可以提供一段生成图片验证码的代码吗

以下是基于Java Servlet API 3.0的生成图片验证码的示例代码：

@WebServlet("/captcha")
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 = 6;

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        BufferedImage image = new BufferedImage(WIDTH, HEIGHT, BufferedImage.TYPE_INT_RGB);
        Graphics2D g = image.createGraphics();
        g.setColor(Color.WHITE);
        g.fillRect(0, 0, WIDTH, HEIGHT);
        String captcha = generateCaptcha();
        request.getSession().setAttribute("captcha", captcha);
        Font font = new Font("Arial", Font.BOLD, 20);
        g.setFont(font);
        g.setColor(Color.BLACK);
        g.drawString(captcha, 10, 25);
        g.dispose();
        response.setContentType("image/jpeg");
        OutputStream out = response.getOutputStream();
        ImageIO.write(image, "jpeg", out);
        out.close();
    }

    private String generateCaptcha() {
        Random random = new Random();
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < LENGTH; i++) {
            sb.append((char) (random.nextInt(26) + 'a'));
        }
        return sb.toString();
    }
}

在上面的示例中，我们使用 Java 的 BufferedImage 类创建了一个图片对象，然后使用 Graphics2D 类绘制验证码。我们使用了随机数生成器来生成验证码，然后将其存储在 Session 中，以便在提交表单时验证验证码。最后，将图片输出到响应中。