java 验证码实例

以下是一个使用Java生成和验证验证码的示例：

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.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class CaptchaServlet extends HttpServlet {

    private static final long serialVersionUID = 1L;
    private static final int WIDTH = 120;
    private static final int HEIGHT = 40;
    private static final int LENGTH = 4;

    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, java.io.IOException {
        response.setContentType("image/jpeg");
        ServletOutputStream out = response.getOutputStream();
        BufferedImage image = new BufferedImage(WIDTH, HEIGHT, BufferedImage.TYPE_INT_RGB);
        Graphics g = image.getGraphics();
        Random random = new Random();
        g.setColor(getRandColor(200, 250));
        g.fillRect(0, 0, WIDTH, HEIGHT);
        g.setFont(new Font("Times New Roman", Font.PLAIN, 18));
        g.setColor(getRandColor(160, 200));
        for (int i = 0; i < 155; i++) {
            int x = random.nextInt(WIDTH);
            int y = random.nextInt(HEIGHT);
            int xl = random.nextInt(12);
            int yl = random.nextInt(12);
            g.drawLine(x, y, x + xl, y + yl);
        }
        StringBuffer sRand = new StringBuffer();
        for (int i = 0; i < LENGTH; i++) {
            String rand = String.valueOf(random.nextInt(10));
            sRand.append(rand);
            g.setColor(new Color(20 + random.nextInt(110), 20 + random.nextInt(110), 20 + random.nextInt(110)));
            g.drawString(rand, 13 * i + 6, 16);
        }
        request.getSession().setAttribute("captcha", sRand.toString());
        g.dispose();
        ImageIO.write(image, "JPEG", out);
        out.close();
    }

    private Color getRandColor(int fc, int bc) {
        Random random = new Random();
        if (fc > 255)
            fc = 255;
        if (bc > 255)
            bc = 255;
        int r = fc + random.nextInt(bc - fc);
        int g = fc + random.nextInt(bc - fc);
        int b = fc + random.nextInt(bc - fc);
        return new Color(r, g, b);
    }
}

这个Servlet会在请求时生成一个验证码并将其写入一个JPEG图像中，然后将该图像发送回浏览器。要验证用户输入的验证码，只需检查该值是否与上一个请求中存储在会话中的验证码相同即可。