Java: Force stopping of ExecutorService threads

Question

My code:

String[] torrentFiles = new File("/root/torrents/").list();

        if(torrentFiles.length == 0 || torrentFiles == null)
        {
            System.exit(0);
        }

        ex = Executors.newFixedThreadPool(3);

        for(String torrentFile : torrentFiles)
        {
            ex.submit(new DownloadTorrent("/root/torrents/" + torrentFile));
        }

        ex.shutdown();

        try
        {
            ex.awaitTermination(30, TimeUnit.MINUTES);
        }
        catch(InterruptedException ex1)
        {
            Logger.getLogger(Main.class.getName()).log(Level.SEVERE, null, ex1);
        }

But sometimes torrent downloading takes unknown time value and «awaitTermination» not works as I want. I need to stop all executed threads instantly after half an hour but as I know «awaitTermination» just only use interrupt() method which works only in loops or waiting. So timeout not works if this moment happens. So, how to?

Answer 1

Instant thread termination is never guaranteed, unless the thread checks periodically for isInterrupted() flag (or is waiting in interruptable method, i.e. which throws InterruptedException).

Consider implementing your worker threads in manner, when they check periodically for isInterrupted(). This may be something like that:

public void run() { 
  byte[] data;
  do {
     data = receiveDataChunk(timeout);
     processData(data);
  } while(!isInterrupted() && data != null);
}

Answer 2

ExecutorService.shutdownNow() will try to stop all the executing threads..

Here is a quote from javadoc

List<Runnable> shutdownNow()

Attempts to stop all actively executing tasks, halts the processing of waiting tasks, and returns a list of the tasks that were awaiting execution.

There are no guarantees beyond best-effort attempts to stop processing actively executing tasks. For example, typical implementations will cancel via Thread.interrupt(), so if any tasks mask or fail to respond to interrupts, they may never terminate.