Java - Stop thead after time without blocking main thread

Question

I'm making a Java application which receives HTTP requests. For each request which comes in, I start a new thread, and in that thread I'm reading the request and performing the necessary actions. However, I want to prevent the users from performing a "Slow Loris Attack", so I was thinking about giving a thread a maxTime value. If the thread takes longer than the maxTime it will terminate no matter what. So it will also block slow connections, which is not a problem.

However, I don't know what the right way is to do this. I've tried the following code, but this code is blocking my main thread. I'm looking for a way to do something similar like this, without blocking the main thread.

Code:

/**
 * Executor which is used for threadpools.
 */
private ExecutorService executor;

/**
 * Constructor for the class RequestReceiver.
 * Initializes fields.
 */
public RequestReceiver() {
    this.executor = Executors.newFixedThreadPool(200);
}

@Override
public void run() {
    try {
        this.serverSocket = new ServerSocket(port);
    } catch (IOException ex) {
        Logger.getInstance().logText("Could not start server on port: " + port);
        return;
    }

    Logger.getInstance().logText("Server running at port: " + port);

    try {
        while (shouldContinue) {
            Socket client = serverSocket.accept();
            HTTPRequestHandler handler = new HTTPRequestHandler(client);
            Thread t = new Thread(handler);
            executor.submit(t).get(10, TimeUnit.SECONDS); //This line is blocking
        }
    } catch (IOException ex) {
        Logger.getInstance().logText("Server is shutdown");
    } catch (InterruptedException | ExecutionException | TimeoutException ex) {
        Logger.getInstance().logText("Thread took too long, it's shutdown");
    }
}

Can you add executor.shutdown(); immediately after blocking statement executor.submit(t).get(10, TimeUnit.SECONDS); and try? — harshavmb
@harshavmb I don't see how this is going to solve my problem.. I did try it do, and like I predicted it crashed. Because when you send a 2nd HTTP request the pool won't accept it, because it's shutdown. java.util.concurrent.RejectedExecutionException — Guido
Why are you calling Future#get on your main thread? You don't even do anything with it. Plus, the TimeOutException will not stop the task. — matt
@matt Because I found multiple answers on SO, indicating that you could use this. stackoverflow.com/a/2733370/4653908 (see comments). — Guido
Answers, or comments? I don't follow what you linked to, the situation seems to be different. I provided an answer with an example so the .get() isn't blocking your main thread. — matt

matt matt · Accepted Answer · 2017-06-15T08:48:52

The least change to your example that could get you something similar to what you want is to submit a new task.

Socket client = serverSocket.accept();
HTTPRequestHandler handler = new HTTPRequestHandler(client);
Future f = executor.submit(handler);
executor.submit(()->{
    try{
        f.get(10, TimeUnit.SECONDS);
    } catch(TimeoutException to){
        //timeout happened, this will cancel/interrupt the task.
        f.cancel(true);
    } catch(Exception e){
        throw new RuntimeException(e);
        //something else went wrong...
    }
});

This would work, but it will block an additional thread waiting on the get call. You'll also need to handle the interrupt in your HTTPRequestHandler code.

Another way could be to use a ScheduledExecutorService.

Java - Stop thead after time without blocking main thread

2 Answers