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completable-futures is a set of utility functions to simplify working with asynchronous code in Java8.


Using completable-futures requires Java 8 but has no additional dependencies. It is meant to be included as a library in other software. To import it with maven, add this to your pom:



Combining more than two things

The builtin CompletableFuture API includes future.thenCombine(otherFuture, function) but if you want to combine more than two things it gets trickier. The CompletableFutures class contains the following APIs to simplify this use-case:


If you want to join a list of futures of uniform type, use allAsList. This returns a future which completes to a list of all values of its inputs:

List<CompletableFuture<String>> futures = asList(completedFuture("a"), completedFuture("b"));
CompletableFuture<List<String>> joined = CompletableFutures.allAsList(futures);


If you want to join a map of key and value-futures, each of uniform type, use allAsMap. This returns a future which completes to a map of all key values of its inputs:

  Map<String, CompletableFuture<String>> futures = new HashMap() {{
    put("key", completedFuture("value"));
  CompletableFuture<Map<String, String>> joined = CompletableFutures.allAsMap(futures);


Works like allAsList, but futures that fail will not fail the joined future. Instead, the defaultValueMapper function will be called once for each failed future and value returned will be put in the resulting list on the place corresponding to the failed future. The default value returned by the function may be anything, such as null or Optional.empty().

List<CompletableFuture<String>> input = asList(
    exceptionallyCompletedFuture(new RuntimeException("boom")));
CompletableFuture<List<String>> joined = CompletableFutures.successfulAsList(input, t -> "default");


joinList is a stream collector that combines multiple futures into a list. This is handy if you apply an asynchronous operation to a collection of entities:


joinMap is a stream collector that applies an asynchronous operation to each element of the stream, and associates the result of that operation to a key derived from the original element. This is useful when you need to keep the association between the entity that triggered the asynchronous operation and the result of that operation:
    .collect(joinMap(this::toKey, this::someAsyncFunc))


If you want to combine more than two futures of different types, use the combine method:

CompletableFutures.combine(f1, f2, (a, b) -> a + b);
CompletableFutures.combine(f1, f2, f3, (a, b, c) -> a + b + c);
CompletableFutures.combine(f1, f2, f3, f4, (a, b, c, d) -> a + b + c + d);
CompletableFutures.combine(f1, f2, f3, f4, f5, (a, b, c, d, e) -> a + b + c + d + e);


If you want to combine multiple futures into another future, use combineFutures:

CompletableFutures.combineFutures(f1, f2, (a, b) -> completedFuture(a + b));
CompletableFutures.combineFutures(f1, f2, f3, (a, b, c) -> completedFuture(a + b + c));
CompletableFutures.combineFutures(f1, f2, f3, f4, (a, b, c, d) -> completedFuture(a + b + c + d));
CompletableFutures.combineFutures(f1, f2, f3, f4, f5, (a, b, c, d, e) -> completedFuture(a + b + c + d + e));

Combine an arbitrary number of futures

If you want to combine more than six futures of different types, use the other combine method. Since it supports vararg usage, the function is now the first argument. The CombinedFutures object that is input to the function can be used to extract values from the input functions.

This is effectively the same thing as calling join() on the input future, but it's safer because calling .get(f) on a future that is not part of the combine, you will get an IllegalArgumentException.

This prevents accidental misuse where you would join on a future that is either not complete, or might never complete at all.

CompletionStage<String> f1;
CompletionStage<String> f2;
CompletionStage<String> result = combine(combined -> combined.get(f1) + combined.get(f2), f1, f2);

If you want to do this in a combineFutures form, you can do that like this:

CompletionStage<String> f1;
CompletionStage<String> f2;
CompletionStage<String> result = dereference(combine(combined -> completedFuture(combined.get(f1) + combined.get(f2)), f1, f2));


Polling an external resource

If you are dealing with a long-running external task that only exposes a polling API, you can transform that into a future like so:

Supplier<Optional<T>> pollingTask = () -> Optional.ofNullable(resource.result());
Duration frequency = Duration.ofSeconds(2);
CompletableFuture<T> result = CompletableFutures.poll(pollingTask, frequency, executor);

Missing parts of the CompletableFuture API

The CompletableFutures class includes utility functions for operating on futures that is missing from the builtin API.


Like CompletableFuture.handle but lets you return a new CompletionStage instead of a direct value.

CompletionStage<String> composed = handleCompose(future, (value, throwable) -> completedFuture("hello"));


Like CompletableFuture.exceptionally but lets you return a new CompletionStage instead of a direct value.

CompletionStage<String> composed = CompletableFutures.exceptionallyCompose(future, throwable -> completedFuture("fallback"));


Unwrap a CompletionStage<CompletionStage<T>> to a plain CompletionStage<T>.

CompletionStage<CompletionStage<String>> wrapped = completedFuture(completedFuture("hello"));
CompletionStage<String> unwrapped = CompletableFutures.dereference(wrapped);


Like CompletableFuture.supplyAsync but unwraps a CompletionStage<CompletionStage<T>> to a plain CompletionStage<T> when the Supplier returns a CompletionStage.

CompletionStage<String> suppliedStage = completedFuture("hello").thenApply(stage -> stage + "-chained");
CompletionStage<String> outputStage = CompletableFutures.supplyAsyncCompose(suppliedStage);


Creates a new future that is already exceptionally completed with the given exception.

return CompletableFutures.exceptionallyCompletedFuture(new RuntimeException("boom"));


Copyright 2016 Spotify AB. Licensed under the Apache License, Version 2.0.

Code of Conduct

This project adheres to the Open Code of Conduct. By participating, you are expected to honor this code.


See the instructions in spotify/foss-root


The Weaver squad is currently owning this project internally. We are currently in the evaluating process of the ownership of this and other OSS Java libraries. The ownership takes into account ONLY security maintenance.

This repo is also co-owned by other people: