用severlet写验证码

下面是一个简单的用Servlet生成验证码的示例代码：

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.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

@WebServlet("/captcha")
public class CaptchaServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;

    private static final int WIDTH = 100; // 图片宽度
    private static final int HEIGHT = 40; // 图片高度
    private static final int LENGTH = 4; // 验证码长度

    private static final String CHARS = "ABCDEFGHJKLMNPQRSTUVWXYZ23456789"; // 验证码字符集

    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, java.io.IOException {
        response.setHeader("Cache-Control", "no-cache");
        response.setContentType("image/jpeg");
        BufferedImage image = new BufferedImage(WIDTH, HEIGHT, BufferedImage.TYPE_INT_RGB);
        Graphics g = image.getGraphics();
        Random random = new Random();
        g.setColor(getRandomColor(200, 250));
        g.fillRect(0, 0, WIDTH, HEIGHT);
        g.setFont(new Font("Times New Roman", Font.PLAIN, 20));
        g.setColor(getRandomColor(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);
        }
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < LENGTH; i++) {
            String s = String.valueOf(CHARS.charAt(random.nextInt(CHARS.length())));
            sb.append(s);
            g.setColor(new Color(20 + random.nextInt(110), 20 + random.nextInt(110), 20 + random.nextInt(110)));
            g.drawString(s, 20 * i + 10, 25);
        }
        request.getSession().setAttribute("captcha", sb.toString());
        ImageIO.write(image, "jpeg", response.getOutputStream());
    }

    private Color getRandomColor(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使用了Java的AWT图形库生成了一个随机的验证码图片，并将验证码存储在Session中。要使用这个Servlet，只需要在web.xml文件中添加以下配置：

<servlet>
    <servlet-name>captcha</servlet-name>
    <servlet-class>your.package.name.CaptchaServlet</servlet-class>
</servlet>

<servlet-mapping>
    <servlet-name>captcha</servlet-name>
    <url-pattern>/captcha</url-pattern>
</servlet-mapping>

然后，在需要生成验证码的地方，只需要向/captcha路径发送GET请求即可得到一个验证码图片。例如：

<img src="/captcha" alt="验证码">