這里是維基百科對網(wǎng)絡(luò)爬蟲的詞條頁面。網(wǎng)絡(luò)爬蟲以叫網(wǎng)絡(luò)蜘蛛，網(wǎng)絡(luò)機(jī)器人，這是一個(gè)程序，其會自動(dòng)的通過網(wǎng)絡(luò)抓取互聯(lián)網(wǎng)上的網(wǎng)頁，這種技術(shù)一般可能用來檢查你的站點(diǎn)上所有的鏈接是否是都是有效的。當(dāng)然，更為高級的技術(shù)是把網(wǎng)頁中的相關(guān)數(shù)據(jù)保存下來，可以成為搜索引擎。

從技相來說，實(shí)現(xiàn)抓取網(wǎng)頁可能并不是一件很困難的事情，困難的事情是對網(wǎng)頁的分析和整理，那是一件需要有輕量智能，需要大量數(shù)學(xué)計(jì)算的程序才能做的事情。下面一個(gè)簡單的流程：

在這里，我們只是說一下如何寫一個(gè)網(wǎng)頁抓取程序。

首先我們先看一下，如何使用命令行的方式來找開網(wǎng)頁。

telnet somesite.com 80
GET /index.html HTTP/1.0
按回車兩次

使用telnet就是告訴你其實(shí)這是一個(gè)socket的技術(shù)，并且使用HTTP的協(xié)議，如 GET方法來獲得網(wǎng)頁，當(dāng)然，接下來的事你就需要解析HTML文法，甚至還需要解析Javascript，因?yàn)楝F(xiàn)在的網(wǎng)頁使用Ajax的越來越多了，而很多網(wǎng)頁內(nèi)容都是通過Ajax技術(shù)加載的，因?yàn)椋皇呛唵蔚亟馕鯤TML文件在未來會遠(yuǎn)遠(yuǎn)不夠。當(dāng)然，在這里，只是展示一個(gè)非常簡單的抓取，簡單到只能做為一個(gè)例子，下面這個(gè)示例的偽代碼：

取網(wǎng)頁
for each 鏈接 in 當(dāng)前網(wǎng)頁所有的鏈接
{
if(如果本鏈接是我們想要的 || 這個(gè)鏈接從未訪問過)
{
處理對本鏈接
把本鏈接設(shè)置為已訪問
}
}

require “rubygems”
require “mechanize”
class Crawler < WWW::Mechanize
attr_accessor :callback
INDEX = 0
DOWNLOAD = 1
PASS = 2
def initialize
super
init
@first = true
self.user_agent_alias = “Windows IE 6″
end
def init
@visited = []
end
def remember(link)
@visited << link
end
def perform_index(link)
self.get(link)
if(self.page.class.to_s == “WWW::Mechanize::Page”)
links = self.page.links.map {|link| link.href } - @visited
links.each do |alink|
start(alink)
end
end
end
def start(link)
return if link.nil?
if(!@visited.include?(link))
action = @callback.call(link)
if(@first)
@first = false
perform_index(link)
end
case action
when INDEX
perform_index(link)
when DOWNLOAD
self.get(link).save_as(File.basename(link))
when PASS
puts “passing on #{link}”
end
end
end
def get(site)
begin
puts “getting #{site}”
@visited << site
super(site)
rescue
puts “error getting #{site}”
end
end
end

上面的代碼就不必多說了，大家可以去試試。下面是如何使用上面的代碼：

require “crawler”
x = Crawler.new
callback = lambda do |link|
if(link =~/\\.(zip|rar|gz|pdf|doc)
x.remember(link)
return Crawler::PASS
elsif(link =~/\\.(jpg|jpeg)/)
return Crawler::DOWNLOAD
end
return Crawler::INDEX;
end
x.callback = callback
x.start(”http://somesite.com”)

下面是一些和網(wǎng)絡(luò)爬蟲相關(guān)的開源網(wǎng)絡(luò)項(xiàng)目

• arachnode.net is a .NET crawler written in C# using SQL 2005 and Lucene and is released under the GNU General Public License.
• DataparkSearch is a crawler and search engine released under the GNU General Public License.
• GNU Wget is a command-line-operated crawler written in C and released under the GPL. It is typically used to mirror Web and FTP sites.
• GRUB is an open source distributed search crawler that Wikia Search ( http://wikiasearch.com ) uses to crawl the web.
• Heritrix is the Internet Archive’s archival-quality crawler, designed for archiving periodic snapshots of a large portion of the Web. It was written in Java.
• ht://Dig includes a Web crawler in its indexing engine.
• HTTrack uses a Web crawler to create a mirror of a web site for off-line viewing. It is written in C and released under the GPL.
• ICDL Crawler is a cross-platform web crawler written in C++ and intended to crawl Web sites based on
  

from:
http://coolshell.cn/?p=27