Kotlin support in the backend

[gilberto-torrezan]

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Description

Although the goal is to have Kotlin support, the idea is to technically support any JVM language. Kotlin would be given special attention in our tutorials and guides, but any JVM language be supported.

This impacts the endpoints generation. Adding support for JVM languages shouldn't come with compromises for the already existing use case for the Java code.

Acceptance criteria

• [ ] The language of choice shouldn't affect the features supported by the framework, as long as the language is supported by the surrounding environment (such as Spring Boot).

General criteria

• [ ] APIs reviewed

• [ ] Design

• [ ] Performance

• [ ] UX/DX tests in Alpha

• [ ] Documentation:

• [ ] How to test?

• [ ] Limitations:

[markusherr]

We are using Kotlin on the server side and are still using Vaadin 14 (so no Fusion Endpoints). Does the issue above mean that Endpoint generation is currently not working with Kotlin? This would really be a blocker for us for switching to Vaadin 19, or even switching to the next LTS (22).