If you want GraphQL and already have your REST, SOAP etc services in Java, I can suggest my library GraphQL-SPQR (it's linked in this project's readme). It's still in beta, but it's pretty usable. The main downside is that it still has almost no docs. If you're in experimentation stage and looking for options you can have a look. It aims to make exposing any Java functionality through GraphQL trivial. You'd slap some annotations or configuration code on top of your existing Java methods, and you instantly have the equivalent GraphQL API.