本篇文章的原理分析围绕着WebMagic的四大组件展开的，不清楚的小伙伴可以看小编的上一篇文章WebMagic初探,原理分析围绕着爬虫的运行展开的，可以运行下方的程序，然后debug跟随小编一起了解四大组件是如何运行的

public class WyNewsProcessor implements PageProcessor {
    //抓取网站的相关配置、编码、抓取间隔、重试次数等

    private Site site = Site.me().setRetryTimes(3).setSleepTime(100);

    //计数器

    private static int count = 0 ;

    @Override
    public void process(Page page) {
        //判断连接是否满足
        boolean flag = page.getUrl().regex("https://www.cnblogs.com/[a-z 0-9 -]+/p/[0-9]{7}.html").match();
        if(!flag){
            //加入满足条件的连接  xpath选取要爬取的div
            page.addTargetRequests(page.getHtml().xpath("//*[@id=\"post_list\"]/div/div[@class='post_item_body']/h3/a/@href").all());
        }else {
            System.out.println("抓到的内容:作者昵称"+page.getHtml().xpath("//*[@id=\"Header1_HeaderTitle\"]/text()").get());
            count++ ;
        }
    }

    @Override
    public Site getSite() {
        return site;
    }

    public static void main(String[] args) {
        Spider.create(new WyNewsProcessor()).addUrl("https://www.cnblogs.com/").thread(5).run();
        System.out.println("结束了"+count);
    }
}

项目运行后，首先是Spider创建，

//程序启动创建 Spider 
 public static Spider create(PageProcessor pageProcessor) {
        return new Spider(pageProcessor);
}
//获取Site
 public Spider(PageProcessor pageProcessor) {
        this.pageProcessor = pageProcessor;
        this.site = pageProcessor.getSite();
 }
//获取url  可变长参数接收
public Spider addUrl(String... urls) {
        for (String url : urls) {
            addRequest(new Request(url));
        }
        signalNewUrl();
        return this;
    }

private void signalNewUrl() {
        try {
            newUrlLock.lock();
            newUrlCondition.signalAll();
        } finally {
            newUrlLock.unlock();
        }
    }

得到抓取网站的相关配置、编码、抓取间隔、重试次数等

页面获取成功后调用 onDownloadSuccess方法

 private void onDownloadSuccess(Request request, Page page) {
        if (site.getAcceptStatCode().contains(page.getStatusCode())){
  //调用自己重写的process()方法   
            pageProcessor.process(page);
            extractAndAddRequests(page, spawnUrl);
            if (!page.getResultItems().isSkip()) {
                for (Pipeline pipeline : pipelines) {
                    pipeline.process(page.getResultItems(), this);
                }
            }
        } else {
            logger.info("page status code error, page {} , code: {}", request.getUrl(), page.getStatusCode());
        }
        sleep(site.getSleepTime());
        return;
    }

Xpath 对下载得到的页面进行筛选，得到有效信息

 @Override
    public Selectable xpath(String xpath) {
        XpathSelector xpathSelector = Selectors.xpath(xpath);
        return selectElements(xpathSelector);
    }

 protected Selectable selectElements(BaseElementSelector elementSelector) {
        ListIterator<Element> elementIterator = getElements().listIterator();
        if (!elementSelector.hasAttribute()) {
            List<Element> resultElements = new ArrayList<Element>();
            while (elementIterator.hasNext()) {
                Element element = checkElementAndConvert(elementIterator);
                List<Element> selectElements = elementSelector.selectElements(element);
                resultElements.addAll(selectElements);
            }
            return new HtmlNode(resultElements);
        } else {
            // has attribute, consider as plaintext
            List<String> resultStrings = new ArrayList<String>();
            while (elementIterator.hasNext()) {
                Element element = checkElementAndConvert(elementIterator);
                List<String> selectList = elementSelector.selectList(element);
                resultStrings.addAll(selectList);
            }
            return new PlainText(resultStrings);

        }
    }
//将匹配的结果赋值给sourceTexts 
 public PlainText(List<String> sourceTexts) {
        this.sourceTexts = sourceTexts;
    }

线程启动，截取了两个方法了解，checkRunningStat方法有并发操作，他有一个值
stat 类型是AtomicInteger 这个类型提供原子操作，具体可以参考其他文章，这里不做深入探讨。
initComponent方法是进行初始化操作

public void run() {
        checkRunningStat();
        initComponent();
      ...省略
}

 private void checkRunningStat() {
        while (true) {
            int statNow = stat.get();
            if (statNow == STAT_RUNNING) {
                throw new IllegalStateException("Spider is already running!");
            }
            if (stat.compareAndSet(statNow, STAT_RUNNING)) {
                break;
            }
        }
    }
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();
    }

致此，程序运行完毕，对代码解读有误请多多指正，多多交流