How to set countdownlatch for unknown number of threads

Question

I have to start unknown number of threads then wait or all threads to finish their job. I am using executor service. I tried using countdownlatch - so that I may wait till countdown is zero. But there is no way I can get number of threads I have started. Can someone give me any idea how may I achieve this ?

Why not use [`Future`s](https://www.baeldung.com/java-future) instead? — Turing85, Dec 25 '19 at 12:43
Can you please share the code you have tried. Not able to understand the requirement. — dassum, Dec 25 '19 at 14:59

score 1 · Accepted Answer · answered Dec 26 '19 at 10:48

1

Thank you for your responses. I came across the answer, and it helped. Sharing a link for reference. Flexible CountDownLatch?

answered Dec 26 '19 at 10:48

kcd

43
4

score 0 · Answer 2 · answered Dec 25 '19 at 12:51

In case that you want combine a List of CompletableFutures, you can do this :

// Waits for *all* futures to complete and returns a list of results.
// If *any* future completes exceptionally then the resulting future will also complete exceptionally.

public static <T> CompletableFuture<List<T>> all(List<CompletableFuture<T>> futures) {
    CompletableFuture[] cfs = futures.toArray(new CompletableFuture[futures.size()]);

    return CompletableFuture.allOf(cfs)
            .thenApply(ignored -> futures.stream()
                                    .map(CompletableFuture::join)
                                    .collect(Collectors.toList())
            );
}

For more details on Future & CompletableFuture, useful links:

Future: https://www.baeldung.com/java-future
CompletableFuture: https://www.baeldung.com/java-completablefuture
CompletableFuture: https://www.callicoder.com/java-8-completablefuture-tutorial/
Waiting on a list of Future

How to set countdownlatch for unknown number of threads

2 Answers2