【转】WebMagic-总体流程源码分析

转自：http://m.blog.csdn.net/article/details?id=51943601

写在前面

前一段时间开发【知了】用到了很多技术（可以看我前面的博文http://blog.csdn.net/wsrspirit/article/details/51751568），这段时间抽空把这些整理一下，WebMagic是一个Java的爬虫，中国人写的，代码很模块化，也很利于二次开发，但是我们在使用的过程中也遇到了一些问题，这些问题我会在最后的博客中介绍，最近的博客将详细的走一下WebMagic的主题流程。 
这是官方的技术文档，写的很好，就是有点简单了： 
https://code4craft.gitbooks.io/webmagic-in-action/content/zh/index.html

走起

我们从一个demo走起：

public static void main(String[] args) {
    Spider.create(new GithubRepoPageProcessor())
            //从https://github.com/code4craft开始抓    
            .addUrl("https://github.com/code4craft")
            //设置Scheduler，使用Redis来管理URL队列
            .setScheduler(new RedisScheduler("localhost"))
            //设置Pipeline，将结果以json方式保存到文件
            .addPipeline(new JsonFilePipeline("D:\data\webmagic"))
            //开启5个线程同时执行
            .thread(5)
            //启动爬虫
            .run();
}

Builder模式添加了总要的组件，然后设置thread，run。我们可以称之为WebMagic四大组件：Pipeline，Scheduler，Downloader和PageProcesser 
 
这里的图是官方网站copy的，如果你是初步使用那可能有点误导性，现在我们已经找到了程序的入口Spider类，看一下代码（有点长，没有关系，我会挑重点的说~）

package us.codecraft.webmagic;

import com.google.common.collect.Lists;
import org.apache.commons.collections.CollectionUtils;
import org.apache.http.HttpHost;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import us.codecraft.webmagic.downloader.Downloader;
import us.codecraft.webmagic.downloader.HttpClientDownloader;
import us.codecraft.webmagic.pipeline.CollectorPipeline;
import us.codecraft.webmagic.pipeline.ConsolePipeline;
import us.codecraft.webmagic.pipeline.Pipeline;
import us.codecraft.webmagic.pipeline.ResultItemsCollectorPipeline;
import us.codecraft.webmagic.processor.PageProcessor;
import us.codecraft.webmagic.scheduler.QueueScheduler;
import us.codecraft.webmagic.scheduler.Scheduler;
import us.codecraft.webmagic.thread.CountableThreadPool;
import us.codecraft.webmagic.utils.UrlUtils;

import java.io.Closeable;
import java.io.IOException;
import java.util.*;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicLong;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;

/**
 * Entrance of a crawler.<br>
 * A spider contains four modules: Downloader, Scheduler, PageProcessor and
 * Pipeline.<br>
 * Every module is a field of Spider. <br>
 * The modules are defined in interface. <br>
 * You can customize a spider with various implementations of them. <br>
 * Examples: <br>
 * <br>
 * A simple crawler: <br>
 * Spider.create(new SimplePageProcessor("http://my.oschina.net/",
 * "http://my.oschina.net/*blog/*")).run();<br>
 * <br>
 * Store results to files by FilePipeline: <br>
 * Spider.create(new SimplePageProcessor("http://my.oschina.net/",
 * "http://my.oschina.net/*blog/*")) <br>
 * .pipeline(new FilePipeline("/data/temp/webmagic/")).run(); <br>
 * <br>
 * Use FileCacheQueueScheduler to store urls and cursor in files, so that a
 * Spider can resume the status when shutdown. <br>
 * Spider.create(new SimplePageProcessor("http://my.oschina.net/",
 * "http://my.oschina.net/*blog/*")) <br>
 * .scheduler(new FileCacheQueueScheduler("/data/temp/webmagic/cache/")).run(); <br>
 *
 * @author code4crafter@gmail.com <br>
 * @see Downloader
 * @see Scheduler
 * @see PageProcessor
 * @see Pipeline
 * @since 0.1.0
 */
public class Spider implements Runnable, Task {

    protected Downloader downloader;

    protected List<Pipeline> pipelines = new ArrayList<Pipeline>();

    protected PageProcessor pageProcessor;

    protected List<Request> startRequests;

    protected Site site;

    protected String uuid;

    protected Scheduler scheduler = new QueueScheduler();

    protected Logger logger = LoggerFactory.getLogger(getClass());

    protected CountableThreadPool threadPool;

    protected ExecutorService executorService;

    protected int threadNum = 1;

    protected AtomicInteger stat = new AtomicInteger(STAT_INIT);

    protected boolean exitWhenComplete = true;

    protected final static int STAT_INIT = 0;

    protected final static int STAT_RUNNING = 1;

    protected final static int STAT_STOPPED = 2;

    protected boolean spawnUrl = true;

    protected boolean destroyWhenExit = true;

    private ReentrantLock newUrlLock = new ReentrantLock();

    private Condition newUrlCondition = newUrlLock.newCondition();

    private List<SpiderListener> spiderListeners;

    private final AtomicLong pageCount = new AtomicLong(0);

    private Date startTime;

    private int emptySleepTime = 30000;

    /**
     * create a spider with pageProcessor.
     *
     * @param pageProcessor pageProcessor
     * @return new spider
     * @see PageProcessor
     */
    public static Spider create(PageProcessor pageProcessor) {
        return new Spider(pageProcessor);
    }

    /**
     * create a spider with pageProcessor.
     *
     * @param pageProcessor pageProcessor
     */
    public Spider(PageProcessor pageProcessor) {
        this.pageProcessor = pageProcessor;
        this.site = pageProcessor.getSite();
        this.startRequests = pageProcessor.getSite().getStartRequests();
    }

    /**
     * Set startUrls of Spider.<br>
     * Prior to startUrls of Site.
     *
     * @param startUrls startUrls
     * @return this
     */
    public Spider startUrls(List<String> startUrls) {
        checkIfRunning();
        this.startRequests = UrlUtils.convertToRequests(startUrls);
        return this;
    }

    /**
     * Set startUrls of Spider.<br>
     * Prior to startUrls of Site.
     *
     * @param startRequests startRequests
     * @return this
     */
    public Spider startRequest(List<Request> startRequests) {
        checkIfRunning();
        this.startRequests = startRequests;
        return this;
    }

    /**
     * Set an uuid for spider.<br>
     * Default uuid is domain of site.<br>
     *
     * @param uuid uuid
     * @return this
     */
    public Spider setUUID(String uuid) {
        this.uuid = uuid;
        return this;
    }

    /**
     * set scheduler for Spider
     *
     * @param scheduler scheduler
     * @return this
     * @Deprecated
     * @see #setScheduler(us.codecraft.webmagic.scheduler.Scheduler)
     */
    public Spider scheduler(Scheduler scheduler) {
        return setScheduler(scheduler);
    }

    /**
     * set scheduler for Spider
     *
     * @param scheduler scheduler
     * @return this
     * @see Scheduler
     * @since 0.2.1
     */
    public Spider setScheduler(Scheduler scheduler) {
        checkIfRunning();
        Scheduler oldScheduler = this.scheduler;
        this.scheduler = scheduler;
        if (oldScheduler != null) {
            Request request;
            while ((request = oldScheduler.poll(this)) != null) {
                this.scheduler.push(request, this);
            }
        }
        return this;
    }

    /**
     * add a pipeline for Spider
     *
     * @param pipeline pipeline
     * @return this
     * @see #addPipeline(us.codecraft.webmagic.pipeline.Pipeline)
     * @deprecated
     */
    public Spider pipeline(Pipeline pipeline) {
        return addPipeline(pipeline);
    }

    /**
     * add a pipeline for Spider
     *
     * @param pipeline pipeline
     * @return this
     * @see Pipeline
     * @since 0.2.1
     */
    public Spider addPipeline(Pipeline pipeline) {
        checkIfRunning();
        this.pipelines.add(pipeline);
        return this;
    }

    /**
     * set pipelines for Spider
     *
     * @param pipelines pipelines
     * @return this
     * @see Pipeline
     * @since 0.4.1
     */
    public Spider setPipelines(List<Pipeline> pipelines) {
        checkIfRunning();
        this.pipelines = pipelines;
        return this;
    }

    /**
     * clear the pipelines set
     *
     * @return this
     */
    public Spider clearPipeline() {
        pipelines = new ArrayList<Pipeline>();
        return this;
    }

    /**
     * set the downloader of spider
     *
     * @param downloader downloader
     * @return this
     * @see #setDownloader(us.codecraft.webmagic.downloader.Downloader)
     * @deprecated
     */
    public Spider downloader(Downloader downloader) {
        return setDownloader(downloader);
    }

    /**
     * set the downloader of spider
     *
     * @param downloader downloader
     * @return this
     * @see Downloader
     */
    public Spider setDownloader(Downloader downloader) {
        checkIfRunning();
        this.downloader = downloader;
        return this;
    }

    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) {
                scheduler.push(request, this);
            }
            startRequests.clear();
        }
        startTime = new Date();
    }

    @Override
    public void run() {
        checkRunningStat();
        initComponent();
        logger.info("Spider " + getUUID() + " started!");
        while (!Thread.currentThread().isInterrupted() && stat.get() == STAT_RUNNING) {
            Request request = scheduler.poll(this);
            if (request == null) {
                if (threadPool.getThreadAlive() == 0 && exitWhenComplete) {
                    break;
                }
                // wait until new url added
                waitNewUrl();
            } else {
                final Request requestFinal = request;
                threadPool.execute(new Runnable() {
                    @Override
                    public void run() {
                        try {
                            processRequest(requestFinal);
                            onSuccess(requestFinal);
                        } catch (Exception e) {
                            onError(requestFinal);
                            logger.error("process request " + requestFinal + " error", e);
                        } finally {
                            pageCount.incrementAndGet();
                            signalNewUrl();
                        }
                    }
                });
            }
        }
        stat.set(STAT_STOPPED);
        // release some resources
        if (destroyWhenExit) {
            close();
        }
    }

    protected void onError(Request request) {
        if (CollectionUtils.isNotEmpty(spiderListeners)) {
            for (SpiderListener spiderListener : spiderListeners) {
                spiderListener.onError(request);
            }
        }
    }

    protected void onSuccess(Request request) {
        if (CollectionUtils.isNotEmpty(spiderListeners)) {
            for (SpiderListener spiderListener : spiderListeners) {
                spiderListener.onSuccess(request);
            }
        }
    }

    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;
            }
        }
    }

    public void close() {
        destroyEach(downloader);
        destroyEach(pageProcessor);
        destroyEach(scheduler);
        for (Pipeline pipeline : pipelines) {
            destroyEach(pipeline);
        }
        threadPool.shutdown();
    }

    private void destroyEach(Object object) {
        if (object instanceof Closeable) {
            try {
                ((Closeable) object).close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }

    /**
     * Process specific urls without url discovering.
     *
     * @param urls urls to process
     */
    public void test(String... urls) {
        initComponent();
        if (urls.length > 0) {
            for (String url : urls) {
                processRequest(new Request(url));
            }
        }
    }

    protected void processRequest(Request request) {
        Page page = downloader.download(request, this);
        if (page == null) {
            throw new RuntimeException("unaccpetable response status");
        }
        // for cycle retry
        if (page.isNeedCycleRetry()) {
            extractAndAddRequests(page, true);
            sleep(site.getRetrySleepTime());
            return;
        }
        pageProcessor.process(page);
        extractAndAddRequests(page, spawnUrl);
        if (!page.getResultItems().isSkip()) {
            for (Pipeline pipeline : pipelines) {
                pipeline.process(page.getResultItems(), this);
            }
        }
        //for proxy status management
        request.putExtra(Request.STATUS_CODE, page.getStatusCode());
        sleep(site.getSleepTime());
    }

    protected void sleep(int time) {
        try {
            Thread.sleep(time);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    protected void extractAndAddRequests(Page page, boolean spawnUrl) {
        if (spawnUrl && CollectionUtils.isNotEmpty(page.getTargetRequests())) {
            for (Request request : page.getTargetRequests()) {
                addRequest(request);
            }
        }
    }

    private void addRequest(Request request) {
        if (site.getDomain() == null && request != null && request.getUrl() != null) {
            site.setDomain(UrlUtils.getDomain(request.getUrl()));
        }
        scheduler.push(request, this);
    }

    protected void checkIfRunning() {
        if (stat.get() == STAT_RUNNING) {
            throw new IllegalStateException("Spider is already running!");
        }
    }

    public void runAsync() {
        Thread thread = new Thread(this);
        thread.setDaemon(false);
        thread.start();
    }

    /**
     * Add urls to crawl. <br>
     *
     * @param urls urls
     * @return this
     */
    public Spider addUrl(String... urls) {
        for (String url : urls) {
            addRequest(new Request(url));
        }
        signalNewUrl();
        return this;
    }

    /**
     * Download urls synchronizing.
     *
     * @param urls urls
     * @return list downloaded
     */
    public <T> List<T> getAll(Collection<String> urls) {
        destroyWhenExit = false;
        spawnUrl = false;
        startRequests.clear();
        for (Request request : UrlUtils.convertToRequests(urls)) {
            addRequest(request);
        }
        CollectorPipeline collectorPipeline = getCollectorPipeline();
        pipelines.add(collectorPipeline);
        run();
        spawnUrl = true;
        destroyWhenExit = true;
        return collectorPipeline.getCollected();
    }

    protected CollectorPipeline getCollectorPipeline() {
        return new ResultItemsCollectorPipeline();
    }

    public <T> T get(String url) {
        List<String> urls = Lists.newArrayList(url);
        List<T> resultItemses = getAll(urls);
        if (resultItemses != null && resultItemses.size() > 0) {
            return resultItemses.get(0);
        } else {
            return null;
        }
    }

    /**
     * Add urls with information to crawl.<br>
     *
     * @param requests requests
     * @return this
     */
    public Spider addRequest(Request... requests) {
        for (Request request : requests) {
            addRequest(request);
        }
        signalNewUrl();
        return this;
    }

    private void waitNewUrl() {
        newUrlLock.lock();
        try {
            //double check
            if (threadPool.getThreadAlive() == 0 && exitWhenComplete) {
                return;
            }
            newUrlCondition.await(emptySleepTime, TimeUnit.MILLISECONDS);
        } catch (InterruptedException e) {
            logger.warn("waitNewUrl - interrupted, error {}", e);
        } finally {
            newUrlLock.unlock();
        }
    }

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

    public void start() {
        runAsync();
    }

    public void stop() {
        if (stat.compareAndSet(STAT_RUNNING, STAT_STOPPED)) {
            logger.info("Spider " + getUUID() + " stop success!");
        } else {
            logger.info("Spider " + getUUID() + " stop fail!");
        }
    }

    /**
     * start with more than one threads
     *
     * @param threadNum threadNum
     * @return this
     */
    public Spider thread(int threadNum) {
        checkIfRunning();
        this.threadNum = threadNum;
        if (threadNum <= 0) {
            throw new IllegalArgumentException("threadNum should be more than one!");
        }
        return this;
    }

    /**
     * start with more than one threads
     *
     * @param executorService executorService to run the spider
     * @param threadNum threadNum
     * @return this
     */
    public Spider thread(ExecutorService executorService, int threadNum) {
        checkIfRunning();
        this.threadNum = threadNum;
        if (threadNum <= 0) {
            throw new IllegalArgumentException("threadNum should be more than one!");
        }
        return this;
    }

    public boolean isExitWhenComplete() {
        return exitWhenComplete;
    }

    /**
     * Exit when complete. <br>
     * True: exit when all url of the site is downloaded. <br>
     * False: not exit until call stop() manually.<br>
     *
     * @param exitWhenComplete exitWhenComplete
     * @return this
     */
    public Spider setExitWhenComplete(boolean exitWhenComplete) {
        this.exitWhenComplete = exitWhenComplete;
        return this;
    }

    public boolean isSpawnUrl() {
        return spawnUrl;
    }

    /**
     * Get page count downloaded by spider.
     *
     * @return total downloaded page count
     * @since 0.4.1
     */
    public long getPageCount() {
        return pageCount.get();
    }

    /**
     * Get running status by spider.
     *
     * @return running status
     * @see Status
     * @since 0.4.1
     */
    public Status getStatus() {
        return Status.fromValue(stat.get());
    }


    public enum Status {
        Init(0), Running(1), Stopped(2);

        private Status(int value) {
            this.value = value;
        }

        private int value;

        int getValue() {
            return value;
        }

        public static Status fromValue(int value) {
            for (Status status : Status.values()) {
                if (status.getValue() == value) {
                    return status;
                }
            }
            //default value
            return Init;
        }
    }

    /**
     * Get thread count which is running
     *
     * @return thread count which is running
     * @since 0.4.1
     */
    public int getThreadAlive() {
        if (threadPool == null) {
            return 0;
        }
        return threadPool.getThreadAlive();
    }

    /**
     * Whether add urls extracted to download.<br>
     * Add urls to download when it is true, and just download seed urls when it is false. <br>
     * DO NOT set it unless you know what it means!
     *
     * @param spawnUrl spawnUrl
     * @return this
     * @since 0.4.0
     */
    public Spider setSpawnUrl(boolean spawnUrl) {
        this.spawnUrl = spawnUrl;
        return this;
    }

    @Override
    public String getUUID() {
        if (uuid != null) {
            return uuid;
        }
        if (site != null) {
            return site.getDomain();
        }
        uuid = UUID.randomUUID().toString();
        return uuid;
    }

    public Spider setExecutorService(ExecutorService executorService) {
        checkIfRunning();
        this.executorService = executorService;
        return this;
    }

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

    public List<SpiderListener> getSpiderListeners() {
        return spiderListeners;
    }

    public Spider setSpiderListeners(List<SpiderListener> spiderListeners) {
        this.spiderListeners = spiderListeners;
        return this;
    }

    public Date getStartTime() {
        return startTime;
    }

    public Scheduler getScheduler() {
        return scheduler;
    }

    /**
     * Set wait time when no url is polled.<br><br>
     *
     * @param emptySleepTime In MILLISECONDS.
     */
    public void setEmptySleepTime(int emptySleepTime) {
        this.emptySleepTime = emptySleepTime;
    }
}

首先就是Spider可以装载四大组件，然后addurl，然后thread，然后run

public Spider addUrl(String... urls) {
        for (String url : urls) {
            addRequest(new Request(url));
        }
        signalNewUrl();
        return this;
    }

    /**
     * Add urls with information to crawl.<br>
     *
     * @param requests requests
     * @return this
     */
    public Spider addRequest(Request... requests) {
        for (Request request : requests) {
            addRequest(request);
        }
        signalNewUrl();
        return this;
    }

    private void addRequest(Request request) {
        if (site.getDomain() == null && request != null && request.getUrl() != null) {
            site.setDomain(UrlUtils.getDomain(request.getUrl()));
        }
        scheduler.push(request, this);
    }

Request是对url的一个包装，我们后面再说，我们发现url经过包装后被scheduler#push，然后执行了signalNewUrl

private void waitNewUrl() {
        newUrlLock.lock();
        try {
            //double check
            if (threadPool.getThreadAlive() == 0 && exitWhenComplete) {
                return;
            }
            newUrlCondition.await(emptySleepTime, TimeUnit.MILLISECONDS);
        } catch (InterruptedException e) {
            logger.warn("waitNewUrl - interrupted, error {}", e);
        } finally {
            newUrlLock.unlock();
        }
    }

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

发现这是一个条件变量，然后如果加入Request后就会唤醒阻塞的task进行处理，这里实现了一个线程池，线程池的线程数量就是thread(5)定义的，threadPool.getThreadAlive()这个的意思是目前正在运行的thread。

ok目前我们走完了addurl和thread，那就是最后的run方法了，我们首先看这里Spider的定义，run方法就是实现的Runnable

public class Spider implements Runnable, Task

@Override
    public void run() {
        checkRunningStat();
        initComponent();
        logger.info("Spider " + getUUID() + " started!");
        while (!Thread.currentThread().isInterrupted() && stat.get() == STAT_RUNNING) {
            Request request = scheduler.poll(this);
            if (request == null) {
                if (threadPool.getThreadAlive() == 0 && exitWhenComplete) {
                    break;
                }
                // wait until new url added
                waitNewUrl();
            } else {
                final Request requestFinal = request;
                threadPool.execute(new Runnable() {
                    @Override
                    public void run() {
                        try {
                            processRequest(requestFinal);
                            onSuccess(requestFinal);
                        } catch (Exception e) {
                            onError(requestFinal);
                            logger.error("process request " + requestFinal + " error", e);
                        } finally {
                            pageCount.incrementAndGet();
                            signalNewUrl();
                        }
                    }
                });
            }
        }
        stat.set(STAT_STOPPED);
        // release some resources
        if (destroyWhenExit) {
            close();
        }
    }

    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) {
                scheduler.push(request, this);
            }
            startRequests.clear();
        }
        startTime = new Date();
    }

首先是checkRunningStat检查一下目前Spider的执行情况，然后是initComponent，我们可以看到默认的Downloader和Pipeline是可以为空，然后初始化threadpool，最后是把starturl加入Request中（现在startUrl是spider.addUrl添加的，这也是符合爬虫的起始地址，而不是site的起始地址的观点）

从这个代码中，我们基本上看懂了scheduler是干什么的：Request request = scheduler.poll(this); 
Scheduler为我们存储了Request并且可以做一些功能（去重，设置优先级等）我们需要pull或者push

真正在线程池中运行的代码是

public void run() {
    try {
            processRequest(requestFinal);
            onSuccess(requestFinal);
        } catch (Exception e) {
            onError(requestFinal);
            logger.error("process request " + requestFinal + " error", e);
        } finally {
            pageCount.incrementAndGet();
            signalNewUrl();
        }
}

    protected void processRequest(Request request) {
        Page page = downloader.download(request, this);
        if (page == null) {
            throw new RuntimeException("unaccpetable response status");
        }
        // for cycle retry
        if (page.isNeedCycleRetry()) {
            extractAndAddRequests(page, true);
            sleep(site.getRetrySleepTime());
            return;
        }
        pageProcessor.process(page);
        extractAndAddRequests(page, spawnUrl);
        if (!page.getResultItems().isSkip()) {
            for (Pipeline pipeline : pipelines) {
                pipeline.process(page.getResultItems(), this);
            }
        }
        //for proxy status management
        request.putExtra(Request.STATUS_CODE, page.getStatusCode());
        sleep(site.getSleepTime());
    }

Request经过Downloader的download方法返回Page，大致经过是Downloader拿到Request的url然后执行http#get方法得到httpResponse组装成Page，然后PageProcesser执行解析，这部分代码是需要自己实现的，例如下面的：

public void process(Page page) {
        List<String> relativeUrl = page.getHtml().xpath("//li[@class='item clearfix']/div/a/@href").all();
        page.addTargetRequests(relativeUrl);
        relativeUrl = page.getHtml().xpath("//div[@id='zh-question-related-questions']//a[@class='question_link']/@href").all();
        page.addTargetRequests(relativeUrl);
        List<String> answers =  page.getHtml().xpath("//div[@id='zh-question-answer-wrap']/div").all();
        boolean exist = false;
        for(String answer:answers){
            String vote = new Html(answer).xpath("//div[@class='zm-votebar']//span[@class='count']/text()").toString();
            if(Integer.valueOf(vote) >= voteNum){
                page.putField("vote",vote);
                page.putField("content",new Html(answer).xpath("//div[@class='zm-editable-content']"));
                page.putField("userid", new Html(answer).xpath("//a[@class='author-link']/@href"));
                exist = true;
            }
        }
        if(!exist){
            page.setSkip(true);
        }
    }

也就是使用正则表达式等东西解析page中的内容，包括哪些是需要的提取的属性，哪些是应该加入Scheduler中继续爬的（如下一页这种），然后将爬出来的属性写入Page的ResultItem一个hashMap中，最后Pipeline会提取ResultItem进行存储等工作。好了，基本的流程就这样了。

新司机开车吧

看完这些你已经对WebMagic有了大致的认识，我最近更新博文速度可能不会很快，所以如果你很想了解WebMagic的全貌那你已经可以自己开车啦~

老司机的报站

后面我将每个组件详细介绍，Scheduler，Downloader，Pipeline和PageProcesser，CountableThreadPool，SpiderMonitor，最后介绍OOSpider也就是使用注解的方式编写爬虫，你会发现这太帅了！