如何等待期货清单完成

Question

I have a list of Futures and I want to have every task completed before doing the next step. Currently I'm doing this:

public static int[] benchmark(){

    List<Future<Integer>> futureNumbers = service.invokeAll(callableList);
    int[] numbers = new int[events.size()];
    int i = 0;

    for (Future<Integer> future : futureNumbers) {
        numbers[i] = future.get();
    }

    return numbers;
}

But then only the first value of the array I get back is right, the others are 0. How can I achieve that numbers is just returned when every task has finished?

Your help would be appreciated.

Answer 1

As per Java specs for invokeAll() method, List<Future<T>> will contain the result of all tasks and you will get Future only when all your threads are finished, either normally or because of exception.
So, your Future object should have all the results.

Executes the given tasks, returning a list of Futures holding their status and results when all complete.

Possibilities:

1. Probably what could be happening is that some of your thread is aborting because of exception, so call() method is not able to return you the desired result. Also, since it is a new thread of execution you will not see the exception propagated.
   So, to counter / manage this, enclose all your code of call() method in try-catch and print the stack-trace from catch. You will know if there is some exception.
2. Another possibility could be that some of your Callable implementations are not returning anything, and could be just void, because of which you are not getting anything in Future object.

Verification step:
isDone() - Returns true if this task completed. Completion may be due to normal termination, an exception, or cancellation -- in all of these cases, this method will return true.

for (Future<Integer> future : futureNumbers) {
      System.out.println(future.isDone());  
      numbers[i] = future.get();
}