Hands-On Reactive Programming in Spring 5

Observer pattern usage example

Now, let's write a simple JUnit test that uses our classes and demonstrates how all of them play together. Also, in the following example, we are using the Mockito library (http://site.mockito.org) in order to verify expectations with the support of the Spies Pattern:

@Testpublic void observersHandleEventsFromSubject() {   // given   Subject<String> subject = new ConcreteSubject();   Observer<String> observerA = Mockito.spy(new ConcreteObserverA());   Observer<String> observerB = Mockito.spy(new ConcreteObserverB());   // when   subject.notifyObservers("No listeners");   subject.registerObserver(observerA);   subject.notifyObservers("Message for A");   subject.registerObserver(observerB); subject.notifyObservers("Message for ...