Example demonstrating how to use PhantomReference to detect memory leak

PhantomReferenceLeakDetector.java

    static class Activity {
        interface Listener {}
        Service service;

        Activity(Service service) {
            this.service = service;
        }
        void onStart() {
            service.registerListener(new Listener() {});//Listener hold a reference to Activity

        }
        void onStop () {
            service.unregisterListener();
            service = null;
        }
    }

    static  class Service {
        Activity.Listener listener;

        void registerListener (Activity.Listener listener) {
            this.listener = listener;
        }
        void unregisterListener () {
            this.listener = null; // comment this to cause a leak
        }
    }
    
    
    // we want to make sure that Service will not leak the Activity 
    // because of the registered anonymous callback 
    public void test () throws InterruptedException {
        Service service = new Service();
        Activity activity = new Activity(service);
        activity.onStart();

        // store a PhantomReference to watch the lifecycle of listener
        ReferenceQueue<Activity.Listener> referenceQueue = new ReferenceQueue<Activity.Listener>();
        PhantomReference<Activity.Listener> reference = 
            new PhantomReference<Activity.Listener>(service.listener, referenceQueue); // no need to store this reference since it's a root object (in stack memory)

        activity.onStop();
//        reference = null; // uncomment to demonstrate why it's important to keep a strong reference to the PhantomReference
        activity = null; // at this point we removed the strong reference to our Activity,
        // service should not leak the Activity & Activity should be GC'd

        // trigger the collection of PhantomReference
        Runtime.getRuntime().gc();

        Reference<?> ref = referenceQueue.remove(TimeUnit.SECONDS.toMillis(10));
        assertNotNull(ref);
        ref.clear();
    }