SpringSource, the company behind the popular Spring Framework announced “Reactor” as:
We’re pleased to announce that, after a long period of internal incubation, we’re releasing a foundational framework for asynchronous applications on the JVM which we’re calling Reactor. It provides abstractions for Java, Groovy and other JVM languages to make building event and data-driven applications easier.
This means Reactor is supposed to be used with the whole variety of JVM languages. For Grails there will be built-in support in the future:
[…] so from version 2.3 on, Grails applications will have a built-in, extremely powerful yet easy to use, convention-based Events API that looks very similar to the current implementation in the platform-core plugin. This Events API will be built on a Reactor foundation.
From my point of view, the following quote summarizes the reason for SpringSource’s development of Reactor very well:
Reactor, as the name suggests, is heavily influenced by the well-known Reactor design pattern. But it is also influenced by other event-driven design practices, as well as several awesome JVM-based solutions that have been developed over the years. Reactor’s goal is to condense these ideas and patterns into a simple and reusable foundation for making event-driven programming much easier.
So this new framework really fits into the Spring world as a general accumulation of solutions for common problems in software development.
The source code can be found on the corresponding GitHub project.

