code: public void addlistener(final ChangeListener listener) {
          synchronized (LISTENER_LOCK) {
              if (listeners.contains(listener)) {
                  LOGGER.warn("Skip listener registration. listener[" + listener + "] is already registered!");
                  return;
              }
              listeners.add(listener);
          }
      }
docs: Registers the given listener to get informed about changes.
  
  @param listener the listener to register.