Can not deserialize instance of java util ArrayList out of VALUE STRING

Question

I have a REST service built with Jersey and deployed in the AppEngine  The REST service implements the verb PUT that consumes an application json media type  The data binding is performed by Jackson   The verb consumes an enterprise-departments relation represented in JSON as     name   myEnterprise    departments    HR   IT   SC      On the client side  I use gson to convert the JSON representation into a java object  Then  I pass the object to my REST service and it works fine   Problem   When my JSON representation has only one item in the collection    name   myEnterprise    departments    HR      the service cannot deserialize the object   ATTENTION   enterprise enterprise  org codehaus jackson map JsonMappingException   Can not deserialize instance of java util ArrayList out of VALUE STRING token at   Source  org mortbay jetty HttpParser Input 5a9c5842  line  1  column  2   As reported by other users  the solution is to add the flag ACCEPT SINGLE VALUE AS ARRAY  e g   Jersey  Can not deserialize instance of ArrayList out of String   Nevertheless  I am not controlling an ObjectMapper because in the service side it is transparently made by Jackson   Question   Is there a way to configure the ObjectMapper on the service side to enable ACCEPT SINGLE VALUE AS ARRAY  annotations  web xml   Code details  Java object    XmlRootElement public class Enterprise       private String name      private List lt String gt  departments       public Enterprise           public String getName             return name            public void setName String name            this name   name            public List lt String gt  getDepartments             return departments            public void setDepartments List lt String gt  departments            this departments   departments            The REST service side        PUT      Consumes MediaType APPLICATION JSON       Produces MediaType APPLICATION JSON       Path   enterprise        public Response putEnterprise Enterprise enterprise               Context HttpServletRequest req                        Client side       String jsonString       name     myEnterprise      departments      HR       Enterprise enterprise   gson fromJson jsonString  Enterprise class   System out println gson toJson enterprise    response   webResource                           type MediaType APPLICATION JSON              put ClientResponse class enterprise   if  response getStatus    gt   400            throw new RuntimeException  Failed   HTTP error code       response getStatus

User · Answer

from Jackson 2 7 x  there is a way to annotate the member variable itself     JsonFormat with   JsonFormat Feature ACCEPT SINGLE VALUE AS ARRAY   private List lt String gt  newsletters    More info here  Jackson  JsonFormat

User · Answer

This is the solution for my old question   I implemented my own ContextResolver in order to enable the DeserializationConfig Feature ACCEPT SINGLE VALUE AS ARRAY feature   package org lig hadas services mapper   import javax ws rs Produces  import javax ws rs core MediaType  import javax ws rs ext ContextResolver  import javax ws rs ext Provider   import org codehaus jackson map DeserializationConfig  import org codehaus jackson map ObjectMapper    Produces MediaType APPLICATION JSON   Provider public class ObjectMapperProvider implements ContextResolver lt ObjectMapper gt       ObjectMapper mapper      public ObjectMapperProvider           mapper   new ObjectMapper           mapper configure DeserializationConfig Feature ACCEPT SINGLE VALUE AS ARRAY  true            Override    public ObjectMapper getContext Class lt   gt  type           return mapper           And in the web xml I registered my package into the servlet definition      lt servlet gt       lt servlet-name gt     lt  servlet-name gt       lt servlet-class gt com sun jersey spi container servlet ServletContainer lt  servlet-class gt       lt init-param gt           lt param-name gt com sun jersey config property packages lt  param-name gt           lt param-value gt     org lig hadas services mapper lt  param-value gt               lt  init-param gt           lt  servlet gt        all the rest is transparently done by jersey jackson

User · Answer

do you try     name   myEnterprise    departments    HR       the square brace is the key point

User · Answer

Setting this attribute to ObjectMapper instance works   objectMapper enable DeserializationFeature ACCEPT SINGLE VALUE AS ARRAY

User · Answer

For people that find this question by searching for the error message  you can also see this error if you make a mistake in your  JsonProperty annotations such that you annotate a List-typed property with the name of a single-valued field    JsonProperty  someSingleValuedField      Oops  should have been  someMultiValuedField  public List lt String gt  getMyField        deserialization fails - single value into List   return myField

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

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