以下是一个Java爬虫程序，它能从指定主页开始，按照指定的深度抓取该站点域名下的网页并维护简单索引。

参数：private static int webDepth = 2;//爬虫深度。主页的深度为1，设置深度后超过该深度的网页不会抓取。

        private int intThreadNum = 10;//线程数。开启的线程数。

抓取时也会在程序源文件目录下生成一个report.txt文件记录爬虫的运行情况，并在抓取结束后生成一个fileindex.txt文件维护网页文件索引。

本程序用到了多线程(静态变量和同步)，泛型，文件操作，URL类和连接，Hashtable类关联数组，正则表达式及其相关类。运行时需使用命令行参数，第一个参数应使用http://开头的有效URL字符串作为爬虫的主页，第二个参数（可选）应输入可转换为int型的字符串（用Integer.parseInt(String s)静态方法可以转换的字符串，如3）作为爬虫深度，如果没有，则默认深度为2。

本程序的不足之处是：只考虑了href= href=' href="后加绝对url的这三种情况(由于url地址在网页源文件中情况比较复杂，有时处理也会出现错误)，还有相对url和window.open('的情况没有考虑。异常处理程序也只是简单处理。如果读者有改进办法可以把源代码帖出，不胜感激。

附上源代码如下（保存名为GetWeb.java）：

import java.io.File;
import java.io.BufferedReader;
import java.io.FileOutputStream;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.net.URL;
import java.net.URLConnection;
import java.util.ArrayList;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
import java.util.Hashtable;

public class GetWeb {
 private int webDepth = 2;//爬虫深度
 private int intThreadNum = 10;//线程数
 private String strHomePage = "";//主页地址
 private String myDomain;//域名
 private String fPath = "web";//储存网页文件的目录名
 private ArrayList<String> arrUrls = new ArrayList<String>();//存储未处理URL
 private ArrayList<String> arrUrl = new ArrayList<String>();//存储所有URL供建立索引
 private Hashtable<String,Integer> allUrls = new Hashtable<String,Integer>();//存储所有URL的网页号
 private Hashtable<String,Integer> deepUrls = new Hashtable<String,Integer>();//存储所有URL深度
 private int intWebIndex = 0;//网页对应文件下标，从0开始
 private String charset = "GB2312";
 private String report = "";
 private long startTime;
 private int webSuccessed = 0;
 private int webFailed = 0;
 
 
 public GetWeb(String s)
 {
  this.strHomePage = s;
 }
 public GetWeb(String s,int i)
 {
  this.strHomePage = s;
  this.webDepth = i;
 }
 
 public synchronized void addWebSuccessed()
 {
  webSuccessed++;
 }
 public synchronized void addWebFailed()
 {
  webFailed++;
 }
 public synchronized void addReport(String s)
 {
  try
  {
   report += s;
   PrintWriter pwReport = new PrintWriter(new FileOutputStream("report.txt"));
   pwReport.println(report);
   pwReport.close();
  }
  catch(Exception e)
  {
   System.out.println("生成报告文件失败!");
  }
 }
 public synchronized String getAUrl()
 {
  String tmpAUrl = arrUrls.get(0);
  arrUrls.remove(0);
  return tmpAUrl;
 }
 public synchronized String getUrl()
 {
  String tmpUrl = arrUrl.get(0);
  arrUrl.remove(0);
  return tmpUrl;
 }
 public synchronized Integer getIntWebIndex()
 {
  intWebIndex++;
  return intWebIndex;
 }
 /**
  * @param args
  */
 public static void main(String[] args)
 {
  if (args.length == 0 || args[0].equals(""))
  {
   System.out.println("No input!");
   System.exit(1);
  }
  else if(args.length == 1)
  {
   GetWeb gw = new GetWeb(args[0]);
   gw.getWebByHomePage();
  }
  else 
  {
   GetWeb gw = new GetWeb(args[0],Integer.parseInt(args[1]));
   gw.getWebByHomePage();
  }
 }
 public void getWebByHomePage()
 {
  startTime = System.currentTimeMillis();
  this.myDomain = getDomain();
  if (myDomain == null)
  {
   System.out.println("Wrong input!");
   //System.exit(1);
   return;
  }
  System.out.println("Homepage = " + strHomePage);
  addReport("Homepage = " + strHomePage + "!\n");
  System.out.println("Domain = " + myDomain);
  addReport("Domain = " + myDomain + "!\n");
  arrUrls.add(strHomePage);
  arrUrl.add(strHomePage);
  allUrls.put(strHomePage,0);
  deepUrls.put(strHomePage,1);
  File fDir = new File(fPath);
        if(!fDir.exists())
        {
         fDir.mkdir();
        }
  System.out.println("Start!");
  this.addReport("Start!\n");
  String tmp = getAUrl();
  this.getWebByUrl(tmp,charset,allUrls.get(tmp)+"");
  int i = 0;
  for (i=0;i<intThreadNum;i++)
  {
   new Thread(new Processer(this)).start();
  }
  while (true)
  {
   if(arrUrls.isEmpty() && Thread.activeCount() == 1)
   {
    long finishTime = System.currentTimeMillis();
    long costTime = finishTime-startTime;
    System.out.println("\n\n\n\n\nFinished!");
    addReport("\n\n\n\n\nFinished!\n");
    System.out.println("Start time = " + startTime + "   " + "Finish time = " + finishTime + "   " + "Cost time = " + costTime + "ms");
    addReport("Start time = " + startTime + "   " + "Finish time = " + finishTime + "   " + "Cost time = " + costTime + "ms" + "\n");
    System.out.println("Total url number = " + (webSuccessed+webFailed) + "   Successed: " + webSuccessed + "   Failed: " + webFailed);
    addReport("Total url number = " + (webSuccessed+webFailed) + "   Successed: " + webSuccessed + "   Failed: " + webFailed + "\n");
    
    String strIndex = "";
    String tmpUrl = "";
    while (!arrUrl.isEmpty())
    {
     tmpUrl = getUrl();
     strIndex += "Web depth:" + deepUrls.get(tmpUrl) + "   Filepath: " + fPath + "/web" + allUrls.get(tmpUrl) + ".htm" + "   url:" + tmpUrl + "\n\n";
    }
    System.out.println(strIndex);
    try
    {
     PrintWriter pwIndex = new PrintWriter(new FileOutputStream("fileindex.txt"));
     pwIndex.println(strIndex);
     pwIndex.close();
    }
    catch(Exception e)
    {
     System.out.println("生成索引文件失败!");
    }
    break;
   }
  }
 }
 public void getWebByUrl(String strUrl,String charset,String fileIndex)
 {
  try
  {
   //if(charset==null||"".equals(charset))charset="utf-8";
   System.out.println("Getting web by url: " + strUrl);
   addReport("Getting web by url: " + strUrl + "\n");
   URL url = new URL(strUrl);
   URLConnection conn = url.openConnection();
   conn.setDoOutput(true);
   InputStream is = null;
   is = url.openStream();
   
   String filePath = fPath + "/web" + fileIndex + ".htm";
   PrintWriter pw = null;
   FileOutputStream fos = new FileOutputStream(filePath);
   OutputStreamWriter writer = new OutputStreamWriter(fos);
   pw = new PrintWriter(writer);
   BufferedReader bReader = new BufferedReader(new InputStreamReader(is));
   StringBuffer sb = new StringBuffer();
   String rLine = null;
   String tmp_rLine = null;
   while ( (rLine = bReader.readLine()) != null)
   {
    tmp_rLine = rLine;
    int str_len = tmp_rLine.length();
    if (str_len > 0)
    {
     sb.append("\n" + tmp_rLine);
     pw.println(tmp_rLine);
     pw.flush();
     if (deepUrls.get(strUrl) < webDepth)getUrlByString(tmp_rLine,strUrl);
    }
    tmp_rLine = null;
   }
   is.close();
   pw.close();
   System.out.println("Get web successfully! " + strUrl);
   addReport("Get web successfully! " + strUrl + "\n");
   addWebSuccessed();
  }
  catch (Exception e)
  {
   System.out.println("Get web failed!       " + strUrl);
   addReport("Get web failed!       " + strUrl + "\n");
   addWebFailed();
  }
 }
 public String getDomain()
 {
  String reg = "(?<=http\\://[a-zA-Z0-9]{0,100}[.]{0,1})[^.\\s]*?\\.(com|cn|net|org|biz|info|cc|tv)";
  Pattern p = Pattern.compile(reg,Pattern.CASE_INSENSITIVE);
  Matcher m = p.matcher(strHomePage);
  boolean blnp = m.find();
  if (blnp == true)
  {
   return m.group(0);
  }
  return null;
 }
 public void getUrlByString(String inputArgs,String strUrl)
 {
  String tmpStr = inputArgs;
  String regUrl = "(?<=(href=)[\"]?[\']?)[http://][^\\s\"\'\\?]*(" + myDomain + ")[^\\s\"\'>]*";
  Pattern p = Pattern.compile(regUrl,Pattern.CASE_INSENSITIVE);
  Matcher m = p.matcher(tmpStr);
  boolean blnp = m.find();
  //int i = 0;
  while (blnp == true)
  {
   if (!allUrls.containsKey(m.group(0)))
   {
    System.out.println("Find a new url,depth:" + (deepUrls.get(strUrl)+1) + " "+ m.group(0));
    addReport("Find a new url,depth:" + (deepUrls.get(strUrl)+1) + " "+ m.group(0) + "\n");
    arrUrls.add(m.group(0));
    arrUrl.add(m.group(0));
    allUrls.put(m.group(0),getIntWebIndex());
    deepUrls.put(m.group(0),(deepUrls.get(strUrl)+1));
   }
   tmpStr = tmpStr.substring(m.end(),tmpStr.length());
   m = p.matcher(tmpStr);
   blnp = m.find();
  }
 }
 
 class Processer implements Runnable
 {
     GetWeb gw;
     public Processer(GetWeb g)
     {
         this.gw = g;
     }
     public void run()
     {
      //Thread.sleep(5000);
      while (!arrUrls.isEmpty())
      {
       String tmp = getAUrl();
       getWebByUrl(tmp,charset,allUrls.get(tmp)+"");
      }
     }
 }
}