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An open source Java client that gives you a simple binding to interact with BigML. You can use it to easily create, retrieve, list, update, and delete BigML resources.
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package org.bigml.binding.utils;

import java.io.*;
import java.net.HttpURLConnection;
import java.net.URL;
import java.net.URLConnection;
import java.security.KeyManagementException;
import java.security.NoSuchAlgorithmException;
import java.util.ArrayList;
import java.util.List;

/**
 * This utility class provides an abstraction layer for sending multipart HTTP
 * POST requests to a web server.
 * @author www.codejava.net
 *
 */
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	the url of the request
     * @param charset		charset
     * 
     * @throws IOException an IO exception
     */
    public MultipartUtility(String requestURL, String charset)
            throws IOException {
        this.charset = charset;

        // creates a unique boundary based on time stamp
        boundary = "===" + System.currentTimeMillis() + "===";

        URL url = new URL(requestURL);
//        httpConn = (HttpURLConnection) url.openConnection();
        try {
            httpConn = Utils.openConnection(url);
        } catch (NoSuchAlgorithmException e) {
            throw new IOException("Unable to open the connection !!!", e);
        } catch (KeyManagementException e) {
            throw new IOException("Unable to open the connection !!!", e);
        }

        httpConn.setRequestMethod("POST");
        httpConn.setUseCaches(false);
        httpConn.setDoOutput(true); // indicates POST method
        httpConn.setDoInput(true);
        httpConn.setRequestProperty("Content-Type",
                "multipart/form-data; boundary=" + boundary);
        httpConn.setRequestProperty("User-Agent", "CodeJava Agent");
        httpConn.setRequestProperty("Test", "Bonjour");
        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("--" + 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);
        writer.flush();
    }

    /**
     * Adds a upload file section to the request
     * 
     * @param fieldName name attribute
     * @param uploadFile a File to be uploaded
     * 
     * @throws IOException an IO exception
     */
    public void addFilePart(String fieldName, File uploadFile)
            throws IOException {
        String fileName = uploadFile.getName();
        writer.append("--" + boundary).append(LINE_FEED);
        writer.append(
                "Content-Disposition: form-data; name=\"" + fieldName
                        + "\"; 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();
    }

    /**
     * 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 + ": " + 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 an IO exception
     */
    public HttpURLConnection finish() throws IOException {
        List response = new ArrayList();

        writer.append(LINE_FEED).flush();
        writer.append("--" + boundary + "--").append(LINE_FEED);
        writer.close();

        return httpConn;
//        // 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);
//        }
//
//        return response;
    }
}