ExecutorService - cancel pending tasks

Question

There are multiple questions about waiting for an Java ExecutorService to finish (shutdown() + awaitTermination()) or alternatively cancel them all (shutdownNow()).

I'd like to go for an early exit without interrupting the tasks that have started already. You could also say I'd like to

• stop allowing further submissions (shutdown())
• cancel all tasks that are still pending
• wait for the in-flight tasks to terminate (awaitTermination())

How do I efficiently implement the cancellation?

Answer 1

Slaw had the right idea. It got me where I am now, and the solution works nicely.

First off, I switched from a generic Executor to a ThreadPoolExecutor. When instantiating it, I defined the queue to use - which allows me to watch progress, or clear it if need be.

In a nutshell, my code looks like this:

    BlockingQueue<Runnable> queue = new LinkedBlockingQueue<>();
    ThreadPoolExecutor executor = new ThreadPoolExecutor(numThreads, numThreads, 10L, TimeUnit.MILLISECONDS, queue);

    // submit all the jobs here

    // wait until all jobs have processed
    executor.shutdown();
    boolean earlyExit = false; // this variable would have to be set by some event
    while (!executor.awaitTermination(5, TimeUnit.SECONDS)) {
        if (earlyExit && !queue.isEmpty()) {
            queue.clear();
        }
    }