private static CompletionStage<?> toCompletionStage(@Nullable Object obj, ExecutorService executor) {
        if (obj instanceof CompletionStage) {
            return (CompletionStage<?>) obj;
        }
        if (obj != null && ScalaUtil.isScalaFuture(obj.getClass())) {
            return ScalaUtil.FutureConverter.toCompletableFuture((scala.concurrent.Future<?>) obj, executor);
        }
        return UnmodifiableFuture.completedFuture(obj);
    }
Converts the specified {@code obj} with {@link CompletableFuture}.
core/src/main/java/com/linecorp/armeria/internal/server/annotation/DefaultAnnotatedService.java