Returning JSON object as response in Spring Boot

Question

I have a sample RestController in Spring Boot    RestController  RequestMapping   api   class MyRestController        GetMapping path     hello       public JSONObject sayHello                 return new JSONObject    aa   bb                I am using the JSON library org json  When I hit API  hello  I get an exception saying        Servlet service   for servlet  dispatcherServlet  in context with path      threw exception  Request processing failed  nested exception is   java lang IllegalArgumentException  No converter found for return   value of type  class org json JSONObject  with root cause      java lang IllegalArgumentException  No converter found for return   value of type  class org json JSONObject   What is the issue  Can someone explain what exactly is happening

User · Answer

you can also use a hashmap for this   GetMapping public HashMap lt String  Object gt  get         HashMap lt String  Object gt  map   new HashMap lt  gt         map put  key1    value1        map put  results   somePOJO       return map

User · Answer

You can either return a response as String as suggested by  vagaasen or you can use ResponseEntity Object provided by Spring as below  By this way you can also return Http status code which is more helpful in webservice call    RestController  RequestMapping   api   public class MyRestController         GetMapping path     hello   produces MediaType APPLICATION JSON VALUE      public ResponseEntity lt Object gt  sayHello                    Get data from service layer into entityList           List lt JSONObject gt  entities   new ArrayList lt JSONObject gt             for  Entity n   entityList                JSONObject entity   new JSONObject                entity put  aa    bb                entities add entity                     return new ResponseEntity lt Object gt  entities  HttpStatus OK

User · Answer

As you are using Spring Boot web  Jackson dependency is implicit and we do not have to define explicitly  You can check for Jackson dependency in your pom xml in the dependency hierarchy tab if using eclipse   And as you have annotated with  RestController there is no need to do explicit json conversion  Just return a POJO and jackson serializer will take care of converting to json  It is equivalent to using  ResponseBody when used with  Controller  Rather than placing  ResponseBody on every controller method we place  RestController instead of vanilla  Controller and  ResponseBody by default is applied on all resources in that controller   Refer this link  https   docs spring io spring docs current spring-framework-reference html mvc html mvc-ann-responsebody  The problem you are facing is because the returned object JSONObject  does not have getter for certain properties  And your intention is not to serialize this JSONObject but instead to serialize a POJO  So just return the POJO    Refer this link   https   stackoverflow com a 35822500 5039001  If you want to return a json serialized string then just return the string  Spring will use StringHttpMessageConverter instead of JSON converter in this case

User · Answer

More correct create DTO for API queries  for example entityDTO   Default response OK with list of entities     GetMapping produces MediaType APPLICATION JSON VALUE   ResponseStatus HttpStatus OK  public List lt EntityDto gt  getAll         return entityService getAllEntities        But if you need return different Map parameters you can use next two examples 2  For return one parameter like map    GetMapping produces MediaType APPLICATION JSON VALUE  public ResponseEntity lt Object gt  getOneParameterMap         return ResponseEntity status HttpStatus CREATED  body              Collections singletonMap  quot key quot    quot value quot          And if you need return map of some parameters since Java 9      GetMapping produces   MediaType APPLICATION JSON VALUE  public ResponseEntity lt Object gt  getSomeParameters         return ResponseEntity status HttpStatus OK  body Map of               quot key-1 quot    quot value-1 quot                quot key-2 quot    quot value-2 quot                quot key-3 quot    quot value-3 quot

User · Answer

use ResponseEntity lt ResponseBean gt    Here you can use ResponseBean or Any java bean as you like to return your api response and it is the best practice  I have used Enum for response  it will return status code and status message of API    GetMapping path     login   public ResponseEntity lt ServiceStatus gt  restApiExample HttpServletRequest request              HttpServletResponse response            String username   request getParameter  username            String password   request getParameter  password             loginService login username  password  request           return new ResponseEntity lt ServiceStatus gt  ServiceStatus LOGIN SUCCESS                  HttpStatus ACCEPTED           for response ServiceStatus or ResponseBody       public enum ServiceStatus        LOGIN SUCCESS 0   Login success         private final int id      private final String message         Enum constructor     ServiceStatus int id  String message            this id   id          this message   message             public int getId             return id             public String getMessage             return message            Spring REST API should have below key in response   Status Code Message   you will get final response below         StatusCode     0        Message   Login success       you can use ResponseBody java POJO  ENUM etc    as per your requirement

User · Answer

The reason why your current approach doesn t work is because Jackson is used by default to serialize and to deserialize objects  However  it doesn t know how to serialize the JSONObject  If you want to create a dynamic JSON structure  you can use a Map  for example   GetMapping public Map lt String  String gt  sayHello         HashMap lt String  String gt  map   new HashMap lt  gt         map put  quot key quot    quot value quot        map put  quot foo quot    quot bar quot        map put  quot aa quot    quot bb quot        return map     This will lead to the following JSON response     quot key quot    quot value quot    quot foo quot    quot bar quot    quot aa quot    quot bb quot     This is a bit limited  since it may become a bit more difficult to add child objects  Jackson has its own mechanism though  using ObjectNode and ArrayNode  To use it  you have to autowire ObjectMapper in your service controller  Then you can use   GetMapping public ObjectNode sayHello         ObjectNode objectNode   mapper createObjectNode        objectNode put  quot key quot    quot value quot        objectNode put  quot foo quot    quot bar quot        objectNode put  quot number quot   42       return objectNode     This approach allows you to add child objects  arrays  and use all various types

User · Answer

RequestMapping   api status   public Map doSomething         return Collections singletonMap  status   myService doSomething         PS  Works only for 1 value

User · Answer

If you need to return a JSON object using a String  then the following should work  import com fasterxml jackson core JsonProcessingException  import com fasterxml jackson databind JsonNode  import com fasterxml jackson databind ObjectMapper  import org springframework http ResponseEntity        RestController  RequestMapping  quot  student quot   public class StudentController         GetMapping      RequestMapping  quot   quot       public ResponseEntity lt JsonNode gt  get   throws JsonProcessingException           ObjectMapper mapper   new ObjectMapper            JsonNode json   mapper readTree  quot    quot id  quot     quot 132  quot     quot name  quot     quot Alice  quot   quot            return ResponseEntity ok json

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