Can not deserialize instance of java util ArrayList out of START OBJECT token

Question

I m trying to POST a List of custom objects   My JSON in request body is this          collection                            name    Test order1                detail    ahk ks                                    name    Test order2                detail    Fisteku                      Server side code that handles the request   import java util Collection   import javax ws rs Consumes  import javax ws rs POST  import javax ws rs Path  import javax ws rs Produces  import javax ws rs core MediaType  import javax ws rs core Response     Path value     rest corder   public class COrderRestService         POST      Produces MediaType APPLICATION JSON       Consumes MediaType APPLICATION JSON      public Response postOrder Collection lt COrder gt  orders            StringBuilder stringBuilder   new StringBuilder            for  COrder c   orders                stringBuilder append c toString                       System out println stringBuilder           return Response ok stringBuilder  MediaType APPLICATION JSON  build              Entity COrder   import javax xml bind annotation XmlRootElement    XmlRootElement public class COrder       String name      String detail        Override     public String toString             return  COrder  name     name      detail     detail                      getClass       getClass        hashCode       hashCode                        toString       super toString                    But an exception is thrown   SEVERE  Failed executing POST  rest corder org jboss resteasy spi ReaderException  org codehaus jackson map JsonMappingException  Can not deserialize instance of java util ArrayList out of START OBJECT token  at  Source  org apache catalina connector CoyoteInputStream 6de8c535  line  1  column  1      at org jboss resteasy core MessageBodyParameterInjector inject MessageBodyParameterInjector java 183      at org jboss resteasy core MethodInjectorImpl injectArguments MethodInjectorImpl java 88      at org jboss resteasy core MethodInjectorImpl invoke MethodInjectorImpl java 111      at org jboss resteasy core ResourceMethodInvoker invokeOnTarget ResourceMethodInvoker java 280      at org jboss resteasy core ResourceMethodInvoker invoke ResourceMethodInvoker java 234      at org jboss resteasy core ResourceMethodInvoker invoke ResourceMethodInvoker java 221      at org jboss resteasy core SynchronousDispatcher invoke SynchronousDispatcher java 356      at org jboss resteasy core SynchronousDispatcher invoke SynchronousDispatcher java 179      at org jboss resteasy plugins server servlet ServletContainerDispatcher service ServletContainerDispatcher java 220      at org jboss resteasy plugins server servlet HttpServletDispatcher service HttpServletDispatcher java 56      at org jboss resteasy plugins server servlet HttpServletDispatcher service HttpServletDispatcher java 51      at javax servlet http HttpServlet service HttpServlet java 728      at org apache catalina core ApplicationFilterChain internalDoFilter ApplicationFilterChain java 305      at org apache catalina core ApplicationFilterChain doFilter ApplicationFilterChain java 210      at org apache tomcat websocket server WsFilter doFilter WsFilter java 51      at org apache catalina core ApplicationFilterChain internalDoFilter ApplicationFilterChain java 243      at org apache catalina core ApplicationFilterChain doFilter ApplicationFilterChain java 210      at org apache catalina core StandardWrapperValve invoke StandardWrapperValve java 222      at org apache catalina core StandardContextValve invoke StandardContextValve java 123      at org apache catalina authenticator AuthenticatorBase invoke AuthenticatorBase java 502      at org apache catalina core StandardHostValve invoke StandardHostValve java 171      at org apache catalina valves ErrorReportValve invoke ErrorReportValve java 100      at org apache catalina valves AccessLogValve invoke AccessLogValve java 953      at org apache catalina core StandardEngineValve invoke StandardEngineValve java 118      at org apache catalina connector CoyoteAdapter service CoyoteAdapter java 408      at org apache coyote http11 AbstractHttp11Processor process AbstractHttp11Processor java 1041      at org apache coyote AbstractProtocol AbstractConnectionHandler process AbstractProtocol java 603      at org apache tomcat util net JIoEndpoint SocketProcessor run JIoEndpoint java 312      at java util concurrent ThreadPoolExecutor runWorker ThreadPoolExecutor java 1145      at java util concurrent ThreadPoolExecutor Worker run ThreadPoolExecutor java 615      at java lang Thread run Thread java 724

User · Accepted Answer

The problem is the JSON - this cannot  by default  be deserialized into a Collection because it s not actually a JSON Array - that would look like this                     name    Test order1            detail    ahk ks                        name    Test order2            detail    Fisteku            Since you re not controlling the exact process of deserialization  RestEasy does  - a first option would be to simply inject the JSON as a String and then take control of the deserialization process    Collection lt COrder gt  readValues   new ObjectMapper   readValue      jsonAsString  new TypeReference lt Collection lt COrder gt  gt             You would loose a bit of the convenience of not having to do that yourself  but you would easily sort out the problem    Another option - if you cannot change the JSON - would be to construct a wrapper to fit the structure of your JSON input - and use that instead of Collection lt COrder gt     Hope this helps

User · Answer

JsonFormat with   JsonFormat Feature ACCEPT SINGLE VALUE AS ARRAY  private List lt  COrder  gt  orders

User · Answer

Dto response   softConvertValue jsonData  Dto class          public static  lt T gt  T softConvertValue Object fromValue  Class lt T gt  toValueType                         ObjectMapper objMapper   new ObjectMapper                return objMapper configure DeserializationFeature FAIL ON UNKNOWN PROPERTIES  false                       convertValue fromValue  toValueType

User · Answer

After struggling with this thing for WAY too long, here is the super easy solution.

My controller was looking for

@RequestBody List<String> ids

and I had the request body as

{
    "ids": [
        "1234",
        "5678"
     ]
}

and the solution was to change the body simply to

["1234", "5678"]

Yup. Just that easy.

User · Answer

In my case  the error was being shown because when I was reading my JSON file using Jackson library  my JSON file contained only 1 object  Hence it started with     and ended with      But while reading it and storing it in a variable  I was storing it in an Array object  As in my case  there could be more than 1 object    Hence  I added     in the start and     in the end of my JSON file to convert it into an array of object and it worked perfectly fine without any error

User · Answer

Related to Eugen s answer  you can solve this particular case by creating a wrapper POJO object that contains a Collection lt COrder gt  as its member variable  This will properly guide Jackson to place the actual Collection data inside the POJO s member variable and produce the JSON you are looking for in the API request   Example   public class ApiRequest        JsonProperty  collection      private Collection lt COrder gt  collection         getters     Then set the parameter type of COrderRestService postOrder   to be your new  ApiRequest wrapper POJO instead of Collection lt COrder gt

User · Answer

As mentioned above the following would solve the problem: mapper.configure(DeserializationFeature.ACCEPT_SINGLE_VALUE_AS_ARRAY, true);

However in my case the provider did this [0..1] or [0..*] serialization rather as a bug and I could not enforce fixing. On the other hand it did not want to impact my strict mapper for all other cases which needs to be validated strictly.

So I did a Jackson NASTY HACK (which should not be copied in general ;-) ), especially because my SingleOrListElement had only few properties to patch:

@JsonProperty(value = "SingleOrListElement", access = JsonProperty.Access.WRITE_ONLY)
private Object singleOrListElement; 

public List<SingleOrListElement> patch(Object singleOrListElement) {
  if (singleOrListElement instanceof List) {
    return (ArrayList<SingleOrListElement>) singleOrListElement;
  } else {
    LinkedHashMap map = (LinkedHashMap) singleOrListElement;
    return Collections.singletonList(SingletonList.builder()
                            .property1((String) map.get("p1"))
                            .property2((Integer) map.get("p2"))
                            .build());
  }

User · Answer

I ran into this same problem these days and maybe some more detail might be helpful to somebody else.

I was looking some security guidelines for REST APIs and crossed a very intriguing issue with json arrays. Check the link for details, but basically, you should wrap them within an Object as we already saw in this post question.

So, instead of:

  [
    {
      "name": "order1"
    },
    {
      "name": "order2"
    }
  ]

It's advisable that we always do:

  {
    "data": [
      {
        "name": "order1"
      },
      {
        "name": "order2"
      }
    ]
  }

This is pretty straight-forward when you doing a GET, but might give you some trouble if, instead, your trying to POST/PUT the very same json.

In my case, I had more than one GET that was a List and more than one POST/PUT that would receive the very same json.

So what I end up doing was to use a very simple Wrapper object for a List:

public class Wrapper<T> {
  private List<T> data;

  public Wrapper() {}

  public Wrapper(List<T> data) {
    this.data = data;
  }
  public List<T> getData() {
    return data;
  }
  public void setData(List<T> data) {
    this.data = data;
  }
}

Serialization of my lists were made with a @ControllerAdvice:

@ControllerAdvice
public class JSONResponseWrapper implements ResponseBodyAdvice<Object> {

  @Override
  public boolean supports(MethodParameter returnType, Class<? extends HttpMessageConverter<?>> converterType) {
    return true;
  }

  @Override
  @SuppressWarnings("unchecked")
  public Object beforeBodyWrite(Object body, MethodParameter returnType, MediaType selectedContentType, Class<? extends HttpMessageConverter<?>> selectedConverterType, ServerHttpRequest request, ServerHttpResponse response) {
    if (body instanceof List) {
      return new Wrapper<>((List<Object>) body);
    }
    else if (body instanceof Map) {
      return Collections.singletonMap("data", body);
    }  
    return body;
  }
}

So, all the Lists and Maps where wrapped over a data object as below:

  {
    "data": [
      {...}
    ]
  }

Deserialization was still default, just using de Wrapper Object:

@PostMapping("/resource")
public ResponseEntity<Void> setResources(@RequestBody Wrapper<ResourceDTO> wrappedResources) {
  List<ResourceDTO> resources = wrappedResources.getData();
  // your code here
  return ResponseEntity
           .ok()
           .build();
}

That was it! Hope it helps someone.

Note: tested with SpringBoot 1.5.5.RELEASE.

User · Answer

Normally we face this issue when there is a problem mapping JSON node with that of Java object. I faced the same issue because in the swagger the node was defined as of Type array and the JSON object was having only one element , hence the system was having difficulty in mapping one element list to an array.

In Swagger the element was defined as

Test:
 "type": "array",
 "minItems": 1,
 "items": {
   "$ref": "#/definitions/TestNew"
  }

While it should be

Test:
    "$ref": "#/definitions/TestNew"

And TestNew should be of type array

User · Answer

This will work   The problem may happen when you re trying to read a list with a single element as a JsonArray rather than a JsonNode or vice versa   Since you can t know for sure if the returned list contains a single element  so the json looks like this        or multiple elements  and the json looks like this                -  you ll have to check in runtime the type of the element   It should look like this    Note  in this code sample I m using com fasterxml jackson    String jsonStr   response readEntity String class   ObjectMapper mapper   new ObjectMapper    JsonNode rootNode   mapper readTree jsonStr       Start by checking if this is a list - gt  the order is important here                        if  rootNode instanceof ArrayNode           Read the json as a list      myObjClass   objects   mapper readValue rootNode toString    myObjClass   class             else if  rootNode instanceof JsonNode           Read the json as a single object      myObjClass object   mapper readValue rootNode toString    myObjClass class             else

User · Answer

I had this issue on a REST API that was created using Spring framework  Adding a  ResponseBody annotation  to make the response JSON  resolved it

User · Answer

Instead of the JSON document  you can update the ObjectMapper object like below     ObjectMapper mapper   new ObjectMapper    mapper configure DeserializationFeature ACCEPT SINGLE VALUE AS ARRAY  true

User · Answer

Same issue   com fasterxml jackson databind exc MismatchedInputException  Cannot deserialize instance of  java util UUID  out of START OBJECT token   What caused it was the following   ResponseEntity lt UUID gt  response   restTemplate postForEntity   example    null  UUID class     In my test I purposely set the request as null  no content POST   As previously mentioned  the cause for the OP was the same because the request didn t contain a valid JSON  so it couldn t be automatically identified as an application json request which was the limitation on the server  consumes    application json    A valid JSON request would be  What fixed it was populating an entity with null body and json headers explicitly    HttpHeaders headers   new HttpHeaders    headers setContentType MediaType APPLICATION JSON   HttpEntity request   new HttpEntity lt  gt  null  headers   ResponseEntity lt UUID gt  response   restTemplate postForEntity   example    request  UUID class

[java] Can not deserialize instance of java.util.ArrayList out of START_OBJECT token

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