Reactive layer
- Java course
- Expert
- Lesson 43/46
- 6 min
Write reactive streams with the Flow API.
The reactive publish layer
The reactive layer builds a push-based model. The Subscriber interface consumes items through its onNext(String item) method. A SimplePublisher holds a subscriber list; publish calls onNext on each subscriber.
This is the core of Reactive Streams: the publisher pushes items to its subscribers. It is built with plain Java, no external library.
Functional interface and lambda
Subscriber is an interface with a single abstract method; that makes it a functional interface. So it can be written with a lambda instead of an anonymous class: s -> System.out.println("got: " + s). The compiler infers from the target type that s is a String.
This shortens the code and improves readability. The subscribe method expects a Subscriber; the lambda is converted automatically.
Practice Write the Subscriber and SimplePublisher classes
Add an interface Subscriber to the structure above with a void onNext(String item); method. Define class SimplePublisher that holds subscribers in a list, has a void subscribe(Subscriber s) method that adds a subscriber, and a void publish(String item) method that calls s.onNext(item) on each subscriber. In main create SimplePublisher p = new SimplePublisher();, subscribe with p.subscribe(s -> System.out.println("got: " + s)); and call p.publish("hello");.
Starter code
@interface Service {}
@Service
class HelloService {
String greet() { return "hello"; }
}
public class Main {
public static void main(String[] args) {
HelloService s = new HelloService();
System.out.println("service ready: " + s.greet());
}
}Sample solution
@interface Service {}
@Service
class HelloService {
String greet() { return "hello"; }
}
interface Subscriber {
void onNext(String item);
}
class SimplePublisher {
java.util.List<Subscriber> subs = new java.util.ArrayList<>();
void subscribe(Subscriber s) { subs.add(s); }
void publish(String item) {
for (Subscriber s : subs) s.onNext(item);
}
}
public class Main {
public static void main(String[] args) {
SimplePublisher p = new SimplePublisher();
p.subscribe(s -> System.out.println("got: " + s));
p.publish("hello");
}
}Expected output
got: hello
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Complete the subscriber method
void ____(String item);void onNext(String item);Explanation: void onNext(String item); is the subscriber method.
2 What does a Subscriber do in a reactive stream?
- It opens files
- It compiles the class
- It collects garbage
- It consumes items Correct answer
Explanation: The Subscriber consumes published items via onNext.
3 Match the reactive elements
| Term | Meaning |
|---|---|
Publisher |
Publishes |
Subscriber |
Subscribes |
onNext |
Item event |
subscribe |
Add subscription |