如何用Java写一个爬虫

如何用Java写一个爬虫

原理即是保存cookie数据保存登陆后的cookie.以后每次抓取页面把cookie在头部信息里面发送过去。系统是根据cookie来判断用户的。有了cookie就有了登录状态，以后的访问都是基于这个cookie对应的用户的。补充：Java是一种可以撰写跨平台应用软件的面向对象的程序设计语言。Java技术具有卓越的通用性、高效性、平台移植性和安全性，广泛应用于PC、数据中心、游戏控制台、科学超级计算机、移动电话和互联网，同时拥有全球最大的开发者专业社群。

以下是一个使用java实现的简单爬虫核心代码：　　 public void crawl() throws Throwable { while (continueCrawling()) { CrawlerUrl url = getNextUrl(); //获取待爬取队列中的下一个URL if (url != null) { printCrawlInfo(); String content = getContent(url); //获取URL的文本信息 //聚焦爬虫只爬取与主题内容相关的网页，这里采用正则匹配简单处理 if (isContentRelevant(content, this.regexpSearchPattern)) { saveContent(url, content); //保存网页至本地 //获取网页内容中的链接，并放入待爬取队列中 Collection urlStrings = extractUrls(content, url); addUrlsToUrlQueue(url, urlStrings); } else { System.out.println(url + " is not relevant ignoring ..."); } //延时防止被对方屏蔽 Thread.sleep(this.delayBetweenUrls); } } closeOutputStream(); } private CrawlerUrl getNextUrl() throws Throwable { CrawlerUrl nextUrl = null; while ((nextUrl == null) && (!urlQueue.isEmpty())) { CrawlerUrl crawlerUrl = this.urlQueue.remove(); //doWeHavePermissionToVisit：是否有权限访问该URL，友好的爬虫会根据网站提供的"Robot.txt"中配置的规则进行爬取 //isUrlAlreadyVisited：URL是否访问过，大型的搜索引擎往往采用BloomFilter进行排重，这里简单使用HashMap //isDepthAcceptable：是否达到指定的深度上限。爬虫一般采取广度优先的方式。一些网站会构建爬虫陷阱（自动生成一些无效链接使爬虫陷入死循环），采用深度限制加以避免 if (doWeHavePermissionToVisit(crawlerUrl) && (!isUrlAlreadyVisited(crawlerUrl)) && isDepthAcceptable(crawlerUrl)) { nextUrl = crawlerUrl; // System.out.println("Next url to be visited is " + nextUrl); } } return nextUrl; } private String getContent(CrawlerUrl url) throws Throwable { //HttpClient4.1的调用与之前的方式不同 HttpClient client = new DefaultHttpClient(); HttpGet httpGet = new HttpGet(url.getUrlString()); StringBuffer strBuf = new StringBuffer(); HttpResponse response = client.execute(httpGet); if (HttpStatus.SC_OK == response.getStatusLine().getStatusCode()) { HttpEntity entity = response.getEntity(); if (entity != null) { BufferedReader reader = new BufferedReader( new InputStreamReader(entity.getContent(), "UTF-8")); String line = null; if (entity.getContentLength() > 0) { strBuf = new StringBuffer((int) entity.getContentLength()); while ((line = reader.readLine()) != null) { strBuf.append(line); } } } if (entity != null) { nsumeContent(); } } //将url标记为已访问 markUrlAsVisited(url); return strBuf.toString(); } public static boolean isContentRelevant(String content, Pattern regexpPattern) { boolean retValue = false; if (content != null) { //是否符合正则表达式的条件 Matcher m = regexpPattern.matcher(content.toLowerCase()); retValue = m.find(); } return retValue; } public List extractUrls(String text, CrawlerUrl crawlerUrl) { Map urlMap = new HashMap(); extractHttpUrls(urlMap, text); extractRelativeUrls(urlMap, text, crawlerUrl); return new ArrayList(urlMap.keySet()); } private void extractHttpUrls(Map urlMap, String text) { Matcher m = (text); while (m.find()) { String url = m.group(); String[] terms = url.split("a href=\""); for (String term : terms) { // System.out.println("Term = " + term); if (term.startsWith("http")) { int index = term.indexOf("\""); if (index > 0) { term = term.substring(0, index); } urlMap.put(term, term); System.out.println("Hyperlink: " + term); } } } } private void extractRelativeUrls(Map urlMap, String text, CrawlerUrl crawlerUrl) { Matcher m = relativeRegexp.matcher(text); URL textURL = crawlerUrl.getURL(); String host = textURL.getHost(); while (m.find()) { String url = m.group(); String[] terms = url.split("a href=\""); for (String term : terms) { if (term.startsWith("/")) { int index = term.indexOf("\""); if (index > 0) { term = term.substring(0, index); } String s = //" + host + term; urlMap.put(s, s); System.out.println("Relative url: " + s); } } } } public static void main(String[] args) { try { String url = ""; Queue urlQueue = new LinkedList(); String regexp = "java"; urlQueue.add(new CrawlerUrl(url, 0)); NaiveCrawler crawler = new NaiveCrawler(urlQueue, 100, 5, 1000L, regexp); // boolean allowCrawl = crawler.areWeAllowedToVisit(url); // System.out.println("Allowed to crawl: " + url + " " + // allowCrawl); crawler.crawl(); } catch (Throwable t) { System.out.println(t.toString()); t.printStackTrace(); } }

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