protected void initComponent() {
        if (downloader == null) {
            this.downloader = new HttpClientDownloader();
        }
        if (pipelines.isEmpty()) {
            pipelines.add(new ConsolePipeline());
        }
        downloader.setThread(threadNum);
        if (threadPool == null || threadPool.isShutdown()) {
            if (executorService != null && !executorService.isShutdown()) {
                threadPool = new CountableThreadPool(threadNum, executorService);
            } else {
                threadPool = new CountableThreadPool(threadNum);
            }
        }
        if (startRequests != null) {
            for (Request request : startRequests) {
                addRequest(request);
            }
            startRequests.clear();
        }
        startTime = new Date();
    }
set the downloader of spider

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