FTP site works but I'm unable to connect from Java program. I get java.net.UnknownHostException

Question

Greetings folks.

I have this weird problem. In the project I'm working on now, I need to upload a file to a FTP site. I have written many programs that do this before and they have worked just fine. But this particular site is giving me trouble. When I try to upload the file from the program, I get this to be specific:

java.net.UnknownHostException: ftp://site.com

However when I try to connect to the same site from within a browser (Chrome, IE) or from windows explorer, I'm able to find the site and login just fine. I tired posting a picture, but I was prevented from doing so as I'm a newbie.

So I'm stumped now. If I could not connect from windows, then I can assume it is a FTP server issue. This happens to me only from the Java program. And I also know that my code works as I have used in numerous occasions before. Here is the code I use:

public void uploadFile(String fileName) throws Exception {
        FileTransferClient ftpClient = null;

        try {
            ftpClient = new FileTransferClient();
            ftpClient.setRemoteHost(gv.ftpHost);
            ftpClient.setRemotePort(21);
            ftpClient.setUserName(gv.ftpUserName);
            ftpClient.setPassword(gv.ftpPassword);
            ftpClient.getAdvancedFTPSettings().setConnectMode(FTPConnectMode.PASV);
            ftpClient.connect();

            ftpClient.uploadFile(gv.irp + fileName, fileName, WriteMode.OVERWRITE);
        }
        catch (Exception e) {
            throw new Exception("Error occured in uploadFile()\n" + e);
        }
        finally {
            if (ftpClient != null) {
                if (ftpClient.isConnected()) {
                    ftpClient.disconnect();
                }
                ftpClient = null;
            }
        }
    }

I use the edtFTPj library. My environment is Eclipse Helios (32 bit) on Java 1.6 (32 bit) running from a Windows 7 64 bit machine.

Any insight on resolving this will be greatly appreciated. Thanks for your time.

Answer 1

The message

java.net.UnknownHostException: ftp://site.com

suggests quite strongly that you're trying to open a connection to a host named "ftp://site.com", which is unfortunately a url rather than a host name and is thus not found.

Try changing your code so that it connects to "site.com".

Answer 2

The UnknownHostException means that the Java networking library is unable to convert the host name supplied into an IP address, via doing a DNS lookup.

Do you have some kind of proxy server configured for your browsers? All I can think of is that you must have some alternative networking configuration set up, if some applications (browsers) can resolve the hostname and others (Java) cannot.

See How do I configure proxy settings for Java? if you do need to set up a proxy for your Java process.