Timeout with default value in Java 8 CompletableFuture

Suppose I have some async computation, such as:

CompletableFuture
        .supplyAsync(() -> createFoo())
        .thenAccept(foo -> doStuffWithFoo(foo));

Is there a nice way to provide a default value for foo if the async supplier times out according to some specified timeout? Ideally, such functionality would attempt to cancel the slow-running supplier as well. For example, is there standard library functionality that is similar to the following hypothetical code:

CompletableFuture
        .supplyAsync(() -> createFoo())
        .acceptEither(
                CompletableFuture.completedAfter(50, TimeUnit.MILLISECONDS, DEFAULT_FOO),
                foo -> doStuffWithFoo(foo));

Or perhaps even better:

CompletableFuture
        .supplyAsync(() -> createFoo())
        .withDefault(DEFAULT_FOO, 50, TimeUnit.MILLISECONDS)
        .thenAccept(foo -> doStuffWithFoo(foo));

I know about get(timeout, unit), but am wondering if there's a nicer standard way of applying a timeout in an asynchronous and reactive fashion as suggested in the code above.

EDIT: Here's a solution that's inspired by Java 8: Mandatory checked exceptions handling in lambda expressions. Why mandatory, not optional?, but unfortunately it blocks a thread. If we rely on createFoo() to asynchronously check for timeout and throw its own timeout exception it would work without blocking a thread, but would place more burden on the creator of the supplier and would still have the cost of creating an exception (which can be expensive without "fast throw")

static <T> Supplier<T> wrapped(Callable<T> callable) {
    return () -> {
        try {
            return callable.call();
        } catch (RuntimeException e1) {
            throw e1;
        } catch (Throwable e2) {
            throw new RuntimeException(e2);
        }
    };
}
CompletableFuture
        .supplyAsync(wrapped(() -> CompletableFuture.supplyAsync(() -> createFoo()).get(50, TimeUnit.MILLISECONDS)))
        .exceptionally(e -> "default")
        .thenAcceptAsync(s -> doStuffWithFoo(foo));

Answers


CompletableFuture.supplyAsync is just a helper method that creates a CompletableFuture for you, and submits the task to the ForkJoin Pool.

You can create your own supplyAsync with your requirements like this:

private static final ScheduledExecutorService schedulerExecutor = 
                                 Executors.newScheduledThreadPool(10);
private static final ExecutorService executorService = 
                                 Executors.newCachedThreadPool();


public static <T> CompletableFuture<T> supplyAsync(
        final Supplier<T> supplier, long timeoutValue, TimeUnit timeUnit,
        T defaultValue) {

    final CompletableFuture<T> cf = new CompletableFuture<T>();

    // as pointed out by Peti, the ForkJoinPool.commonPool() delivers a 
    // ForkJoinTask implementation of Future, that doesn't interrupt when cancelling
    // Using Executors.newCachedThreadPool instead in the example
    // submit task
    Future<?> future = executorService.submit(() -> {
        try {
            cf.complete(supplier.get());
        } catch (Throwable ex) {
            cf.completeExceptionally(ex);
        }
    });

    //schedule watcher
    schedulerExecutor.schedule(() -> {
        if (!cf.isDone()) {
            cf.complete(defaultValue);
            future.cancel(true);
        }

    }, timeoutValue, timeUnit);

    return cf;
}

Creating the CompletableFuture with that helper is as easy as using the static method in CompletableFuture:

    CompletableFuture<String> a = supplyAsync(() -> "hi", 1,
            TimeUnit.SECONDS, "default");

To test it:

    a = supplyAsync(() -> {
        try {
            Thread.sleep(2000);
        } catch (InterruptedException e1) {
            // ignore
        }
        return "hi";
    }, 1, TimeUnit.SECONDS, "default");

In Java 9, there will be completeOnTimeout(T value, long timeout, TimeUnit unit), which does what you want, although it does not cancel the slow supplier.

There is also a orTimeout(long timeout, TimeUnit unit), which completes exceptionally in case on a timeout.


DZone has a good article how to solve this: https://dzone.com/articles/asynchronous-timeouts

I'm not sure about the copyright of the code, hence I can't copy it here. The solution is very much like the one from Dane White but it uses a thread pool with a single thread plus schedule() to avoid wasting a thread just to wait for the timeout.

It also throws a TimeoutException instead of returning a default.


I think you'll always need an extra thread monitoring when its time to supply the default value. I'd probably go the route of having two supplyAsync calls, with the default wrapped in a utility API, linked by an acceptEither. If you'd rather wrap your Supplier, then you could use a utility API that makes the 'either' call for you:

public class TimeoutDefault {
    public static <T> CompletableFuture<T> with(T t, int ms) {
        return CompletableFuture.supplyAsync(() -> {
            try {
                Thread.sleep(ms);
            } catch (InterruptedException e) { }
            return t;
        });
    }

    public static <T> Supplier<T> with(Supplier<T> supplier, T t, int ms) {
        return () -> CompletableFuture.supplyAsync(supplier)
            .applyToEither(TimeoutDefault.with(t, ms), i -> i).join();
    }
}

CompletableFuture<Void> future = CompletableFuture
        .supplyAsync(Example::createFoo)
        .acceptEither(
            TimeoutDefault.with("default", 1000),
            Example::doStuffWithFoo);

CompletableFuture<Void> future = CompletableFuture
        .supplyAsync(TimeoutDefault.with(Example::createFoo, "default", 1000))
        .thenAccept(Example::doStuffWithFoo);

There's no standard library method for constructing a CompletableFuture supplied with a value after a timeout. That said, it's really simple to roll your own with minimal resource overhead:

private static final ScheduledExecutorService EXECUTOR
        = Executors.newSingleThreadScheduledExecutor();

public static <T> CompletableFuture<T> delayedValue(final T value,
                                                    final Duration delay) {
    final CompletableFuture<T> result = new CompletableFuture<>();
    EXECUTOR.schedule(() -> result.complete(value),
                      delay.toMillis(), TimeUnit.MILLISECONDS);
    return result;
}

It can be used with the "either" methods of CompleteableFuture:

  • accceptEither, acceptEitherAsync
  • applyToEither, applyToEitherAsync
  • runAfterEither, runAfterEitherAsync

One application is using a cached value if a remote service call exceeds some latency threshold:

interface RemoteServiceClient {
    CompletableFuture<Foo> getFoo();
}

final RemoteServiceClient client = /* ... */;
final Foo cachedFoo = /* ... */;
final Duration timeout = /* ... */;

client.getFoos()
    .exceptionally(ignoredException -> cachedFoo)
    .acceptEither(delayedValue(cachedFoo, timeout),
        foo -> /* do something with foo */)
    .join();

In case the remote client call completes exceptionally (e.g. SocketTimeoutException), we can fail fast and use the cached value immediately.

CompletableFuture.anyOf(CompletableFuture<?>...) can be combined with this delayedValue primitive to wrap a CompletableFuture with the above semantics:

@SuppressWarnings("unchecked")
public static <T> CompletableFuture<T> withDefault(final CompletableFuture<T> cf,
                                                   final T defaultValue,
                                                   final Duration timeout) {
    return (CompletableFuture<T>) CompletableFuture.anyOf(
        cf.exceptionally(ignoredException -> defaultValue),
        delayedValue(defaultValue, timeout));
}

This nicely simplifies the remote service call example above:

withDefault(client.getFoos(), cachedFoo, timeout)
    .thenAccept(foo -> /* do something with foo */)
    .join();

CompletableFutures are more accurately termed promises, as they decouple creation of the Future from its completion. Be sure to use dedicated thread pools for heavy CPU work. To create a CompletableFuture for an expensive computation, you should use the CompletableFuture#supplyAsync(Supplier, Executor) overload, as the #supplyAsync(Supplier) overload defaults to the common ForkJoinPool. The returned CompletableFuture could not cancel its task, as this functionality isn't exposed by the Executor interface. More generally, dependent CompletableFutures don't cancel their parents, e.g. cf.thenApply(f).cancel(true) does not cancel cf. I'd recommend sticking to the Futures returned by ExecutorServices if you need that functionality.


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