基于crawler4j、jsoup、javacsv的爬虫实践

1. crawler4j基础

crawler4j是一个基于Java的爬虫开源项目，其官方地址如下：

http://code.google.com/p/crawler4j/

crawler4j的使用主要分为两个步骤：

1. 实现一个继承自WebCrawler的爬虫类；
2. 通过CrawlController调用实现的爬虫类。

WebCrawler是一个抽象类，继承它必须实现两个方法：shouldVisit和visit。其中：

• shouldVisit是判断当前的URL是否已经应该被爬取（访问）；
• visit则是爬取该URL所指向的页面的数据，其传入的参数即是对该web页面全部数据的封装对象Page。

另外，WebCrawler还有其它一些方法可供覆盖，其方法的命名规则类似于Android的命名规则。如getMyLocalData方法可以返回WebCrawler中的数据；onBeforeExit方法会在该WebCrawler运行结束前被调用，可以执行一些资源释放之类的工作。

相对而言，CrawlController的调用就比较格式化了。一般地，它的调用代码如下：

            String crawlStorageFolder = "data/crawl/root";
            int numberOfCrawlers = 7;

            CrawlConfig config = new CrawlConfig();
            config.setCrawlStorageFolder(crawlStorageFolder);

            /*
             * Instantiate the controller for this crawl.
             */
            PageFetcher pageFetcher = new PageFetcher(config);
            RobotstxtConfig robotstxtConfig = new RobotstxtConfig();
            RobotstxtServer robotstxtServer = new RobotstxtServer(robotstxtConfig, pageFetcher);
            CrawlController controller = new CrawlController(config, pageFetcher, robotstxtServer);

            /*
             * For each crawl, you need to add some seed urls. These are the first
             * URLs that are fetched and then the crawler starts following links
             * which are found in these pages
             */
            controller.addSeed("http://www.ics.uci.edu/~welling/");
            controller.addSeed("http://www.ics.uci.edu/~lopes/");
            controller.addSeed("http://www.ics.uci.edu/");

            /*
             * Start the crawl. This is a blocking operation, meaning that your code
             * will reach the line after this only when crawling is finished.
             */
            controller.start(MyCrawler.class, numberOfCrawlers);   

CrawlController自带多线程功能，start方法中的第二个参数numberOfCrawlers即是同时开启的线程数。

另外，由于CrawlController对WebCrawler特殊的调用方式——反射（上述代码最后一行），因此WebCrawler的实现类必须拥有无参的构造方法，且有参的构造方法不会生效。对WebCrawler的实现类的私有成员的赋值需要通过静态方法来实现，示例参见crawler4j提供的例子：Image Crawler

更多信息请参见crawler4j的代码和示例。

2. jsoup基础

jsoup是一个基于Java的开源HTML解析器，其官网地址如下：

http://jsoup.org/

jsoup最大的特点，或者说，它比使用DOM4J进行HTML解析更好的原因，是它可以采用jQuery选择器的语法。

例如：

Document doc = Jsoup.connect("http://en.wikipedia.org/").get();
Elements newsHeadlines = doc.select("#mp-itn b a");

上述代码就是获取了http://en.wikipedia.org/页面中id为mp-itn的元素下的标签中的标签，与jQuery选择器的结果一致。

更多jsoup的使用方法请参见jsoup的示例（在主页的右侧Cookbook Content中）。

需要特别说明的是，jsoup中主要有三种操作对象：Document、Elements及Element。其中：

• Document继承自Element类，它包含页面中的全部数据，通过Jsoup类的静态方法获得；
• Elements是Element的集合类；
• Element是页面元素的实体类，包含了诸多操作页面元素的方法，其中除了类似于jQuery选择器的select方法外，还有大量类似于JS和jQuery的对DOM元素进行操作的方法，如getElementById，text，addClass，等等。
  

3. javacsv基础

javacsv是一个基于Java的开源CSV文件读写工具，其官方地址如下：

http://www.csvreader.com/java_csv.php

CSV文件的读写其实很简单，可以自己实现，网上也有诸多示例。使用javacsv的原因在于其代码简洁易用。

javacsv的使用示例参见其官方示例：

http://www.csvreader.com/java_csv_samples.php

需要说明的是，读写CSV文件时，若存在中文，请尽量使用FileReader（读）及FileWriter（写），而非FileInputStream和FileOutputStream，以免出现乱码。

4. 爬虫实践

下面的实践的目标是爬取搜车网的全部二手车信息，并作为CSV文件输出。代码如下：

Maven pom.xml

		
			edu.uci.ics
			crawler4j
			3.5
			jar
			compile
		

		
			org.jsoup
			jsoup
			1.7.3
		

		
			net.sourceforge.javacsv
			javacsv
			2.0
		

MyCrawler.java

import java.io.File;
import java.io.FileWriter;
import java.io.IOException;
import java.util.regex.Pattern;

import org.jsoup.Jsoup;
import org.jsoup.nodes.Document;
import org.jsoup.nodes.Element;
import org.jsoup.select.Elements;

import com.csvreader.CsvWriter;

import edu.uci.ics.crawler4j.crawler.Page;
import edu.uci.ics.crawler4j.crawler.WebCrawler;
import edu.uci.ics.crawler4j.parser.HtmlParseData;
import edu.uci.ics.crawler4j.url.WebURL;

public class MyCrawler extends WebCrawler {

	private final static Pattern FILTERS = Pattern
			.compile(".*(\\.(css|js|bmp|gif|jpe?g|ico"
					+ "|png|tiff?|mid|mp2|mp3|mp4"
					+ "|wav|avi|mov|mpeg|ram|m4v|pdf"
					+ "|rm|smil|wmv|swf|wma|zip|rar|gz))$");

	private final static String URL_PREFIX = "http://www.souche.com/pages/onsale/sale_car_list.html?";
	private final static Pattern URL_PARAMS_PATTERN = Pattern
			.compile("carbrand=brand-\\d+(&index=\\d+)?");

	private final static String CSV_PATH = "data/crawl/data.csv";
	private CsvWriter cw;
	private File csv;

	public MyCrawler() throws IOException {
		csv = new File(CSV_PATH);

		if (csv.isFile()) {
			csv.delete();
		}

		cw = new CsvWriter(new FileWriter(csv, true), ',');
		cw.write("title");
		cw.write("brand");
		cw.write("newPrice");
		cw.write("oldPrice");
		cw.write("mileage");
		cw.write("age");
		cw.write("stage");
		cw.endRecord();
		cw.close();
	}

	/**
	 * You should implement this function to specify whether the given url
	 * should be crawled or not (based on your crawling logic).
	 */
	@Override
	public boolean shouldVisit(WebURL url) {
		String href = url.getURL().toLowerCase();
		if (FILTERS.matcher(href).matches() || !href.startsWith(URL_PREFIX)) {
			return false;
		}

		String[] strs = href.split("\\?");
		if (strs.length < 2) {
			return false;
		}

		if (!URL_PARAMS_PATTERN.matcher(strs[1]).matches()) {
			return false;
		}
		
		return true;
	}

	/**
	 * This function is called when a page is fetched and ready to be processed
	 * by your program.
	 */
	@Override
	public void visit(Page page) {
		String url = page.getWebURL().getURL();

		if (page.getParseData() instanceof HtmlParseData) {
			HtmlParseData htmlParseData = (HtmlParseData) page.getParseData();
			String html = htmlParseData.getHtml();

			Document doc = Jsoup.parse(html);
			String brand = doc.select("div.choose_item").first().text();

			Elements contents = doc.select("div.list_content");
			
			if (contents.size() == 20 && !url.contains("index=")) {
				return;
			} else {
				System.out.println("URL: " + url);
			}
			
			for (Element c : contents) {
				Element info = c.select(".list_content_carInfo").first();
				String title = info.select("h1").first().text();

				Elements prices = info.select(".list_content_price div");
				String newPrice = prices.get(0).text();
				String oldPrice = prices.get(1).text();

				Elements others = info.select(".list_content_other div");
				String mileage = others.get(0).select("ins").first().text();
				String age = others.get(1).select("ins").first().text();

				String stage = "unknown";
				if (c.select("i.car_tag_zhijian").size() != 0) {
					stage = c.select("i.car_tag_zhijian").text();
				} else if (c.select("i.car_tag_yushou").size() != 0) {
					stage = "presell";
				}

				try {
					cw = new CsvWriter(new FileWriter(csv, true), ',');
					cw.write(title);
					cw.write(brand);
					cw.write(newPrice.replaceAll("[￥万]", ""));
					cw.write(oldPrice.replaceAll("[￥万]", ""));
					cw.write(mileage);
					cw.write(age);
					cw.write(stage);
					cw.endRecord();
					cw.close();
				} catch (IOException e) {
					e.printStackTrace();
				}
			}
		}
	}
}

Controller.java

import edu.uci.ics.crawler4j.crawler.CrawlConfig;
import edu.uci.ics.crawler4j.crawler.CrawlController;
import edu.uci.ics.crawler4j.fetcher.PageFetcher;
import edu.uci.ics.crawler4j.robotstxt.RobotstxtConfig;
import edu.uci.ics.crawler4j.robotstxt.RobotstxtServer;

public class Controller {
    public static void main(String[] args) throws Exception {
            String crawlStorageFolder = "data/crawl/root";
            int numberOfCrawlers = 7;

            CrawlConfig config = new CrawlConfig();
            config.setCrawlStorageFolder(crawlStorageFolder);

            /*
             * Instantiate the controller for this crawl.
             */
            PageFetcher pageFetcher = new PageFetcher(config);
            RobotstxtConfig robotstxtConfig = new RobotstxtConfig();
            RobotstxtServer robotstxtServer = new RobotstxtServer(robotstxtConfig, pageFetcher);
            CrawlController controller = new CrawlController(config, pageFetcher, robotstxtServer);

            /*
             * For each crawl, you need to add some seed urls. These are the first
             * URLs that are fetched and then the crawler starts following links
             * which are found in these pages
             */
            controller.addSeed("http://www.ics.uci.edu/~welling/");
            controller.addSeed("http://www.ics.uci.edu/~lopes/");
            controller.addSeed("http://www.ics.uci.edu/");

            /*
             * Start the crawl. This is a blocking operation, meaning that your code
             * will reach the line after this only when crawling is finished.
             */
            controller.start(MyCrawler.class, numberOfCrawlers);    
    }
}