code: public void addSourceFolder(File folder) {
  		Assert.notNull(folder, "Folder must not be null");
  		Assert.isTrue(!folder.isFile(), "Folder '" + folder + "' must not be a file");
  		synchronized (this.monitor) {
  			checkNotStarted();
  			this.folders.put(folder, null);
  		}
  	}
docs: Add a source folder to monitor. Cannot be called after the watcher has been
  {@link #start() started}.
  @param folder the folder to monitor