What are the steps to implement Flow API in Java 9?

Flow API corresponds to the Reactive Streams specification in Java 9, which is a de facto standard. It contains a minimal set of interfaces that capture the core of asynchronous publishing and subscription.

The following are the key interfaces of Flow API:

1) Flow.Publisher: It generates items for subscribers to consume, and it contains only one method: subscribe(Subscriber), whose purpose should be obvious.

Syntax

<strong>void subscribe(Flow.Subscriber<? super T><!--? super T--> subscriber)</strong>

2) Flow.Subscriber: It subscribes to publishers (usually only one) to receive items (via method onNext(T)), error messages (onError(Throwable)), or a signal that no more items are to be expected (onComplete()). Before any of those things happen, the publisher calls onSubscription(Subscription) method.

Syntax

<strong>void onSubscribe(Flow.Subscription subscription)
void onNext(T item)
void onError(Throwable throwable)
void onComplete()</strong>

3) Flow.Subscription: The connection between a single publisher and a single subscriber. The subscriber can use it to request more items (request (long)) or break the connection (cancel()).

Syntax

<strong>void request(long n)
void cancel()</strong>

Flow API execution steps:

• First, we need to create a Publisher and a Subscriber.
• Use Publisher::subscribe to subscribe to Subscriber.
• Publisher creates a Subscription and calls Subscriber::onSubscription so that Subscriber can store the subscription.
• At a certain moment, Subscriber calls Subscription::request to request a certain number of items.
• Publisher passes items to Subscriber by calling Subscriber::onNext. It will not publish more than the requested number of items.
• Publisher may encounter a problem at some point and call Subscriber::onComplete or Subscriber::onError respectively.
• Subscriber can request more items at intervals or disconnect by calling Subscription::cancel.

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