Java 9 Flow API – Reactive Streams


Java 9 introduces Reactive Streams under java.util.concurrent.Flow that supports an interoperable publish-subscribe framework. In the tutorial, we’re gonna look at a general view of Reactive Streams and how it comes to Java 9 with some new Flow API Components.

Related Articles:
Java 9 Flow API example – Publisher and Subscriber
Java 9 Flow API example – Processor
Java 9 FLow SubmissionPublisher – A Concrete Publisher

I. Reactive Streams

1. Overview

Reactive Streams aims to improve concurrency workflows for developers by solving the pain of back-pressure (when fast data source doesn’t overwhelm the stream destination).


In the image above, we can see that if Destination can not deal with incoming data from Source, all future data could be blocked until the existing ones are processed.

If the Source checks when Destination is not overflowed to send data, the problem could be solved. But in this case, it is still synchronous communication and we don’t take advantage of an asynchronous system which enables the parallel use of computing resources, on collaborating network hosts or multi-core processor.

Reactive Streams processes an asynchronous stream data across an asynchronous boundary (passing elements on to another thread or thread-pool), and receiving side (Destination) is not forced to buffer arbitrary amounts of data, then buffer overflow will not occur.

In summary, Reactive Streams:
– process a potentially unbounded number of elements
– in sequence,
– asynchronously passing elements between components,
– with mandatory non-blocking back-pressure.

These things are possible by flow control and the publish-subscribe pattern which allows the subscriber to be able to understand limitation and show publisher its capacity.

2. Publish-Subscribe Pattern with Flow Control

It is a bi-directional transport:
– signal for demand emitted from subscriber to publisher.
– data flow emitted from publisher to subscriber.


If the subscriber emits a demand request, the publisher can push up to the number of request elements safety. It helps to prevent wasting resources. Because demand is signalled asynchronously, the subscriber, by its capacity, can send requests as much as possible for future work.

When the subscriber is slower than publisher, it works like a pull-based system and a push-based system when the subscriber is faster.

For example, if a subscriber requests few items (2,3 items or less, only 1), it’s effectively a pull. If that subscriber requests more elements than the publisher is ready to emit (100 items or more), flow is changed to push.

Reactive Streams doesn’t provide the strategy for spinning up new subscribers on different threads to handle the volume and load balance elements between those subscribers, we should do these things ourselves or use a specific Reactive Streams library.

To understand the behaviour, we will look at new Java 9 Flow API for Reactive Streams.

II. Java 9 Flow API

1. Components

Java 9 provides java.util.concurrent.Flow API that supports the Reactive Streams publish-subscribe framework. There are 4 components: Publisher, Subscriber, Subscription and Processor.

At a glance:

public static interface Publisher {
	public void subscribe(Subscriber subscriber);

public static interface Subscriber {
	public void onSubscribe(Subscription subscription);
	public void onNext(T item);
	public void onError(Throwable throwable);
	public void onComplete();

public static interface Subscription {
	public void request(long n);
	public void cancel();

public static interface Processor extends Subscriber, Publisher {

2. Behavior

Now look at the diagram below:

Publisher uses its subscribe() method with Subscriber object as input parameter. That Subscriber now subscribes the Publisher.
Publisher defines its own Subscription implementation and produces data elements for that Subscription.
– When a Subscriber subscribes a Publisher, onSubscribe() method is invoked. Then Subscriber now can use Subscription to link to the Publisher by request(numberItems) or cancel() method.
Publisher uses Subscription to invoke Subscriber‘s methods:
+ onNext() if publishing items to the Subscriber asynchronously, normally using an Executor.
+ onComplete() when no more elements are available,
+ onError() if there is a failure.

– So, what is a Processor?
A Processor is a component that sits between the Publisher and Subscriber. It acts as:
+ a Subscriber when emitting a request signal to Publisher
+ a Publisher when pushing items to Subscriber.

We can create one or more Processors in chain which link a Publisher to a Subscriber.

JDK 9 provides concrete Publisher named SubmissionPublisher but there is no concrete Processor.

For more methods’ details, please visit: Flow (Java Platform SE 9)

For deep understanding, please check some examples:
Java 9 Flow API example – Publisher and Subscriber
Java 9 Flow API example – Processor
Java 9 FLow SubmissionPublisher – A Concrete Publisher

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