终极解决方案：org.apache.jasper.JasperException: java.lang.IllegalStateException: getOutputStream() has alr

    @Override
    public void showCodeImg(HttpServletRequest request, HttpServletResponse response)  throws IOException {
        response.setHeader("Expires","-1");//控制缓存的失效日期
        response.setHeader("Cache-Control","no-cache");//必须先与服务器确认返回的响应是否被更改，然后才能使用该响应来满足后续对同一个网址的请求
        response.setHeader("Pragma","-1");
        CaptchaUtil util=CaptchaUtil.Instance();
        // 将验证码输入到session中，用来验证
        String code=util.getString();
        request.setCharacterEncoding("UTF-8");
        request.getSession().setAttribute(SecurityCodeConstant.CODE_NUM,code);
        // 输出到web页面
        ImageIO.write(util.getImage(), "jpg", response.getOutputStream());
    }