Tuple to Json (with Java & EntityManager)

Question

For a project, i needed to fetch records from a SQL database into a list of objects (using java for backend). The exchange between the backend and the frontend is made with webservices that use JSON as data encoding.

With simple queries (like a 'list all') it works, as EntityManager.createQuery uses SELECT * by default , but for more complex ones it doesn't return anything, even when i checked that the query itself returns the respective entries.

My friend thought of using EntityManager.createNativeQuery instead of EntityManager.createQuery to use sql queries without SELECT * or more complex queries. The problem is, when we get to the backend-frontend exchanges (tested with Postman), it sends tuples instead of JSON. And i'm fairly sure the front-end needs JSON.

I checked what EntityManager.createQuery and EntityManager.createNativeQuery return and it seems to be mostly the same. So what is the issue?

Is there a way to turn a tuple into a JSON?