Shutdown Daemon Thread which is reading on a queue

Question

I have a thread which is looping a queue, reading tasks and delegating them to a CachedThreadPool. This queue is continuously being filled with client-submitted tasks.

I want the reader thread to terminate if the client programs also terminates. At first I thought that setting the reader thread as a daemon would solve the problem. However, there are two problems with this:

The reader thread needs to read all the events still left in the queue before it exits.
The reader thread has a CachedThreadPool. Since the threads in it are not daemon, the program will no exit. If I set all the threads in the pool as daemon, somehow they do not process all the work (probably because the reader will terminate before processing all the queue).

I also tried to solve this with a shutdown hook, however the hook is never called at the right time (maybe because I'm running the client from a build tools? (Scala's SBT)).

Any hints on getting around this?

score 2 · Accepted Answer · answered Jan 28 '11 at 11:37

2

You could use an ExecutorService instead. It manages the queue and the thread(s) and can be shutdown().

answered Jan 28 '11 at 11:37

Peter Lawrey

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Agreed! Why re-invent the wheel. – cheekoo Jan 28 '11 at 17:06

score 0 · Answer 2 · answered Jan 28 '11 at 10:02

0

You can use a 'poison task' to signal reader thread that it should stop its work and shutdown the CachedThreadPool.

answered Jan 28 '11 at 10:02

Boris Pavlović

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score 0 · Answer 3 · answered Jan 28 '11 at 11:06

When you have a continuous loop such as the one your reader goes round, you should provide some way to notify it to stop. This can be as simple as a boolean on whatever class you implemented the reader in.

public class QueueReader implements Runnable
{
  private boolean runIndicator = false;

  public void run()
  {
    runIndicator = true;
    while (runIndicator)
    {
      //poll the queue with a timeout
      //submit any task from the queue to the cached thread pool
    }

    //shutdown the cached thread pool
  }

  public void stop()
  {
    runIndicator = false;
  }

}

Note that when you call stop() it will still wait for the queue poll to time out and will still submit any task it finds on there.

For completeness, you should probably go ahead and declare the boolean as `volatile` (see http://stackoverflow.com/questions/106591/do-you-ever-use-the-volatile-keyword-in-java) — J. Dimeo, Aug 27 '16 at 01:21

Shutdown Daemon Thread which is reading on a queue

3 Answers3