Poniendo ese error en google esto es lo que aparece
http://efreedom.com/Question/1-33355...ception-Thread Cita: There is code which eventually needs to touch a graphics card, or at least a working graphics subsystem in Java. If you run this code on a system which doesn't have graphics enabled, then you will throw a HeadlessException.
En ese caso en concreto están intentando utilizar swing en un servlet, auqnue parece que ese no es tu caso.