code: public void addCancelListener(final CancelListener<T> listener) {
  		final Object v = this.value.get();
  		if (v == null) {
  			this.listeners.add(listener);
  		} else if (FutureImpl.CANCELLED_FLAG.equals(v)) {
  			listener.wasCancelled(this.getId());
  		}
  	}
docs: IMPORTANT: In the event that the Cancel listener is added after cancel() was called the event is immediately fired.<br>
  IMPORTANT2: Keep in mind, that the Cancel listener may not be called from your own thread, so you must care about thread-safety!
  
  @param listener Cancel listener