用Java写一个验证码功能

好的，以下是一个简单的 Java 验证码功能的示例代码：

import java.awt.Color;
import java.awt.Font;
import java.awt.Graphics;
import java.awt.image.BufferedImage;
import java.util.Random;

import javax.imageio.ImageIO;
import javax.servlet.ServletOutputStream;
import javax.servlet.http.Cookie;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

public class CaptchaUtil {
    
    private static final String CAPTCHA_COOKIE_NAME = "captcha_key";
    private static final String CAPTCHA_SESSION_ATTRIBUTE_NAME = "captcha_key";
    private static final int CAPTCHA_LENGTH = 6;
    private static final int CAPTCHA_WIDTH = 120;
    private static final int CAPTCHA_HEIGHT = 40;
    private static final int CAPTCHA_FONT_SIZE = 20;
    private static final int CAPTCHA_EXPIRATION_TIME = 5 * 60; // 5 minutes
    
    public static void generateCaptcha(HttpServletRequest request, HttpServletResponse response) throws Exception {
        // Generate random captcha text
        String captchaKey = generateRandomCaptchaKey(CAPTCHA_LENGTH);
        
        // Set captcha key in session
        HttpSession session = request.getSession();
        session.setAttribute(CAPTCHA_SESSION_ATTRIBUTE_NAME, captchaKey);
        
        // Set captcha key in cookie with expiration time
        Cookie captchaCookie = new Cookie(CAPTCHA_COOKIE_NAME, captchaKey);
        captchaCookie.setMaxAge(CAPTCHA_EXPIRATION_TIME);
        captchaCookie.setPath(request.getContextPath() + "/");
        response.addCookie(captchaCookie);
        
        // Generate captcha image
        BufferedImage captchaImage = generateCaptchaImage(captchaKey);
        
        // Write captcha image to response output stream
        ServletOutputStream outputStream = response.getOutputStream();
        ImageIO.write(captchaImage, "png", outputStream);
        outputStream.close();
    }
    
    public static boolean validateCaptcha(HttpServletRequest request) {
        // Get captcha key from session and cookie
        HttpSession session = request.getSession();
        String captchaSessionKey = (String) session.getAttribute(CAPTCHA_SESSION_ATTRIBUTE_NAME);
        Cookie[] cookies = request.getCookies();
        String captchaCookieKey = null;
        if (cookies != null) {
            for (Cookie cookie : cookies) {
                if (cookie.getName().equals(CAPTCHA_COOKIE_NAME)) {
                    captchaCookieKey = cookie.getValue();
                    break;
                }
            }
        }
        
        // Validate captcha key
        if (captchaSessionKey == null || captchaCookieKey == null || !captchaSessionKey.equals(captchaCookieKey)) {
            return false;
        }
        
        // Remove captcha key from session and cookie
        session.removeAttribute(CAPTCHA_SESSION_ATTRIBUTE_NAME);
        Cookie captchaCookie = new Cookie(CAPTCHA_COOKIE_NAME, null);
        captchaCookie.setMaxAge(0);
        captchaCookie.setPath(request.getContextPath() + "/");
        response.addCookie(captchaCookie);
        
        return true;
    }
    
    private static String generateRandomCaptchaKey(int length) {
        String captchaKey = "";
        Random random = new Random();
        for (int i = 0; i < length; i++) {
            captchaKey += (char) (random.nextInt(26) + 'a');
        }
        return captchaKey;
    }
    
    private static BufferedImage generateCaptchaImage(String captchaKey) {
        BufferedImage captchaImage = new BufferedImage(CAPTCHA_WIDTH, CAPTCHA_HEIGHT, BufferedImage.TYPE_INT_RGB);
        Graphics graphics = captchaImage.getGraphics();
        
        // Draw background
        graphics.setColor(Color.WHITE);
        graphics.fillRect(0, 0, CAPTCHA_WIDTH, CAPTCHA_HEIGHT);
        
        // Draw captcha text
        graphics.setColor(Color.BLACK);
        graphics.setFont(new Font("Arial", Font.BOLD, CAPTCHA_FONT_SIZE));
        graphics.drawString(captchaKey, 10, 25);
        
        // Draw noise
        graphics.setColor(Color.GRAY);
        for (int i = 0; i < 50; i++) {
            graphics.drawLine(random.nextInt(CAPTCHA_WIDTH), random.nextInt(CAPTCHA_HEIGHT), random.nextInt(CAPTCHA_WIDTH), random.nextInt(CAPTCHA_HEIGHT));
        }
        
        return captchaImage;
    }
    
}

这个验证码功能使用了 Servlet API 和 Java 2D 图形库，可以在 Servlet 环境中使用。它生成一个随机的长度为 6 的验证码字符串，并在 session 和 cookie 中保存验证码字符串。它还生成一个包含验证码字符串的 PNG 图片，可以直接写入 HTTP 响应输出流中。在验证验证码时，它从 session 和 cookie 中检索验证码字符串，并比较它们是否相等。如果验证成功，它将从 session 和 cookie 中删除验证码字符串。