code: public ServiceRefType<WebAppDescriptor> getOrCreateServiceRef()
     {
        List<Node> nodeList = model.get("service-ref");
        if (nodeList != null &&  nodeList.size() > 0)
        {
           return new ServiceRefTypeImpl<WebAppDescriptor>(this, "service-ref", model, nodeList.get(0));
        }
        return createServiceRef();
     }
docs: If not already created, a new <code>service-ref</code> element will be created and returned.
  Otherwise, the first existing <code>service-ref</code> element will be returned.
  @return the instance defined for the element <code>service-ref</code>