Java ExecutorService - Monitor Task Completion/Status bar

Question

So I have an ExecutorService successfully blocking and running linearly right now. My trouble is, I am trying to add a status update and I can't figure out how to get Futures to settle one-item at a time. It seems that by the time the first item in my Future<> is ready so is the last. I'm hoping to find a place where I can know how many tasks my executorService has remaining/total so I can calculate a simple percentage indicator. Please note I intend on recycling my Executor and don't want to shut it down.

ExecutorService updateService = Executors.newSingleThreadExecutor();
Callable<String> callHour = () -> {
  //doStuff, unaware of total number of hourCalls
  return "done";
};
private void startMe(int hours){
  List<Future<String>> futureHours;
  List<Callable<String>> hourCalls = new ArrayList<>(hours);
    for (int hour = 0; hour < hours; ++hour) {
      hourCalls.add(callHour); //queue list (not running yet)
    }
    try {
      //executes queue and blocks thread
      futureHours = updateService.invokeAll(hourCalls);
      futureHours.get(0).get();//performs blocking
    } catch (Exception e) {
      e.printStackTrace();
    }
  }
}

Answer 1

There are two things at work here.

---

Firstly, if we take a look at the documentation of ExecutorService#invokeAll(...), we see that it returns

[...] a list of Futures holding their status and results when all complete. [...]

(emphasis added by me)

You most probably want to use Executor#submit(...) instead.

---

Secondly, you have no guarantee that the task coupled to futureHours.get(0) is executed first. I would suggest using Future#isDone() with some additional logic:

private void startMe(int hours) {
  [...]
  try {
    [...]
    ArrayList<Future<String>> futureHoursDone = new ArrayList<>();
    final int numTasks = futureHours.size();
    int done = 0;
    double percentageDone = 0.0d;
    while (futureHours.isEmpty() == false) {
      for (int index = 0; index < futureHours.size(); ++index) {
        Future<String> futureHour = futureHours.get(index);
        if (futureHour.isDone()) {
          futureHours.remove(index);
          futureHoursDone.add(futureHour);
          --index;
          ++done;
          percentageDone = done / (double) numTasks;
        }
      }
    }
  } catch (Exception e) {
    // TODO: don't forget to HCF (https://en.wikipedia.org/wiki/Halt_and_Catch_Fire) :)
    e.printStackTrace();
  }
}

(This is a rough sketch. To make the progress, i.e. percentage, visible to the outside, you would have to make it an attribute and accessible through, e.g., some getter)