ListenableFuture to scala Future

The second option is best, it keeps everything asynchronous. but... you can do one better and abstract the solution into a reusable pattern:

implicit class RichListenableFuture[T](lf: ListenableFuture[T]) {
  def asScala: Future[T] = {
    val p = Promise[T]()
    Futures.addCallback(lf, new FutureCallback[T] {
      def onFailure(t: Throwable): Unit = p failure t
      def onSuccess(result: T): Unit    = p success result
    })
    p.future
  }    
}

You can then simply call:

executor.asyncExecute(query).asScala

Another, slightly shorter solution:

implicit class ListenableFutureDecorator[T](val f: ListenableFuture[T]) extends AnyVal {
  def asScala(implicit e: Executor): Future[T] = {
    val p = Promise[T]()
    f.addListener(() => p.complete(Try(f.get())), e)
    p.future
  }
}