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 · 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

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 lt T gt      private List lt T gt  data     public Wrapper         public Wrapper List lt T gt  data        this data   data        public List lt T gt  getData         return data        public void setData List lt T gt  data        this data   data          Serialization of my lists were made with a  ControllerAdvice    ControllerAdvice public class JSONResponseWrapper implements ResponseBodyAdvice lt Object gt        Override   public boolean supports MethodParameter returnType  Class lt   extends HttpMessageConverter lt   gt  gt  converterType        return true          Override    SuppressWarnings  unchecked     public Object beforeBodyWrite Object body  MethodParameter returnType  MediaType selectedContentType  Class lt   extends HttpMessageConverter lt   gt  gt  selectedConverterType  ServerHttpRequest request  ServerHttpResponse response        if  body instanceof List          return new Wrapper lt  gt   List lt Object gt   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 lt Void gt  setResources  RequestBody Wrapper lt ResourceDTO gt  wrappedResources      List lt ResourceDTO gt  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

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

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

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

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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

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

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 lt SingleOrListElement gt  patch Object singleOrListElement      if  singleOrListElement instanceof List        return  ArrayList lt SingleOrListElement gt   singleOrListElement      else       LinkedHashMap map    LinkedHashMap  singleOrListElement      return Collections singletonList SingletonList builder                                property1  String  map get  p1                                 property2  Integer  map get  p2                                 build

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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

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

User · Answer

After struggling with this thing for WAY too long  here is the super easy solution  My controller was looking for  RequestBody List lt String gt  ids  and I had the request body as        quot ids quot              quot 1234 quot            quot 5678 quot            and the solution was to change the body simply to   quot 1234 quot    quot 5678 quot    Yup  Just that easy

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

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

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

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