spring框架写一个验证码

在Spring框架中，可以使用Java Servlet API提供的类库来生成和验证验证码。下面是一个简单的示例：

1. 在web.xml文件中添加servlet和servlet-mapping配置

<servlet>
    <servlet-name>captchaServlet</servlet-name>
    <servlet-class>com.example.CaptchaServlet</servlet-class>
</servlet>

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

2. 创建CaptchaServlet类

package com.example;

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 IMG_WIDTH = 120;
    private static final int IMG_HEIGHT = 40;
    private static final int CODE_LENGTH = 4;
    private static final String SESSION_KEY = "captcha_code";

    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        // 创建验证码图片
        BufferedImage bi = new BufferedImage(IMG_WIDTH, IMG_HEIGHT, BufferedImage.TYPE_INT_RGB);
        Graphics g = bi.getGraphics();
        Random r = new Random();
        g.setColor(new Color(r.nextInt(255), r.nextInt(255), r.nextInt(255)));
        g.fillRect(0, 0, IMG_WIDTH, IMG_HEIGHT);
        g.setColor(Color.BLACK);
        g.setFont(new Font("Arial", Font.BOLD, 20));
        String code = "";
        for (int i = 0; i < CODE_LENGTH; i++) {
            String s = String.valueOf(r.nextInt(10));
            code += s;
            g.drawString(s, 20 * i + 10, 25);
        }
        HttpSession session = request.getSession();
        session.setAttribute(SESSION_KEY, code);
        response.setContentType("image/png");
        ImageIO.write(bi, "png", response.getOutputStream());
    }

}

3. 在JSP页面中添加验证码输入框和图片

<form method="post" action="/login">
    <label for="captcha">验证码：</label>
    <input type="text" name="captcha" id="captcha" />
    <img src="/captcha" alt="验证码" />
    <button type="submit">登录</button>
</form>

4. 在登录请求处理方法中验证验证码

@RequestMapping("/login")
public String login(@RequestParam("username") String username,
                    @RequestParam("password") String password,
                    @RequestParam("captcha") String captcha,
                    HttpSession session) {
    String expected = (String) session.getAttribute("captcha_code");
    if (!captcha.equals(expected)) {
        return "login_failed";
    }
    // 验证用户名和密码
    // ...
}

以上就是一个简单的Spring框架下生成和验证验证码的示例。