Unable to deserialize a Set of polymorphic objects (Jackson 2.1.2)

Question

I’m trying to deserialize the following:

{
 "type":"GIQuestionnaire",
 "id":6051,   
 "questions":[
      {
        "type":"IncludeExcludeQuestion",
        "id":24057,
        "answers":[
            {
              "type":"IncludeExcludeAnswer",
               "id":101497
            },
            {
              "type":"IncludeExcludeAnswer",
               "id":101496
            }
         ]
      }
   ]
}

Which results in this error:

java.lang.AssertionError: Can not handle managed/back reference 'questionnaire-questions': value deserializer is of type ContainerDeserializerBase, but content type is not handled by a BeanDeserializer  (instead it's of type com.foo.questionnaire.json.QuestionDeserializer)

The Junit test:

@Test
public void testDeserializeQuestionnaire() {
    ObjectMapper mapper = new ObjectMapper();
    GIQuestionnaire q = manager.createGIQuestionnaire(…);
    try
    {
        String json = mapper.writeValueAsString(q);
        assertTrue(q.equals(mapper.readValue(json, GIQuestionnaire.class)));
    } catch (Exception e) {
        fail(e.getMessage());
    }
}

A Questionnaire contains a Set (implemented as TreeSet) of Question objects. A Question contains a Set (TreeSet) of Answer objects.

Here are the relevant snippets of code that show how I used Jackson annotations:

// Questionnaire abstract base class
@JsonTypeInfo(use=JsonTypeInfo.Id.NAME, include=JsonTypeInfo.As.PROPERTY, property="type")
@JsonSubTypes({
    @JsonSubTypes.Type(value=GIQuestionnaire.class, name="GIQuestionnaire"),
    @JsonSubTypes.Type(value=PolicyQuestionnaire.class, name="PolicyQuestionnaire")    
})
public abstract class Questionnaire implements Serializable {
    @JsonManagedReference("questionnaire-questions")
    @JsonDeserialize(as = TreeSet.class, contentAs = Question.class)

    private Set<Question> questions = new TreeSet<>();

    // … remainder omitted for brevity 
}

// Question abstract base class
@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.PROPERTY, property = "type")
@JsonSubTypes({
    @JsonSubTypes.Type(value = EntityQuestion.class, name = "EntityQuestion"),
    @JsonSubTypes.Type(value = IncludeExcludeQuestion.class, name = "IncludeExcludeQuestion")
})
@JsonDeserialize(using = QuestionDeserializer.class)
public abstract class Question implements Comparable<Question>, Serializable {
   @JsonBackReference("questionnaire-questions")
   private Questionnaire questionnaire;

   @JsonManagedReference("question-answers")
   @JsonDeserialize(as = TreeSet.class, contentAs = Answer.class)
   private Set<Answer> answers = new TreeSet<>();

    // … remainder omitted for brevity
}

// Answer abstract base class
@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, include = JsonTypeInfo.As.PROPERTY, property = "type")
@JsonSubTypes({
    @JsonSubTypes.Type(value = EntityAnswer.class, name = "EntityAnswer"),         
    @JsonSubTypes.Type(value = IncludeExcludeAnswer.class, name = "IncludeExcludeAnswer")
})
@JsonDeserialize(using = AnswerDeserializer.class)
public abstract class Answer implements Comparable<Answer>, Serializable {

   @JsonBackReference("question-answers")
   private Question question;
   // … remainder omitted for brevity 
}

public class QuestionDeserializer extends JsonDeserializer<Question> {
    @Override
    public Question deserialize(JsonParser jsonParser, DeserializationContext     deserializationContext) throws IOException, JsonProcessingException {
        //deserialize a Question to a concrete instance
    }
}

public class AnswerDeserializer extends JsonDeserializer<Answer> {
    @Override
    public Answer deserialize(JsonParser jsonParser, DeserializationContext deserializationContext) throws IOException, JsonProcessingException {
        //deserialize an Answer to a concrete instance
    }
}

Where am I going wrong with my annotations and/or Deserializers?

Answer 1

What if you try, very handy interface implementation @JsonIdentityInfo which is provided in jackson 2 library?

@Entity
@JsonIdentityInfo(generator=ObjectIdGenerators.PropertyGenerator.class, property="id")
public class Answer { ....

@Entity
@JsonIdentityInfo(generator=ObjectIdGenerators.PropertyGenerator.class, property="id")
public class Question { ....

in maven

<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-core</artifactId>
    <version>2.0.2</version>
</dependency>

Answer 2

According to the Jackson WIKI the combination of polymorphic deserialization and @JsonManagedReference / @JsonBackReference is not supported.

Currently (as of version 1.6.0) combination of abstract types (with polymorphic handling using @JsonTypeInfo) do not work with this feature

See also a recent issue JIRA-890 in the Jackson issue tracker.