gistfile1.java

import java.io.IOException;
import java.io.PrintWriter;
import java.io.OutputStream;
import java.io.OutputStreamWriter;
import java.io.InputStream;
import java.io.FileInputStream;
import java.io.File;
import java.net.URLConnection;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;

class MultipartPost {
    public static void main(String[] args) throws IOException, MalformedURLException {
        String charset = "UTF-8";
        URL url = new URL("http://localhost:3000");
        HttpURLConnection connection = (HttpURLConnection) url.openConnection();
        connection.setDoOutput(true);
        String boundary = Long.toHexString(System.currentTimeMillis()); // Just generate some unique random value.
        connection.setRequestProperty("Content-Type", "multipart/form-data; boundary=" + boundary);
        PrintWriter writer = null;

        String param = "value";
        File binaryFile = new File("[some file here]");
        String CRLF = "\r\n"; // Line separator required by multipart/form-data.

        try {
            OutputStream output = connection.getOutputStream();
            writer = new PrintWriter(new OutputStreamWriter(output, charset), true); // true = autoFlush, important!

            // Send normal param.
            writer.append("--" + boundary).append(CRLF);
            writer.append("Content-Disposition: form-data; name=\"param\"").append(CRLF);
            writer.append("Content-Type: text/plain; charset=" + charset).append(CRLF);
            writer.append(CRLF);
            writer.append(param).append(CRLF).flush();

            // Send binary file.
            writer.append("--" + boundary).append(CRLF);
            writer.append("Content-Disposition: form-data; name=\"file\"; filename=\"" + binaryFile.getName() + "\"").append(CRLF);
            writer.append("Content-Type: " + URLConnection.guessContentTypeFromName(binaryFile.getName())).append(CRLF);
            writer.append(CRLF).flush();
            InputStream input = null;
            try {
                input = new FileInputStream(binaryFile);
                byte[] buffer = new byte[1024];
                for (int length = 0; (length = input.read(buffer)) > 0;) {
                    output.write(buffer, 0, length);
                }
                output.flush(); // Important! Output cannot be closed. Close of writer will close output as well.
            } finally {
                if (input != null) try {
                    input.close();
                } catch (IOException logOrIgnore) {
                    System.out.println(logOrIgnore);
                }
            }
            writer.append(CRLF).flush(); // CRLF is important! It indicates end of binary boundary.

            // End of multipart/form-data.
            writer.append("--" + boundary + "--").append(CRLF);
        } finally {
            if (writer != null) writer.close();
        }
    }
}