code: protected final void crawl(final CrawlRequest request) {
          Validate.validState(!isStopped,
                  "The crawler is not started. Maybe you meant to add this request as a crawl seed?");
          Validate.validState(!isStopping, "Cannot add request when the crawler is stopping.");
          Validate.notNull(request, "The request cannot be null.");
  
          crawlFrontier.feedRequest(request, false);
      }
docs: Feeds a crawl request to the crawler. The crawler should be running, otherwise the request
  has to be added as a crawl seed instead.
  
  @param request the crawl request