code: public void add(Listener listener) {
          logger.trace("Adding listener [{}]", listener);
          synchronized (listeners) {
              listeners.add(listener);
          }
      }
docs: Adds the specified {@link org.vaadin.spring.events.internal.ListenerCollection.Listener} to the listener
  collection.
  
  @param listener the listener to add, never {@code null}.
  @see #remove(org.vaadin.spring.events.internal.ListenerCollection.Listener)