code: public void addListener(FileChangeListener fileChangeListener) {
  		Assert.notNull(fileChangeListener, "FileChangeListener must not be null");
  		synchronized (this.monitor) {
  			checkNotStarted();
  			this.listeners.add(fileChangeListener);
  		}
  	}
docs: Add listener for file change events. Cannot be called after the watcher has been
  {@link #start() started}.
  @param fileChangeListener the listener to add