I have an appointment json which I need to cast to DSTU2 HAPI FHIR json object. Any standard library available for the same ? Google gson library works but does not give value to object in fields

```
{
  "resourceType": "Appointment",
  "id": "",
  "status": "proposed",
  "reason": {
    "text": "Regular checkup"
  },
  "description": "",
  "slot": [
    {
      "reference": "bfgf5dfdf4e45g"
    }
  ],
  "comment": "Regular yearly visit",
  "participant": [
    {
      "actor": {
        "reference": "9sdfsndjkfnksdfu3yyugbhjasbd"
      },
      "required": "required"
    },
    {
      "actor": {
        "reference": "78hjkdfgdfg223vg"
      },
      "required": "required"
    },
    {
      "actor": {
        "reference": "sdfs3df5sdfdfgdf"
      },
      "required": "required"
    }
  ]
}

```

Need to convert above json to ca.uhn.fhir.model.dstu2.resource.Appointment class for which I use 

```
Appointment appointment = new Gson().fromJson(map.get("appointment"), Appointment.class);

```

but it gives appointment object with empty fields