public void onEvent(ServiceInstancesChangedEvent event) {
        if (destroyed.get() || !accept(event) || isRetryAndExpired(event)) {
            return;
        }
        doOnEvent(event);
    }
@Test
    @Order(12)
    public void testInstanceWithoutRevision() {
        Set<String> serviceNames = new HashSet<>();
        serviceNames.add("app1");
        ServiceDiscovery serviceDiscovery = Mockito.mock(ServiceDiscovery.class);
        listener = new ServiceInstancesChangedListener(serviceNames, serviceDiscovery);
        ServiceInstancesChangedListener spyListener = Mockito.spy(listener);
        Mockito.doReturn(null).when(metadataService).getMetadataInfo(eq(null));
        ServiceInstancesChangedEvent event = new ServiceInstancesChangedEvent("app1", app1InstancesWithNoRevision);
        spyListener.onEvent(event);
        // notification succeeded
        assertTrue(true);
    }