Question

I’m trying to use ExecutorService with BlockingQueue<Runnable> but I have problems in exiting the script. It finishes without problems, but then keep waiting I don’t know what.

First of all I have a class

public class GenericTask implements Runnable {
    public void run() {
        // do stuff
    }
}

then this is the code

BlockingQueue<Runnable> queue = new ArrayBlockingQueue<Runnable>(10000, true);
ExecutorService myExecutor = Executors.newFixedThreadPool(numThreads);

new Thread(new Runnable() {
    public void run() {
        for (; ; ) {
            try {
                myExecutor.execute(queue.take());
            } catch (InterruptedException ignored) {
            }
        }
    }
}).start();

while (...) {
    queue.put(new GenericTask());
}

int waitTime = 500;
myExecutor.shutdown();
try {
    while (!myExecutor.awaitTermination(waitTime, TimeUnit.MILLISECONDS)) {
        logger.info("Waiting...");
        Thread.sleep(waitTime);
    }
} catch (Exception e) {
    e.printStackTrace();
}

System.out.println("Finished!");

When it prints “Finished!”, it is really completed, but the script continues going on unless I add a System.exit(0), but I think this is not correct.

Answer 1

In the end you are correctly shutting down all threads in thread pool. But there is yet another non-daemon thread that stops JVM from terminating. Can you spot it? It’s your anonymous producer thread with infinite loop inside: for (;;).

Use Thread.setDaemon(true):

Thread t = new Thread(new Runnable() {
  //...
});
t.setDaemon(true);
t.start();

Now when all threads in ExecutorService terminate after shutdown, main thread terminates as well and JVM will stop because your only remaining thread is a daemon.