WindowedInvoker.java

public class WindowedInvoker<T, R> {
    private final Semaphore semaphore = new Semaphore(windowSize);

    private class ReleasingCallback<R> implements Consumer<R> {
        private final Consumer<R> delegate;

        @Override
        public void accept(R result) {
            semaphore.release();
            delegate.accept(t);
        }
    }

    public void invoke(Consumer<List<T>> consumer, List<T> batch, Consumer<R> callback) {
        semaphore.acquire();
        consumer.accept(batch, new ReleasingCallback(semaphore, callback));
    }
}