List<Future<?>> results = new ArrayList<>();
    Runnable[] manualExecutorTask = new Runnable[1];
    Executor manualExecutor =
        new Executor() {
          @Override
          public void execute(Runnable task) {
            manualExecutorTask[0] = task;
          }
        };

    results.add(serializer.submit(Callables.returning(null), manualExecutor));

    List<Future<?>> results = new ArrayList<>();
    Runnable[] manualExecutorTask = new Runnable[1];
    Executor manualExecutor =
        new Executor() {
          @Override
          public void execute(Runnable task) {
            manualExecutorTask[0] = task;
          }
        };

    results.add(serializer.submit(Callables.returning(null), manualExecutor));