如何结合两个ListenableFutures并从第一个中获得结果以完成

Question

Is there a simple way (library provided) to combine two com.google.common.util.concurrent.ListenableFuture and get the result from the first one to complete.

Something like a coalesce in time:

ListenableFuture f1 = ...;
ListenableFuture f2 = ...;
ListenableFuture f3 = Future.firstToComplete(f1,f2)

I guess this would be the equivalent to CompletionStage. applyToEither :

f3 = f1.applyToEither(f2, res->res);

Answer 1

Sounds like you want to use Futures.inCompletionOrder and simply wait for the first future in the list.

Once the first future completes, you will likely want to cancel() all the remaining futures in the list, but there is a (mostly benign) race there where the futures could be completing while you're cancelling them; it depends on how close the finish is. In the worst case, you might end up doing all the work for all the other futures anyway, but if your primary concern is latency and not the underlying work items, that's likely not going to be an issue for you.