Transform Java Future into a CompletableFuture

Question

Java 8 introduces CompletableFuture, a new implementation of Future that is composable (includes a bunch of thenXxx methods). I'd like to use this exclusively, but many of the libraries I want to use return only non-composable Future instances.

Is there a way to wrap up a returned Future instances inside of a CompleteableFuture so that I can compose it?

Answer 1

There is a way, but you won't like it. The following method transforms a Future<T> into a CompletableFuture<T>:

public static <T> CompletableFuture<T> makeCompletableFuture(Future<T> future) {
    return CompletableFuture.supplyAsync(() -> {
        try {
            return future.get();
        } catch (InterruptedException|ExecutionException e) {
            throw new RuntimeException(e);
        }
    });
}

Obviously, the problem with this approach is, that for each Future, a thread will be blocked to wait for the result of the Future--contradicting the idea of futures. In some cases, it might be possible to do better. However, in general, there is no solution without actively wait for the result of the Future.

Answer 2

If the library you want to use also offers a callback style method in addition to the Future style, you can provide it a handler that completes the CompletableFuture without any extra thread blocking. Like so:

    AsynchronousFileChannel open = AsynchronousFileChannel.open(Paths.get("/some/file"));
    // ... 
    CompletableFuture<ByteBuffer> completableFuture = new CompletableFuture<ByteBuffer>();
    open.read(buffer, position, null, new CompletionHandler<Integer, Void>() {
        @Override
        public void completed(Integer result, Void attachment) {
            completableFuture.complete(buffer);
        }

        @Override
        public void failed(Throwable exc, Void attachment) {
            completableFuture.completeExceptionally(exc);
        }
    });
    completableFuture.thenApply(...)

Without the callback the only other way I see solving this is to use a polling loop that puts all your Future.isDone() checks on a single thread and then invoking complete whenever a Future is gettable.

Answer 3

I published a little futurity project that tries to make better than the straightforward way in the answer.

The main idea is to use the only one thread (and of course with not just a spin loop) to check all Futures states inside, which helps to avoid blocking a thread from a pool for each Future -> CompletableFuture transformation.

Usage example:

Future oldFuture = ...;
CompletableFuture profit = Futurity.shift(oldFuture);

Answer 4

Let me suggest another (hopefully, better) option: https://github.com/vsilaev/java-async-await/tree/master/com.farata.lang.async.examples/src/main/java/com/farata/concurrent

Briefly, the idea is the following:

1. Introduce CompletableTask<V> interface -- the union of the CompletionStage<V> + RunnableFuture<V>
2. Warp ExecutorService to return CompletableTask from submit(...) methods (instead of Future<V>)
3. Done, we have runnable AND composable Futures.

Implementation uses an alternative CompletionStage implementation (pay attention, CompletionStage rather than CompletableFuture):

Usage:

J8ExecutorService exec = J8Executors.newCachedThreadPool();
CompletionStage<String> = exec
   .submit( someCallableA )
   .thenCombineAsync( exec.submit(someCallableB), (a, b) -> a + " " + b)
   .thenCombine( exec.submit(someCallableC), (ab, b) -> ab + " " + c); 

Answer 5

Suggestion:

http://www.thedevpiece.com/converting-old-java-future-to-completablefuture/

But, basically:

public class CompletablePromiseContext {
    private static final ScheduledExecutorService SERVICE = Executors.newSingleThreadScheduledExecutor();

    public static void schedule(Runnable r) {
        SERVICE.schedule(r, 1, TimeUnit.MILLISECONDS);
    }
}

And, the CompletablePromise:

public class CompletablePromise<V> extends CompletableFuture<V> {
    private Future<V> future;

    public CompletablePromise(Future<V> future) {
        this.future = future;
        CompletablePromiseContext.schedule(this::tryToComplete);
    }

    private void tryToComplete() {
        if (future.isDone()) {
            try {
                complete(future.get());
            } catch (InterruptedException e) {
                completeExceptionally(e);
            } catch (ExecutionException e) {
                completeExceptionally(e.getCause());
            }
            return;
        }

        if (future.isCancelled()) {
            cancel(true);
            return;
        }

        CompletablePromiseContext.schedule(this::tryToComplete);
    }
}

Example:

public class Main {
    public static void main(String[] args) {
        final ExecutorService service = Executors.newSingleThreadExecutor();
        final Future<String> stringFuture = service.submit(() -> "success");
        final CompletableFuture<String> completableFuture = new CompletablePromise<>(stringFuture);

        completableFuture.whenComplete((result, failure) -> {
            System.out.println(result);
        });
    }
}