@Test(expected = IllegalStateException.class)
    public void testSubscribeAfterNextConfig() {
        ConfigSubscriber subscriber = new ConfigSubscriber();
        subscriber.subscribe(AppConfig.class, CONFIG_ID);
        subscriber.nextConfig(false);
        subscriber.subscribe(AppConfig.class, CONFIG_ID);
        subscriber.close();
    }
Verifies that it is not possible to to subscribe again after calling nextConfig()
config/src/test/java/com/yahoo/config/subscription/ConfigApiTest.java