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Asked: 2026-05-25T11:54:18+00:00

I’m using a ConcurrentLinkedQueue to store computational steps, and an ExecutorService created by Executors.newFixedThreadPool

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I’m using a ConcurrentLinkedQueue to store computational steps, and an ExecutorService created by Executors.newFixedThreadPool to execute them out. My problem is that the application never terminates. Here’s some code:

public class Run {

    public static void main (String[] args) throws Exception {
        ParallelExecutor executor = new ParallelExecutor();
        executor.execute();

        // manual shutdown
        Thread.sleep(30 * 1000);
        executor.stop();

        // the main thread dies, the application keeps running
}

 

public class ParallelExecutor implements Runnable {

    // stores executions steps to
    private ConcurrentLinkedQueue<ExecutionStep> queue = new ConcurrentLinkedQueue<ExecutionStep>();

    private ExecutorService threadPoolExecutor = Executors.newFixedThreadPool(10);

    private Thread feedingThread = new Thread(this);

    public void execute() {
        ExecutionStep step = new ConcreteExecutionStep();
        this.queue.add(step);
        this.feedingThread.start();
    }

    public void stop() {
        if (log.isInfoEnabled()) {
            log.info("Shutting down");
        }
        this.queue = new ConcurrentLinkedQueue<ExecutionStep>();

        try {
            this.feedingThread.join(0);
            this.threadPoolExecutor.shutdownNow();
        } catch (InterruptedException e) {
            log.warn("Something happened!", e);
        }
    }

    public void run() {
        while (true) {

            if (this.threadPoolExecutor.isShutdown()) {
                return;
            }

            try {
                Thread.sleep(this.waitMillis);
            } catch (InterruptedException e) {
                // NOP
            }

            if (!this.queue.isEmpty()) {
                this.threadPoolExecutor.execute(new Runnable() {

                    @Override
                    public void run() {
                        ExecutionStep step = this.queue.poll();
                        List<ExecutionStep> nextSteps = Collections.emptyList();
                        try {
                            nextSteps = step.execute();
                        } catch (Exception e) {
                            // NOP
                        }

                        // here we feed the queue
                        this.queue.addAll(nextSteps);
                    }
                });
            }
        }
    }

EDIT: Here’s the thread dump after the main thread died:

2011-09-06 17:20:56
Full thread dump Java HotSpot(TM) 64-Bit Server VM (20.1-b02-384 mixed mode):

"DestroyJavaVM" prio=5 tid=10f874800 nid=0x100501000 waiting on condition [00000000]
   java.lang.Thread.State: RUNNABLE

"Poller SunPKCS11-Darwin" daemon prio=1 tid=101a4d800 nid=0x11362e000 waiting on condition [11362d000]
   java.lang.Thread.State: TIMED_WAITING (sleeping)
    at java.lang.Thread.sleep(Native Method)
    at sun.security.pkcs11.SunPKCS11$TokenPoller.run(SunPKCS11.java:692)
    at java.lang.Thread.run(Thread.java:680)

"pool-2-thread-1" prio=5 tid=103db3000 nid=0x11341a000 waiting on condition [113419000]
   java.lang.Thread.State: WAITING (parking)
    at sun.misc.Unsafe.park(Native Method)
    - parking to wait for  <7f42e8f38> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)
    at java.util.concurrent.locks.LockSupport.park(LockSupport.java:158)
    at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(AbstractQueuedSynchronizer.java:1987)
    at java.util.concurrent.LinkedBlockingQueue.take(LinkedBlockingQueue.java:399)
    at java.util.concurrent.ThreadPoolExecutor.getTask(ThreadPoolExecutor.java:947)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:907)
    at java.lang.Thread.run(Thread.java:680)

"Timer-0" daemon prio=5 tid=101a63000 nid=0x113317000 in Object.wait() [113316000]
   java.lang.Thread.State: TIMED_WAITING (on object monitor)
    at java.lang.Object.wait(Native Method)
    - waiting on <7f49cc200> (a java.util.TaskQueue)
    at java.util.TimerThread.mainLoop(Timer.java:509)
    - locked <7f49cc200> (a java.util.TaskQueue)
    at java.util.TimerThread.run(Timer.java:462)

"Low Memory Detector" daemon prio=5 tid=10180a000 nid=0x10f607000 runnable [00000000]
   java.lang.Thread.State: RUNNABLE

"C2 CompilerThread1" daemon prio=9 tid=101809800 nid=0x10f504000 waiting on condition [00000000]
   java.lang.Thread.State: RUNNABLE

"C2 CompilerThread0" daemon prio=9 tid=103951000 nid=0x10f401000 waiting on condition [00000000]
   java.lang.Thread.State: RUNNABLE

"JDWP Command Reader" daemon prio=5 tid=103950000 nid=0x10df01000 runnable [00000000]
   java.lang.Thread.State: RUNNABLE

"JDWP Event Helper Thread" daemon prio=5 tid=10394f800 nid=0x10dc0a000 runnable [00000000]
   java.lang.Thread.State: RUNNABLE

"JDWP Transport Listener: dt_socket" daemon prio=5 tid=10394e800 nid=0x10db07000 runnable [00000000]
   java.lang.Thread.State: RUNNABLE

"Signal Dispatcher" daemon prio=9 tid=10e000000 nid=0x10da04000 waiting on condition [00000000]
   java.lang.Thread.State: RUNNABLE

"Surrogate Locker Thread (Concurrent GC)" daemon prio=5 tid=101808800 nid=0x10d901000 waiting on condition [00000000]
   java.lang.Thread.State: RUNNABLE

"Finalizer" daemon prio=8 tid=10393f000 nid=0x10c204000 in Object.wait() [10c203000]
   java.lang.Thread.State: WAITING (on object monitor)
    at java.lang.Object.wait(Native Method)
    - waiting on <7f44e3d70> (a java.lang.ref.ReferenceQueue$Lock)
    at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:118)
    - locked <7f44e3d70> (a java.lang.ref.ReferenceQueue$Lock)
    at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:134)
    at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:159)

"Reference Handler" daemon prio=10 tid=10393e000 nid=0x10c101000 in Object.wait() [10c100000]
   java.lang.Thread.State: WAITING (on object monitor)
    at java.lang.Object.wait(Native Method)
    - waiting on <7f44e5220> (a java.lang.ref.Reference$Lock)
    at java.lang.Object.wait(Object.java:485)
    at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:116)
    - locked <7f44e5220> (a java.lang.ref.Reference$Lock)

"VM Thread" prio=9 tid=103939800 nid=0x10b6fc000 runnable 

"Gang worker#0 (Parallel GC Threads)" prio=9 tid=103801800 nid=0x102601000 runnable 

"Gang worker#1 (Parallel GC Threads)" prio=9 tid=103802000 nid=0x102704000 runnable 

"Gang worker#2 (Parallel GC Threads)" prio=9 tid=103803000 nid=0x107102000 runnable 

"Gang worker#3 (Parallel GC Threads)" prio=9 tid=103803800 nid=0x107205000 runnable 

"Gang worker#4 (Parallel GC Threads)" prio=9 tid=103804000 nid=0x107308000 runnable 

"Gang worker#5 (Parallel GC Threads)" prio=9 tid=103804800 nid=0x10740b000 runnable 

"Gang worker#6 (Parallel GC Threads)" prio=9 tid=103805800 nid=0x10750e000 runnable 

"Gang worker#7 (Parallel GC Threads)" prio=9 tid=103806000 nid=0x107611000 runnable 

"Concurrent Mark-Sweep GC Thread" prio=9 tid=1038e3800 nid=0x10b408000 runnable 
"Gang worker#0 (Parallel CMS Threads)" prio=9 tid=1038e2000 nid=0x10aa02000 runnable 

"Gang worker#1 (Parallel CMS Threads)" prio=9 tid=1038e2800 nid=0x10ab05000 runnable 

"VM Periodic Task Thread" prio=10 tid=10181c000 nid=0x10f70a000 waiting on condition 

"Exception Catcher Thread" prio=10 tid=103801000 nid=0x1017f9000 runnable 
JNI global references: 13614

Heap
 par new generation   total 19136K, used 16017K [7f3000000, 7f44c0000, 7f44c0000)
  eden space 17024K,  81% used [7f3000000, 7f3d944a8, 7f40a0000)
  from space 2112K, 100% used [7f42b0000, 7f44c0000, 7f44c0000)
  to   space 2112K,   0% used [7f40a0000, 7f40a0000, 7f42b0000)
 concurrent mark-sweep generation total 63872K, used 6053K [7f44c0000, 7f8320000, 7fae00000)
 concurrent-mark-sweep perm gen total 30656K, used 30476K [7fae00000, 7fcbf0000, 800000000)

EDIT: The problem was due to another executor being (silently) started in the code I cut out before posting the question, as I thought it irrelevant. Anyway, Peter Lawrey answer is still valuable input, hence it got accepted.

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1 Answer

  1. Editorial Team

    This will shut down provided there is no work being done and it won’t pause from time to time which is slowing down your application and could be making it appear nothing is happening.

    public class ParallelExecutor {
  interface ExecutionStep {
    List<ExecutionStep> execute();
  }

  public static final int N_THREADS = Runtime.getRuntime().availableProcessors();
  private final ExecutorService executorService = Executors.newFixedThreadPool(N_THREADS);

  public void execute(final ExecutionStep step) {
    executorService.submit(new Runnable() {
      @Override
      public void run() {
        for (ExecutionStep es : step.execute())
          execute(es);
      }
    });
  }

  public void stop() {
    this.executorService.shutdownNow();
  }
}

    run with

    ParallelExecutor executor = new ParallelExecutor();
executor.execute(new ConcreteExecutionStep());

// manual shutdown
Thread.sleep(30 * 1000);
executor.stop();
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