@Override
  public Future<BoxedUnit> apply(Seq<Span> input) {
    List<io.zipkin.Span> spans =
        JavaConversions.asJavaCollection(input)
            .stream()
            .map(ScalaSpanStoreAdapter::invert)
            .filter(i -> i != null)
            .collect(toList());

    this.spanStore.accept(spans);
    return Future.Unit();
  }