Future Task rejected from ThreadPoolExecutor

Question

I have a ThreadPoolExecutor and I submit a task to it.

private ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(1, 1, 0L, TimeUnit.MILLISECONDS, new ArrayBlockingQueue<Runnable>(1));

This code submits the Runnable to the ThreadPoolExecutor.

 protected void waitAndSweep(final String symbol) {

    runnable = new Runnable() {
      public void run() { /* irrelevant code */ }
    };

    try {
      Future<?> self = threadPoolExecutor.submit(runnable);
      futures.add(self);
    } catch (RejectedExecutionException re) {
      /* this exception will be thrown when wait and sweep is called more than twice.
       * threadPoolExecutor can have one running task and one waiting task.
       */
    } catch (Exception e) {
      logEvent(StrategyEntry.ERROR, "waitAndSweep", symbol, "Exception caught...", e);
    }
  }

The following code stops the task.

protected synchronized void stop(StrategyEntry entry) throws Exception {
    for (Object future : futures) {
      ((Future<?>) future).cancel(true);
    }
    futures.clear();

    threadPoolExecutor.shutdown();
}

The problem here is: When I try to stop the task, I am getting following exception:

Task java.util.concurrent.FutureTask@3a475611 rejected from java.util.concurrent.ThreadPoolExecutor@216393fb[Terminated, pool size = 0, active threads = 0, queued tasks = 0, completed tasks = 1]

Answer 1

The problem is that you shutdown() the excutor in the stop method. If you just want to wait for the task to complete, use Future.get(). When a executor is shut down, tasks can no longer be submitted to it.

shutdown() should only be used when you actually want to terminate the application.