Sending an MMS via Zipwhip's API using Java.

Main.java

import java.io.*;
import java.nio.file.Files;
import java.util.List;

public class Main {

    public static void main(String[] args) {

        String charset = "UTF-8";
        File uploadFile1 = new File("/Users/alancapps/Desktop/test.jpg");
        String requestURL = "https://api.zipwhip.com/messaging/send";

        String session = "sessionKey";
        String to = "+12065551212";
        String body = "testBody";

        StringBuilder stringBuilder = new StringBuilder();
        stringBuilder.append(requestURL).append("?session=").append(session).append("&to=").append(to).append("&body=").append(body);

        try {
            MultipartUtility multipart = new MultipartUtility(stringBuilder.toString(), charset);

//            multipart.addFilePart("image", uploadFile1);
            multipart.addByteArrayFilePart("image", "test", Files.readAllBytes(uploadFile1.toPath()));

            List<String> response = multipart.finish();

            System.out.println("SERVER REPLIED:");

            for (String line : response) {
                System.out.println(line);
            }
        } catch (IOException ex) {
            System.err.println(ex);
        }
    }
}

MultipartUtility.java

import java.io.*;
import java.net.HttpURLConnection;
import java.net.URL;
import java.net.URLConnection;
import java.util.ArrayList;
import java.util.List;
import java.util.UUID;

/**
 * This utility class provides an abstraction layer for sending multipart HTTP`
 * POST requests to a web server.
 *
 * @author www.codejava.net
 * http://www.codejava.net/java-se/networking/upload-files-by-sending-multipart-request-programmatically
 */
public class MultipartUtility {
    private final String boundary;
    private static final String LINE_FEED = "\r\n";
    private HttpURLConnection httpConn;
    private String charset;
    private OutputStream outputStream;
    private PrintWriter writer;

    /**
     * This constructor initializes a new HTTP POST request with content type
     * is set to multipart/form-data
     *
     * @param requestURL
     * @param charset
     * @throws IOException
     */
    public MultipartUtility(final String requestURL, final String charset)
            throws IOException {
        this.charset = charset;

        // creates a unique boundary based on time stamp
        boundary = "------" + UUID.randomUUID().toString() + "------";

        URL url = new URL(requestURL);
        httpConn = (HttpURLConnection) url.openConnection();
        httpConn.setUseCaches(false);
        httpConn.setDoOutput(true); // indicates POST method
        httpConn.setDoInput(true);
        httpConn.setRequestProperty("Content-Type",
                "multipart/form-data; boundary=" + boundary);
        outputStream = httpConn.getOutputStream();
        writer = new PrintWriter(new OutputStreamWriter(outputStream, charset),
                true);
    }

    /**
     * Adds a form field to the request
     *
     * @param name  field name
     * @param value field value
     */
    public void addFormField(String name, String value) {
        writer.append("--")
                .append(boundary)
                .append(LINE_FEED);
        writer.append("Content-Disposition: form-data; name=\"")
                .append(name)
                .append("\"")
                .append(LINE_FEED);
        writer.append("Content-Type: text/plain; charset=")
                .append(charset)
                .append(
                LINE_FEED);
        writer.append(LINE_FEED);
        writer.append(value).append(LINE_FEED);
        writer.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(final String fieldName, final File uploadFile)
            throws IOException {
        final String fileName = uploadFile.getName();
        writeContentDisposition(fieldName, fileName);

        final 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();
    }

    public void addByteArrayFilePart(final String fieldName, final String fileName, final byte[] file) throws IOException {

        writeContentDisposition(fieldName, fileName);

        outputStream.write(file);
        outputStream.flush();

        writer.append(LINE_FEED);
        writer.flush();
    }

    public void writeContentDisposition(final String fieldName, final String fileName) {
        writer.append("--")
                .append(boundary)
                .append(LINE_FEED);
        writer.append("Content-Disposition: form-data; name=\"")
                .append(fieldName)
                .append("\"; filename=\"")
                .append(fileName)
                .append("\"")
                .append(LINE_FEED);
        writer.append("Content-Type: ")
                .append(URLConnection.guessContentTypeFromName(fileName))
                .append(LINE_FEED);
        writer.append("Content-Transfer-Encoding: binary").append(LINE_FEED);
        writer.append(LINE_FEED);
        writer.flush();
    }

    /**
     * Adds a header field to the request.
     *
     * @param name  - name of the header field
     * @param value - value of the header field
     */
    public void addHeaderField(String name, String value) {
        writer.append(name)
                .append(": ")
                .append(value)
                .append(LINE_FEED);
        writer.flush();
    }

    /**
     * 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 List<String> finish() throws IOException {
        List<String> response = new ArrayList<String>();

        writer.append(LINE_FEED).flush();
        writer.append("--")
                .append(boundary)
                .append("--")
                .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("Server returned non-OK status: " + status + "\nBody: " + httpConn.getResponseMessage());
        }

        return response;
    }
}