ParallelLimit.java

public class ParallelLimit {


    private static class Parallelizer<T> {

        private final LinkedList<Supplier<Future<T>>> pendingTasks;
        private final int limit;
        private final int totalTasks;
        private int concurrency;
        private final List<T> completedTasks = new ArrayList<>();
        private final Promise<List<T>> promise = Promise.promise();
        private boolean failed = false;

        Parallelizer(List<Supplier<Future<T>>> tasks, int limit) {
            if (limit < 0) {
                throw new IllegalArgumentException("limit must be positive");
            }
            this.pendingTasks = new LinkedList<>(tasks);
            this.totalTasks = tasks.size();
            this.limit = limit;
            this.concurrency = 0;
        }

        Future<List<T>> run() {
            if (pendingTasks.isEmpty())
                return Future.succeededFuture(Collections.emptyList());
            runInternal();
            return promise.future();
        }

        private void runInternal() {
            while (!pendingTasks.isEmpty() && concurrency < limit && !failed) {
                Supplier<Future<T>> futureSupplier = pendingTasks.removeFirst();
                Future<T> future = futureSupplier.get();
                concurrency++;
                future.onSuccess(res -> {
                    completedTasks.add(res);
                    concurrency--;
                    if (completedTasks.size() == totalTasks)
                        promise.complete(completedTasks);
                    else if (concurrency < limit)
                        runInternal();
                });
                future.onFailure(t -> {
                    failed = true;
                    promise.fail(t);
                });
            }
        }


    }
  
    public static <T> Future<List<T>> parallelLimit(
            List<Supplier<Future<T>>> tasks, int limit) {
        return new Parallelizer<T>(tasks, limit).run();
    }

  
    // ----------------- test code ------------------------
  
    public static void main(String[] args) throws InterruptedException {
        Vertx vertx = Vertx.vertx();
        ArrayList<Supplier<Future<Integer>>> suppliers = new ArrayList<>();
        for (int i=0; i<50 ; i++) {
            int nb = i;
            suppliers.add(() -> asyncMultiplier(vertx, nb));
        }
        CountDownLatch latch = new CountDownLatch(1);
        parallelLimit(suppliers, 10).onComplete(ar -> {
            if (ar.succeeded()) {
                System.out.println("done!");
            } else {
                System.err.println("error");
                ar.cause().printStackTrace();
            }
            latch.countDown();
        });
        latch.await();
        vertx.close();
    }

    public static Future<Integer> asyncMultiplier(Vertx vertx, int n) {
        System.out.println("starting task: " + n);
        Promise<Integer> result = Promise.promise();
        vertx.setTimer(1000, it -> {
            System.out.println("task complete: " + n);
            result.complete(n * 2);
        });
        return result.future();
    }

}

test code can remove CountDownLatch:

  // ----------------- test code ------------------------

  public static void main(String[] args) throws InterruptedException {
    Vertx vertx = Vertx.vertx();
    ArrayList<Supplier<Future<Integer>>> suppliers = new ArrayList<>();
    for (int i=0; i<50 ; i++) {
      int nb = i;
      suppliers.add(() -> asyncMultiplier(vertx, nb));
    }
    parallelLimit(suppliers, 10).onComplete(ar -> {
      if (ar.succeeded()) {
        System.out.println("done!");
      } else {
        System.err.println("error");
        ar.cause().printStackTrace();
      }
      vertx.close();
    });
  }

  public static Future<Integer> asyncMultiplier(Vertx vertx, int n) {
    System.out.println("starting task: " + n);
    Promise<Integer> result = Promise.promise();
    vertx.setTimer(1000, it -> {
      System.out.println("task complete: " + n);
      result.complete(n * 2);
    });
    return result.future();
  }

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