Why HTTP POST is not working unless we read the POST response from server?

Question

I am writing a uploader class for a site and in that, after uploading the files, I am reading the upload response from that site. If I didn't read the response, then the file is not getting uploaded. My code is as follows,

   String charset = "UTF-8";


        File binaryFile = new File("C:\\TestVideoFile.flv");
        String boundary = Long.toHexString(System.currentTimeMillis());
        System.out.println(boundary);// Just generate some unique random value.
        String CRLF = "\r\n"; // Line separator required by multipart/form-data.
        URLConnection connection = new URL(UPLOAD_URL).openConnection();
        connection.setDoInput(true); 
        connection.setDoOutput(true);
        connection.setRequestProperty("Content-Type", "multipart/form-data; boundary=" + boundary);
        PrintWriter writer = null;
        try {
            OutputStream output = connection.getOutputStream();
            writer = new PrintWriter(new OutputStreamWriter(output, charset), true); // true = autoFlush, important!
            writer.append("--" + boundary).append(CRLF);
            writer.append("Content-Disposition: form-data; name=\"file\"; filename=\"" + binaryFile.getName() + "\"").append(CRLF);
            writer.append("Content-Type: " + URLConnection.guessContentTypeFromName(binaryFile.getName())).append(CRLF);
            writer.append("Content-Transfer-Encoding: binary").append(CRLF);
            writer.append(CRLF).flush();
            InputStream input = null;
            try {
                input = new FileInputStream(binaryFile);
                long filelen = binaryFile.length();
                System.out.println("Length : " + filelen);
                int dataRead = 0;
                byte[] buffer = new byte[1024];
                for (int length = 0; (length = input.read(buffer)) > 0;) {

                    output.write(buffer, 0, length);
                }
                System.out.println("Now only terminating the file write loop");
                output.flush(); // Important! Output cannot be closed. Close of writer will close output as well.
            } catch (Exception e) {
                System.out.println(e);
            } finally {
                if (input != null) {
                    try {
                        input.close();
                    } catch (IOException logOrIgnore) {
                        System.out.println(logOrIgnore);
                    }
                }
            }
            writer.append(CRLF).flush(); // CRLF is important! It indicates end of binary boundary.

            // End of multipart/form-data.
            writer.append("--" + boundary + "--").append(CRLF);

            System.out.println("Sending username");
            // Send normal param.
            writer.append("--" + boundary).append(CRLF);
            writer.append("Content-Disposition: form-data; name=\"user\"").append(CRLF);
            writer.append("Content-Type: text/plain; charset=" + charset).append(CRLF);
            writer.append(CRLF);
            writer.append(username).append(CRLF).flush();
            System.out.println("Sending password");
            writer.append("--" + boundary).append(CRLF);
            writer.append("Content-Disposition: form-data; name=\"password\"").append(CRLF);
            writer.append("Content-Type: text/plain; charset=" + charset).append(CRLF);
            writer.append(CRLF);
            writer.append(password).append(CRLF).flush();


            System.out.println("Reading response from server");

            BufferedReader br = new BufferedReader(new InputStreamReader(connection.getInputStream()));
            String k = "", tmp = "";
            while ((tmp = br.readLine()) != null) {
                System.out.println(tmp);
                k += tmp;
            }
            if (k.contains("Successfully")) {
                System.out.println("File Uploaded successfully into PutLocker :)");
                String downloadLink = parseResponse(k, "<link>", "</link>");
                System.out.println("Download Link : " + downloadLink);
            } else {
                System.out.println("Upload failed :(");
            }

          } catch (Exception e) {
            System.out.println(e);
        } finally {
            if (writer != null) {
                writer.close();
            }
        }

As you can see, I am writing the data into the server in the following line,

for (int length = 0; (length = input.read(buffer)) > 0;) {

                        output.write(buffer, 0, length);
                    }

But after this, I have to do the following,

 BufferedReader br = new BufferedReader(new InputStreamReader(connection.getInputStream()));
                String k = "", tmp = "";
                while ((tmp = br.readLine()) != null) {
                    System.out.println(tmp);
                    k += tmp;
                }

Why do I have to read the response from server to make the upload successful?

Can anybody explain on this?

Thanks in advance.

Answer 1

If you try to post something to the server and the connection get interrupted before you fully send your message, then the server will stop processing the incomplete request. Once the server is done receiving the request and you don't wait to get its response, then you will never know if your request is sent successfully or not. Therefore, the URLConnection is designed to wait until you get the response.

Another reason is that you may want build and configure your URLConnection first and then send it later when you want to by invoking the getInputStream or getResponseCode. You have more control of when you want the transaction to happen.

You do not need to always getInputStream, just calling getResponseCode is good enough to complete the request. However, the whole input stream is still sent to your code but they are discarded.