@Override
                public void run() {
                    try {
                        processRequest(request);
                        onSuccess(request);
                    } catch (Exception e) {
                        onError(request, e);
                        logger.error("process request " + request + " error", e);
                    } finally {
                        pageCount.incrementAndGet();
                        scheduler.signalNewUrl();
                    }
                }
set the downloader of spider

@param downloader downloader
@return this
@see Downloader
webmagic-core/src/main/java/us/codecraft/webmagic/Spider.java