}

    /**
     * Adds a property to the extractor.
     *
     * @param key   the key of the property
     * @param value the value of the property
     */
    public void addProperty(final String key, final String value) {
        if (StringUtil.isBlank(key) || StringUtil.isBlank(value)) {
            throw new CrawlerSystemException("key or value is null.");
        }

     * Adds a parser property configuration.
     *
     * @param key the property key
     * @param value the property value
     * @throws CrawlerSystemException if key or value is null
     */
    public void addProperty(final String key, final String value) {
        if (StringUtil.isBlank(key) || StringUtil.isBlank(value)) {
            throw new CrawlerSystemException("key or value is null.");
        }