code: private static String simpleHashOf(String resourceRealPath) {
          if (resourceRealPath == null) return null;
          File resource = new File(resourceRealPath);
          if (!resource.exists()) return null;
          long lastModified = resource.lastModified();
          long size = resource.length();
          return String.format("%s#%s", lastModified, size);
      }
docs: Calculates simple hash using file size and last modified time.
  
  @param resourceRealPath - file path, whose has to be calculated
  @return - hash string as lastmodified#size