Is there an easy way to turn Future<Future<T>> into Future<T>?

Question

I've got some code that submits a request to another thread which may or may not submit that request to yet another thread. That yields a return type of Future<Future<T>>. Is there some non-heinous way to immediately turn this into Future<T> that waits on the completion of the entire future chain?

I'm already using the Guava library to handle other fun concurrency stuff and as a replacement for Google Collections and its working well but I can't seem to find something for this case.

Answer 1

Another possible implementation that uses the guava libraries and is a lot simpler.

import java.util.concurrent.*;
import com.google.common.util.concurrent.*;
import com.google.common.base.*;

public class FFutures {
  public <T> Future<T> flatten(Future<Future<T>> future) {
    return Futures.chain(Futures.makeListenable(future), new Function<Future<T>, ListenableFuture<T>>() {
      public ListenableFuture<T> apply(Future<T> f) {
        return Futures.makeListenable(f);
      }
    });
  }
}

Answer 2

Guava 13.0 adds Futures.dereference to do this. It requires a ListenableFuture<ListenableFuture>, rather than a plain Future<Future>. (Operating on a plain Future would require a makeListenable call, each of which requires a dedicated thread for the lifetime of the task (as is made clearer by the method's new name, JdkFutureAdapters.listenInPoolThread).)