When applet is created, surface is also created. As I see in the code P2D/P3D/Java2D modes always somewhere call getScreenDevices or getDefaultScreenDevice functions. They throw HeadlessException when called in headless mode. So in your case you should use virtual framebuffer.

The second option is to use Clojure2d as an alternative. I created this library having exactly your problem in mind. Downside is Java2d mode only.