遇到一個問題，我們的服務只做代理功能，有時候需要轉發檔案，又不能把檔案先儲存下來後再讀取檔案上傳到第三方伺服器，這時候就要直接傳送流檔案。

restTemplate(親測有效，很費勁才找到的):

  class MultipartInputStreamFileResource extends InputStreamResource {

    private final String filename;

    MultipartInputStreamFileResource(InputStream inputStream, String filename) {
        super(inputStream);
        this.filename = filename;
    }

    @Override
    public String getFilename() {
        return this.filename;
    }

    @Override
    public long contentLength() throws IOException {
        return -1; // we do not want to generally read the whole stream into memory ...
    }
}

LinkedMultiValueMap<String, Object> map = new LinkedMultiValueMap<>();
   try {
            if (!file.isEmpty()) {
                map.add("images", new MultipartInputStreamFileResource(file.getInputStream(), file.getOriginalFilename()));
            }
}
 

HttpClient(親測有效,但可以能與restTemplate某些地方衝突，因為RestTemplate底層實現是有多種的，
引入HttpClient包之後會用新的實現，導致原先RestTemplate發的請求都不會重寫Host了，這個坑至今詳細原因不明，
所以專案中還是用統一的Http Client吧)

        CloseableHttpClient httpClient = HttpClients.createDefault();
        HttpResponse response = null;
        String result = "";
        try {
            String fileName = multipartFile.getOriginalFilename();
            HttpPost httpPost = new HttpPost(url);
            MultipartEntityBuilder builder = MultipartEntityBuilder.create();
            builder.addBinaryBody("file", multipartFile.getInputStream(), ContentType.MULTIPART_FORM_DATA, fileName);
            if (cuser == null) {
                cuser = "0";
            }
            builder.addTextBody("cuser", cuser);
            org.apache.http.HttpEntity entity = builder.build();
            httpPost.setEntity(entity);
            response = httpClient.execute(httpPost);
            result = EntityUtils.toString(response.getEntity(), Charset.forName("UTF-8"));
        } catch (ClientProtocolException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }