code: public boolean isRequestedSessionIdValid()
      {
          if (WCCustomProperties.CHECK_REQUEST_OBJECT_IN_USE){
              checkRequestObjectInUse();
          }
          // 321485
          boolean sessionInvalid = _requestContext.isRequestedSessionIdValid(((WebAppDispatcherContext) this.getDispatchContext()).getWebApp());
          if (TraceComponent.isAnyTracingEnabled()&&logger.isLoggable (Level.FINE)) {  //306998.15
              logger.logp(Level.FINE, CLASS_NAME,"isRequestedSessionIdValid", " " + String.valueOf(sessionInvalid));
          }
          return sessionInvalid;
      }
docs: If the session doesn't exist, then the Id that came
  in is invalid. If there is no sessionID in the request, then
  it's not valid.