Jackson - JsonMappingException due to constructor

Question

I am having the following exception when trying to deserialize a JSON

No suitable constructor found for type [simple type, class MyObj$obj$Card]: can not instantiate from JSON object (need to add/enable type information?) at [Source: java.io.StringReader@4344ee21; line: 1, column: 201] (through reference chain: MyObj["obj"]->Obj["cards"])

And the JSON is

{   "obj":{   "api":"OK",   "cache":false,   "cards":[      {         "id":1232995897,         "items":[            {               "id":"vmdSJLpnY",               "cat":50,               "rating":0.0            }         ]      },      {         "id":0005897,         "items":[            {               "id":"vxdSJLpnY",               "cat":50,               "rating":0.0            }         ]      }  ]  } } 

And within the Obj class I have the following statement

@JsonProperty("cards") private Card[] cards;

Which produces the exception above. Changing the type Card[] to Object[] does not produce an exception, but it lacks of the correct mapping I desire to get.

Any clue how can I resolve it? A snippet will be GREAT! What this error means anyhow?

UPDATE

I have included the Java class.

import com.fasterxml.jackson.annotation.JsonCreator; import com.fasterxml.jackson.annotation.JsonIgnoreProperties; import com.fasterxml.jackson.annotation.JsonProperty; import com.google.gson.Gson;  @JsonIgnoreProperties(ignoreUnknown=true) public final class MyObj {     @JsonIgnoreProperties(ignoreUnknown=true)     public final class Obj {         @JsonIgnoreProperties(ignoreUnknown=true)         public final class Card {             @JsonIgnoreProperties(ignoreUnknown=true)             public final class Item {                 @JsonProperty("id") private String id;                 @JsonProperty("cat") private String cat;                 @JsonProperty("rating") private String rating;                 public final String getId() { return id; }                 public final String getCat() { return cat; }                 public final String getRating() { return ranting; }                 public final String toString() { return new Gson().toJson(this); }             }              @JsonProperty("items") private Item[] items;             public final Item[] getItems() { return items; }             public final String toString() { return new Gson().toJson(this); }         }          @JsonProperty("cards") private Card[] cards;         public Card[] getCards() { return cards; }         public final String toString() { return new Gson().toJson(this); }           }       @JsonProperty("obj") MyObj obj;     public final Card[] getCards(){ return apiServiceResultsNoLists.getCards(); } } 

Answer 1

I think the problem is most likely with the Card object

• it might not have a default constructor
• if it doesn't have a default constructor, it should be annotated using @JsonCreator

EDIT I have two things:
* you don't have setters.
* you don't have a public constructor that would be allowed to set your fileds.

How should the deserializer populate your fields if you don't give it any (legal*) means for it?

Solutions:
-> add public setters to the classes
-> or create parametrized constructors annotated with @JsonCreator

*: of course, the parser could do the reflective "mofidy the visibility" trick, but come on, this is not "the way it's meant to be played"

EDIT2 I think this should work, but I can't test it - I don't have a project at hand with Jackson properly set up now (this is just a part of it, but I think it is easy to interpret what I wanted to show.) Note, I changed the array to a List:

    @JsonIgnoreProperties(ignoreUnknown=true)      public final class Card {          @JsonIgnoreProperties(ignoreUnknown=true)      public final class Item {              @JsonProperty("id") private String id;              @JsonProperty("cat") private String cat;              @JsonProperty("rating") private String rating;               @JsonCreator         public Item(@JsonProperty("id") String id, @JsonProperty("cat") String cat, @JsonProperty("rating") String rating) {             this.id = id;             this.cat = cat;             this.rating = rating;         }              public final String getId() { return id; }              public final String getCat() { return cat; }              public final String getRating() { return ranting; }              public final String toString() { return new Gson().toJson(this); }          }          @JsonProperty("items") private List<Item> items;           @JsonCreator     public Card(@JsonProperty("items") List<Item> items) {         this.items = items;     }             public final List<Item> getItems() { return items; }           public final String toString() { return new Gson().toJson(this); }      }