Controlling Task execution order with ExecutorService

Question

I have a process which delegates asynch tasks to a pool of threads. I need to ensure that certain tasks are executed in order. So for example

Tasks arrive in order<

Answer 1

You can use Executors.newSingleThreadExecutor(), but it will use only one thread to execute your tasks. Another option is to use CountDownLatch. Here is a simple example:

public class Main2 {

public static void main(String[] args) throws InterruptedException {

    final CountDownLatch cdl1 = new CountDownLatch(1);
    final CountDownLatch cdl2 = new CountDownLatch(1);
    final CountDownLatch cdl3 = new CountDownLatch(1);

    List list = new ArrayList();
    list.add(new Runnable() {
        public void run() {
            System.out.println("Task 1");

            // inform that task 1 is finished
            cdl1.countDown();
        }
    });

    list.add(new Runnable() {
        public void run() {
            // wait until task 1 is finished
            try {
                cdl1.await();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }

            System.out.println("Task 2");

            // inform that task 2 is finished
            cdl2.countDown();
        }
    });

    list.add(new Runnable() {
        public void run() {
            // wait until task 2 is finished
            try {
                cdl2.await();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }

            System.out.println("Task 3");

            // inform that task 3 is finished
            cdl3.countDown();
        }
    });

    ExecutorService es = Executors.newFixedThreadPool(200);
    for (int i = 0; i < 3; i++) {
        es.submit(list.get(i));
    }

    es.shutdown();
    es.awaitTermination(1, TimeUnit.MINUTES);
}
}