How to Upload Files with Additional Parameters from a Java Client Using `multipart/form-data` Encoding Without Third-Party Libraries?

Uploading Files with Additional Parameters from Java Client to HTTP Server

In the pursuit of uploading files along with additional parameters from a Java client to an HTTP server, let's explore a scenario and its solution.

Imagine you want to pass a file and a parameter called "username" to a server. How would you achieve this using a POST request with multipart/form-data encoding?

To keep things straightforward, let's steer clear of third-party libraries and rely on Java's built-in tools.

<code class="java">import java.io.File;
import java.io.OutputStream;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.net.URLConnection;
import java.nio.file.Files;
import java.nio.file.Path;
import java.util.Scanner;

public class HttpFileUploadWithParameters {

    private static final String BOUNDARY = Long.toHexString(System.currentTimeMillis());
    private static final String CRLF = "\r\n";
    private static final String CHARSET = "UTF-8";

    public static void main(String[] args) throws Exception {
        String url = "http://example.com/upload";

        File file = new File("/path/to/file.txt");
        String parameter = "value";

        URLConnection connection = new URL(url).openConnection();
        connection.setDoOutput(true);
        connection.setRequestProperty("Content-Type", "multipart/form-data; boundary=" + BOUNDARY);

        try (OutputStream output = connection.getOutputStream();
             PrintWriter writer = new PrintWriter(new OutputStreamWriter(output, CHARSET), true)) {

            // Write parameter
            writer.append("--" + BOUNDARY).append(CRLF);
            writer.append("Content-Disposition: form-data; name=\"parameter\"").append(CRLF);
            writer.append("Content-Type: text/plain; charset=" + CHARSET).append(CRLF);
            writer.append(CRLF).append(parameter).append(CRLF).flush();

            // Write file
            writer.append("--" + BOUNDARY).append(CRLF);
            writer.append("Content-Disposition: form-data; name=\"file\"; filename=\"" + file.getName() + "\"").append(CRLF);
            writer.append("Content-Type: application/octet-stream").append(CRLF);
            writer.append(CRLF).flush();
            Files.copy(file.toPath(), output);
            output.flush(); // Important before continuing with writer!
            writer.append(CRLF).flush(); // CRLF is important! It indicates end of boundary.

            // End of multipart/form-data.
            writer.append("--" + BOUNDARY + "--").append(CRLF).flush();
        }

        // Request is lazily fired whenever you need to obtain information about response.
        int responseCode = ((java.net.HttpURLConnection) connection).getResponseCode();
        System.out.println(responseCode); // Should be 200
    }
}</code>

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