RxJava was introduced by Netflix engineers to support the Reactive model in Java 8, with the bridge to Reactive Streams. However, Java started supporting the Reactive model with Java 9, and Reactive Streams have been incorporated into the JDK as java.util.concurrent.Flow in Java 9.
Also, Pivotal introduced the Reactor framework, which is built directly on Reactive Streams, avoiding the external bridge to Reactive Streams. A Reactor is considered as a 4th generation library.
Finally, Spring Framework 5.0 added Reactive features built into it, including the tools for HTTP servers and clients. Spring users find annotations and controllers handy when they deal with HTTP requests, especially dispatching Reactive requests and back pressure concerns to the framework.
The Reactive model seems to be efficient in resource utilization, as it can process higher loads with fewer threads. However, the Reactive model may not be the right solution for all problems. In some cases, Reactor may make things worse if we use it in the wrong section.
