How to check if all tasks running on ExecutorService are completed

Question

I'v got ConcurrentLinkedDeque which I'm using for synchronic push/pop elements, and I'v got some async tasks which are taking one element from stack and if this element has neighbors It's pushing it to stack.

Example code:

private ConcurrentLinkedDeque<Item> stack = new ConcurrentLinkedDeque<>(); private ExecutorService exec = Executors.newFixedThreadPool(5);      while ((item = stack.pollFirst()) != null) {                 if (item == null) {                 } else {                     Runnable worker = new Solider(this, item);                     exec.execute(worker);                 }             }     class Solider{          public void run(){              if(item.hasNeighbors){                 for(Item item:item.neighbors){                     stack.push(item)                 }              }           }     } 

I would like to have additional statement in while loop which answers the question - "any task in Executor is working?"

Answer 1

There isn't a clean way to check if all Runnables are done if you use ExecutorService.execute(Runnable). Unless you build a mechanism to do so in the Runnable itself (which is sloppy in my opinion).

Instead:
Use ExecutorService.submit(Runnable). This method will return a Future<?> which is a handle to the result of a Runnable. Using Futures provides a clean way to check results.

All you have to do is maintain a list of Futures that you submit, and then you can iterate over the whole list of Futures and either:
  A) wait for all the futures to be done in a blocking way or
  B) check if all the futures are done in a non-blocking way.

Here is a code example:

List<Future<?>> futures = new ArrayList<Future<?>>(); ExecutorService exec = Executors.newFixedThreadPool(5);  // Instead of using exec.execute() use exec.submit() // because it returns a monitorable future while((item = stack.pollFirst()) != null){     Runnable worker = new Solider(this, item);     Future<?> f = exec.submit(worker);     futures.add(f); }  // A) Await all runnables to be done (blocking) for(Future<?> future : futures)     future.get(); // get will block until the future is done  // B) Check if all runnables are done (non-blocking) boolean allDone = true; for(Future<?> future : futures){     allDone &= future.isDone(); // check if future is done } 

Answer 2

Update: with Java 8+ CompletableFutures you can manage this with its new callback functions. First you will need to create all of the CompletableFutures you need which will also start running, eg:

We need to accumulate all the futures generated in an Array in order to pass them later to CompletableFuture.allOf(CompletableFutures...)

So let's say you have a list of people you want to calculate its days until birthday asynchronously:

First we create all those needed futures and collect them together in an array:

CompletableFuture<?>[] completables = people.stream()     .map(p -> createCompletableFuture(p))     .toArray(CompletableFuture<?>[]::new);  private CompletableFuture createCompletableFuture(Person p) {         return CompletableFuture.runAsync(daysUntillBirthday(p));     } 

Then you pass those completables to a new CompletableFuture:

CompletableFuture c = CompletableFuture.allOf(completables) 

And you can now check if there are still futures running with:

c.isDone()