The document discusses reactive programming concepts in Java using RxJava. It defines key RxJava terms like Observable, Observer, and Operator. It explains how RxJava uses asynchronous message passing with Observables that push data to Observer subscribers, unlike traditional Java APIs that use synchronous pulling of data. It provides examples of creating Observables and applying operators like map, filter, and flatMap. It also covers multi-threading in RxJava using schedulers, backpressure, and other advanced topics.

1. Reactive Thinking in Java
Yakov Fain, Farata Systems
@yfain

2. Reactive thinking is not new

3. Бди!
Козьма Прутков
Reactive thinking is not new

4. Reactive thinking is not new

5. The reactive style app
• Message-driven - components communicates via notifications
• Non-imperative - the app logic is coded in async, non-blocking,
composable functions
• The data moves through your app’s algorithm
www.reactivemanifesto.org

6. Java concurrency
• Blocking I/O is the problem
• Future.get()
- blocks till all threads are complete
• CompletableFuture.supplyAsync(task).thenAccept(action)
- what if the tasks need to fetch millions of records?

7. Some open-source Rx libraries
• RxJava
• RxAndroid, RxJavaFX, RxSwing
• Rx.NET, RxCpp, RxJS, Rx.rb, Rx.py, RxSwift,
RxScala, RxPHP
http://reactivex.io
JDK 9 will include reactive streams in java.util.concurrent.Flow

8. Main RxJava players
• Observable - producer of data
• Observer - consumer of observable sequences
• Subscriber - connects observer with observable
• Operator - en-route data transformation
• Scheduler - multi-threading support

9. beers.forEach(brr -> {
if ("USA".equals(brr.country)){
americanBeers.add(brr);
}
});
Java Iterable: a pull

10. beers.stream()
.skip(1)
.limit(3)
.filter(b -> "USA".equals(b.country))
.map(b -> b.name + ": $" + b.price)
.forEach(beer -> System.out.println(beer));
Java 8 Stream: a pull

11. A fool with a tool is still a fool
Американская народная мудрость

12. A pull with a tool is still a pull
A fool with a tool is still a fool
Американская народная мудрость
Yakov Fain

13. Observable is an Iterable inside out

14. Word Antonym
Iterable Observable
Iterator Observer
Pull Push

15. Data
Source
Data Flow

16. Observable Data
Source
Data Flow

17. Observable Data
Source
Data Flow

18. Observable SubscriberObserver Data
Source
Data Flow

19. Observable
onNext()
SubscriberObserver
onError()
onCompleted()
Data
Source
Data Flow

20. Event-driven push
Subscribe to messages from Observable and handle them by Observer
Observable
Observer
Subscriber
Observer
Subscriber
push
push
push
Observer
Subscriber

21. Observable.subscribe(Subscriber)
class Subscriber implements Observer {}

22. observableBeer
.skip(1)
.take(3)
.filter(b -> "USA".equals(b.country))
.map(b -> b.name + ": $" + b.price)
.subscribe(
beer -> System.out.println(beer),
err -> System.out.println(err),
() -> System.out.println("Streaming is complete")
);
Rx Observable: a push

23. Demo
HelloObservable
Get rxjava.jar on http://search.maven.org

24. An Operator
Observable Observable
A transforming
function

25. An Operator
Observable Observable
A transforming
function
observableBeer
.filter(b -> "USA".equals(b.country))

26. An Operator
Observable Observable
A transforming
function
An operator is a higher-order function
observableBeer
.filter(b -> "USA".equals(b.country))
pure function

27. Marble Diagrams
http://rxmarbles.com

28. Observable map(function){}

29. Observable filter(function){}

30. Operator chaining: map and filter

31. RX: the data moves across your algorithm

32. Observable
Operator
Observable
Operator
Observable
Operator
Observable
Operator

33. Creating an Observable
• Observable.create() - returns Observable that can invoke methods on Observer
• Observable.from() - converts an Iterable or Future into Observable
• Observable.fromCallable() - converts a Callable into Observable
• Observable.empty() - returns empty Observable that invokes onCompleted()
• Observable.range() - returns a sequence of integers in the specified range
• Observable.just() - converts up to 10 items into Observable

34. Demo
BeerClient

35. Functions with side effects
• doOnNext()
• doOnError()
• doOnCompleted()
• doOnEach()
• doOnSubscribe()
Affect environment outside the function.

36. Error Handling
Observer Observable
next
error
completed

37. Error-handling operators
• Errors sent using onError() kill the subscription
• retryWhen() - intercept, anylize the error, resubscribe
• onErrorResumeNext() - used for failover to another Observable
• onResumeReturn() - returns an app-specific value

38. Demo
BeerClientWithFailover

39. The flatMap() operator

40. .flatMap()Observable

41. Demo
composingObservables/ObservableDrinks

42. Schedulers

43. Concurrency with Schedulers
• An observable stream is single-threaded by default
• subscribeOn(strategy) - run Observable in a separate thread
• observeOn(strategy) - run Observer in a separate thread

44. Multi-threading strategies
• Schedulers.computation() - for computations: # of threads <= # of cores
• Schedulers.io() - for long running communications; backed by a thread pool
• Schedulers.newThread() - new thread fo each unit of work
• Schedulers.from(Executor) - a wrapper for Java Executor
• Scedulers.trampoline() - queues the work on the current thread
• AndroidSchedulers.mainThread() - handle data on the main thread (RxAndroid)

45. Switching threads
Operator1() Operator2() ObserveOn()
Observable
Subscriber
Thread 1
Thread 2

46. Parallel processing
Operator1() Operator2() flatMap()
Observable
Subscriber
Thread 1
Observable/Thr2
Observable/Thr3
Observable/ThrN

47. Demo
schedulers/SubscribeOnObserveOn
schedulers/ParallelStreams

48. Hot and cold observables
• Cold: emits items only when someone subscribes to it
• Hot: emits items when it’s created regardless if there is a
subscriber or not

49. Backpressure
• Throttling: sample(), throttleFirst(), debounce()
• Buffers: buffer()
• Window: window(n)
• Reactive pull: Subscriber.request(n)
https://github.com/ReactiveX/RxJava/wiki/Backpressure

50. Links
• Code samples and slides:
https://github.com/yfain/rxjava
• Our company: faratasystems.com
• Blog: yakovfain.com
• Twitter:@yfain discount code: faindz