Tuesday March 27, 2007

Spring cleaning and FTP

With OpenJDK upon us I figured it's about time I dusted my blog and start posting again since I intend for it to be one of the sources of information and discussion concerning the evolution of the networking stack (more on that later).

Anyway, since it's a question that comes up quite often these days, it occurred to me that the state of FTP support in J2SE would be a good subject for jump starting my writing efforts. So, without further ado:

FTP support in J2SE

Let's get something out of the way first: Yes I'm fully (dare I say painfully) aware that the current FTP API is somewhat lacking, to say the least. There is an RFE filed against it, 4650689 to be exact . It has rather high vote count (119 last time I checked and is on the top 25 RFEs), and, believe me, it hasn't gone unnoticed. I have been pushing for that RFE for a long time now and I have hopes that it will finally get approved soon. When that happens (fingers crossed) I'll post here the details of what we intend to do. And I'd like to personally thank all and everyone of you who voted for it and/or posted comments. It does help. A lot. So thanks again.

Meanwhile, let's have a look at we already have.

FTP is one of the protocol handlers mandated for the URL API. Which means that, for the time being, the only public FTP API in J2SE is through the URL and URLConnection API. In short, it means that to access a file on an FTP server you create a URL as specified in RFC 1738, get a URLConnection out of it, then use getInputStream() or getOutputStream() to get or put, respectively, the document. Did I say 'put'? Yep, I sure did! How many of you did know that you can do puts as well as gets? <counting...> Really, that many? I'm surprised. So, how many now knew that you can also provide a user name and password? An absolute file path? Or a type (binary for instance) of transfer?

Yes you can do all that. Let's look at a quick example:

URL url = new URL(“ftp://user:password@host.dom/%2Fetc/motd;type=a”);
URLConnection conn = url.openConnection();
PrintWriter out = new PrintWriter(conn.getOutputStream());
out.println(“Message of the day”);
out.close();

And voila! You just logged to an FTP server using a user name and password, then wrote to the /etc/motd file by doing a PUT in ASCII mode.

The important things to notice here are:

• Calling getOutputStream() instead of getInputStream() will trigger a PUT operation instead of a GET.

• If you need to authenticate when logging onto the server, prefix the hostname with user:password@

• If you need to use an absolute path use %2F (code for a slash) at the start of the file path.

• If you need to specify a type of transfer (default is binary) add ';type=<a,i,d>' at the end of the URL. 'a' means ASCII, 'i' Image (or binary) and 'd' means directory.

I'd like to add that the FTP protocol handler will also automatically try to use passive mode when possible, knows about IPv6 extensions and will, if the URL points to a directory, give you a listing of its content.

So, I hope you can see now that you can do many FTP operations through the URL API.

Like I said earlier, I know that many features are missing, like being able to do more than one operation per session, or rename a file, or use FTP over SSL and so on and so forth. But hopefully, not for much longer.