Futures-extra

Futures-extra is a set of small utility functions to simplify working with Guava's ListenableFuture class

Build status

Maven central

Build dependencies

• Java 8 or higher
• Maven

Runtime dependencies

• Java 6 or higher
• Guava 18.0 or higher

Usage

Futures-extra is meant to be used as a library embedded in other software. To import it with maven, use this:

<dependency>
  <groupId>com.spotify</groupId>
  <artifactId>futures-extra</artifactId>
  <version>1.4.1</version>
</dependency>

Examples

Cleaner transforms for Java 8.

Java 8 introduced lambdas which can greatly reduce verbosity in code, which is great when using futures and transforms. One drawback with lambdas though is that when a lambda is supplied as an argument to a method with overloaded parameters, the compiler may fail to figure out which variant of a method call that is intended to be used.

Ideally, applying java 8 lambdas to Guava's Futures.transform() would look something like this:

public static <A, B> ListenableFuture<B> example(ListenableFuture<A> future) {
  return Futures.transform(future, a -> toB(a));
}

Unfortunately this doesn't actually work because Futures.transform has two variants: one that takes a Function as its second parameter and one that takes an AsyncFunction. The compiler can't determine which variant to use without additional type information.

You could work around that by casting it like this:

public static <A, B> ListenableFuture<B> example(ListenableFuture<A> future) {
  return Futures.transform(future, (Function<A, B>) a -> toB(a));
}

With futures-extra you can do this instead:

public static <A, B> ListenableFuture<B> example(ListenableFuture<A> future) {
  return FuturesExtra.syncTransform(future, a -> toB(a));
}

This is just a simple delegating method that explicitly calls Futures.transform(future, Function). There is also a corresponding FuturesExtra.asyncTransform that calls Futures.transform(future, AsyncFunction).

Joining multiple futures

A common use case is waiting for two or more futures and then transforming the result to something else. You can do this in a couple of different ways, here are two of them:

The examples are for Java 8, but they also work for Java 6 and 7 (though it becomes more verbose).

final ListenableFuture<A> futureA = getFutureA();
final ListenableFuture<B> futureB = getFutureB();

ListenableFuture<C> ret = Futures.transform(Futures.allAsList(futureA, futureB),
    (Function<List<?>, C>)list -> combine((A) list.get(0), (B) list.get(1));

where combine is a method with parameters of type A and B returning C.

This one has the problem that you have to manually make sure that the casts and ordering are correct, otherwise you will get ClassCastException.

You could also access the futures directly to avoid casts:

final ListenableFuture<A> futureA = getFutureA();
final ListenableFuture<B> futureB = getFutureB();

ListenableFuture<C> ret = Futures.transform(Futures.allAsList(futureA, futureB),
    (Function<List<?>, C>)list -> combine(Futures.getUnchecked(futureA), Futures.getUnchecked(futureB));

Now you instead need to make sure that the futures in the transform input are the same as the ones you getUnchecked. If you fail to do this, things may work anyway (which is a good way of hiding bugs), but block the thread, actually removing the asynchronous advantage. Even worse - the future may never finish, blocking the thread forever.

To simplify these use cases we have a couple of helper functions:

final ListenableFuture<A> futureA = getFutureA();
final ListenableFuture<B> futureB = getFutureB();

ListenableFuture<C> ret = FuturesExtra.syncTransform2(futureA, futureB,
    (a, b) -> combine(a, b));

This is much clearer! We don't need any type information because the lambda can infer it, and we avoid the potential bugs that can occur as a result of the first to examples.

The tuple transform can be used up to 6 arguments named syncTransform2() through syncTransform6(). If you need more than that you could probably benefit from some refactoring, but you can also use FuturesExtra.join():

final ListenableFuture<A> futureA = getFutureA();
final ListenableFuture<B> futureB = getFutureB();

final ListenableFuture<JoinedResults> futureJoined = FuturesExtra.join(futureA, futureB);
return Futures.transform(futureJoined,
    joined -> combine(joined.get(futureA), joined.get(futureB)));

This supports an arbitrary number of futures, but is slightly more complex. However, it is much safer than the first two examples, because joined.get(...) will fail if you try to get the value of a future that was not part of the input.

Timeouts

Sometimes you want to stop waiting for a future after a specific timeout and to do this you generally need to have some sort of scheduling involved. To simplify that, you can use this:

final ListenableFuture<A> future = getFuture();
final ListenableFuture<A> futureWithTimeout = FuturesExtra.makeTimeoutFuture(scheduledExecutor, future, 100, TimeUnit.MILLISECONDS);

Select

If you have some futures and want to succeed as soon as the first one succeeds, you can use select:

final List<ListenableFuture<A>> futures = getFutures();
final ListenableFuture<A> firstSuccessful = FuturesExtra.select(futures);

Success/Failure callbacks

If you are only interested in either successful or failed results of a future, you can use these callbacks:

final ListenableFuture<A> future = getFuture();
FuturesExtra.addSuccessCallback(future, a -> System.out.println(a));

final ListenableFuture<B> future = getFuture();
FuturesExtra.addFailureCallback(future, e -> e.printStackTrace());

Completed futures

In some cases you want to extract the value (or exception) from the future and you know that the future is completed so it won't be a blocking operation.

You could use these methods for that, but they will also block if the future is not complete which may lead to hard to find bugs.

T value = future.get();
T value = Futures.getUnchecked(future);

Instead you can use these methods which will never block but instead immediately throw an exception if the future is not completed. This is typically useful in unit tests (where futures should be immediate) and in general future callbacks/transforms where you know that a specific future must be completed for this codepath to be triggered.

T value = FuturesExtra.getCompleted(future);
Throwable exc = FuturesExtra.getException(future);

JDK 8 CompletableFuture <-> ListenableFuture Conversion

• From ListenableFuture To JDK 8 CompletableFuture

ListenableFuture<V> listenable = getFuture();
CompletableFuture<V> completable = CompletableFuturesExtra.toCompletableFuture(listenable);

• From JDK 8 CompletableFuture To ListenableFuture

CompletableFuture<V> completable = getFuture();
ListenableFuture<V> listenable = CompletableFuturesExtra.toListenableFuture(completable);