CompletableFuture: like anyOf() but return a new CompletableFuture that is completed when any of the given CompletableFutures return not null

Question

I have multiple CompletableFuture

CompletableFuture<String> comp1 = CompletableFuture.supplyAsync(() -> ...);
CompletableFuture<String> comp2 = CompletableFuture.supplyAsync(() -> ...);
...
CompletableFuture<String> compN = CompletableFuture.supplyAsync(() -> ...);

I need something like

CompletableFuture<Object> firstResult = CompletableFuture.anyOf(comp1, comp2, ..., compN);

but this returns the result from the first one that completes.

I want to have something that returns the first non-null result from the first one that completes. Is it possible?

Answer 1

There is no helper function in CompletableFuture that returns first future to complete with a non-null value.

You can implement your own:

public static <T> CompletableFuture<T> anyNotNull(Collection<CompletableFuture<? extends T>> cfs) {
    var cfsCopy = new ArrayList<>(cfs);

    if (cfsCopy.isEmpty()) {
        return CompletableFuture.failedFuture(new NoSuchElementException("No futures provided"));
    }

    var result = new CompletableFuture<T>();
    var remaining = new AtomicInteger(cfsCopy.size());

    for (CompletableFuture<? extends T> cf : cfsCopy) {
        cf.whenComplete((v, ex) -> {
            if (Objects.nonNull(v)) {
                result.complete(v);
                return;
            }
            if (remaining.decrementAndGet() == 0) {
                result.completeExceptionally(new NoSuchElementException("No future completed with a non-null value"));
            }
        });
    }

    return result;
}

Few notes about the code:

• make a copy of the input collection to avoid issues if the caller mutates the list of passed futures
• return a failed future if:
  • the collection is empty
  • all futures complete with null or exceptionally

Possible changes of the code that could make sense. Some of them are inspired by implementation of CompletableFuture.anyOf:

• check for already completed futures with non-null results before attaching callbacks via whenComplete
• cancel remaining futures once an acceptable result is found
• accept varargs instead of a Collection parameter
• log exceptions from futures that complete exceptionally instead of ignoring them