Core Java

Java 8: CompletableFuture in action

After thoroughly exploring CompletableFuture API in Java 8 we are prepared to write a simplistic web crawler. We solved similar problem already using ExecutorCompletionService, Guava ListenableFuture and Scala/Akka. I choose the same problem so that it’s easy to compare approaches and implementation techniques.

First we shall define a simple, blocking method to download the contents of a single URL:

private String downloadSite(final String site) {
    try {
        log.debug("Downloading {}", site);
        final String res = IOUtils.toString(new URL("http://" + site), UTF_8);
        log.debug("Done {}", site);
        return res;
    } catch (IOException e) {
        throw Throwables.propagate(e);

Nothing fancy. This method will be later invoked for different sites inside thread pool. Another method parses the String into an XML Document (let me leave out the implementation, no one wants to look at it):

private Document parse(String xml)  //...

Finally the core of our algorithm, function computing relevance of each website taking Document as input. Just as above we don’t care about the implementation, only the signature is important:

private CompletableFuture<Double> calculateRelevance(Document doc) //...

Let’s put all the pieces together. Having a list of websites our crawler shall start downloading the contents of each web site asynchronously and concurrently. Then each downloaded HTML string will be parsed to XML Document and later relevance will be computed. As a last step we take all computed relevance metrics and find the biggest one. This sounds pretty straightforward to the moment when you realize that both downloading content and computing relevance is asynchronous (returns CompletableFuture) and we definitely don’t want to block or busy wait. Here is the first piece:

ExecutorService executor = Executors.newFixedThreadPool(4);
List<String> topSites = Arrays.asList(
        "", "", "", ""
List<CompletableFuture<Double>> relevanceFutures =
        map(site -> CompletableFuture.supplyAsync(() -> downloadSite(site), executor)).
        map(contentFuture -> contentFuture.thenApply(this::parse)).
        map(docFuture -> docFuture.thenCompose(this::calculateRelevance)).

There is actually a lot going on here. Defining thread pool and sites to crawl is obvious. But there is this chained expression computing relevanceFutures. The sequence of map() and collect() in the end is quite descriptive. Starting from a list of web sites we transform each site (String) into CompletableFuture<String> by submitting asynchronous task (downloadSite()) into thread pool.

So we have a list of CompletableFuture<String>. We continue transforming it, this time applying parse() method on each of them. Remember that thenApply() will invoke supplied lambda when underlying future completes and returns CompletableFuture<Document> immediately. Third and last transformation step composes each CompletableFuture<Document> in the input list with calculateRelevance(). Note that calculateRelevance() returns CompletableFuture<Double> instead of Double, thus we use thenCompose() rather than thenApply(). After that many stages we finally collect() a list of CompletableFuture<Double>.

Now we would like to run some computations on all results. We have a list of futures and we would like to know when all of them (last one) complete. Of course we can register completion callback on each future and use CountDownLatch to block until all callbacks are invoked. I am too lazy for that, let us utilize existing CompletableFuture.allOf(). Unfortunately it has two minor drawbacks – takes vararg instead of Collection and doesn’t return a future of aggregated results but Void instead. By aggregated results I mean: if we provide List<CompletableFuture<Double>> such method should return CompletableFuture<List<Double>>, not CompletableFuture<Void>! Luckily it’s easy to fix with a bit of glue code:

private static <T> CompletableFuture<List<T>> sequence(List<CompletableFuture<T>> futures) {
    CompletableFuture<Void> allDoneFuture =
        CompletableFuture.allOf(futures.toArray(new CompletableFuture[futures.size()]));
    return allDoneFuture.thenApply(v ->
                    map(future -> future.join()).

Watch carefully sequence() argument and return types. The implementation is surprisingly simple, the trick is to use existing allOf() but when allDoneFuture completes (which means all underlying futures are done), simply iterate over all futures and join() (blocking wait) on each. However this call is guaranteed not to block because by now all futures completed! Equipped with such utility method we can finally complete our task:

CompletableFuture<List<Double>> allDone = sequence(relevanceFutures);
CompletableFuture<OptionalDouble> maxRelevance = allDone.thenApply(relevances ->

This one is easy – when allDone completes, apply our function that counts maximal relevance in whole set. maxRelevance is still a future. By the time your JVM reaches this line, probably none of the websites are yet downloaded. But we encapsulated business logic on top of futures, stacking them in an event-driven manner. Code remains readable (version without lambda and with ordinary Futures would be at least twice as long) but avoids blocking main thread. Of course allDone can as well be an intermediate step, we can further transform it, not really having the result yet.


CompletableFuture in Java 8 is a huge step forward. From tiny, thin abstraction over asynchronous task to full-blown, functional, feature rich utility. However after few days of playing with it I found few minor disadvantages:

  • CompletableFuture.allOf() returning CompletableFuture<Void> discussed earlier. I think it’s fair to say that if I pass a collection of futures and want to wait for all of them, I would also like to extract the results when they arrive easily. It’s even worse with CompletableFuture.anyOf(). If I am waiting for any of the futures to complete, I can’t imagine passing futures of different types, say CompletableFuture<Car> and CompletableFuture<Restaurant>. If I don’t care which one completes first, how am I suppose to handle return type? Typically you will pass a collection of homogeneous futures (e.g. CompletableFuture<Car>) and then anyOf() can simply return future of that type (instead of CompletableFuture<Void> again).
  • Mixing settable and listenable abstractions. In Guava there is ListenableFuture and SettableFuture extending it. ListenableFuture allows registering callbacks while SettableFuture adds possibility to set value of the future (resolve it) from arbitrary thread and context. CompletableFuture is equivalent to SettableFuture but there is no limited version equivalent to ListenableFuture. Why is it a problem? If API returns CompletableFuture and then two threads wait for it to complete (nothing wrong with that), one of these threads can resolve this future and wake up other thread, while it’s only the API implementation that should do it. But when API tries to resolve the future later, call to complete() is ignored. It can lead to really nasty bugs which are avoided in Guava by separating these two responsibilities.
  • CompletableFuture is ignored in JDK. ExecutorService was not retrofitted to return CompletableFuture. Literally CompletableFuture is not referenced anywhere in JDK. It’s a really useful class, backward compatible with Future, but not really promoted in standard library.
  • Bloated API (?) Fifty methods in total, most in three variants. Splitting settable and listenable (see above) would help. Also some methods like runAfterBoth() or runAfterEither() IMHO do not really belong to any CompletableFuture. Is there a difference between fast.runAfterBoth(predictable, ...) and predictable.runAfterBoth(fast, ...)? No, but API favours one or the other. Actually I believe runAfterBoth(fast, predictable, ...) much better expresses my intention.
  • CompletableFuture.getNow(T) should take Supplier<T> instead of raw reference. In the example below expensiveAlternative() is always code, irrespective to whether future finished or not:

    However we can easily tweak this behaviour (I know, there is a small race condition here, but the original getNow() works this way as well):

    public static <T> T getNow(
                CompletableFuture<T> future,
                Supplier<T> valueIfAbsent) throws ExecutionException, InterruptedException {
        if (future.isDone()) {
            return future.get();
        } else {
            return valueIfAbsent.get();

    With this utility method we can avoid calling expensiveAlternative() when it’s not needed:

    getNow(future, () -> expensiveAlternative());
    getNow(future, this::expensiveAlternative);

In overall CompletableFuture is a wonderful new tool in our JDK belt. Minor API issues and sometimes too verbose syntax due to limited type inference shouldn’t stop you from using it. At least it’s a solid foundation for better abstractions and more robust code.

Reference: Java 8: CompletableFuture in action from our JCG partner Tomasz Nurkiewicz at the Java and neighbourhood blog.

Tomasz Nurkiewicz

Java EE developer, Scala enthusiast. Enjoying data analysis and visualization. Strongly believes in the power of testing and automation.
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