code: private void processExternalListener(Class<?> listenerClass) {
      ExternalListenerMetadata listenerMetadata = ExternalListenerIntrospector
          .introspect(listenerClass);
      Map<CallbackType, Method> callbacks = listenerMetadata.getCallbacks();
      if (callbacks != null) {
        for (Map.Entry<CallbackType, Method> entry : callbacks.entrySet()) {
          validateExternalCallback(entry.getValue(), entry.getKey());
        }
      }
    }
docs: Introspects the given listener class and finds all methods that should receive callback event
  notifications.
  
  @param listenerClass
  the listener class