Multipart transmission library on Android

There weren't many examples of simple and easy multi-part transmission, so I posted it instead of a memo.

Library body

PostMultipart.java

package com.app.sample;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.net.HttpURLConnection;
import java.net.URL;
import java.net.URLConnection;
import java.util.ArrayList;
import java.util.List;

/**
 * Created by solid_red on 2017/03/13.
 */

public class PostMultipart {
    private final String boundary;
    private static final String LINE_FEED = "\r\n";
    private HttpURLConnection httpConn;
    private String charset;
    private OutputStream outputStream;
    private PrintWriter writer;

    public PostMultipart(String requestURL, String charset)
            throws IOException {
        this.charset = charset;

        boundary = "===" + System.currentTimeMillis() + "===";
        URL url = new URL(requestURL);
        httpConn = (HttpURLConnection) url.openConnection();
        httpConn.setUseCaches(false);
        httpConn.setDoOutput(true);
        httpConn.setDoInput(true);
        httpConn.setRequestProperty("Content-Type",
                "multipart/form-data; boundary=" + boundary);
        outputStream = httpConn.getOutputStream();
        writer = new PrintWriter(new OutputStreamWriter(outputStream, charset),
                true);
    }
    
    //Add form field
    public void addField(String name, String value) {
        writer.append("--" + boundary).append(LINE_FEED);
        writer.append("Content-Disposition: form-data; name=\"" + name + "\"")
                .append(LINE_FEED);
        writer.append("Content-Type: text/plain; charset=" + charset).append(
                LINE_FEED);
        writer.append(LINE_FEED);
        writer.append(value).append(LINE_FEED);
        writer.flush();
    }

　　　　　　　　//Add file
    public void addFile(String name, File uploadFile)
            throws IOException {
        String fileName = uploadFile.getName();
        writer.append("--" + boundary).append(LINE_FEED);
        writer.append(
                "Content-Disposition: form-data; name=\"" + name
                        + "\"; filename=\"" + fileName + "\"")
                .append(LINE_FEED);
        writer.append(
                "Content-Type: "
                        + URLConnection.guessContentTypeFromName(fileName))
                .append(LINE_FEED);
        writer.append("Content-Transfer-Encoding: binary").append(LINE_FEED);
        writer.append(LINE_FEED);
        writer.flush();

        FileInputStream inputStream = new FileInputStream(uploadFile);
        byte[] buffer = new byte[4096];
        int bytesRead = -1;
        while ((bytesRead = inputStream.read(buffer)) != -1) {
            outputStream.write(buffer, 0, bytesRead);
        }
        outputStream.flush();
        inputStream.close();
        writer.append(LINE_FEED);
        writer.flush();
    }
　　　　　　　　
　　　　　　　　//Add header
    public void addHeader(String name, String value) {
        writer.append(name + ": " + value).append(LINE_FEED);
        writer.flush();
    }
    
    //Run
    public List<String> post() throws IOException {
        List<String> response = new ArrayList<String>();
        writer.append(LINE_FEED).flush();
        writer.append("--" + boundary + "--").append(LINE_FEED);
        writer.close();

        // checks server's status code first
        int status = httpConn.getResponseCode();
        if (status == HttpURLConnection.HTTP_OK) {
            BufferedReader reader = new BufferedReader(new InputStreamReader(
                    httpConn.getInputStream()));
            String line = null;
            while ((line = reader.readLine()) != null) {
                response.add(line);
            }
            reader.close();
            httpConn.disconnect();
        } else {
            throw new IOException("Send Fail: " + status);
        }
        return response;
    }
}

How to use

try {
    //Initialize with upload destination and character code
    PostMultipart multipart = new PostMultipart("http://sample/upload/url", "UTF-8");

    //Normal parameter addition
    multipart.addField("hoge", "fuga");

    //For image files
    File picture_one = new File("/sample/picture/path.jpg ");
    multipart.addFile("file1", picture_one);

    //For multiple image files(It is assumed that the imageList contains multiple file paths)
    for (int i = 0; i < imageList.size(); i++) {
        File picture_multi = new File(imageList.get(i));
        multipart.addFile("file2[]", picture_multi);
    }
    
    //Get list for each response line
    List<String> response = multipart.post();
} catch (IOException e) {
    Log.e("ERROR", e.getMessage());
}

Released under the MIT license