docs: @Override
      public Optional<String> hash(final Optional<URL> url) throws IOException {
          if (!url.isPresent()) {
              return Optional.absent();
          }
          logger.debug("Calculating md5 hash, url:{}", url);
          if (isHotReloadModeOff()) {
              final String md5 = cache.getIfPresent(url.get());
  
              logger.debug("md5 hash:{}", md5);
  
              if (md5 != null) {
                  return Optional.of(md5);
              }
          }
  
          final InputStream is = url.get().openStream();
          final String md5 = getMD5Checksum(is);
  
          if (isHotReloadModeOff()) {
              logger.debug("caching url:{} with hash:{}", url, md5);
  
              cache.put(url.get(), md5);
          }
  
          return Optional.fromNullable(md5);
      }