code: public Resource createRelative(String relativePath) {
          if (relativePath.startsWith("/")) {
              relativePath = relativePath.substring(1);
          }
          try {
              return new UrlResource(new URL(url, relativePath));
          } catch (MalformedURLException e) {
              return null;
          }
      }
docs: This implementation creates a UrlResource, applying the given path
  relative to the path of the underlying URL of this resource descriptor.
  
  @param relativePath The path
  @return The resource for the path
  @see java.net.URL#URL(java.net.URL, String)