When adding multiplayer to your game, you may find that your server needs to know about game state (e.g. where are players, objects? Was that a direct hit? etc.) You can code all this up yourself, but there’s an easier way.

It’s very easy to change your current (client) game to function as a server as well.

What Does Headless Mean?

A headless server…

  • does not display any output – no window opens, no audio plays, no graphics are rendered.

  • ignores all input – no input handling.

  • keeps game state – you can attach to, transform, and save the rootNode, although the scene is not displayed.

  • calls the simpleUpdate() loop – you can run tests and trigger events as usual.

Client Code

First, let’s take a look at the default way of creating a new game (in its simplest form):

public static void main(String[] args) {
  Application app = new Main();
  app.start();
}

Headless Server Code

Now, with a simple change you can start your game in Headless mode. This means that all input and audio/visual output will be ignored. That’s a good thing for a server.

import com.jme3.system.JmeContext;
import com.jme3.system.JmeContext.Type;

public static void main(String[] args) {
  Application app = new Main();
  app.start(JmeContext.Type.Headless);
}

Next steps

Okay, so you can now start your game in a headless 'server mode', where to go from here?

  • Parse String[] args from the main-method to enable server mode on demand (e.g. start your server like java -jar mygame.jar –server.

  • Integrate SpiderMonkey, to provide game updates to the server over a network.

  • Only execute code that’s needed. (E.g. place all rendering code inside an if (servermode)-block) (or if (!servermode) for the client).

  • Add decent logging so your server actually makes sense.