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,

Answer 1

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 2

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);
        });
    }
}

Answer 3

If your Future is the result of a call to an ExecutorService method (e.g. submit()), the easiest would be to use the CompletableFuture.runAsync(Runnable, Executor) method instead.

From

Runnbale myTask = ... ;
Future<?> future = myExecutor.submit(myTask);

to

Runnbale myTask = ... ;
CompletableFuture<?> future = CompletableFuture.runAsync(myTask, myExecutor);

The CompletableFuture is then created "natively".

EDIT: Pursuing comments by @SamMefford corrected by @MartinAndersson, if you want to pass a Callable, you need to call supplyAsync(), converting the Callable<T> into a Supplier<T>, e.g. with:

CompletableFuture.supplyAsync(() -> {
    try { return myCallable.call(); }
    catch (Exception ex) { throw new RuntimeException(ex); } // Or return default value
}, myExecutor);

Because T Callable.call() throws Exception; throws an exception and T Supplier.get(); doesn't, you have to catch the exception so prototypes are compatible.

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

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) {
  if (future.isDone())
    return transformDoneFuture(future);
  return CompletableFuture.supplyAsync(() -> {
    try {
      if (!future.isDone())
        awaitFutureIsDoneInForkJoinPool(future);
      return future.get();
    } catch (ExecutionException e) {
      throw new RuntimeException(e);
    } catch (InterruptedException e) {
      // Normally, this should never happen inside ForkJoinPool
      Thread.currentThread().interrupt();
      // Add the following statement if the future doesn't have side effects
      // future.cancel(true);
      throw new RuntimeException(e);
    }
  });
}

private static <T> CompletableFuture<T> transformDoneFuture(Future<T> future) {
  CompletableFuture<T> cf = new CompletableFuture<>();
  T result;
  try {
    result = future.get();
  } catch (Throwable ex) {
    cf.completeExceptionally(ex);
    return cf;
  }
  cf.complete(result);
  return cf;
}

private static void awaitFutureIsDoneInForkJoinPool(Future<?> future)
    throws InterruptedException {
  ForkJoinPool.managedBlock(new ForkJoinPool.ManagedBlocker() {
    @Override public boolean block() throws InterruptedException {
      try {
        future.get();
      } catch (ExecutionException e) {
        throw new RuntimeException(e);
      }
      return true;
    }
    @Override public boolean isReleasable() {
      return future.isDone();
    }
  });
}

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 6

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);