Ratpack

Ratpack is set of Java libraries which are event driven, non-blocking, high performance, and asynchronous in nature to build scalable services with HTTP. It conforms to the Reactive Streams specification, meaning it comes with interoperability out of the box. It is built on Netty—a framework to build a client-server application over the network with quick and easy development.

It is a web framework to develop efficient and lightweight JVM-based applications. It has its own testing library to easily set up test cases. Spring provides support for Ratpack. You can get more information about Ratpack from its official site:  https://ratpack.io.