code: public static String getCurrentUser() {
          ServiceContext ctx = ServiceContextStore.get();
          String currentUser;
          if (ctx == null) {
              currentUser = ServiceContextFactory.SYSTEM_USER;
          } else {
              currentUser = ctx.getUserId();
          }
  
          if (currentUser == null) {
              currentUser = ServiceContextFactory.UNKNOWN_USER;
          }
          return currentUser;
      }
docs: Current user from ServiceContext. If ServiceContext is not set then
  {@link ServiceContextFactory.SYSTEM_USER} is returned.