经过后台输出js脚本

private Boolean checkImg(File upload) {
        // 上传图片以前先判断图片大小
        Boolean flag = false;
        String info = "";
        if (upload == null) {
            info = "<script>alert('添加失败，上传文件不存在');history.back();</script>";
        } else if (upload.length() > MERIMG_MAX_SIZE) {
            info = "<script>alert('添加失败，上传文件大小不能超过20M!');history.back();</script>";
        } else if (upload.length() == 0) {
            info = "<script>alert('添加失败，上传文件大小为0');history.back();</script>";
        } else {
            flag = true;
        }
        if (!flag) {
            ServletActionContext.getResponse().setContentType(
                    "text/html;charset=UTF-8");
            PrintWriter out;
            try {
                out = ServletActionContext.getResponse().getWriter();
                out.print(info);
                out.close();
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
        return flag;
    }