code: public static boolean unregisterExtension(String key) {
          if (key == null) {
              throw new IllegalArgumentException(
                              "Parameter 'key' can not be null");
          }
          w.lock();
          try {
              return extensionMap.remove(key) != null;
          } finally {
              w.unlock();
          }
      }
docs: Removes context extension registration.
  
  @param key
  String key associated with the registered extension.
  @return <code>true</code> if key had extension associated with it.
  @throws IllegalArgumentException
  if parameter <code>key</code> is <code>null</code>.