How to implement an ExecutorService to execute tas

Question

I'm using java.util.concurrent.ExecutorService with fixed thread pool to execute list of tasks. My list of tasks will typically be around 80 - 150 and I've limited the number of threads running at any time to 10 as shown below:

ExecutorService threadPoolService = Executors.newFixedThreadPool(10);

for ( Runnable task : myTasks ) 
{     
    threadPoolService.submit(task); 
}

My use case demands that even the completed task should be re-submitted again to the ExecutorService but it should be executed/taken again only when all the already submitted tasks are serviced/completed. That is basically, the tasks submitted should be executed on a rotation-basis. Hence, there will not be either threadPoolService.shutdown() or threadPoolService.shutdownNow() call in this case.

My question is, how do I implement ExecutorService servicing rotation-basis tasks?

Answer 1

The answer is more related to the implementation of the work queue used for the instance of ExecutorService. So, I'd suggest:

1. First choose an implementation of java.util.concurrent.BlockingQueue (an example) that provides a circular queue functionality. NOTE, the reason BlockingQueue has been chosen is that to wait until the next task is provided to queue; so, in case of circular + blocking queue, you should be careful how to provide the same behavior and functionality.

2. Instead of using Executors.new... to create a new ThreadPoolExecutor use a direct constructor such as

public ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue)

This way, unless you command the executor to shutdown, it will try to fetch the next task from the queue for execution from its work queue which is a circular container for tasks.

Answer 2

I suggest the following solution which completely uses functionality existing in the standard library concurrency utils. It uses a CyclicBarrier with a task decorator class and a barrier action which re-submits all tasks:

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class Rotation {

    private static final class RotationDecorator implements Runnable {
        private final Runnable          task;
        private final CyclicBarrier barrier;


        RotationDecorator( Runnable task, CyclicBarrier barrier ) {
            this.task = task;
            this.barrier = barrier;
        }


        @Override
        public void run() {
            this.task.run();
            try {
                this.barrier.await();
            } catch(InterruptedException e) {
                ; // Consider better exception handling
            } catch(BrokenBarrierException e) {
                ; // Consider better exception handling
            }
        }
    }


    public void startRotation( List<Runnable> tasks ) {
        final ExecutorService threadPoolService = Executors.newFixedThreadPool( 10 );
        final List<Runnable> rotatingTasks = new ArrayList<Runnable>( tasks.size() );
        final CyclicBarrier barrier = new CyclicBarrier( tasks.size(), new Runnable() {
            @Override
            public void run() {
                Rotation.this.enqueueTasks( threadPoolService, rotatingTasks );
            }
        } );
        for(Runnable task : tasks) {
            rotatingTasks.add( new RotationDecorator( task, barrier ) );
        }
        this.enqueueTasks( threadPoolService, rotatingTasks );
    }


    private void enqueueTasks( ExecutorService service, List<Runnable> tasks ) {
        for(Runnable task : tasks) {
            service.submit( task );
        }
    }

}

Answer 3

ThreadPoolExecutor provides an extension point for afterExecution where you can put the job back at the end of the queue.

public class TaskRepeatingThreadPoolExecutor extends ThreadPoolExecutor {

    public TaskRepeatingThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue) {
        super(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue);
    }

    @Override
    protected void afterExecute(Runnable r, Throwable t) {
        super.afterExecute(r, t);
        this.submit(r);
    }
}

You'll have to do a little more work of course to instantiate it yourself without the help of ExecutorService's handy factory method, but the constructors are simple enough to grok.

Answer 4

You could simply check that all the tasks have been executed and resubmit them once it is the case, like this for example:

    List<Future> futures = new ArrayList<>();
    for (Runnable task : myTasks) {
        futures.add(threadPoolService.submit(task));
    }
    //wait until completion of all tasks
    for (Future f : futures) {
        f.get();
    }
    //restart
    ......

EDIT
It seems you want to resubmit a task as soon as it gets completed. You could use an ExecutorCompletionService which enables you to retrieve tasks as and when they get executed, - see below a simple example with 2 tasks that get resubmitted a few times as soon as they are completed. Sample output:

Task 1 submitted pool-1-thread-1
Task 2 submitted pool-1-thread-2
Task 1 completed pool-1-thread-1
Task 1 submitted pool-1-thread-3
Task 2 completed pool-1-thread-2
Task 1 completed pool-1-thread-3
Task 2 submitted pool-1-thread-4
Task 1 submitted pool-1-thread-5
Task 1 completed pool-1-thread-5
Task 2 completed pool-1-thread-4

public class Test1 {

    public final ConcurrentMap<String, String> concurrentMap = new ConcurrentHashMap<>();
    public final AtomicInteger retries = new AtomicInteger();
    public final Object lock = new Object();

    public static void main(String[] args) throws InterruptedException, ExecutionException {
        int count = 0;
        List<Runnable> myTasks = new ArrayList<>();
        myTasks.add(getRunnable(1));
        myTasks.add(getRunnable(2));
        ExecutorService threadPoolService = Executors.newFixedThreadPool(10);
        CompletionService<Runnable> ecs = new ExecutorCompletionService<Runnable>(threadPoolService);
        for (Runnable task : myTasks) {
            ecs.submit(task, task);
        }
        //wait until completion of all tasks
        while(count++ < 3) {
            Runnable task = ecs.take().get();
            ecs.submit(task, task);
        }
        threadPoolService.shutdown();
    }

    private static Runnable getRunnable(final int i) {
        return new Runnable() {

            @Override
            public void run() {
                System.out.println("Task " + i + " submitted " + Thread.currentThread().getName() + "  ");
                try {
                    Thread.sleep(500 * i);
                } catch (InterruptedException ex) {
                    System.out.println("Interrupted");
                }
                System.out.println("Task " + i + " completed " + Thread.currentThread().getName() + "  ");
            }
        };
    }
}