servlet实操之图片验证码

    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //如何让浏览器五秒自动刷新一次
        resp.setHeader("refresh","3");
        //在内存中创建一个图片
        BufferedImage bufferedImage = new BufferedImage(60,20,BufferedImage.TYPE_INT_RGB);
        //得到图片
        Graphics graphics = (Graphics2D)bufferedImage.getGraphics();
        graphics.setColor(Color.white);
        graphics.fillRect(0,0,80,20);
        //给图片写数据
        graphics.setColor(Color.BLUE);
        graphics.setFont(new Font(null,Font.BOLD,20));
        graphics.drawString(makeNum(),0,20);

        //告诉浏览器这个请求用图片的方式打开
        resp.setContentType("image/jpeg");
        //网站存在缓存，不让浏览器缓存
        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(bufferedImage,"jpg",resp.getOutputStream());

    }
    //生成随机数
    private String makeNum(){
        Random random = new Random();
        String num = random.nextInt(9999) + "";
        StringBuffer stringBuffer = new StringBuffer();
        //保证位数
        for (int i = 0; i < 4-num.length(); i++) {
            stringBuffer.append("0");
        }
        String s = stringBuffer + num;
        return  s;

    }

最后添加web.xml映射，启动tomcat，本地访问即可