Question

ex) Authenticate users on their desktop app from a server.

Questions about server: Do I have to use Tomcat ? Are there any other solutions ? could I even use Apache ?

Answer 1

You need some way in your Java app to download the pages at specific given URLs.

URL url;
InputStream is = null;
DataInputStream dis;
String line;

try {
    url = new URL("https://stackoverflow.com/");
    is = url.openStream();  // throws an IOException
    dis = new DataInputStream(new BufferedInputStream(is));

    while ((line = dis.readLine()) != null) {
        System.out.println(line);
    }
} catch (MalformedURLException mue) {
     mue.printStackTrace();
} catch (IOException ioe) {
     ioe.printStackTrace();
} finally {
    try {
        is.close();
    } catch (IOException ioe) {
        // nothing to see here
    }
}

(taken from How do you Programmatically Download a Webpage in Java)

After that, it seems to me that any web server that will accept URLs with arguments will work, with a script on the server (any language) to accept requests. All you need is a response, and to maintain the session on the server.

Example: http://myserver.com/myscript.php?action=login&username=blah&password=blah would return a page saying if the action succeeded or not, and you would parse this.