Reactive programming has gained significant popularity in recent years, enabling developers to build responsive and robust applications. With the release of Java 9, the java.util.concurrent.Flow API was introduced, providing native support for reactive programming in the Java language. In this article, we will explore the Java 9 Flow API and discover how it can be used for reactive programming.

What is Reactive Programming?

Reactive programming is a programming paradigm that focuses on the asynchronous processing of events and data streams. It enables developers to write more efficient and responsive code by allowing them to react to events as they occur. This is particularly useful in applications that involve real-time data processing, such as IoT devices, financial systems, and streaming services.

Introduction to Java 9 Flow API

The Java 9 Flow API introduces a set of interfaces and classes that enable developers to work with reactive streams. It is built upon the Reactive Streams specification, which provides a common API for reactive programming across different languages and frameworks.

The key interfaces provided by the Java 9 Flow API include:

1. Flow.Publisher: Represents a provider of a potentially unbounded number of reactive streams.
2. Flow.Subscriber: Represents a consumer of reactive streams.

These two interfaces, along with other supporting classes, allow developers to create and handle reactive streams in Java.

Example Usage

Let’s consider a simple example where we have a stream of numbers that we want to process asynchronously using reactive programming principles.

import java.util.concurrent.Flow.*;

public class NumberPublisher implements Publisher<Integer> {

    private final List<Integer> numbers = List.of(1, 2, 3, 4, 5);

    @Override
    public void subscribe(Subscriber<? super Integer> subscriber) {
        Subscriber<Integer> numberSubscriber = new NumberSubscriber(subscriber);
        subscriber.onSubscribe(new NumberSubscription(numberSubscriber, numbers));
    }
}

public class NumberSubscriber implements Subscriber<Integer> {

    private final Subscriber<? super Integer> downstream;

    public NumberSubscriber(Subscriber<? super Integer> downstream) {
        this.downstream = downstream;
    }

    @Override
    public void onSubscribe(Subscription subscription) {
        downstream.onSubscribe(subscription);
        subscription.request(Long.MAX_VALUE);
    }

    @Override
    public void onNext(Integer item) {
        // Process the number asynchronously
        // ...
        downstream.onNext(item);
    }

    @Override
    public void onError(Throwable throwable) {
        downstream.onError(throwable);
    }

    @Override
    public void onComplete() {
        downstream.onComplete();
    }
}

public class NumberSubscription implements Subscription {

    private final Subscriber<? super Integer> subscriber;
    private final List<Integer> numbers;
    private int currentIndex = 0;

    public NumberSubscription(Subscriber<? super Integer> subscriber, List<Integer> numbers) {
        this.subscriber = subscriber;
        this.numbers = numbers;
    }

    @Override
    public void request(long n) {
        for (int i = currentIndex; i < currentIndex + n && i < numbers.size(); i++) {
            subscriber.onNext(numbers.get(i));
        }
        currentIndex += n;

        if (currentIndex >= numbers.size()) {
            subscriber.onComplete();
        }
    }

    @Override
    public void cancel() {
        // implementation for canceling the subscription if needed
    }
}

In this example, we define a custom NumberPublisher that implements the Flow.Publisher interface. This publisher provides a stream of numbers that the subscriber can consume. We also define a NumberSubscriber that implements the Flow.Subscriber interface to consume the numbers asynchronously and process them accordingly.

To use the reactive stream, we can subscribe to the NumberPublisher and define what actions to perform on each number using the onNext method. The reactive stream handles backpressure by allowing the subscriber to request a specific number of items using the Subscription interface.

Conclusion

The Java 9 Flow API provides native support for reactive programming in Java, enabling developers to build responsive applications that can handle asynchronous data streams efficiently. By leveraging the interfaces and classes provided by the Java 9 Flow API, developers can create reactive streams and work with them seamlessly. It is a powerful addition to the Java language, opening up new possibilities for building modern and reactive applications.

#reactiveprogramming #javaprogramming