I tried with your code with a sample spring boot project and I get the error,

No converter for [class org.json.JSONObject]

The reason for this error is explained clearly here. To reiterate the answer, JSONObject classes don't have getters and hence the error. By default spring-boot starter web dependency has Jackson web support which can convert any POJO class to JSON object. So as the answer by @süleyman-can using a POJO is the right way to handle this.

In case, you can't use a POJO class because the fields in the response will be different for each request. For example, you have to send

{"a": "b"}

for one response and

{"c": "d"}

for another response, you can always use Map<String, String> like this,

@RequestMapping(value = "/test", method = RequestMethod.GET)
@ResponseBody
public Map<String, String> test() {
  Map<String, String> test = new HashMap<>();
  test.put("name","caroline");
  return test;
}

and the response would come like this,

{"name":"caroline"}                                                                                                                      

As you are using Spring Boot web, Jackson dependency is implicit and we do not have to define explicitly. You can check for Jackson dependency in your pom.xml in the dependency hierarchy tab if using eclipse.

And as you have annotated with @RestController there is no need to do explicit json conversion. Just return a POJO and jackson serializer will take care of converting to json. It is equivalent to using @ResponseBody when used with @Controller. Rather than placing @ResponseBody on every controller method we place @RestController instead of vanilla @Controller and @ResponseBody by default is applied on all resources in that controller.
Refer this link: https://docs.spring.io/spring/docs/current/spring-framework-reference/html/mvc.html#mvc-ann-responsebody

The problem you are facing is because the returned object(JSONObject) does not have getter for certain properties. And your intention is not to serialize this JSONObject but instead to serialize a POJO. So just return the POJO.
Refer this link: https://stackoverflow.com/a/35822500/5039001

If you want to return a json serialized string then just return the string. Spring will use StringHttpMessageConverter instead of JSON converter in this case.

I think using jackson library you can do something like below.

Copy@RequestMapping(value = "/getjson", method = RequestMethod.GET, produces="application/json")
@ResponseBody
public String getJson() {
   //your logic
    ObjectMapper mapper = new ObjectMapper();
    return mapper.writeValueAsString(json);
}

@RequestMapping(value = "/post", method = RequestMethod.POST, produces="application/json", consumes="application/json")
@ResponseBody
public String post(@RequestBody String json) {
    POJO pj = new POJO();
    ObjectMapper mapper = new ObjectMapper();
    pj = mapper.readValue(json, POJO.class);

    //do some things with json, put some header information in json
    return mapper.writeValueAsString(pj);
}