Advanced Techniques for concurrency and memory management

Code samples:
Looper: https://gist.github.com/nhachicha/e993fe9b5f09f0595ccd
PhantomReference: https://gist.github.com/nhachicha/4ba780712a7ae179cc67
Synchronizers: https://gist.github.com/nhachicha/7596e912c81aae38d721

Managing memory & concurrency are one of the biggest challenges we face when it comes to writing testable & efficient apps.
based on the experience of building Realm, We'll dig into some patterns & tricks used to control memory footprint & multithreading.

Some of the topics this talks will cover are:

How to use the Java Reference API (WeakReference & ReferenceQueue) to work with the GC
Writing tests involving the GC
Working & testing code using the Handler/Looper api
Alternative asynchronous models (Messenger/ResultReceiver)
Threadpool customisation & pitfalls
Handler (mocking & testing, using IdleHandler)

Transcript

1. Advanced techniques for concurrency & memory management +NabilHachicha @nabil_hachicha Droidcon – October 2015
2. Agenda • Item 1: GC & Reference API • Item 2: Asynchronous communication • Item 3: Looper & Handler • Item 4: Synchronizers • Q/A
3. Item 1: GC & Reference API
4. WeakHashMap aka LeakMap for some Map<Object, String> map = new WeakHashMap<Object, String>(); Object key = new Object(); map.put(key, "xyz"); key = null;
5. WeakHashMap class Key {} class Value { Key key; } Map<Key, Value> map = new WeakHashMap<Key, Value>(); Key key = new Key(); Value value = new Value(); map.put(key, value); // this will leak (value is a strong reference) value.key = key; leaking the key
6. WeakHashMap defensive copy (aka copy constructor) com.google.android.gms.maps.model.Polygon polygon = ...; Map<List<LatLng>, Object> map = new WeakHashMap<List<LatLng>, Object>(); map.put(polygon.getPoints(), new Object());
7. WeakHashMap literal String as Key WeakHashMap<String, Object> map = new WeakHashMap<String, Object>(); String key = "I'm pooled!"; map.put(key, new Object()); key = null;
8. WeakHashMap Boxed type private static class IntegerCache { static final int low = -128; static final int high; static final Integer cache[]; Map<Integer, Object> map = new WeakHashMap<Integer, Object>(); for (int i=0; i < 128; i++) { map.put(i, new Object()); }
9. java.lang.ref.Reference • Object pointing to another Object (referent) • Can be GC'ed anytime • Used to observe the lifecycle of an Object • Soft, Weak & Phantom for different Use Cases
10. java.lang.ref.Reference Object referent = new Object (); Reference<Object> reference = new WeakReference<Object>(referent);
11. java.lang.ref.Reference Object referent = new Object (); Reference<Object> reference = new WeakReference<Object>(referent); referent = null; Runtime.getRuntime().gc(); // Hint to run the GC
12. java.lang.ref.Reference Object referent = new Object (); Reference<Object> reference = new WeakReference<Object>(referent); referent = null; Runtime.getRuntime().gc(); // Hint to run the GC if (reference.get() == null) { //The garbage collector deleted my referent } else { // referent object is still here }
13. ReferenceQueue • BlockingQueue • GC place Reference to it when cleared
14. ReferenceQueue ReferenceQueue<Object> queue = new ReferenceQueue<Object>(); for (int i = 0 ; i < 100 ; i++) { WeakReference<Object> ref = new WeakReference<Object>(new Object(), queue); }
15. ReferenceQueue ReferenceQueue<Object> queue = new ReferenceQueue<Object>(); for (int i = 0 ; i < 100 ; i++) { WeakReference<Object> ref = new WeakReference<Object>(new Object(), queue); Reference<?> r; while ((r = queue.poll()) != null) { // polling to discover GC'ed referent // reference 'r' cleared } }
16. ReferenceQueue Set<WeakReference<Object>> refs = new HashSet<WeakReference<Object>>(); ReferenceQueue<Object> queue = new ReferenceQueue<Object>(); for (int i = 0 ; i < 100 ; i++) { WeakReference<Object> ref = new WeakReference<Object>(new Object(), queue); refs.add(ref); Reference<?> r; while ((r = queue.poll()) != null) { // polling to discover GC'ed referent // reference 'r' cleared refs.remove(r); } }
17. Finalizer • Can resurrect the original object • Require 2 GC cycles • Slow, one thread performing finalisation
18. PhantomReference • Always used with a ReferenceQueue • Signal that the Referent has been finalized • Referent is always null (prevent resurrection)
19. PhantomReference Scenario: detect a memory leak
20. 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; } }
21. class Service { Activity.Listener listener; void registerListener (Activity.Listener listener) { this.listener = listener; } void unregisterListener () { this.listener = null; } }
22. Service service = new Service(); Activity activity = new Activity(service); activity.onStart();
23. Service service = new Service(); Activity activity = new Activity(service); activity.onStart(); ReferenceQueue<Activity.Listener> referenceQueue = new ReferenceQueue<Activity.Listener>(); PhantomReference<Activity.Listener> reference = new PhantomReference<Activity.Listener>(service. listener, referenceQueue);
24. Service service = new Service(); Activity activity = new Activity(service); activity.onStart(); ReferenceQueue<Activity.Listener> referenceQueue = new ReferenceQueue<Activity.Listener>(); PhantomReference<Activity.Listener> reference = new PhantomReference<Activity.Listener>(service. listener, referenceQueue); activity.onStop(); activity = null; // at this point we removed the strong reference to our Activity, service should not leak the Activity & Activity should be GC'd Runtime.getRuntime().gc();
25. Service service = new Service(); Activity activity = new Activity(service); activity.onStart(); ReferenceQueue<Activity.Listener> referenceQueue = new ReferenceQueue<Activity.Listener>(); PhantomReference<Activity.Listener> reference = new PhantomReference<Activity.Listener>(service. listener, referenceQueue); activity.onStop(); activity = null; // at this point we removed the strong reference to our Activity, service should not leak the Activity & Activity should be GC'd Runtime.getRuntime().gc(); Reference<?> ref = referenceQueue.remove(TimeUnit.SECONDS.toMillis(10)); assertNotNull(ref); ref.clear();
26. Write ~deterministic tests Involving GC • Used by AOSP FinalizationTester.java & LeakCanary lib • Prefer Runtime.getRuntime().gc(); https://android.googlesource.com/platform/libcore/+/master/support/src/test/java/libcore/java/lang/ref/FinalizationTester.java https://github.com/square/leakcanary/blob/master/leakcanary-watcher/src/main/java/com/squareup/leakcanary/GcTrigger.java https://github.com/square/leakcanary/blob/master/leakcanary-watcher/src/main/java/com/squareup/leakcanary/RefWatcher.java
27. IdentityHashMap • Regular Map
28. IdentityHashMap • Regular Map • Reference-equality in place of object-equality for key & value • Faster if you have a complex hashCode it uses System.identityHashCode()
29. IdentityHashMap • Regular Map • Reference-equality in place of object-equality for key & value • Faster if you have a complex hashCode it uses System.identityHashCode() • Support mutable key
30. Item 2: Asynchronous models
31. Activity - Service communication • Activity start service for background work • Service need to communicate results • Service may be in another process
32. PendingIntent
33. 1. Activity pass the PendingIntent as a Bundle argument (it's Parcelable)
34. 1. Activity pass the PendingIntent as a Bundle argument (it's Parcelable) 2. Service invoke PendingIntent.send
35. 1. Activity pass the PendingIntent as a Bundle argument (it's Parcelable) 2. Service invoke PendingIntent.send 3. onActivityResult will be called
36. Messenger
37. 1. Activity create a Handler
38. 1. Activity create a Handler 2. create a messenger from a Handler new Messenger(handler)
39. 1. Activity create a Handler 2. create a messenger from a Handler new Messenger(handler) 3. pass the Messenger as a Bundle argument
40. 1. Activity create a Handler 2. create a messenger from a Handler new Messenger(handler) 3. pass the Messenger as a Bundle argument 4. Service invoke Messenger.send
41. 1. Activity create a Handler 2. create a messenger from a Handler new Messenger(handler) 3. pass the Messenger as a Bundle argument 4. Service invoke Messenger.send 5. Handler will receive the Message
42. ResultReceiver
43. 1. Activity create a Handler
44. 1. Activity create a Handler 2. create a ResultReceiver using the Handler new ResultReceiver(handler)
45. 1. Activity create a Handler 2. create a ResultReceiver using the Handler new ResultReceiver(handler) 3. override onReceiveResult to receive msg
46. 1. Activity create a Handler 2. create a ResultReceiver using the Handler new ResultReceiver(handler) 3. override onReceiveResult to receive msg 4. pass the ResultReceiver as a Bundle args
47. 1. Activity create a Handler 2. create a ResultReceiver using the Handler new ResultReceiver(handler) 3. override onReceiveResult to receive msg 4. pass the ResultReceiver as a Bundle args 5. Service invoke ResultReceiver.send
48. 1. Activity create a Handler 2. create a ResultReceiver using the Handler new ResultReceiver(handler) 3. override onReceiveResult to receive msg 4. pass the ResultReceiver as a Bundle args 5. Service invoke ResultReceiver.send 6. onReceiveResult will be called
49. EventBus • Don't solve everything with EventBus • Global state • YoYo problem • No IPC https://en.wikipedia.org/wiki/Yo-yo_problem
50. Item 3: Looper/Handler
51. Test Runner • Test run inside an InstrumentationThread • InstrumentationThread prepare a Looper but doesn't start looping • Can't use it with Handler (Message will not be processed)
52. Looper & Unit test Code examples full code here: https://gist.github.com/nhachicha/e993fe9b5f09f0595ccd
53. Item 4: Synchronizers https://gist.github.com/nhachicha/7596e912c81aae38d721
54. CountDownLatch • Initialize counter with # participants • await() sleep until counter = 0 • every thread call countDown()
55. CyclicBarrier • Similar to CountDownLatch • await() sleep until all participants reaches the barrier
56. Phaser • Thread register() to increment # of participants • Thread arrive() similar to countDown() • Barrier/Phase is crossed when Number(registered thread) = Number(arrived thread)
57. Conclusion • Understand your problem, "your architecture is not the list of tools/lib" - Uncle Bob
58. Conclusion • Understand your problem, "your architecture is not the list of tools/lib" - Uncle Bob • Understanding implementation & Time/Space complexity
59. Conclusion • Understand your problem, "your architecture is not the list of tools/lib" - Uncle Bob • Understanding implementation & Time/Space complexity • Be opinionated don't follow trends
60. We are hiring https://realm.io/jobs/ Q/A +NabilHachicha @nabil_hachicha