Spring Framework is the de facto standard for building Java applications. Over the last decade, it has matured with every major release. Spring Framework 5 became generally available as 5.0.0. in September 2017; this is an important release (major) for the framework since its previous version, which was released in 2013.

One of the major additions to Spring 5 is the introduction of a functional web framework, Spring WebFlux, built on the core reactive foundation. Reactive programming is slowly creeping into the framework and many core modules within the framework are inherently supporting reactive programming in a big way. Since the framework has started supporting reactive programming natively, core aspects of this programming are fully implemented and followed by many of the modules. Also, many reactive concepts have become common language within the framework.

It's important to note that Spring's reactive concepts have been taken as is from Java 8's Reactor Core library, which implements the reactive programming paradigm. Reactor Core is built on top of Reactive Streams Specification, which is the industry standard for building reactive applications in the Java world.

Another important feature is the inclusion of new way by which such applications can be tested. We have a dedicated chapter for Spring WebFlux in (Chapter 5, Integrating with Spring WebFlux) where these aspects will be covered in more detail.

Being a major release, it has loads of stuff either added or enhanced. But we are not going to list all of its features. The full list can be found at this link: https://github.com/spring-projects/spring-framework/wiki/What%27s-New-in-Spring-Framework-5.x.