Why Choose Java Over Python for Web Crawling? A Practical Guide

Why Java for Web Crawling?

When the author needed large amounts of Q&A data for a thesis, manual copy‑paste was painful. Later, during graduate research, a senior assigned a web‑data collection task, prompting the author to learn web crawling. After evaluating Python and Java, the author chose Java for its mature ecosystem, strong typing, and abundant libraries such as Jsoup, HttpClient, Crawler4j, and WebMagic.

1. Web Crawling Process

The typical crawling workflow consists of five steps:

Select seed URLs and add them to a queue (e.g., List, LinkedList, Queue in Java).

Check whether the URL queue is empty; if so, terminate.

Dequeue a URL, request the page, and verify the HTTP status (e.g., 200, 403). If the request fails, re‑queue the URL after filtering invalid ones.

Parse the successful response to extract required fields (e.g., post ID, title, timestamp).

Store the extracted data.

2. Required Java Fundamentals

Developing a Java crawler requires familiarity with basic data types, arrays, control statements, collections, objects, strings, date/time handling, regular expressions, Maven project setup, multithreading, and logging. The following examples illustrate how these concepts are applied in a crawler.

Queue<String> urlQueue = new LinkedList<String>();
urlQueue.offer("https://ccm.net/download/?page=1");
urlQueue.offer("https://ccm.net/download/?page=2");
urlQueue.offer("https://ccm.net/download/?page=3");
boolean t = true;
while (t) {
    if (urlQueue.isEmpty()) {
        t = false;
    } else {
        String url = urlQueue.poll();
        // getHtml = ...
        if (requestSuccessful) {
            // parse data
        } else {
            urlQueue.offer(url);
        }
    }
}

Different websites use various timestamp formats. The author provides utilities to normalize them:

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
public class TimeTest {
    public static void main(String[] args) {
        System.out.println(parseStringTime("2016-05-19 19:17", "yyyy-MM-dd HH:mm", "yyyy-MM-dd HH:mm:ss"));
        System.out.println(parseStringTime("2018-06-19", "yyyy-MM-dd", "yyyy-MM-dd HH:mm:ss"));
    }
    public static String parseStringTime(String inputTime, String inputTimeFormat, String outTimeFormat) {
        String outputDate = null;
        try {
            Date inputDate = new SimpleDateFormat(inputTimeFormat).parse(inputTime);
            outputDate = new SimpleDateFormat(outTimeFormat).format(inputDate);
        } catch (ParseException e) {
            e.printStackTrace();
        }
        return outputDate;
    }
}

public static String TimeStampToDate(String timestampString, String formats) {
    Long timestamp = Long.parseLong(timestampString) * 1000;
    String date = new SimpleDateFormat(formats, Locale.CHINA).format(new Date(timestamp));
    return date;
}

3. HTTP Basics and Network Capture

Understanding HTTP is essential for crawling. Key points include URL structure, request/response messages, common methods (GET, POST), status codes (e.g., 200 for success), headers (User‑Agent, Referer), and response bodies (HTML, XML, JSON). Network sniffing helps developers see how browsers interact with servers, which is the starting point for building a crawler.

The article concludes by promoting the author's book "Java Web Crawling in Practice", which systematically covers crawler theory, Java fundamentals, HTTP protocol, data extraction tools, and advanced topics such as Selenium for dynamic pages.