Inject field into JSON response object

Question

I'd like to inject a field into the response object during serialization. Is it possible to inject "success": "true" into the JSON response object? This needs to be a general solution for all parent response objects that are serialized.

For example with object:

public class UserResponse {
    private int id;
    private String firstName;
    private String lastName;
    private Organisation organisation;

    // getters setters
}

Jackson should return:

{
    "success": "true",
    "id": 1, 
    "firstName": "tom",
    "lastName": "jeffrey",
    "organisation": {
        // etc.
    }
}

I tried already with

public class CustomJsonSerializer extends StdSerializer<Object> {

    public CustomJsonSerializer() {
        this(null);
    }

    public CustomJsonSerializer(Class<Object> t) {
        super(t);
    }

    @Override
    public void serialize(Object o, JsonGenerator jsonGenerator, SerializerProvider serializerProvider) throws IOException {
        jsonGenerator.writeStartObject();
        jsonGenerator.writeStringField("success", "true");
        jsonGenerator.writeObject(o);
        jsonGenerator.writeEndObject();
    }
}

But didn't succeed:

Could not write JSON: Can not start an object, expecting field name (context: Object); nested exception is com.fasterxml.jackson.core.JsonGenerationException: Can not start an object, expecting field name (context: Object)

Answer 1

You can use a ResponseBodyAdvice to modify the object returned by a controller before being serialized into JSON. Here is a trivial example which is applied by default to all responses:

The view your controller returns

public class Greeting {
    private static final String message = "Hello World!";

    public String getMessage() {
        return message;
    }
}

The response wrapper for setting a status

public class ResponseWrapper {
    private Object data;
    private boolean success;

    public Object getData() {
        return data;
    }

    public void setData(Object data) {
        this.data = data;
    }

    public boolean isSuccess() {
        return success;
    }

    public void setSuccess(boolean success) {
        this.success = success;
    }
}

The controller

@RestController
@RequestMapping("/api/hello")
public class HelloController {
    @GetMapping
    public Greeting hello() {
        return new Greeting();
    }
}

Test that validates the behavior

@RunWith(SpringJUnit4ClassRunner.class)
@WebMvcTest
public class HelloControllerTest {
    @Autowired
    private MockMvc mockMvc;
    private static final String GREETING_ENDPOINT = "/api/hello";


    @Test
    public void returnsGreetingWithStatus() throws Exception {
        mockMvc.perform(get(GREETING_ENDPOINT))
                .andExpect(jsonPath("$.data.message").value("Hello World!"))
                .andExpect(jsonPath("$.success").value(true));
    }
}

The ResponseBodyAdvice

@ControllerAdvice
public class ResponseStatusAdvice implements ResponseBodyAdvice {
    @Override
    public boolean supports(MethodParameter returnType, Class converterType) {
        return converterType.equals(MappingJackson2HttpMessageConverter.class);
    }

    @Override
    public Object beforeBodyWrite(Object body, MethodParameter returnType, MediaType selectedContentType, Class selectedConverterType, ServerHttpRequest request, ServerHttpResponse response) {
        ResponseWrapper wrapper = new ResponseWrapper();
        wrapper.setData(body);
        wrapper.setSuccess(true);
        return wrapper;
    }
}

This should give you a head start. On a side note, you can always take advantage of HTTP status codes. It is part of the protocol and will always be set.