How to post files with HttpURLConnection [Android/Java]

This article describes “How to post file data(multipart) with java.net.HttpURLConnection”.

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

public class MultipartUtility {
    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 
     * @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;
    }
}

🐡 Special Thanks

android - Sending files using POST with HttpURLConnection - Stack Overflow

server {
listen 8080;
return 301 https://$host$request_uri;
}

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