Spark is a micro-framework founded by Per Wendel for creating web applications in Java with minimal effort, and it's a free and open source Java Web Framework, released under the Apache 2 license.
The Spark Framework is a rapid-development web framework built with Java 8 Lambda Expression (based on the lambda philosophy), so it can help to build a web application with fewer verbose, in fact you can build a REST API with a JSON response in less than 10 lines of code and provide Node.js-like experience when developing a web API. Cool, isn't it?