Key technologies to become an expert in the field of Java crawlers!

Master these technologies and become an expert in the field of Java crawlers!

In today's era of information explosion, obtaining and processing large amounts of data has become a need for many people, and the amount of data on the Internet is even greater. As a Java developer, if you want to become an expert in the field of crawlers, mastering some basic crawler technologies is essential. In this article, we will introduce several commonly used Java crawler technologies and give specific code examples.

1. HTML parsing technology

When crawling web pages, one of the most common situations is the need to extract specific information from web pages. This requires the use of HTML parsing technology to convert the HTML code of the web page into a readable data structure to facilitate subsequent processing.

The following is an example of using the Jsoup library for HTML parsing:

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

public class HtmlParserExample {

    public static void main(String[] args) throws Exception {
        String url = "https://example.com";
        Document document = Jsoup.connect(url).get();

        Elements links = document.select("a[href]");
        for (Element link : links) {
            System.out.println("Link: " + link.attr("href"));
        }

        Elements images = document.select("img[src~=(?i)\.(png|jpe?g|gif)]");
        for (Element image : images) {
            System.out.println("Image: " + image.attr("src"));
        }
    }
}

This code uses the Jsoup library to parse web pages and extract links and image addresses.

2. HTTP request technology

When crawling a web page, you need to send an HTTP request to the server and receive the server's response. There are many ways to send HTTP requests in Java, you can use the native HttpURLConnection class, or you can use third-party libraries, such as Apache HttpClient.

The following is an example of using the Apache HttpClient library to send an HTTP request:

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 HttpRequestExample {

    public static void main(String[] args) throws Exception {
        String url = "https://example.com";
        HttpClient client = HttpClientBuilder.create().build();
        HttpGet request = new HttpGet(url);

        HttpResponse response = client.execute(request);
        String content = EntityUtils.toString(response.getEntity());
        System.out.println(content);
    }
}

This code uses the Apache HttpClient library to send a GET request and output the content returned by the server to the console.

3. Data storage technology

After crawling the data, we usually need to store it in a database or file for subsequent analysis and processing. Java provides a variety of ways to store data, such as using JDBC to access relational databases, using the JPA framework to operate object databases, or using file IO streams for file operations.

The following is an example of storing crawled data into a MySQL database (using JDBC):

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;

public class DataStorageExample {

    public static void main(String[] args) throws Exception {
        String url = "jdbc:mysql://localhost:3306/test";
        String username = "root";
        String password = "password";

        Class.forName("com.mysql.jdbc.Driver");
        Connection connection = DriverManager.getConnection(url, username, password);

        String sql = "INSERT INTO data (title, content) VALUES (?, ?)";
        PreparedStatement statement = connection.prepareStatement(sql);
        statement.setString(1, "Example title");
        statement.setString(2, "Example content");
        statement.executeUpdate();

        statement.close();
        connection.close();
    }
}

This code uses JDBC to connect to the MySQL database and store the crawled data Insert into the data table.

By mastering these technologies, you can crawl web pages and process data more efficiently and become an expert in the field of Java crawlers! But in practical applications, we also need to pay attention to legality and ethics, follow relevant regulations, and respect the crawling rules of the website. Only under the premise of legality and compliance can the convenience and benefits brought by crawler technology be better utilized.

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