等待两个 Futures/Runnables/Callables 中的任何一个完成？

Question

How can I add tasks, two at a time, to an Executor (or similar) and then wait for any of the two to finish (ie the fastest), while the other one, as well as previously started tasks, continues in the background?

I know that a CompletionService offers something similar but with that one all I can do is wait for the next one to complete, with .take() . In my case that might be from a previous scheduling and not one of the ones I need to wait for.

In pseudo code I'd like

ExecutorService executorService = Executors.newFixedThreadPool(100);
Future<?> one = executorService.submit(() -> oneWay());
Future<?> two = executorService.submit(() -> orAnother());

Future theFirstOneToFinish = waitFor(one, two);
// one done, the other one keeps on working
return theFirstOneToFinish;

Answer 1

A CompletionService oversees only those tasks that it submits. In other words, you can create a new one for every pair of tasks you submit and call take() to retrieve the first one that completes.

If you're using ExecutorCompletionService , create a new instance wrapping your ExecutorService , submit two tasks, and call take() .

For example:

public Future<String> submitPair(ExecutorService executorService) throws InterruptedException {
    ExecutorCompletionService<String> ecs = new ExecutorCompletionService<>(executorService);
    ecs.submit(() -> oneWay());
    ecs.submit(() -> orAnother());
    return ecs.take();
}

No additional cleanup is required for the ExecutorCompletionService .

Answer 2

Use CompletableFuture .

ExecutorService executorService = Executors.newFixedThreadPool(100);
CompletableFuture<?> one = CompletableFuture.supplyAsync(() -> oneWay(),  executorService);
CompletableFuture<?> two = CompletableFuture.supplyAsync(() -> orAnother(),  executorService);

return CompletableFuture.anyOf(one, two);