code: private <R> CompletableFuture<R> execute(Callable<R> operation) {
          return CompletableFuture.supplyAsync(() -> {
              try {
                  return operation.call();
              } catch (Exception ex) {
                  throw new CompletionException(ex);
              }
          }, this.executor);
      }
docs: Executes the given Callable synchronously and invokes cleanup when done.
  
  @param operation    The Callable to execute.