@SuppressWarnings("unchecked")
    public <T> T getExtra(String key) {
        if (extras == null) {
            return null;
        }
        return (T) extras.get(key);
    }
Set the priority of request for sorting.<br>
Need a scheduler supporting priority.<br>
@see us.codecraft.webmagic.scheduler.PriorityScheduler

@param priority priority
@return this
webmagic-core/src/main/java/us/codecraft/webmagic/Request.java