How to test if java Future is complete?

Question

I'm tracking execution of a task with standard Future object. There are the following valid states of that task (as I see in Guava code of AbstractFuture class):

• Running;
• Completing;
• Completed successfully or with exception;
• Cancelled;
• Interrupted.

Future.isDone() returns true if and only if the state is completed, cancelled or interrupted. Future.isCancelled() returns true if and only if the state is either interrupted or cancelled.

Ok, but I need to check if the task is completed. There is an obvious way to do this:

boolean isCompleted = future.isDone() && !future.isCancelled();

Unfortunatelly, a nasty concurrency bug hides there.

1. Thread #1 invokes future.isCancelled(). The result is false, because the task is still in progress.
2. Thread #2 cancels the task calling future.cancel().
3. Thread #1 invokes future.isDone(). The result is true now.
4. Evaluation of expression above yields true, and it's incorrect answer.

How to avoid this issue?

Answer 1

You can try calling get(..) with an extremely short timeout. If it returns a value, it was completed. If you get a TimeoutException, it wasn't. If you get any other of the possible exceptions, it was either cancelled, failed, or was interrupted.