Implementing a web crawler using Crawler4j in Java
Crawler4j is an open-source Java based web crawler framework that provides a simple and efficient way to quickly develop web crawler applications. Here are some advantages and disadvantages of this framework.
Advantages:
1. Highly configurable: Crawler4j provides various configuration options to flexibly define the logic and rules for crawling pages, allowing developers to customize development according to their own needs.
2. Multi threading support: Crawler4j uses a multi threading mechanism to crawl pages in parallel, improving crawling efficiency and speed.
3. Low memory usage: Crawler4j adopts a memory based URL queue and page caching mechanism, which can effectively manage memory and reduce memory usage.
4. Highly scalable: Crawler4j provides a rich plugin mechanism that allows for easy extension and customization of one's own crawler applications.
Disadvantages:
1. Lack of comprehensive documentation and examples: Crawler4j has relatively few official documents and examples, which may pose some learning difficulties for beginners to use.
The following is a sample Java code for implementing a simple web crawler using Crawler4j.
Firstly, the following dependencies need to be added to the pom.xml file of the Maven project:
<dependencies>
<dependency>
<groupId>edu.uci.ics</groupId>
<artifactId>crawler4j</artifactId>
<version>x.y.z</version>
</dependency>
</dependencies>
Among them, 'x.y.z' represents the latest version number.
Next, we will create a simple crawler class' MyCrawler 'that inherits from' WebCrawler 'and rewrites some core methods. The code is as follows:
import java.util.regex.Pattern;
import edu.uci.ics.crawler4j.crawler.WebCrawler;
import edu.uci.ics.crawler4j.url.WebURL;
public class MyCrawler extends WebCrawler {
Private final static Pattern FILTERS=Pattern. compile (". * ( . (css | js | gif | jpg)
+| png | mp3 | mp4 | zip | gz) $);
@Override
public boolean shouldVisit(Page referringPage, WebURL url) {
String href = url.getURL().toLowerCase();
return !FILTERS.matcher(href).matches()
&&Href.startsWith（“ http://www.example.com/ Define URL rules to crawl based on requirements
}
@Override
public void visit(Page page) {
//Parse the page and process the data, you can process the data according to your own needs
String url = page.getWebURL().getURL();
System.out.println("URL: " + url);
if (page.getParseData() instanceof HtmlParseData) {
HtmlParseData htmlParseData = (HtmlParseData) page.getParseData();
String text = htmlParseData.getText();
String html = htmlParseData.getHtml();
Set<WebURL> links = htmlParseData.getOutgoingUrls();
//Here for data processing, you can save to a database or write to a file
System.out.println("Text length: " + text.length());
System.out.println("Html length: " + html.length());
System.out.println("Number of outgoing links: " + links.size());
}
}
}
Finally, we create a main class' CrawlerApp 'to configure and start the crawler, with the following code:
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 CrawlerApp {
public static void main(String[] args) throws Exception {
String crawlStorageFolder = "data/crawl";
int numberOfCrawlers = 7;
CrawlConfig config = new CrawlConfig();
config.setCrawlStorageFolder(crawlStorageFolder);
PageFetcher pageFetcher = new PageFetcher(config);
RobotstxtConfig robotstxtConfig = new RobotstxtConfig();
RobotstxtServer robotstxtServer = new RobotstxtServer(robotstxtConfig, pageFetcher);
CrawlController controller = new CrawlController(config, pageFetcher, robotstxtServer);
Controller. addSeed（“ http://www.example.com/ );//Set the start URL of the crawler
Controller. start (MyCrawler. class, numberOfCrawlers)// Start crawler
}
}
In the above code, we use 'CrawlConfig' to configure the relevant parameters of the crawler, such as storage location, crawling speed, etc. Then create 'PageFetcher' and 'RobotstxtConfig' objects to implement page download and Robots.txt processing. Finally, use 'CrawlController' to control the start and stop of the crawler, and configure the start URL and concurrency of the crawler.
Summary: Crawler4j is a powerful, highly configurable, and high-performance Java crawler framework. It provides rich functions and flexible extension mechanisms, which can meet the needs of various crawler applications. However, due to the lack of comprehensive documentation and examples, beginners may need to spend some time learning and using the framework.
