Generating Captcha in Java

A comprehensive guide on how to create a Captcha in Java to protect your application from automated activities and enhance security.

This article explains how to generate a Captcha code using Java's java.awt and javax.imageio libraries. The Captcha consists of random characters converted into an image with noise to prevent automated recognition.

Java Code

import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics2D;
import java.awt.image.BufferedImage;
import java.util.Random;
import javax.imageio.ImageIO;
import java.io.File;
import java.io.IOException;

public class CaptchaGenerator {
    // Generate random Captcha text
    private static String generateCaptchaText(int length) {
        String chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";
        Random random = new Random();
        StringBuilder captchaText = new StringBuilder();
        
        for (int i = 0; i < length; i++) {
            captchaText.append(chars.charAt(random.nextInt(chars.length())));
        }
        
        return captchaText.toString();
    }

    // Generate Captcha image
    private static BufferedImage generateCaptchaImage(String captchaText) {
        int width = 160;
        int height = 50;
        BufferedImage image = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
        Graphics2D g2d = image.createGraphics();

        // Set background and font
        g2d.setColor(Color.WHITE);
        g2d.fillRect(0, 0, width, height);
        g2d.setFont(new Font("Arial", Font.BOLD, 24));

        // Draw Captcha text
        g2d.setColor(Color.BLACK);
        g2d.drawString(captchaText, 20, 35);

        // Add noise
        Random random = new Random();
        for (int i = 0; i < 100; i++) {
            int x = random.nextInt(width);
            int y = random.nextInt(height);
            int rgb = random.nextInt(0xFFFFFF);
            image.setRGB(x, y, rgb);
        }
        
        g2d.dispose();
        return image;
    }

    public static void main(String[] args) {
        String captchaText = generateCaptchaText(6);
        BufferedImage captchaImage = generateCaptchaImage(captchaText);
        
        try {
            ImageIO.write(captchaImage, "png", new File("captcha.png"));
            System.out.println("Captcha generated: " + captchaText);
        } catch (IOException e) {
            System.err.println("Error saving Captcha image: " + e.getMessage());
        }
    }
}

Detailed explanation:

1. generateCaptchaText(int length): Generates a random Captcha string with the specified length.
2. generateCaptchaImage(String captchaText): Creates a Captcha image with the generated text and adds noise.
3. main(String[] args):
   • Generates a random Captcha string and creates the corresponding image.
   • Saves the image as "captcha.png" in the current directory.

System Requirements:

• Java 8 or later
• Libraries java.awt and javax.imageio are integrated with Java.

How to install the libraries needed to run the Java code above:

No additional libraries are needed since they are included in the JDK.

Tips:

• Add more noise or distortions to the Captcha image to enhance security.
• Avoid using easily confused characters such as '0', 'O', '1', and 'l'.

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