Javalin is described as 'A simple web framework for Java and Kotlin' and is an app. There are three alternatives to Javalin for a variety of platforms, including Linux, Mac, Windows, Online / Web-based and Python. The best alternative is Flask, which is both free and Open Source. Other great apps like Javalin are Sinatra and Roll.
Sinatra is a free and open source software web application library and domain-specific language written in Ruby. It is an alternative to other Ruby web application frameworks such as Ruby on Rails, Merb, Nitro, Camping, and Rango. It is dependent on the Rack web server interface.