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

Question

My question is essentially a follow-up to this question    RestController public class TestController        RequestMapping   getString       public String getString                 return  Hello World             In the above  Spring would add  Hello World  into the response body  How can I return a String as a JSON response   I understand that I could add quotes  but that feels more like a hack   Please provide any examples to help explain this concept      Note  I don t want this written straight to the HTTP Response body  I want to return the String in JSON format  I m using my Controller   with RestyGWT which requires the response to be in valid JSON   format

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

Add this annotation to your method   RequestMapping value     getString   method   RequestMethod GET  produces    application json

User · Answer

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 lt HttpMessageConverter lt   gt  gt  converters        converters stream          filter c - gt  c instanceof StringHttpMessageConverter         findFirst   ifPresent converters  remove           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

User · Answer

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 lt List lt T gt  gt  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 lt String gt  as json    example

User · Answer

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   quot Don t do JSON deserialization  quot   or something else   quot Do JSON deserialization  quot     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 lt Any  gt             override fun supports returnType  MethodParameter  converterType  Class lt out HttpMessageConverter lt   gt  gt    Boolean           converterType     StringHttpMessageConverter  class java      override fun beforeBodyWrite          body  Any           returnType  MethodParameter          selectedContentType  MediaType          selectedConverterType  Class lt out HttpMessageConverter lt   gt  gt           request  ServerHttpRequest          response  ServerHttpResponse        Any            return if  request uri path contains  quot api quot                  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

User · Answer

Add  ResponseBody annotation  which will write return data in output stream

User · Answer

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

User · Answer

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 lt String  Object gt  endPointExample             Map lt String  Object gt  rtn   new LinkedHashMap lt  gt          rtn put  pic   image       rtn put  potato    King Potato         return rtn       This will return     pic   a17fefab83517fb   beb8ac5a2ae8f0449   potato   King Potato

User · Answer

Make simple        GetMapping   health       public ResponseEntity lt String gt  healthCheck             LOG info  REST request health check            return new ResponseEntity lt  gt      status       UP      HttpStatus OK

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

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