code: @Override
      public Object preInvoke() throws SecurityViolationException {
          Subject invokedSubject = subjectManager.getInvocationSubject();
          Subject receivedSubject = subjectManager.getCallerSubject();
  
          subjectManager.clearSubjects();
  
          return null;
      }
docs: This preInvoke is called at the start of every request in
  WebContainer.handleRequest to null out the thread. {@inheritDoc}