[android] Sending files using POST with HttpURLConnection

The solution of Jaydipsinh Zala didn't work for me, I don't know why but it seems to be close to the solution.

So merging this one with the great solution and explanation of Mihai Todor, the result is this class that currently works for me. If it helps someone:

MultipartUtility2V.java

import java.io.*;
import java.net.HttpURLConnection;
import java.net.URL;
import java.nio.file.Files;

public class MultipartUtilityV2 {
    private HttpURLConnection httpConn;
    private DataOutputStream request;
    private final String boundary =  "*****";
    private final String crlf = "\r\n";
    private final String twoHyphens = "--";

    /**
     * This constructor initializes a new HTTP POST request with content type
     * is set to multipart/form-data
     *
     * @param requestURL
     * @throws IOException
     */
    public MultipartUtilityV2(String requestURL)
            throws IOException {

        // creates a unique boundary based on time stamp
        URL url = new URL(requestURL);
        httpConn = (HttpURLConnection) url.openConnection();
        httpConn.setUseCaches(false);
        httpConn.setDoOutput(true); // indicates POST method
        httpConn.setDoInput(true);

        httpConn.setRequestMethod("POST");
        httpConn.setRequestProperty("Connection", "Keep-Alive");
        httpConn.setRequestProperty("Cache-Control", "no-cache");
        httpConn.setRequestProperty(
                "Content-Type", "multipart/form-data;boundary=" + this.boundary);

        request =  new DataOutputStream(httpConn.getOutputStream());
    }

    /**
     * Adds a form field to the request
     *
     * @param name  field name
     * @param value field value
     */
    public void addFormField(String name, String value)throws IOException  {
        request.writeBytes(this.twoHyphens + this.boundary + this.crlf);
        request.writeBytes("Content-Disposition: form-data; name=\"" + name + "\""+ this.crlf);
        request.writeBytes("Content-Type: text/plain; charset=UTF-8" + this.crlf);
        request.writeBytes(this.crlf);
        request.writeBytes(value+ this.crlf);
        request.flush();
    }

    /**
     * Adds a upload file section to the request
     *
     * @param fieldName  name attribute in <input type="file" name="..." />
     * @param uploadFile a File to be uploaded
     * @throws IOException
     */
    public void addFilePart(String fieldName, File uploadFile)
            throws IOException {
        String fileName = uploadFile.getName();
        request.writeBytes(this.twoHyphens + this.boundary + this.crlf);
        request.writeBytes("Content-Disposition: form-data; name=\"" +
                fieldName + "\";filename=\"" +
                fileName + "\"" + this.crlf);
        request.writeBytes(this.crlf);

        byte[] bytes = Files.readAllBytes(uploadFile.toPath());
        request.write(bytes);
    }

    /**
     * Completes the request and receives response from the server.
     *
     * @return a list of Strings as response in case the server returned
     * status OK, otherwise an exception is thrown.
     * @throws IOException
     */
    public String finish() throws IOException {
        String response ="";

        request.writeBytes(this.crlf);
        request.writeBytes(this.twoHyphens + this.boundary +
                this.twoHyphens + this.crlf);

        request.flush();
        request.close();

        // checks server's status code first
        int status = httpConn.getResponseCode();
        if (status == HttpURLConnection.HTTP_OK) {
            InputStream responseStream = new
                    BufferedInputStream(httpConn.getInputStream());

            BufferedReader responseStreamReader =
                    new BufferedReader(new InputStreamReader(responseStream));

            String line = "";
            StringBuilder stringBuilder = new StringBuilder();

            while ((line = responseStreamReader.readLine()) != null) {
                stringBuilder.append(line).append("\n");
            }
            responseStreamReader.close();

            response = stringBuilder.toString();
            httpConn.disconnect();
        } else {
            throw new IOException("Server returned non-OK status: " + status);
        }

        return response;
    }
}