@Override
    public String getUUID() {
        if (uuid != null) {
            return uuid;
        }
        if (site != null) {
            return site.getDomain();
        }
        uuid = UUID.randomUUID().toString();
        return uuid;
    }
Whether add urls extracted to download.<br>
Add urls to download when it is true, and just download seed urls when it is false. <br>
DO NOT set it unless you know what it means!

@param spawnUrl spawnUrl
@return this
@since 0.4.0
webmagic-core/src/main/java/us/codecraft/webmagic/Spider.java