Using Swing applet to write data to file on SERVER

Question

Hello,
I'm in the process of writing an applet using Swing (SDK v1.4.2). Here's the deal. The JApplet calls a JPanel, which will be used by customers on my site to enter data. I'm adding a <Save> button to the "form" which is supposed to write data to a file on the server (in order to preserve the customer's preferences).
Please note that I am NOT attempting to write data to the customer's hard disk. That requires digital certificates, etc. which I am trying to avoid for now.
Instead, I am using the URL class to access the file:
URL page = new URL("http://www.whatever.com/mycustomers/preferences.txt")
I then use the URLConnection class to establish the connection:
URLConnection conn = this.page.openConnection();
conn.setDoInput(true);
conn.setDoOutput(true);
conn.connect();
etc...
I've created a text file (preferences.txt) on my web site. Using the classes InputStreamReader, BufferedStreamReader, and StringBuffer, I can successfully read the file into a JOptionPane in my applet.
The problem comes when I try to write data TO this file. I know the file exists because the applet can read it. I've set the permissions on the file to 666. I've got all of the appropriate syntax within a try statment that catches an IOException. I also have JOptionPanes all over the place to let me know where the program is. I've tried different combinations of output streams like BufferedWriter, BufferedOutputStream, StringWriter, but the file does not get updated. When the applet runs, it does not throw any exceptions, not even when I change the URL from "HTTP://www.whatever.com/prefs.txt" to "HTTP:/www.whatever.com/prefs.txt" (only one slash on HTTP, shouldn't I get a MalformedURLException?)
I apologize for all the background, but I thought you might need it. The bottom line is:
1) Can an applet write to a file on a remote server (not local hard disk)?
2) If so, what (if any) caveats are there?
3) Is there a way to check for file existence or be able to create a new file?
4) I'm using the HTTP protocol - is there some restriction that prevents an applet from writing to a file using that protocol? If so, is there a workaround?
5) Assuming that creating/writing a file using the method I've described is possible, what would be the appropriate output streams to use? (Currently, I'm using OutputStreamWriter with BufferedWriter).
I've been struggling with this for a while. Any help/suggestions would be appreciated.
Thanks
P.S. I also posted this message on the Applet development forum, but I've received no response as of yet.

Answer 1

Http servers support PUT as a mechanism to upload data to a specified URL. Get on the other hand which is what most people are familiar with is how you retrieve that data. The basic URLConnection is an abstraction of the Http connection which can be used for GET and POST operations by default based on doInput(true|false).. If you which to use any of the http methods other than GET|POST you will have to cast the URLConnection to HttpURLConnection so you can gain access to the specific Http functionaility that could not be abstracted.
Since you are using a hosting service the chances are that you won't be able to use HTTP PUT on their server. Most servers do not support HTTP PUT without configuring them todo so. Now Apache allows localized config through the .htacess file. It might be possible (keep in mind I am not an apache expert) to configure a particular directory to allow HTTP PUT using this .htacess file it may not be possible. You will have to consult the Apache web server documentation for that answer.
But regardless you can use the HttpURLConnection and the PUT method to send data from your applet to the server. In fact that is the preferred way to do this. If you can not configure your web server to support that method then you will have to develop a Servlet to do it. A servlet has several methods such as doGet(), doPost(), and doPut(). You would override the doPut() method get the URI path create a file and a FileOutputStream to that file, get from the request the inputstream, possibly skip the http headers, and then write byte for byte the incoming data to your OutputStream until you reach the end at which point you would close the OutputStream and send an Http Response of some sort. Typically in HTTP it would be 200 OK plus some web content. You content can be empty so long as your applet recognizes that is what it should expect...
And away you go...