Use the results of two Guava ListenableFutures of different types

Question

I have two ListenableFutures which are completed on other threads. Each future is of a different type, and I wish to use both of their results when they are both complete.

Is there an elegant way to handle this using Guava?

Answer 1

Runnable listener = new Runnable() {
    private boolean jobDone = false;

    @Override
    public synchronized void run() {
        if (jobDone || !(future1.isDone() && future2.isDone())) {
            return;
        }
        jobDone = true;
        // TODO do your job
    }
};
future1.addListener(listener);
future2.addListener(listener);

Not really elegant, but should do the job.

Or, more elegant, but you'll need casts:

ListenableFuture<List<Object>> composedFuture = 
    Futures.allAsList(future1, future2);

Answer 2

Since Guava v20.0 you can use:

ListenableFuture<CombinedResult> resultFuture =
   Futures.whenAllSucceed(future1, future2)
       .call(callableThatCombinesAndReturnsCombinedResult, executor);

Look at java docs example here

Answer 3

If you want some sort of type safety you can do the following:

class Composite {
  public A a;
  public B b;
}

public ListenableFuture<Composite> combine(ListenableFuture<A> futureA, 
                                           final ListenableFuture<B> futureB) {

  return Futures.transform(futureA, new AsyncFunction<A, Composite>() {
    public ListenableFuture<Composite> apply(final A a) throws Exception {
      return Futures.transform(futureB, new Function<B, Compisite>() {
        public Composite apply(B b) {
          return new Composite(a, b);
        }
      }
    } 
  }

}

ListenableFuture<A> futureA = ...
ListenableFuture<B> futureB = ...

ListenableFuture<Composite> result = combine(futureA, futureB);

In this case Composite can be a Pair<A, B> from Apache Commons if you like.

Also, a failure in either future will result in a failure in the resulting combined future.

Another solution would be to take a look at Trickle from the team at Spotify. The GitHub README has an example which shows a solution to a similar problem.

There are undoubtedly other solutions but this is the one that popped into my head.