Java Servlet 验证码生成器 - 图形验证码生成和验证 - 常规

package code;

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

import javax.imageio.ImageIO;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

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 FONT_SIZE = 20;
    private static final int CODE_LENGTH = 4;

    private static final String SESSION_CAPTCHA_KEY = 'captcha';

    private static final char[] CHARS = { 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o',
            'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z', 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J',
            'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z', '0', '1', '2', '3', '4',
            '5', '6', '7', '8', '9' };

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        HttpSession session = request.getSession();

        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, FONT_SIZE));

        // 生成随机验证码
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < CODE_LENGTH; i++) {
            char c = CHARS[random.nextInt(CHARS.length)];
            sb.append(c);
            g.setColor(new Color(20 + random.nextInt(110), 20 + random.nextInt(110), 20 + random.nextInt(110)));
            g.drawString(String.valueOf(c), i * (WIDTH / CODE_LENGTH) + 5, HEIGHT / 2 + FONT_SIZE / 2 - 5);
        }

        // 将验证码存入 Session
        session.setAttribute(SESSION_CAPTCHA_KEY, sb.toString());

        // 绘制干扰线
        for (int i = 0; i < 8; i++) {
            g.setColor(getRandColor(40, 150));
            int x1 = random.nextInt(WIDTH);
            int y1 = random.nextInt(HEIGHT);
            int x2 = random.nextInt(WIDTH);
            int y2 = random.nextInt(HEIGHT);
            g.drawLine(x1, y1, x2, y2);
        }

        g.dispose();

        // 输出图片
        response.setContentType('image/jpeg');
        ImageIO.write(image, 'JPEG', response.getOutputStream());
    }

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        HttpSession session = request.getSession();

        // 获取用户输入的验证码
        String captcha = request.getParameter('captcha');

        // 获取 Session 中的验证码
        String sessionCaptcha = (String) session.getAttribute(SESSION_CAPTCHA_KEY);

        // 验证码比较不区分大小写
        if (captcha != null && captcha.equalsIgnoreCase(sessionCaptcha)) {
            response.getWriter().write('验证码正确');
        } else {
            response.getWriter().write('验证码错误');
        }
    }

    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);
    }

}


在jsp界面中显示不出来图片，给出jsp示例代码内容：
```html
<!DOCTYPE html>
<html>
<head>
	<meta charset='UTF-8'>
	<title>Captcha Demo</title>
</head>
<body>
	<h1>Captcha Demo</h1>
	<form action='CaptchaServlet' method='post'>
		<label for='captcha'>验证码：</label>
		<input type='text' id='captcha' name='captcha'>
		<img src='CaptchaServlet' alt='captcha'>
		<button type='submit'>提交</button>
	</form>
</body>
</html>
注意：img标签的src属性指向CaptchaServlet，这样才能够显示出验证码图片。