Elegantly implementing queue length indicators to ExecutorServices

Question

Why, oh why doesn't java.util.concurrent provide a queue length indicators for its ExecutorServices? Recently I found myself doing something like this:

ExecutorService queue = Executors.newSingleThreadExecutor();
AtomicInteger queueLength = new AtomicInteger();
...

public void addTaskToQueue(Runnable runnable) {
    if (queueLength.get() < MAX_QUEUE_LENGTH) {
        queueLength.incrementAndGet(); // Increment queue when submitting task.
        queue.submit(new Runnable() {
            public void run() {
                runnable.run();
                queueLength.decrementAndGet(); // Decrement queue when task done.
            }
        });
    } else {
        // Trigger error: too long queue
    }
}

Which works ok, but... I think this really should be implemented as a part of the ExecutorService. It's dumb and error prone to carry around a counter separated from the actual queue, whose length the counter is supposed to indicate (reminds me of C arrays). But, ExecutorServices are obtained via static factory methods, so there's no way to simply extend the otherwise excellent single thread executor and add a queue counter. So what should I do:

1. Reinvent stuff already implemented in JDK?
2. Other clever solution?

Answer 1

There is a more direct way:

ThreadPoolExecutor executor = (ThreadPoolExecutor) Executors.newSingleThreadExecutor();
// add jobs
// ...
int size = executor.getQueue().size();

Although you might consider not to use the convenience create methods of Executor, but rather create the executor directly to get rid of the cast and thus be sure that the executor will always actually be a ThreadPoolExecutor, even if the implementation of Executors.newSingleThreadExecutor would change some day.

ThreadPoolExecutor executor = new ThreadPoolExecutor( 1, 1, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>() );

This is directly copied from Executors.newSingleThreadExecutor in JDK 1.6. The LinkedBlockingQueue that is passed to the constructor is actually the very object that you will get back from getQueue.

Answer 2

While you can check the queue size directly. Another way of dealing with a queue that's getting too long is making the internal queue bounded.

public static
ExecutorService newFixedThreadPoolWithQueueSize(int nThreads, int queueSize) {
  return new ThreadPoolExecutor(nThreads, nThreads,
                              5000L, TimeUnit.MILLISECONDS,
                              new ArrayBlockingQueue<Runnable>(queueSize, true));
}

This will cause RejectedExecutionExceptions (see this) when you exceed the limit.

If you want to avoid the exception the calling thread can be hijacked to execute the function. See this SO question for a solution.

References

• ThreadPoolExecutor