解析CrawlController

来源：互联网发布：怎么成为一个程序员编辑：程序博客网时间：2024/05/20 01:11

当我们以Web UI方式使用Heritrix时,点击任务开始(start)按钮时,Heritrix就开始了它的爬取工作.但它的内部

执行流程是怎样的呢?别急,下面将慢慢道来.

(一)CrawlJobHandler

当点击任务开始(start)按钮时,将执行它的startCrawler()方法:

if(sAction.equalsIgnoreCase("start"))
{
// Tell handler to start crawl job
handler.startCrawler();

}

再来看看startCrawler()方法的执行:

public class CrawlJobHandler implements CrawlStatusListener {

public void startCrawler() {
 running = true;
 if (pendingCrawlJobs.size() > 0 && isCrawling() == false) {
 // Ok, can just start the next job
 startNextJob();
 }
 }

protected final void startNextJob() {
 synchronized (this) {
 if(startingNextJob != null) {
 try {
 startingNextJob.join();
 } catch (InterruptedException e) {
 e.printStackTrace();
 return;
 }
 }
 startingNextJob = new Thread(new Runnable() {
 public void run() {
 startNextJobInternal();
 }
 }, "StartNextJob");
 //当前任务线程开始执行
 startingNextJob.start();
 }
 }

protected void startNextJobInternal() {
 if (pendingCrawlJobs.size() == 0 || isCrawling()) {
 // No job ready or already crawling.
 return;
 }
 //从待处理的任务列表取出一个任务
 this.currentJob = (CrawlJob)pendingCrawlJobs.first();
 assert pendingCrawlJobs.contains(currentJob) :
 "pendingCrawlJobs is in an illegal state";
 //从待处理列表中删除
 pendingCrawlJobs.remove(currentJob);
 try {
 this.currentJob.setupForCrawlStart();
 // This is ugly but needed so I can clear the currentJob
 // reference in the crawlEnding and update the list of completed
 // jobs. Also, crawlEnded can startup next job.
 this.currentJob.getController().addCrawlStatusListener(this);
 // now, actually start
 //控制器真正开始执行的地方
 this.currentJob.getController().requestCrawlStart();
 } catch (InitializationException e) {
 loadJob(getStateJobFile(this.currentJob.getDirectory()));
 this.currentJob = null;
 startNextJobInternal(); // Load the next job if there is one.
 }
 }
}

由以上代码不难发现整个流程如下:

可以看出,最终将启动CrawlController的requestCrawlStart()方法.

(二)CrawlController

该类是一次抓取任务中的核心组件。它将决定整个抓取任务的开始和结束.

先看看它的源代码:在构造 CrawlController实例,需要先做以下工作:

package org.archive.crawler.framework;

public class CrawlController implements Serializable, Reporter {

    // key subcomponents which define and implement a crawl in progress
    private transient CrawlOrder order;
    private transient CrawlScope scope;
    private transient ProcessorChainList processorChains;

    private transient Frontier frontier;

    private transient ToePool toePool;

    private transient ServerCache serverCache;

    // This gets passed into the initialize method.
    private transient SettingsHandler settingsHandler;

}

CrawlOrder：它保存了对该次抓取任务中order.xml的属性配置。
CrawlScope：决定当前抓取范围的一个组件。
ProcessorChainList：从名称上可知，其表示处理器链。
Frontier：它是一个URL的处理器，决定下一个要被处理的URL是什么。
ToePool：它表示一个线程池，管理了所有该抓取任务所创建的子线程。
ServerCache：它表示一个缓冲池，保存了所有在当前任务中，抓取过的Host名称和Server名称。

(1)首先构造一个XMLSettingsHandler对象，将order.xml内的属性信息装入，并调用它的initialize方法进行初始化。

(2)调用CrawlController构造函数，构造一个CrawlController实例

(3)调用CrawlController的initilize（SettingsHandler）方法，初始化CrawlController实例。其中，传入的参数就是

在第一步里构造的XMLSettingsHandler实例。

(4 )当上述3步完成后，CrawlController就具备了运行的条件。此时，只需调用它的requestCrawlStart（）方法，就

可以启动线程池和Frontier，然后开始不断的抓取网页。

先来看看initilize（SettingsHandler）方法：

public void initialize(SettingsHandler sH)
 throws InitializationException {
 sendCrawlStateChangeEvent(PREPARING, CrawlJob.STATUS_PREPARING);

 this.singleThreadLock = new ReentrantLock();
 this.settingsHandler = sH;

 //从XMLSettingsHandler中取出Order
 this.order = settingsHandler.getOrder();

 this.order.setController(this);
 this.bigmaps = new Hashtable<String,CachedBdbMap<?,?>>();
 sExit = "";
 this.manifest = new StringBuffer();
 String onFailMessage = "";
 try {
 onFailMessage = "You must set the User-Agent and From HTTP" +
 " header values to acceptable strings. \n" +
 " User-Agent: [software-name](+[info-url])[misc]\n" +
 " From: [email-address]\n";

 //检查了用户设定的UserAgent等信息，看是否符合格式
 order.checkUserAgentAndFrom();

 onFailMessage = "Unable to setup disk";
 if (disk == null) {
 setupDisk(); //设定了开始抓取后保存文件信息的目录结构
 }

 onFailMessage = "Unable to create log file(s)";
 //初始化了日志信息的记录工具
 setupLogs();

 onFailMessage = "Unable to test/run checkpoint recover";
 this.checkpointRecover = getCheckpointRecover();
 if (this.checkpointRecover == null) {
 this.checkpointer =
 new Checkpointer(this, this.checkpointsDisk);
 } else {
 setupCheckpointRecover();
 }

 onFailMessage = "Unable to setup bdb environment.";

 //初始化使用Berkley DB的一些工具
 setupBdb();

 onFailMessage = "Unable to setup statistics";
 setupStatTracking();

 onFailMessage = "Unable to setup crawl modules";
 //初始化了Scope、Frontier以及ProcessorChain
 setupCrawlModules();
 } catch (Exception e) {
 String tmp = "On crawl: "
 + settingsHandler.getSettingsObject(null).getName() + " " +
 onFailMessage;
 LOGGER.log(Level.SEVERE, tmp, e);
 throw new InitializationException(tmp, e);
 }


 Lookup.getDefaultCache(DClass.IN).setMaxEntries(1);
 //dns.getRecords("localhost", Type.A, DClass.IN);

 //实例化线程池
 setupToePool();
 setThresholds();

 reserveMemory = new LinkedList<char[]>();
 for(int i = 1; i < RESERVE_BLOCKS; i++) {
 reserveMemory.add(new char[RESERVE_BLOCK_SIZE]);
 }
 }

可以看出在initilize()方法中主要做一些初始化工作,但这些对于Heritrix的运行是必需的.

再来看看CrawlController的核心,requestCrawlStart()方法：

public void requestCrawlStart() {
 //初始化处理器链
 runProcessorInitialTasks();

 sendCrawlStateChangeEvent(STARTED, CrawlJob.STATUS_PENDING);
 String jobState;
 state = RUNNING;
 jobState = CrawlJob.STATUS_RUNNING;
 sendCrawlStateChangeEvent(this.state, jobState);

 // A proper exit will change this value.
 this.sExit = CrawlJob.STATUS_FINISHED_ABNORMAL;

 Thread statLogger = new Thread(statistics);
 statLogger.setName("StatLogger");
 //开始日志线程
 statLogger.start();
 //启运Frontier，抓取工作开始
 frontier.start();
 }

可以看出，做了那么多工作，最终将启动Frontier的start方法，而Frontier将为线程池的线程提供URI，真正开始

抓取任务．至此，抓取任务开始.

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