Hi, Please let me have a chance to work with you
1. JWT JSON Web Token, I'm using use jwt for my api system too, if you use jwt, the main problem is how to secure the key to generate and validate a jwt
2. kotin file is the files with .kt extension. I don't really know koltin but it a new programming language for jvm so you can trust me, I can learn and work with koltin very fast
3. As you said, you have only three small spring controller, I dont really understand your question but if you want to use jwt with your controllers, at least you will have to use login controller(validate jwt) and register controller(create jwt)
4. I don't need to work with hibernate when I use JWT, but I think use Hibernate or not depends on the design of your application
Thanks