Java 网络爬虫的基本实现

这是一个web搜索的基本程序，从命令行输入搜索条件（起始的URL、处理url的最大数、要搜索的字符串),
它就会逐个对Internet上的URL进行实时搜索,查找并输出匹配搜索条件的页面。 这个程序的原型来自《java编程艺术》，
为了更好的分析，站长去掉了其中的GUI部分，并稍作修改以适用jdk1.5。以这个程序为基础，可以写出在互联网上搜索
诸如图像、邮件、网页下载之类的“爬虫”。
先请看程序运行的过程：


D:/java>javac  SearchCrawler.java（编译）

D:/java>java   SearchCrawler http://127.0.0.1:8080/zz3zcwbwebhome/index.jsp 20 java

Start searching...
result:
searchString=java
http://127.0.0.1:8080/zz3zcwbwebhome/index.jsp
http://127.0.0.1:8080/zz3zcwbwebhome/reply.jsp
http://127.0.0.1:8080/zz3zcwbwebhome/learn.jsp
http://127.0.0.1:8080/zz3zcwbwebhome/download.jsp
http://127.0.0.1:8080/zz3zcwbwebhome/article.jsp
http://127.0.0.1:8080/zz3zcwbwebhome/myexample/jlGUIOverview.htm
http://127.0.0.1:8080/zz3zcwbwebhome/myexample/Proxooldoc/index.html
http://127.0.0.1:8080/zz3zcwbwebhome/view.jsp?id=301
http://127.0.0.1:8080/zz3zcwbwebhome/view.jsp?id=297
http://127.0.0.1:8080/zz3zcwbwebhome/view.jsp?id=291
http://127.0.0.1:8080/zz3zcwbwebhome/view.jsp?id=286
http://127.0.0.1:8080/zz3zcwbwebhome/view.jsp?id=285
http://127.0.0.1:8080/zz3zcwbwebhome/view.jsp?id=284
http://127.0.0.1:8080/zz3zcwbwebhome/view.jsp?id=276
http://127.0.0.1:8080/zz3zcwbwebhome/view.jsp?id=272  

又如：
D:/java>java    SearchCrawler http://www.sina.com  20 java
Start searching...
result:
searchString=java
http://sina.com
http://redirect.sina.com/WWW/sinaCN/www.sina.com.cn class=a2
http://redirect.sina.com/WWW/sinaCN/www.sina.com.cn class=a8
http://redirect.sina.com/WWW/sinaHK/www.sina.com.hk class=a2
http://redirect.sina.com/WWW/sinaTW/www.sina.com.tw class=a8
http://redirect.sina.com/WWW/sinaUS/home.sina.com class=a8
http://redirect.sina.com/WWW/smsCN/sms.sina.com.cn/ class=a2
http://redirect.sina.com/WWW/smsCN/sms.sina.com.cn/ class=a3
http://redirect.sina.com/WWW/sinaNet/www.sina.net/ class=a3


D:/java>
下面是这个程序的源码

import java.util.*;import java.net.*;import java.io.*;import java.util.regex.*;// 搜索Web爬行者public class SearchCrawler implements Runnable{ /* disallowListCache缓存robot不允许搜索的URL。 Robot协议在Web站点的根目录下设置一个robots.txt文件, *规定站点上的哪些页面是限制搜索的。 搜索程序应该在搜索过程中跳过这些区域,下面是robots.txt的一个例子: # robots.txt for http://somehost.com/ User-agent: * Disallow: /cgi-bin/ Disallow: /registration # /Disallow robots on registration page Disallow: /login */ private HashMap< String,ArrayList< String>> disallowListCache = new HashMap< String,ArrayList< String>>(); ArrayList< String> errorList= new ArrayList< String>();//错误信息 ArrayList< String> result=new ArrayList< String>(); //搜索到的结果 String startUrl;//开始搜索的起点 int maxUrl;//最大处理的url数 String searchString;//要搜索的字符串(英文) boolean caseSensitive=false;//是否区分大小写 boolean limitHost=false;//是否在限制的主机内搜索 public SearchCrawler(String startUrl,int maxUrl,String searchString){ this.startUrl=startUrl; this.maxUrl=maxUrl; this.searchString=searchString; } public ArrayList< String> getResult(){ return result; } public void run(){//启动搜索线程 crawl(startUrl,maxUrl, searchString,limitHost,caseSensitive); } //检测URL格式 private URL verifyUrl(String url) { // 只处理HTTP URLs. if (!url.toLowerCase().startsWith("http://")) return null; URL verifiedUrl = null; try { verifiedUrl = new URL(url); } catch (Exception e) { return null; } return verifiedUrl; } // 检测robot是否允许访问给出的URL. private boolean isRobotAllowed(URL urlToCheck) { String host = urlToCheck.getHost().toLowerCase();//获取给出RUL的主机 //System.out.println("主机="+host); // 获取主机不允许搜索的URL缓存 ArrayList< String> disallowList =disallowListCache.get(host); // 如果还没有缓存,下载并缓存。 if (disallowList == null) { disallowList = new ArrayList< String>(); try { URL robotsFileUrl =new URL("http://" + host + "/robots.txt"); BufferedReader reader =new BufferedReader(new InputStreamReader(robotsFileUrl.openStream())); // 读robot文件，创建不允许访问的路径列表。 String line; while ((line = reader.readLine()) != null) { if (line.indexOf("Disallow:") == 0) {//是否包含"Disallow:" String disallowPath =line.substring("Disallow:".length());//获取不允许访问路径 // 检查是否有注释。 int commentIndex = disallowPath.indexOf("#"); if (commentIndex != - 1) { disallowPath =disallowPath.substring(0, commentIndex);//去掉注释 } disallowPath = disallowPath.trim(); disallowList.add(disallowPath); } } // 缓存此主机不允许访问的路径。 disallowListCache.put(host, disallowList); } catch (Exception e) { return true; //web站点根目录下没有robots.txt文件,返回真 } } String file = urlToCheck.getFile(); //System.out.println("文件getFile()="+file); for (int i = 0; i < disallowList.size(); i++) { String disallow = disallowList.get(i); if (file.startsWith(disallow)) { return false; } } return true; } private String downloadPage(URL pageUrl) { try { // Open connection to URL for reading. BufferedReader reader = new BufferedReader(new InputStreamReader(pageUrl.openStream())); // Read page into buffer. String line; StringBuffer pageBuffer = new StringBuffer(); while ((line = reader.readLine()) != null) { pageBuffer.append(line); } return pageBuffer.toString(); } catch (Exception e) { } return null; } // 从URL中去掉"www" private String removeWwwFromUrl(String url) { int index = url.indexOf("://www."); if (index != -1) { return url.substring(0, index + 3) + url.substring(index + 7); } return (url); } // 解析页面并找出链接 private ArrayList< String> retrieveLinks(URL pageUrl, String pageContents, HashSet crawledList, boolean limitHost) { // 用正则表达式编译链接的匹配模式。 Pattern p =Pattern.compile("<a//s+href//s*=//s*/"?(.*?)[/"|>]",Pattern.CASE_INSENSITIVE); Matcher m = p.matcher(pageContents); ArrayList< String> linkList = new ArrayList< String>(); while (m.find()) { String link = m.group(1).trim(); if (link.length() < 1) { continue; } // 跳过链到本页面内链接。 if (link.charAt(0) == '#') { continue; } if (link.indexOf("mailto:") != -1) { continue; } if (link.toLowerCase().indexOf("javascript") != -1) { continue; } if (link.indexOf("://") == -1){ if (link.charAt(0) == '/') {//处理绝对地 link = "http://" + pageUrl.getHost()+":"+pageUrl.getPort()+ link; } else { String file = pageUrl.getFile(); if (file.indexOf('/') == -1) {//处理相对地址 link = "http://" + pageUrl.getHost()+":"+pageUrl.getPort() + "/" + link; } else { String path =file.substring(0, file.lastIndexOf('/') + 1); link = "http://" + pageUrl.getHost() +":"+pageUrl.getPort()+ path + link; } } } int index = link.indexOf('#'); if (index != -1) { link = link.substring(0, index); } link = removeWwwFromUrl(link); URL verifiedLink = verifyUrl(link); if (verifiedLink == null) { continue; } /* 如果限定主机，排除那些不合条件的URL*/ if (limitHost && !pageUrl.getHost().toLowerCase().equals( verifiedLink.getHost().toLowerCase())) { continue; } // 跳过那些已经处理的链接. if (crawledList.contains(link)) { continue; } linkList.add(link); } return (linkList); } // 搜索下载Web页面的内容，判断在该页面内有没有指定的搜索字符串 private boolean searchStringMatches(String pageContents, String searchString, boolean caseSensitive){ String searchContents = pageContents; if (!caseSensitive) {//如果不区分大小写 searchContents = pageContents.toLowerCase(); } Pattern p = Pattern.compile("[//s]+"); String[] terms = p.split(searchString); for (int i = 0; i < terms.length; i++) { if (caseSensitive) { if (searchContents.indexOf(terms[i]) == -1) { return false; } } else { if (searchContents.indexOf(terms[i].toLowerCase()) == -1) { return false; } } } return true; } //执行实际的搜索操作 public ArrayList< String> crawl(String startUrl, int maxUrls, String searchString,boolean limithost,boolean caseSensitive ) { System.out.println("searchString="+searchString); HashSet< String> crawledList = new HashSet< String>(); LinkedHashSet< String> toCrawlList = new LinkedHashSet< String>(); if (maxUrls < 1) { errorList.add("Invalid Max URLs value."); System.out.println("Invalid Max URLs value."); } if (searchString.length() < 1) { errorList.add("Missing Search String."); System.out.println("Missing search String"); } if (errorList.size() > 0) { System.out.println("err!!!"); return errorList; } // 从开始URL中移出www startUrl = removeWwwFromUrl(startUrl); toCrawlList.add(startUrl); while (toCrawlList.size() > 0) { if (maxUrls != -1) { if (crawledList.size() == maxUrls) { break; } } // Get URL at bottom of the list. String url = toCrawlList.iterator().next(); // Remove URL from the to crawl list. toCrawlList.remove(url); // Convert string url to URL object. URL verifiedUrl = verifyUrl(url); // Skip URL if robots are not allowed to access it. if (!isRobotAllowed(verifiedUrl)) { continue; } // 增加已处理的URL到crawledList crawledList.add(url); String pageContents = downloadPage(verifiedUrl); if (pageContents != null && pageContents.length() > 0){ // 从页面中获取有效的链接 ArrayList< String> links =retrieveLinks(verifiedUrl, pageContents, crawledList,limitHost); toCrawlList.addAll(links); if (searchStringMatches(pageContents, searchString,caseSensitive)) { result.add(url); System.out.println(url); } } } return result; } // 主函数 public static void main(String[] args) { if(args.length!=3){ System.out.println("Usage:java SearchCrawler startUrl maxUrl searchString"); return; } int max=Integer.parseInt(args[1]); SearchCrawler crawler = new SearchCrawler(args[0],max,args[2]); Thread search=new Thread(crawler); System.out.println("Start searching..."); System.out.println("result:"); search.start(); }}