Spring MVC - How to return simple String as JSON in Rest Controller

JSON is essentially a String in PHP or JAVA context. That means string which is valid JSON can be returned in response. Following should work.

  @RequestMapping(value="/user/addUser", method=RequestMethod.POST)
  @ResponseBody
  public String addUser(@ModelAttribute("user") User user) {

    if (user != null) {
      logger.info("Inside addIssuer, adding: " + user.toString());
    } else {
      logger.info("Inside addIssuer...");
    }
    users.put(user.getUsername(), user);
    return "{\"success\":1}";
  }

This is okay for simple string response. But for complex JSON response you should use wrapper class as described by Shaun.

In one project we addressed this using JSONObject (maven dependency info). We chose this because we preferred returning a simple String rather than a wrapper object. An internal helper class could easily be used instead if you don't want to add a new dependency.

Example Usage:

@RestController
public class TestController
{
    @RequestMapping("/getString")
    public String getString()
    {
        return JSONObject.quote("Hello World");
    }
}

You can easily return JSON with String in property response as following

@RestController
public class TestController {
    @RequestMapping(value = "/getString", produces = MediaType.APPLICATION_JSON_VALUE)
    public Map getString() {
        return Collections.singletonMap("response", "Hello World");
    }
}

Simply unregister the default StringHttpMessageConverter instance:

@Configuration
public class WebMvcConfiguration extends WebMvcConfigurationSupport {
  /**
   * Unregister the default {@link StringHttpMessageConverter} as we want Strings
   * to be handled by the JSON converter.
   *
   * @param converters List of already configured converters
   * @see WebMvcConfigurationSupport#addDefaultHttpMessageConverters(List)
   */
  @Override
  protected void extendMessageConverters(List<HttpMessageConverter<?>> converters) {
    converters.removeIf(c -> c instanceof StringHttpMessageConverter);
  }
}

Tested with both controller action handler methods and controller exception handlers:

@RequestMapping("/foo")
public String produceFoo() {
  return "foo";
}

@ExceptionHandler(FooApiException.class)
public String fooException(HttpServletRequest request, Throwable e) {
  return e.getMessage();
}

Final notes:

• extendMessageConverters is available since Spring 4.1.3, if are running on a previous version you can implement the same technique using configureMessageConverters, it just takes a little bit more work.
• This was one approach of many other possible approaches, if your application only ever returns JSON and no other content types, you are better off skipping the default converters and adding a single jackson converter. Another approach is to add the default converters but in different order so that the jackson converter is prior to the string one. This should allow controller action methods to dictate how they want String to be converted depending on the media type of the response.

Setting the return type as a Map will serialize into JSON.

@GetMapping("...")
@ResponseBody
public Map<String, Object> endpointExample(...) {
    Map<String, Object> rtn = new LinkedHashMap<>();

    rtn.put("pic", image);
    rtn.put("potato", "King Potato");

    return rtn;

}

This will return:

{"pic":"a17fefab83517fb...beb8ac5a2ae8f0449", "potato":"King Potato"}

Make simple:

    @GetMapping("/health")
    public ResponseEntity<String> healthCheck() {
        LOG.info("REST request health check");
        return new ResponseEntity<>("{\"status\" : \"UP\"}", HttpStatus.OK);
    }

Add produces = "application/json" in @RequestMapping annotation like:

@RequestMapping(value = "api/login", method = RequestMethod.GET, produces = "application/json")

Hint: As a return value, i recommend to use ResponseEntity<List<T>> type. Because the produced data in JSON body need to be an array or an object according to its specifications, rather than a single simple string. It may causes problems sometimes (e.g. Observables in Angular2).

Difference:

returned String as json: "example"

returned List<String> as json: ["example"]

Add @ResponseBody annotation, which will write return data in output stream.

This issue has driven me mad: Spring is such a potent tool and yet, such a simple thing as writing an output String as JSON seems impossible without ugly hacks.

My solution (in Kotlin) that I find the least intrusive and most transparent is to use a controller advice and check whether the request went to a particular set of endpoints (REST API typically since we most often want to return ALL answers from here as JSON and not make specializations in the frontend based on whether the returned data is a plain string ("Don't do JSON deserialization!") or something else ("Do JSON deserialization!")). The positive aspect of this is that the controller remains the same and without hacks.

The supports method makes sure that all requests that were handled by the StringHttpMessageConverter(e.g. the converter that handles the output of all controllers that return plain strings) are processed and in the beforeBodyWrite method, we control in which cases we want to interrupt and convert the output to JSON (and modify headers accordingly).

@ControllerAdvice
class StringToJsonAdvice(val ob: ObjectMapper) : ResponseBodyAdvice<Any?> {
    
    override fun supports(returnType: MethodParameter, converterType: Class<out HttpMessageConverter<*>>): Boolean =
        converterType === StringHttpMessageConverter::class.java

    override fun beforeBodyWrite(
        body: Any?,
        returnType: MethodParameter,
        selectedContentType: MediaType,
        selectedConverterType: Class<out HttpMessageConverter<*>>,
        request: ServerHttpRequest,
        response: ServerHttpResponse
    ): Any? {
        return if (request.uri.path.contains("api")) {
            response.getHeaders().contentType = MediaType.APPLICATION_JSON
            ob.writeValueAsString(body)
        } else body
    }
}

I hope in the future that we will get a simple annotation in which we can override which HttpMessageConverter should be used for the output.

Simple and Straightforward send any object or return simple List

@GetMapping("/response2")
    @ResponseStatus(HttpStatus.CONFLICT)
    @ResponseBody List<String> Response2() {
        List<String> response = new ArrayList<>(Arrays.asList("Response2"));
        
        return response;
        
    }

I have added HttpStatus.CONFLICT as Random response to show how to pass RequestBody also the HttpStatus

One way to make it work for a single controller is to wrap it as a TextNode:

@RestController
public class TestController {

    @RequestMapping("/get-json-string")
    public TextNode getStringAsValidJson() {

        return TextNode.valueOf("Hello world!");

    }
}

The TextNode won't be handled by the StringHttpMessageConverter, but will be passed through to the MappingJackson2HttpMessageConverter.

Annotate your method with the @ResponseBody annotation to tell spring you are not trying to render a view and simple return the string plain