*
     * @param sessionId unique identifier for this crawling session
     */
    public void crawl(final String sessionId) {
        final List<DataConfig> configList = ComponentUtil.getCrawlingConfigHelper().getAllDataConfigList();

        if (configList.isEmpty()) {
            // nothing
            if (logger.isInfoEnabled()) {
                logger.info("No crawling target data.");
            }
            return;