IPersistenceService ps = SERVICES.getService(IPersistenceService.class);
Map<String, Object> user = ps.loadSimple("User", "Username", username);

ServerJob job = new ServerJob("security-filter-job", new ServerSession()) {

  @Override
  protected IStatus runTransaction(IProgressMonitor monitor) throws Exception {
    //Here you can call SERVICES.getService(..)

    //Return a status depending on your query in your IPersistenceService
    return Status.OK_STATUS;
  }
};
IStatus status = job.runNow(new NullProgressMonitor());
job.throwOnError();

//Use the status to decide if the user is valid or not.