javaweb Response验证码实现

public class ImageServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //如何让浏览器5秒刷新一次
        resp.setHeader("refresh","5");
        //在内存中创建一个图片
        BufferedImage image = new BufferedImage(80,20,BufferedImage.TYPE_INT_RGB);
        //得到图片
        Graphics2D g =(Graphics2D) image.getGraphics();
        //设置图片的背景颜色
        g.setColor(Color.white);
        g.fillRect(0,0,80,20);
        //给图片写数据
        g.setColor(Color.blue);
        g.setFont(new Font(null,Font.BOLD,20));
        g.drawString(makeNumber(),0,20);
        //告诉浏览器这个请求用图片的形式打开
        resp.setContentType("image/png");
        //网站存在缓存，不让浏览器缓存
        resp.setDateHeader("expires",-1);
        //把图写给浏览器
        ImageIO.write(image,"jpg",resp.getOutputStream())
    }