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.

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" }