
try 
{
    inputLine = in.readLine();
    if (inputLine == null)
    {
        System.out.println("Client Disconnected!");
    }
    else 
    {
        // I have input, do something with it
    }
}
catch(java.net.SocketTimeoutException e)
{
    System.out.println("Timed out trying to read from socket");
}

Everything is already done. Try to use java.util.concurrent.

    //
    // 1. construct reading task
    //
    final FutureTask<String> readLine = new FutureTask<String> (
        new Callable<String>() {
            @Override public String call() throws Exception {
                return socketIn.readLine();
            }
        }
    );
    //
    // 2. wrap it with "timed logic"
    //    *** remember: you expose to your users only this task
    //
    final FutureTask<String> timedReadLine = new FutureTask<String> (
        new Callable<String>() {
            @Override public String call() throws Exception {
                try {
                    //
                    // you give reading task a time budget:
                    //      try to get a result for not more than 1 minute
                    //
                    return readLine.get( 1, TimeUnit.MINUTES );
                } finally {
                    //
                    // regardless of the result you MUST interrupt readLine task
                    // otherwise it might run forever
                    //      *** if it is already done nothing bad will happen 
                    //
                    readLine.cancel( true );
                }
            }
        }
    )
    {
        //
        // you may even protect this task from being accidentally interrupted by your users:
        //      in fact it is not their responsibility
        //
        @Override
        public boolean cancel(boolean mayInterruptIfRunning) {
            return false;
        }
    };

    Executor executor = Executors.newCachedThreadPool();

    // 3. execute both
    executor.execute( readLine );
    executor.execute( timedReadLine );

    // 4. ...and here comes one of your users who can wait for only a second
    try {
        String answer = timedReadLine.get(1, TimeUnit.SECONDS);
        //
        // finally user got his (her) answer
        //
    } catch (InterruptedException e) {
        //
        // someone interrupted this thread while it was blocked in timedReadLine.get(1, TimeUnit.SECONDS)
        //
    } catch (ExecutionException e) {
         //
        // e.getCause() probably is an instance of IOException due to I/O failure
        //
    } catch (TimeoutException e) {
        //
        // it wasn't possible to accomplish socketIn.readLine() in 1 second
        //
    }

I figured out a solution:

answer = "";
try{
    Thread d = new Thread(new Runnable() {
        public void run() {
            try {
                answer = socketIn.readLine();
            }   
            catch (IOException ex) {
                System.out.println("IO exception occurred");
            }
        }
    });
    d.join(10000); //Not sure if this is superfluous or not, but it didn't seem to work without it.
    d.start();
    i = 0;
    while (true){
        if (i == 10000){
            if (d.isAlive()) throw InterruptedException;
        }
        if (answer.equals("")){
            Thread.sleep(1);
        }
        else{
            break;
        }
        i++;
    }
    //This essentially acts as Thread.sleep(10000), but the way I 
    //implemented it, it checks to see if answer is modified or not every  
    //.001 seconds. It will run for just over 10 seconds because of these checks. 
    //The number in Thread.sleep could probably be higher, as it is 
    //unnecessary to check so frequently and it will make the code more efficient
    //Once it hits 10000, it throws an exception to move to the catch block below
    //where the catch block returns everything to its original state and 
    //returns the client to the beginning
    }
catch (Exception e){
    socketOut.println("Timeout Occurred. Returning you to the beginning...");
    socketOut.flush();
    pointer = tree.root;
    //reset class members to their original state
    return;
}

Thanks for looking at it, Brian Roach. Your post was informative, but I accidentally omitted some key bits of information. I will try to be more careful in the future.