I am trying to send a multipart/related message through http post method using URLConnection.

Here is some code that might help you
It can POST params and files together.
Try to change application/octet-stream by application/xml

Julien Guisset, paris FRANCE

/*
 * EssaiPostURLConnection.java
 *
 * Created on 21 janvier 2002, 12:14
 */

/**
 *
 * @author  guisset
 */

import java.net.*;
import java.io.*;

public class EssaiPostURLConnection {
    
    private static void writeParam(String name, String value, DataOutputStream out, String boundary) {
        try {
            out.writeBytes("content-disposition: form-data; name="" + name + ""

");
            out.writeBytes(value);
            out.writeBytes("
" + "--" + boundary + "
");
        } catch (Exception e) {  System.out.println(e.toString());      }
    }
    
    private static void writeFile(String name, String filePath, DataOutputStream out, String boundary) {
        try {
        out.writeBytes("content-disposition: form-data; name="" + name + ""; filename=""
                            + filePath + ""
");
            out.writeBytes("content-type: application/octet-stream" + "

");
            FileInputStream fis = new FileInputStream(filePath);
            while (true) {
                synchronized (buffer) {
                    int amountRead = fis.read(buffer);
                        if (amountRead == -1) {
                            break;
                        }
                    out.write(buffer, 0, amountRead); 
                    }
            }
            fis.close();
            out.writeBytes("
" + "--" + boundary + "
");
             } catch (Exception e) {  System.out.println(e.toString());      }
    }
    
    static final int BUFF_SIZE = 1024;
    static final byte[] buffer = new byte[BUFF_SIZE];
    
    /** Creates a new instance of EssaiPostURLConnection */
    public EssaiPostURLConnection() {
    }

    /**
    * @param args the command line arguments
    */
    public  static void main (String args[]) {
        try {
            URL servlet = new URL("http://127.0.0.1/pu");            
            URLConnection conn=servlet.openConnection();
            conn.setDoOutput(true);
            conn.setDoInput(true);
            conn.setUseCaches(false);
            String boundary = "---------------------------7d226f700d0";
            conn.setRequestProperty("Content-type","multipart/form-data; boundary=" + boundary);
            conn.setRequestProperty("Referer", "http://127.0.0.1/index.jsp");
            conn.setRequestProperty("Cache-Control", "no-cache");
             
            DataOutputStream out = new DataOutputStream(conn.getOutputStream());
            out.writeBytes("--" + boundary + "
");
            writeParam("nom", "julien", out, boundary);
            writeParam("poste", "stagiaire", out, boundary);
            writeFile("superFichier", "C:	mpfoo.txt", out, boundary);
            out.flush();
            out.close();
            
            InputStream stream = conn.getInputStream();
            BufferedInputStream in = new BufferedInputStream(stream);
            int i = 0;            
            while ((i = in.read()) != -1) {
                System.out.write(i);            
            }            
            in.close();
        } catch (Exception e) {  
            System.out.println(e.toString());
        }
    }
}