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

Question

Either return text/plain (as in Return only string message from Spring MVC 3 Controller) OR wrap your String is some object

public class StringResponse {

    private String response;

    public StringResponse(String s) { 
       this.response = s;
    }

    // get/set omitted...
}

Set your response type to MediaType.APPLICATION_JSON_VALUE (= "application/json")

@RequestMapping(value = "/getString", method = RequestMethod.GET,
                produces = MediaType.APPLICATION_JSON_VALUE)

and you'll have a JSON that looks like

{  "response" : "your string value" }

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.

I know that this question is old but i would like to contribute too:

The main difference between others responses is the hashmap return.

@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"}