Here's a very simple example of writing to file in a servlet inside the doPost method:

public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException
{
    String filepath = "C:/somepath/that/you/want/to/write/to.txt";
    File filetowriteto = new File(filepath);

    //open up a stream; using printwriter because it easy
    PrintWriter outtofile = new PrintWriter(new FileOutputStream(filetowriteto));

    //write something
    outtofile.println("Hello, World!");

    //be nice a put your toys away when you are done playing with them
    outtofile.close();

    //write out something to the user
    .
    .
    .
}

-John

There are variour methods on the http request object such as getPort getServerName etc.. u can also get the ServletContext and using the File Object create the File on the Server