java编写一个简单的验证码

public class ImageServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.setHeader("refresh","3");
        //在内存中创建一个画布
        BufferedImage image = new BufferedImage(80,20,BufferedImage.TYPE_INT_RGB);
        //得到画笔
        Graphics2D g = (Graphics2D) image.getGraphics();
        //设置画笔颜色
        g.setColor(Color.white);
        //画一个填充矩形
        g.fillRect(0,0,80,20);
        //写数据
        g.setColor(Color.BLUE);
        g.setFont(new Font(null,Font.BOLD,20));
        g.drawString(makeNum(),0,20);
        //告诉浏览器文件类型
        resp.setContentType("image/jpg");
        //网站有缓存，要清除缓存
        resp.setDateHeader("expires",-1);
        resp.setHeader("Cache-Control","no-cache");
        resp.setHeader("Pragma","no-cache");
        //把图片写给浏览器
        ImageIO.write(image,"jpg", resp.getOutputStream());
    }
    public String makeNum(){
        Random random = new Random();
//        100000--999999
        String i = random.nextInt((999999-100000)+1)+100000+"";
        return i;
    }
}