Simple string as JSON return value in spring rest controller

Question

Let's take a look at the following simple test controller (Used with Spring 4.0.3):

@RestController
public class TestController
{
    @RequestMapping("/getList")
    public List<String> getList()
    {
        final List<String> list = new ArrayList<>();
        list.add("1");
        list.add("2");
        return list;
    }

    @RequestMapping("/getString")
    public String getString()
    {
        return "Hello World";
    }
}

In theory both controller methods should return valid JSON. Calling the first controller method indeed does return the following JSON array:

$ curl -i -H "Accept: application/json" http://localhost:8080/getList
HTTP/1.1 200 OK
Content-Type: application/json;charset=UTF-8

["1","2"]

But the second controller method returns the string without quotes which is not a valid JSON string:

$ curl -i -H "Accept: application/json" http://localhost:8080/getString
HTTP/1.1 200 OK
Content-Type: application/json;charset=UTF-8

Hello World

Why is that so? Can it be configured? Is it a bug? Or a feature I don't understand?

Answer 1

When you return a String object, Spring MVC interprets that as the content to put in the response body and doesn't modify it further. If you're wanting an actual string to be the JSON response, you'll need to either quote it yourself or run it through Jackson explicitly.

Answer 2

You can remove the StringHttpMessageConverter which is registered before the jackson converter, - like mentioned in the comment.

/**
 * Unregister the default {@link StringHttpMessageConverter} as we want
 * Strings to be handled by the JSON converter.
 *
 * Our MappingJackson2HttpMessageConverter will deal with strings.
 *
 * @param converters
 *            List of already configured converters
 */
@Override
public void extendMessageConverters(List<HttpMessageConverter<?>> converters) {
    converters.removeIf(converter -> converter instanceof StringHttpMessageConverter);
}

Answer 3

If you want to return a String object, Spring MVC interprets that as the content to put in the response body and doesn't modify it further. If you're wanting an actual string to be the JSON response, you'll need to either quote it yourself or run it through Jackson explicitly.

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

Answer 4

Here are the steps that I did to achieve this :

1. Add dependency in pom file:

   <dependency>
       <groupId>com.fasterxml.jackson.core</groupId>
       <artifactId>jackson-databind</artifactId>
       <version>2.9.3</version>
   </dependency>
   

2. Put @ResponseBody annotation on your method like this:

   @RequestMapping(value = "/getCountries", method = RequestMethod.GET)    
   @ResponseBody    
   public List<Country> getCountries() {    
       return countryDAO.list();    
   }