Kerb5Authenticator auth = new Kerb5Authenticator((Subject) null);
        SmbUnsupportedOperationException ex = assertThrows(SmbUnsupportedOperationException.class, auth::refresh);
        assertTrue(ex.getMessage().contains("Refreshing credentials is not supported"));
    }

    @Test
    @DisplayName("clone: copies all relevant fields")
    void clone_copiesFields() {
        Subject subj = new Subject();

            Subject copySubj = copy.getSubject();
            assertNotNull(copySubj);
            assertSame(cachedOrig, copySubj, "Clone should share the cached Subject reference");

            // Refreshing the original must not clear the clone's cached subject
            orig.refresh();
            // Clone should retain its cached Subject

    }

    @Override
    public void refresh() throws CIFSException {
        // custom Kerb5Authenticators need to override this method for support
        throw new SmbUnsupportedOperationException("Refreshing credentials is not supported by this authenticator");
    }

    @Override
    public Kerb5Authenticator clone() {
        Kerb5Authenticator auth = new Kerb5Authenticator(getSubject());

    }

    /**
     * {@inheritDoc}
     *
     * @see jcifs.smb.Kerb5Authenticator#refresh()
     */
    @Override
    public void refresh() throws CIFSException {
        log.debug("Refreshing JAAS credentials");
        this.cachedSubject = null;
    }

    @Override
    public synchronized Subject getSubject() {
        if (this.cachedSubject != null) {
            return this.cachedSubject;