How to check if java future is complete?

Question

I track the progress of a task with a standard Future object. The following valid states for this task exist (as I see in the Guava code of the AbstractFuture class):

• Launch;
• Completion;
• Completed successfully or with an exception;
• Canceled;
• Interrupted.

Future.isDone() returns true if and only if the state is complete, canceled, or aborted. Future.isCancelled() returns true if and only if the state is interrupted or canceled.

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

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

Unfortunately, the nasty concurrency error lurks here.

• Topic # 1 calls Future.isCancelled() . The result is false because the task is still in progress.
• Topic # 2 cancels the call to future.cancel() .
• Topic # 1 calls Future.isDone() . The result is true now.
• Evaluating the expression above gives true , and this is the wrong answer.

How to avoid this problem?

Answer 1

You can try get(..) with a very short wait time. If it returns a value, it will be completed. If you get a TimeoutException , that is not the case. If you receive any of the possible exceptions, it was either canceled, or failed, or was interrupted.