BlockingCallable blockingCallable = new BlockingCallable();
    ListenableFuture<@Nullable Void> unused = serializer.submit(blockingCallable, executor);
    ListenableFuture<Boolean> future2 =
        serializer.submit(
            new Callable<Boolean>() {
              @Override
              public Boolean call() {
                return blockingCallable.isRunning();
              }
            },