protected static final Traverser[] EMPTY_ARRAY = new Traverser[0];

    private static final Logger logger = Logger.getLogger(TraversalUtil.class);

    /** URLのプロトコルをキー、{@link TraverserFactory}を値とするマッピングです。 */
    protected static final ConcurrentMap<String, TraverserFactory> traverserFactories = newConcurrentHashMap();
    static {