if (urlQueueList == null) {
            urlQueueList = new LinkedList<>();
            urlQueueMap.put(sessionId, urlQueueList);
        }
        return urlQueueList;
    }

    public synchronized void addUrlQueueList(final String sessionId, final Queue<UrlQueueImpl<Long>> urlQueueList) {
        final Queue<UrlQueueImpl<Long>> uqList = getUrlQueueList(sessionId);
        uqList.addAll(urlQueueList);
        urlQueueMap.put(sessionId, uqList);

        // not MT-safe
        final Queue<UrlQueueImpl<Long>> urlQueueList = dataHelper.getUrlQueueList(oldSessionId);
        // overwrite
        dataHelper.addUrlQueueList(newSessionId, urlQueueList);
        dataHelper.removeUrlQueueList(oldSessionId);
    }

    /*
     * (non-Javadoc)
     *
     * @see org.codelibs.fess.crawler.service.UrlQueueService#add(java.lang.String,