It is simplicity itself from an (JWS or non-JWS) applet..

<sscce>
import java.applet.Applet;
import java.awt.Label;
import java.net.*;

public class LaunchWebStartApp extends Applet {

 static URL url;
 static String address = "http://www.physci.org/jnlp/Slider.jnlp";

 public void init() {
   add( new Label("Install Slider") );
   try {
     url = new URL(address);
     getAppletContext().showDocument(url);
   } catch(MalformedURLException murle) {
     murle.printStackTrace();
   }
 }
}
</sscce>

You could do the same thing from an application by calling
BrowserLauncher, but that is rather kludgy.

If you discover better ways to do it, I'd love to hear hem.