Reactive programming is a paradigm used to describe a certain pattern for the propagation of changes in a system. Reactiveness is not built in Java itself, but reactive data flows can be established using third-party libraries, such as RxJava or Project Reactor (part of the Spring Framework). JDK 9 also addressed the need for an API that aids the development of highly responsive applications built around the idea of reactive streams by providing the java.util.concurrent.Flow class for the purpose. The Flow class, along with other related changes introduced in JDK 9, will be covered in Chapter 12, Concurrency Enhancements.