Learning Java Crawling: An Indispensable Guide to Technologies and Tools

Java Crawler Getting Started Guide: Necessary technologies and tools, specific code examples are required

1. Introduction

With the rapid development of the Internet, People's demand for obtaining information on the Internet is increasing. As a technology for automatically obtaining network information, crawlers are becoming more and more important. As a powerful programming language, Java is also widely used in the crawler field. This article will introduce the necessary technologies and tools for Java crawlers, and provide specific code examples to help readers get started.

2. Necessary Technology

1. HTTP Request

The primary task of the crawler is to simulate the browser sending HTTP requests to obtain web page content. Java provides a variety of HTTP request libraries, the commonly used ones are HttpClient and URLConnection. The following is a sample code for using HttpClient to send a GET request:

import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.HttpClientBuilder;
import org.apache.http.util.EntityUtils;

public class HttpUtils {
    public static String sendGetRequest(String url) {
        HttpClient httpClient = HttpClientBuilder.create().build();
        HttpGet httpGet = new HttpGet(url);
        try {
            HttpResponse response = httpClient.execute(httpGet);
            HttpEntity entity = response.getEntity();
            return EntityUtils.toString(entity);
        } catch (IOException e) {
            e.printStackTrace();
            return null;
        }
    }
}

2. HTML parsing

After obtaining the web page content, you need to extract the required information from the HTML. Java has a variety of HTML parsing libraries to choose from, the most commonly used of which is Jsoup. The following is a sample code for using Jsoup to parse HTML:

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

public class HtmlParser {
    public static void parseHtml(String html) {
        Document doc = Jsoup.parse(html);
        Elements links = doc.select("a[href]"); // 解析出所有的链接
        for (Element link : links) {
            System.out.println(link.attr("href"));
        }
    }
}

3. Data storage

The data obtained by the crawler needs to be stored. Java provides a variety of database operation libraries, such as JDBC, Hibernate and MyBatis, etc. In addition, files can also be used to store data. Common file formats include CSV and JSON. The following is a sample code for storing data in CSV format:

import java.io.FileWriter;
import java.io.IOException;
import java.util.List;

public class CsvWriter {
    public static void writeCsv(List<String[]> data, String filePath) {
        try (FileWriter writer = new FileWriter(filePath)) {
            for (String[] row : data) {
                writer.write(String.join(",", row));
                writer.write("
");
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

3. Necessary tools

1. Development environment

Write and run Java crawler programs A suitable development environment is required. It is recommended to use an integrated development environment (IDE) such as Eclipse or Intellij IDEA. They provide rich editor and debugger functions, which can greatly improve development efficiency.

2. Version Control Tool

Use version control tools to easily manage code and collaborate with team members. Git is currently the most popular version control tool, which can easily create and merge code branches, making it convenient for multiple people to develop.

3. Log tool

In the process of developing a crawler, you are likely to encounter some problems, such as page parsing failure or data storage exception. Using logging tools can help locate problems and debug them. The most commonly used logging tools in Java are Log4j and Logback.

4. Code Example

The following is a complete Java crawler example, which uses HttpClient to send HTTP requests, uses Jsoup to parse HTML, and saves the parsed results as a CSV file:

import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.HttpClientBuilder;
import org.apache.http.util.EntityUtils;
import org.jsoup.Jsoup;
import org.jsoup.nodes.Document;
import org.jsoup.nodes.Element;
import org.jsoup.select.Elements;

import java.io.FileWriter;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

public class WebCrawler {
    public static void main(String[] args) {
        String url = "http://example.com";
        String html = HttpUtils.sendGetRequest(url);
        HtmlParser.parseHtml(html);
        CsvWriter.writeCsv(data, "data.csv");
    }
}

The above example code is only used as a starting guide. In actual applications, it may need to be modified and expanded appropriately according to the situation. I hope that through the introduction of this article, readers can have a preliminary understanding of the basic technologies and tools of Java crawlers and apply them in actual projects.

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