用java写一段生成动态验证码的代码

以下是用 Java 生成动态验证码的示例代码：

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.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

public class CaptchaUtil {
    private static final int WIDTH = 120;
    private static final int HEIGHT = 40;
    private static final int CODE_LENGTH = 4;
    private static final int LINE_COUNT = 20;
    private static final int FONT_SIZE = 20;

    public static void generateCaptcha(HttpServletRequest request, HttpServletResponse response) {
        HttpSession session = request.getSession();

        // 生成验证码
        String captchaCode = generateCode(CODE_LENGTH);
        session.setAttribute("captchaCode", captchaCode);

        // 生成图片
        BufferedImage captchaImage = generateImage(captchaCode);

        // 输出图片到客户端
        response.setContentType("image/png");
        try (ServletOutputStream out = response.getOutputStream()) {
            ImageIO.write(captchaImage, "png", out);
        } catch (Exception e) {
            throw new RuntimeException(e);
        }
    }

    private static String generateCode(int length) {
        String chars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";
        StringBuilder sb = new StringBuilder();
        Random random = new Random();
        for (int i = 0; i < length; i++) {
            sb.append(chars.charAt(random.nextInt(chars.length())));
        }
        return sb.toString();
    }

    private static BufferedImage generateImage(String captchaCode) {
        BufferedImage image = new BufferedImage(WIDTH, HEIGHT, BufferedImage.TYPE_INT_RGB);
        Graphics g = image.getGraphics();

        // 填充背景色
        g.setColor(Color.WHITE);
        g.fillRect(0, 0, WIDTH, HEIGHT);

        // 画干扰线
        Random random = new Random();
        for (int i = 0; i < LINE_COUNT; i++) {
            g.setColor(new Color(random.nextInt(256), random.nextInt(256), random.nextInt(256)));
            g.drawLine(random.nextInt(WIDTH), random.nextInt(HEIGHT), random.nextInt(WIDTH), random.nextInt(HEIGHT));
        }

        // 写验证码
        g.setFont(new Font("Arial", Font.PLAIN, FONT_SIZE));
        for (int i = 0; i < captchaCode.length(); i++) {
            g.setColor(new Color(random.nextInt(256), random.nextInt(256), random.nextInt(256)));
            g.drawString("" + captchaCode.charAt(i), 25 * i + 10, HEIGHT / 2 + 5);
        }

        g.dispose();
        return image;
    }
}

这段代码会生成一个包含随机验证码的 PNG 图片，并输出到客户端。在生成图片的过程中，会生成一些干扰线以及随机颜色的验证码字符。通过 `session` 存储生成的验证码，在客户端提交表单时验证验证码是否正确即可。