public class MultipartRequestSender {
private static final String BOUNDARY = "----WebKitFormBoundary7MA4YWxkTrZu0gW";

public static void sendMultipartRequest(String requestURL, File xmlFile) throws IOException {
    URL url = new URL(requestURL);
    HttpURLConnection connection = (HttpURLConnection) url.openConnection();
    connection.setUseCaches(false);
    connection.setDoOutput(true); // Indicates POST method
    connection.setDoInput(true);
    connection.setRequestMethod("POST");
    connection.setRequestProperty("Content-Type", "multipart/form-data; boundary=" + BOUNDARY);

    try (OutputStream outputStream = connection.getOutputStream();
         PrintWriter writer = new PrintWriter(new OutputStreamWriter(outputStream, "UTF-8"), true);
         FileInputStream fileInputStream = new FileInputStream(xmlFile)) {

        // Send XML file.
        writer.append("--" + BOUNDARY).append("rn");
        writer.append("Content-Disposition: form-data; name="xmlFile"; filename="" + xmlFile.getName() + """).append("rn");
        writer.append("Content-Type: text/xml; charset=UTF-8").append("rn");
        writer.append("rn").flush();
        byte[] buffer = new byte[4096];
        int bytesRead;
        while ((bytesRead = fileInputStream.read(buffer)) != -1) {
            outputStream.write(buffer, 0, bytesRead);
        }
        outputStream.flush(); // Important before continuing with writer!
        writer.append("rn").flush(); // CRLF is important! It indicates end of boundary.

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

    // Check server's response.
    System.out.println("Response: " + connection.getResponseCode() + " " + connection.getResponseMessage());

    try (BufferedReader reader = new BufferedReader(new InputStreamReader(connection.getInputStream()))) {
        String line = null;
        while ((line = reader.readLine()) != null) {
            System.out.println(line);
        }
    }

    connection.disconnect();
}

}

see the xml loaded in the java. Step-by-Step Guide to Sending Multipart Form Data with XML in Java

1. Add Required Libraries:

   • Java doesn’t provide built-in support for creating multipart form data easily, so it’s common to use org.apache.http libraries, which greatly simplify the process. However, if you want to stick with HttpsURLConnection, you will need to manually construct the multipart body.

2. Prepare Your Java Environment:

   • If you choose to use Apache libraries, you can add them via Maven. If not, ensure your Java environment is set up to compile and run HttpsURLConnection code.

3. Java Method Using HttpsURLConnection: Here is a simple Java method that constructs and sends a POST request with multipart/form-data containing an XML file: