Open-source Java and Kotlin frameworks

11 Java and Kotlin frameworks whose code is public under an OSI-approved license, rated for privacy with evidence. Picks first, then by score.

1. Eclipse Vert.x Grade B

   Eclipse Foundation toolkit for building reactive, event-driven applications on the JVM with non-blocking I/O, usable from Java, Kotlin and other JVM languages.

   Dual-licensed under the Eclipse Public License 2.0 and Apache 2.0. Source code

2. Helidon Grade B

   Java framework from Oracle for building microservices, offered as a lightweight functional API (Helidon SE) and a MicroProfile implementation (Helidon MP).

   Apache 2.0-licensed. Source code

3. Javalin Grade B

   Lightweight web framework for Java and Kotlin built on the Jetty server, with a small handler-based API for REST APIs and WebSockets.

   Apache 2.0-licensed. Source code

4. Play Framework Grade B

   Web framework for Java and Scala with a stateless, non-blocking architecture, hot reloading during development and type-safe templates.

   Apache 2.0-licensed. Source code

5. Dropwizard Grade C

   Java framework for building RESTful web services that bundles Jetty, Jersey and Jackson into one application package with built-in metrics and health checks.

   Apache 2.0-licensed. Source code

6. Quarkus Grade C

   Java framework for cloud and Kubernetes applications, designed for fast startup and low memory use, with GraalVM native compilation and a live-reload dev mode.

   Apache 2.0-licensed. Source code

7. Apache Struts Grade D

   Apache MVC framework for building Java web applications, based on actions, interceptors and result views.

   Apache 2.0-licensed. Source code

8. Grails Grade D

   Web application framework for the Groovy language built on Spring Boot, with convention over configuration, the GORM data layer and GSP templates.

   Apache 2.0-licensed. Source code

9. Ktor Grade D

   Kotlin framework from JetBrains for building asynchronous servers and HTTP clients with coroutines, extended through plugins.

   Apache 2.0-licensed. Source code

10. Micronaut Grade D

    JVM framework for microservices and serverless applications in Java, Kotlin and Groovy, using compile-time dependency injection instead of reflection.

    Apache 2.0-licensed. Source code

11. Spring Boot Grade D

    Java framework from the Spring project for building standalone applications, with auto-configuration, embedded web servers and starter dependencies.

    Apache 2.0-licensed. Source code

Compare all 12 Java and Kotlin frameworks, open source or not.