@Test(expected = IllegalStateException.class)
    public void testSubscribeAfterClose() {
        ConfigSubscriber subscriber = new ConfigSubscriber();
        subscriber.subscribe(AppConfig.class, CONFIG_ID);
        subscriber.nextConfig(false);
        subscriber.close();
        subscriber.subscribe(AppConfig.class, CONFIG_ID);
    }
Verifies that we get an exception when trying to subscribe after close() has been called
for a ConfigSubscriber
config/src/test/java/com/yahoo/config/subscription/ConfigApiTest.java