code: public void requestCancellation() {
          Collection<CompletableFuture<?>> toInvoke;
          synchronized (this.futures) {
              this.cancellationRequested = true;
              toInvoke = new ArrayList<>(this.futures);
          }
  
          toInvoke.forEach(f -> f.cancel(true));
          synchronized (this.futures) {
              this.futures.clear();
          }
      }
docs: Cancels all registered futures.