Fork me on GitHub

About Javalin

Javalin started as a fork of the Java and Kotlin web framework Spark, but quickly turned into a ground-up rewrite influenced by koa.js. Both of these web frameworks are inspired by the modern micro web framework grandfather: Sinatra, so if you’re coming from Ruby then Javalin shouldn’t feel too unfamiliar.


Like Sinatra, Javalin is not aiming to be a full web framework, but rather just a lightweight REST API library (or a micro framework, if you must). There is no concept of MVC, but there is support for template engines, WebSockets, and static file serving for convenience. This allows you to use Javalin for both creating your RESTful API backend, as well as serving an index.html with static resources (in case you’re creating an SPA). This is practical if you don’t want to deploy an apache or nginx server in addition to your Javalin service. If you wish to use Javalin to create a more traditional website instead of a REST APIs, there are several template engine wrappers available for a quick and easy setup.

API design

All the methods on the Javalin instance return this, making the API fully fluent. This will let you create a declarative and predictive REST API, that will be very easy to reason about for new developers joining your project.

Java and Kotlin interoperability

Javalin is both a Kotlin web framework and a Java web framework, meaning the API is being developed with focus on great interoperability between the two languages. The library itself is written primarily in Kotlin, but has a few core classes written in Java to achieve the best interoperability between the two languages. Javalin is intended as a “foot in the door” to Kotlin development for companies that already write a lot of Java.

When moving a Javalin project from Java to Kotlin, you shouldn’t need to learn a new way of doing things. To maintain this consistent API for both languages is an important goal of the project.


If you want to get in touch, please create an issue on our GitHub tracker.

Like Javalin?
Star us 😊