synchronized (urlQueueList) {
            return !isNewUrl(urlQueue, urlQueueList);
        }
    }

    @Override
    public void generateUrlQueues(final String previousSessionId, final String sessionId) {
        final Queue<UrlQueueImpl<Long>> urlQueueList = dataHelper.getUrlQueueList(sessionId);

    public void clear() {
        urlQueueMap.clear();
        sessionMap.clear();
    }

    public synchronized Queue<UrlQueueImpl<Long>> getUrlQueueList(final String sessionId) {
        Queue<UrlQueueImpl<Long>> urlQueueList = urlQueueMap.get(sessionId);
        if (urlQueueList == null) {
            urlQueueList = new LinkedList<>();