Android HTTP Post返回“E / Response：来自服务器的响应：null”

Question

我正在尝试使用Android将图像上传到Web服务器，以下代码在logcat中返回“E / Response：来自服务器的响应：null”。

        protected String doInBackground(String... args) {
        HttpURLConnection.setFollowRedirects(false);
        HttpURLConnection connection = null;
        String fileName = sourceFile.getName();

        try {
            connection = (HttpURLConnection) new URL(FILE_UPLOAD_URL).openConnection();
            connection.setRequestMethod("POST");
            String boundary = "---------------------------boundary";
            String tail = "\r\n--" + boundary + "--\r\n";
            connection.setRequestProperty("Content-Type", "multipart/form-data; boundary=" + boundary);
            connection.setDoOutput(true);

            String metadataPart = "--" + boundary + "\r\n"
                    + "Content-Disposition: form-data; name=\"metadata\"\r\n\r\n"
                    + "" + "\r\n";

            String fileHeader1 = "--" + boundary + "\r\n"
                    + "Content-Disposition: form-data; name=\"fileToUpload\"; filename=\""
                    + fileName + "\"\r\n"
                    + "Content-Type: application/octet-stream\r\n"
                    + "Content-Transfer-Encoding: binary\r\n";

            long fileLength = sourceFile.length() + tail.length();
            String fileHeader2 = "Content-length: " + fileLength + "\r\n";
            String fileHeader = fileHeader1 + fileHeader2 + "\r\n";
            String stringData = metadataPart + fileHeader;

            long requestLength = stringData.length() + fileLength;
            connection.setRequestProperty("Content-length", "" + requestLength);
            connection.setFixedLengthStreamingMode((int) requestLength);
            connection.connect();

            DataOutputStream out = new DataOutputStream(connection.getOutputStream());
            out.writeBytes(stringData);
            out.flush();

            int progress = 0;
            int bytesRead = 0;
            byte buf[] = new byte[1024];
            BufferedInputStream bufInput = new BufferedInputStream(new FileInputStream(sourceFile));
            while ((bytesRead = bufInput.read(buf)) != -1) {
                // write output
                out.write(buf, 0, bytesRead);
                out.flush();
                progress += bytesRead; // Here progress is total uploaded bytes

                publishProgress(""+(int)((progress*100)/totalSize)); // sending progress percent to publishProgress
            }

            // Write closing boundary and close stream
            out.writeBytes(tail);
            out.flush();
            out.close();

            // Get server response
            BufferedReader reader = new BufferedReader(new InputStreamReader(connection.getInputStream()));
            String line = "";
            StringBuilder builder = new StringBuilder();
            while((line = reader.readLine()) != null) {
                builder.append(line);
            }

        } catch (Exception e) {
            // Exception
        } finally {
            if (connection != null) connection.disconnect();
        }
        return null;
    }

此代码取自以下教程： https://www.androstock.com/tutorials/create-a-file-uploader-with-progress-in-android-android-studio.html

你知道我为什么会收到这个错误吗？