Returning JSON object as response in Spring Boot

I have a sample RestController in Spring Boot:

@RestController
@RequestMapping("/api")
class MyRestController
{
    @GetMapping(path = "/hello")
    public JSONObject sayHello()
    {
        return new JSONObject("{'aa':'bb'}");
    }
}

I am using the JSON library org.json

When I hit API /hello, I get an exception saying :

Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is java.lang.IllegalArgumentException: No converter found for return value of type: class org.json.JSONObject] with root cause

java.lang.IllegalArgumentException: No converter found for return value of type: class org.json.JSONObject

What is the issue? Can someone explain what exactly is happening?

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.

The reason why your current approach doesn't work is because Jackson is used by default to serialize and to deserialize objects. However, it doesn't know how to serialize the JSONObject. If you want to create a dynamic JSON structure, you can use a Map, for example:

@GetMapping
public Map<String, String> sayHello() {
    HashMap<String, String> map = new HashMap<>();
    map.put("key", "value");
    map.put("foo", "bar");
    map.put("aa", "bb");
    return map;
}

This will lead to the following JSON response:

{ "key": "value", "foo": "bar", "aa": "bb" }

This is a bit limited, since it may become a bit more difficult to add child objects. Jackson has its own mechanism though, using ObjectNode and ArrayNode. To use it, you have to autowire ObjectMapper in your service/controller. Then you can use:

@GetMapping
public ObjectNode sayHello() {
    ObjectNode objectNode = mapper.createObjectNode();
    objectNode.put("key", "value");
    objectNode.put("foo", "bar");
    objectNode.put("number", 42);
    return objectNode;
}

This approach allows you to add child objects, arrays, and use all various types.

You can either return a response as String as suggested by @vagaasen or you can use ResponseEntity Object provided by Spring as below. By this way you can also return Http status code which is more helpful in webservice call.

@RestController
@RequestMapping("/api")
public class MyRestController
{

    @GetMapping(path = "/hello", produces=MediaType.APPLICATION_JSON_VALUE)
    public ResponseEntity<Object> sayHello()
    {
         //Get data from service layer into entityList.

        List<JSONObject> entities = new ArrayList<JSONObject>();
        for (Entity n : entityList) {
            JSONObject entity = new JSONObject();
            entity.put("aa", "bb");
            entities.add(entity);
        }
        return new ResponseEntity<Object>(entities, HttpStatus.OK);
    }
}