"service" is null @WebServiceRef

Hello Experts,

I'm using '@WebServiceRef' in my Web Module.
Sometimes the 'service' object is null.

public class WsStaffMobile {
 @WebServiceRef (name="WsAuthUserWebService") WsAuthUserService service;

 public UserDetails getUserDetails(@WebParam(name = "userName") String userName, @WebParam(name = "password") String password) {
  WsAuthUser port = service.getWsAuthUserPort();
  UserDetails userDetails = port.getUserDetails(userName, password);
  return userDetails;
 }

As a workaround I'm using JNDI (from this thread: http://scn.sap.com/thread/1716058) and it's working but how can I use annotation and injection and reinitialize the object (if needed)?

J2EE version is 7.31 SP7.

Regards,
Omri