Captcha示例

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);
}

}

配置Servlet步骤：

1. 在web.xml中添加Servlet配置：

2. 将CaptchaServlet.java编译成class文件，并将class文件放置在WEB-INF/classes/code目录下（如果该目录不存在则需要手动创建）。

3. 将captcha.jsp放置在WebContent目录下。

4. 启动Tomcat服务器，在浏览器中访问http://localhost:8080/项目名/captcha.jsp即可看到验证码图片和输入框。