Java is not a reactive language. This does not mean that we cannot create reactive programs in Java. There are libraries that support different reactive programming approaches. It is to mention the Akka framework and the ReactiveX that also exist for other languages as well. With Java 9, the JDK starts to support reactive programming, providing a few classes and interfaces for the purpose. We will focus on these features.

The JDK contains the java.util.concurrent.Flow class, which contains related interfaces and some static methods to support flow controlled programs. The model that this class supports is based on Publisher, Subscriber, and Subscription.

As a very simple explanation, a Publisher accepts a subscription from a Subscriber. A Subscriber gets the data it subscribed to when the data is available. The interfaces focus on the very core of the data flow control of the communication and are a bit abstract. No surprise, they are interfaces. However, it may...