根据java网络编程相关的内容，使用jdk提供的相关类可以得到url对应网页的html页面代码。
针对得到的html代码，通过使用正则表达式即可得到我们想要的内容。
比如，我们如果想得到一个网页上所有包括“java”关键字的文本内容，就可以逐行对网页代码进行正则表达式的匹配。最后达到去除html标签和不相关的内容，只得到包括“java”这个关键字的内容的效果。
从网页上爬取图片的流程和爬取内容的流程基本相同，但是爬取图片的步骤会多一步。
需要先用img标签的正则表达式匹配获取到img标签，再用src属性的正则表达式获取这个img标签中的src属性的图片url，然后再通过缓冲输入流对象读取到这个图片url的图片信息，配合文件输出流将读到的图片信息写入到本地即可。
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()
}
}
