請求方面：
@RequestMapping(value = "/fileUpload", method = RequestMethod.POST)
public String upload(MultipartHttpServletRequest request, HttpServletResponse response){
   String s = fileUpdateUtil.fileUpload(request, response);
       try {
      String referer = request.getHeader("referer");
      Pattern p = Pattern.compile("([a-z]*:(//[^/?#]+)?)?", Pattern.CASE_INSENSITIVE);
      Matcher mathcer = p.matcher(referer);
      if (mathcer.find()) {
         String callBackPath = mathcer.group();// 請求來源a.com
         MultipartHttpServletRequest multipartRequest = (MultipartHttpServletRequest) request;
         MultipartFile imgFile = multipartRequest.getFile("fileupload");//k4中fileupload
         //JsonObject json =自己的處理imgFile
         // 同域時直接返回json.toString()即可無需redirect
         String url = "redirect:" + callBackPath + "/redirect?s=" + s + "#" + s;
         return url;
      } else {
         System.out.println("upload referer not find");
      }
   } catch (Exception e) {
      System.out.println("upload error"+e);
   }
   return null;
}

工具類：
public static String fileUpload(MultipartHttpServletRequest request, HttpServletResponse response){
    String rootPath = Thread.currentThread().getContextClassLoader().getResource("").getPath();//獲取專案絕對路徑
    String substring = rootPath.substring(1, rootPath.length()).replace("/", "\\");
    String savePath = substring + "static\\attached\\";
    // 檔案儲存目錄URL
    // 定義允許上傳的副檔名
    HashMap<String, String> extMap = new HashMap<String, String>();
    extMap.put("image", "gif,jpg,jpeg,png,bmp");
    extMap.put("flash", "swf,flv");
    extMap.put("media", "swf,flv,mp3,wav,wma,wmv,mid,avi,mpg,asf,rm,rmvb");
    extMap.put("file", "doc,docx,xls,xlsx,ppt,htm,html,txt,zip,rar,gz,bz2");

    // 最大檔案大小
    long maxSize = 100000000;

    response.reset();
    response.setHeader("P3P","CP=CAO PSA OUR");
    response.setCharacterEncoding("UTF-8");
    response.setContentType("text/html");
    // writer.println(json);  //想辦法把map轉成json

    if (!ServletFileUpload.isMultipartContent(request)) {
        System.out.println("請選擇檔案。");
        return null;
    }
    File uploadDir = new File(savePath);
    if (!uploadDir.exists()) {
        uploadDir.mkdirs();
    }
    // 檢查目錄寫許可權
    if (!uploadDir.canWrite()) {
        System.out.println("上傳目錄沒有寫許可權。");
        return null;
    }

    String dirName = request.getParameter("dir");
    if (dirName == null) {
        dirName = "image";
    }
    if (!extMap.containsKey(dirName)) {
        System.out.println("目錄名不正確。");
        return null;
    }
    // 建立資料夾
    savePath += dirName + "/";
    File saveDirFile = new File(savePath);
    if (!saveDirFile.exists()) {
        saveDirFile.mkdirs();
    }
    SimpleDateFormat sdf = new SimpleDateFormat("yyyyMMdd");
    String ymd = sdf.format(new Date());
    savePath += ymd + "/";
    File dirFile = new File(savePath);
    if (!dirFile.exists()) {
        dirFile.mkdirs();
    }
    List<MultipartFile> files = request.getFiles("imgFile");

    Iterator<MultipartFile> itr = files.iterator();
    while (itr.hasNext()) {
        MultipartFile item = itr.next();
        String fileName = item.getOriginalFilename();
        // 檢查檔案大小
        if (item.getSize() > maxSize) {
            System.out.println("上傳檔案大小超過限制。");
            return null;
        }
        // 檢查副檔名
        String fileExt = fileName.substring(
                fileName.lastIndexOf(".") + 1).toLowerCase();
        if (!Arrays.<String>asList(extMap.get(dirName).split(","))
                .contains(fileExt)) {
            System.out.println("上傳副檔名是不允許的副檔名。只允許" + extMap.get(dirName) + "格式。");
            return null;
        }

        SimpleDateFormat df = new SimpleDateFormat("yyyyMMddHHmmss");
        String newFileName = df.format(new Date()) + "_"
                + new Random().nextInt(1000) + "." + fileExt;
        try {
            File uploadedFile = new File(savePath, newFileName);
            item.transferTo(uploadedFile);

        } catch (Exception e) {
            System.out.println("上傳檔案失敗");
        }
        //http://192.168.100.157:8080/attached/image/20190923/20190923094646_10.png
        int fast = savePath.indexOf("static")+6;
        String savePatha = savePath.substring(fast, savePath.length());
        String replace = savePatha.replace("\\", "/");
        String path = "http://192.168.100.157:8080"+replace + newFileName;
        String url = "{\"error\":0,\"url\":\""+path+"\"}";
        return url;
    }
    return null;
}

都是整合別的博主的，還有其他的