FULL PRODUCT VERSION :
java version "1.6.0"
Java(TM) SE Runtime Environment (build 1.6.0-b105)
Java HotSpot(TM) Client VM (build 1.6.0-b105, mixed mode, sharing)

ADDITIONAL OS VERSION INFORMATION :
Microsoft Windows [Version 5.2.3790]

EXTRA RELEVANT SYSTEM CONFIGURATION :
Internet - Standard FTP Server
DMZ - Apache HTTP Server, using mod_proxy as HTTP proxy server
Local network - Client machine running the java app where exception occur.

A DESCRIPTION OF THE PROBLEM :
I'm trying to write a file from my client app via an HTTP proxy to a FTP server on the Internet.


STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Use supplied test program using your own URL and HTTP proxy server.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Create a new file on the FTP server named text.txt containing "Test".
ACTUAL -
The error message.

ERROR MESSAGES/STACK TRACES THAT OCCUR :
java.net.ProtocolException: cannot write to a URLConnection if doOutput=false -
call setDoOutput(true)
        at sun.net.www.protocol.http.HttpURLConnection.getOutputStream(Unknown Source)
        at sun.net.www.protocol.ftp.FtpURLConnection.getOutputStream(Unknown Source)
        at FtpTest.main(FtpTest.java:24)

REPRODUCIBILITY :
This bug can be reproduced always.

---------- BEGIN SOURCE ----------
import java.io.OutputStream;
import java.net.InetSocketAddress;
import java.net.Proxy;
import java.net.URL;
import java.net.URLConnection;
import java.net.Proxy.Type;

public class FtpTest
{

   private static final String FILE_URL = "ftp://user:###@###.###/test.txt";

   public static void main(String[] args)
   {
      try
      {
         URL url = new URL(FILE_URL);
         Proxy proxy = new Proxy(Type.HTTP, new InetSocketAddress("proxy", 8080));
         URLConnection connection = url.openConnection(proxy);
         connection.setDoOutput(true);
         OutputStream outputStream = connection.getOutputStream();
         outputStream.write("Test".getBytes());
         outputStream.close();
      }
      catch (Exception e)
      {
         e.printStackTrace();
      }
   }
}

---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Using a 3rd party ftp package.