Spring MVC @ResponseBody问题

Question

I just learn about spring mvc for few days and there are some codes I wrote :

@RequestMapping(value = "/login", method = {RequestMethod.POST, RequestMethod.GET}, consumes = "*/*", produces = "application/json")
@ResponseBody
public  Object userLogin(@RequestParam(value = "userName") String userName,@RequestParam(value = "password") String password) {

    String password2 = userService.selectUserPassword(userName);

    JSONObject object = new JSONObject();
    if(password2.equals(password)){
        object.put("login", "true");
    }else{
        object.put("login", "false");
    }
    return object;  

}

It doesn't work, but if I change the code to this

public  Map<String, String> userLogin(@RequestParam(value = "userName") String userName,
        @RequestParam(value = "password") String password) {

    String password2 = userService.selectUserPassword(userName);

    Map<String, String> map = new HashMap<String, String>(1);
    //JSONObject object = new JSONObject();
    if(password2.equals(password)){
        map.put("login", "true");
    }else{
        map.put("login", "false");
    }
    return map; 

}

then it works. So I was wondering why?

Answer 1

To understand a difference between your two methods, note the following

• The @ResponseBody annotation indicates that the return type is written to the response body.

• Spring converts the returned object to a response body by using an appropriate HttpMessageConverter

• Since you've indicated the produces = "application/json" the converter that will kick in is MappingJackson2HttpMessageConverter An HttpMessageConverter implementation that can read and write JSON using Jackson's ObjectMapper

this is a powerfull feature that Spring MVC does automatically, it resolve the representation, and converts automatically.

In your first method you've attempted doing this manually which is not needed and wrong. What the framework attempted is to convert your JsonObject to JSON representation, which either fails or gives you JSON properties containing the fileds of the JsonObject class.

In the later method you poplated the values to a Map, and let the Spring MVC convert to JSON. As Jackson libraries knows how to convert the Map to JSON, your later method works without issues