后台生成一个随机的英文与数字组合的验证码

@CrossOrigin(origins = "http://192.168.10.110:2501",maxAge = 3600,allowCredentials = "true")
@RestController
@RequestMapping("/checkCode")
public class CheckCode extends HttpServlet {
    @PostMapping("/check")
    public void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {
        response.setHeader("Access-Control-Allow-Origin",request.getHeader("Origin"));//支持跨域请求
        response.setHeader("Access-Control-Allow-Methods", "*");
        response.setHeader("Access-Control-Allow-Credentials", "true");//是否支持cookie跨域
        //response.setHeader("Access-Control-Allow-Headers", "Authorization,Origin, X-Requested-With, Content-Type, Accept,Access-Token");
        response.setHeader("Access-Control-Allow-Headers", "Content-Type, Access-Control-Allow-Headers, Authorization, X-Requested-With");
        //服务器通知浏览器不要缓存
        response.setHeader("Pragma","no-cache");
        response.setHeader("Cache-Control","no-cache");
        response.setHeader("expires","0");
        response.setDateHeader("Expires",0);
        //在内存中创建一个长80，宽30的图片，默认黑色背景
        //参数一：长
        //参数二：宽
        //参数三：颜色
        int width = 80;
        int height = 30;
        BufferedImage image = new BufferedImage(width,height,BufferedImage.TYPE_INT_RGB);
        //获取画笔
        Graphics g = image.getGraphics();
        //设置画笔颜色为灰色
        g.setColor(Color.GRAY);
        //填充图片
        g.fillRect(0,0, width,height);
        //产生4个随机验证码，12Ey
        String checkCode = getCheckCode();
        //将验证码放入HttpSession中
        request.getSession().setAttribute("CHECKCODE_SERVER",checkCode);
        HttpSession session = request.getSession();
        System.out.println(session.getId());
        HttpPost httpPost=new HttpPost();
        httpPost.setHeader("SessionId",session.getId());
        //设置画笔颜色为黄色
        g.setColor(Color.YELLOW);
        //设置字体的小大
        g.setFont(new Font("黑体",Font.BOLD,24));
        //向图片上写入验证码
        g.drawString(checkCode,15,25);
        //将内存中的图片输出到浏览器
        //参数一：图片对象
        //参数二：图片的格式，如PNG,JPG,GIF
        //参数三：图片输出到哪里去
        ImageIO.write(image,"PNG",response.getOutputStream());
    }
    /**
     * 产生4位随机字符串
     */
    private String getCheckCode() {
        String base = "0123456789ABCDEFGabcdefg";
        int size = base.length();
        Random r = new Random();
        StringBuffer sb = new StringBuffer();
        for(int i=1;i<=4;i++){
            //产生0到size-1的随机值
            int index = r.nextInt(size);
            //在base字符串中获取下标为index的字符
            char c = base.charAt(index);
            //将c放入到StringBuffer中去
            sb.append(c);
        }
        return sb.toString();
    }
    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        this.doGet(request,response);
    }
}