code: public XExtension getByUri(URI uri) {
                  XExtension extension = extensionMap.get(uri);
                  if (extension == null) {
                          try {
                                  extension = XExtensionParser.instance().parse(uri);
                                  register(extension);
                                  XLogging.log("Imported XES extension '" + extension.getUri()
                                          + "' from remote source", XLogging.Importance.DEBUG);
                          } catch (IOException e) {
                                  // Now do something if the Internet is down...
  
                          } catch (ParserConfigurationException | SAXException e) {
                                  e.printStackTrace();
                                  return null;
                          }
                          cacheExtension(uri);
                  }
                  return extension;
          }
docs: Retrieves an extension instance by its unique URI. If the extension
  has not been registered before, it is looked up in the local cache.
  If it cannot be found in the cache, the manager attempts to download
  it from its unique URI, and add it to the set of managed extensions.
  
  @param uri The unique URI of the requested extension.
  @return The requested extension.