pass a list of objects containing other objects to Json in Java

I have a web service hosted on Java in Netbeans (NO MAVEN) with several classes that are related to foreign keys. For example, I have a Notes class that has two foreign keys (idUsuari and idIncidencia). From the Postman, I do a GET with the method that comes by default when generating the service, which returns a List and shows it to me in Json format because that's how I have it in the get method:

Method:

GET
    @Override
    @Produces({MediaType.APPLICATION_JSON})
    public List<Notes> findAll() {
        return super.findAll();
    }

And he returns me:

[
  {
    "dataNota": "2017-04-01",
    "descripcio": "nota de la incidencia",
    "id": 1,
    "idIncidencia": { //objeto Incidencia
      "dataFi": "2017-04-06",
      "dataIncidencia": "2017-03-25",
      "dataInici": "2017-04-06",
      "descCurta": "Aixeta trencada",
      "descLlarga": "La aixeta del lavabo situat a la dreta està bessant aigua continuament",
      "id": 1,
      "idEstat": {
        "id": 2,
        "nomEstat": "assignada"
      },
      "idLocalitzacio": {//objeto Localitzacio
        "id": 12,
        "nomLloc": "dpt. exportació"
      },
      "idPrioritat": {//objeto Prioritat
        "id": 2,
        "nomPrioritat": "normal"
      },
      "idUsuariObertura": { //objeto Usuari
        "bloquejat": false,
        "cognoms": "Olivera Cortes",
        "contrasenya": "e10adc3949ba59abbe56e057f20f883e",
        "correu": "[email protected]",
        "id": 2,
        "idTipus": { //objeto TipusUsuari
          "id": 2,
          "nomTipus": "gestor"
        },
        "idUsuari": "EulaliaOli2",
        "mobil": "698899663",
        "nom": "Calatina"
      },
      "idUsuariOperari": { //objeto Usuari
        "bloquejat": false,
        "cognoms": "Olivera Cortes",
        "contrasenya": "e10adc3949ba59abbe56e057f20f883e",
        "correu": "[email protected]",
        "id": 3,
        "idTipus": { //objeto TipusUsuari
          "id": 2,
          "nomTipus": "gestor"
        },
        "idUsuari": "EulaliaOli",
        "mobil": "698899663",
        "nom": "Eulalia"
      }
    },
    "idUsuari": { //objeto Usuari
      "bloquejat": false,
      "cognoms": "Campmany",
      "contrasenya": "0c924da91233daf35ad47a7745d10edf",
      "correu": "[email protected]",
      "id": 7,
      "idTipus": { //objeto TipusUsuari
        "id": 2,
        "nomTipus": "gestor"
      },
      "idUsuari": "Joan",
      "mobil": "",
      "nom": "Joan"
    },
    "metodologia": "manual"
  },

...mas registros con la misma estructura...

What I want is to convert this list of objects (which contains other objects and at the same time other objects) to a String in Json format. I've tried with Gson, JSONObject, Jackson and it does not return anything, the Postman is left standing as if thinking.

I do not know if the problem is that these libraries do not recognize the different levels within the hierarchy of objects or the EntityManager behind these restful methods.

Any suggestions?