Jackson - @JsonTypeInfo property is being mapped as null?

I have this response:

{  
    "id":"decaa828741611e58bcffeff819cdc9f",
    "statement":"question statement",
    "exercise_type":"QUESTION"
}

Then, based on exercise_type attribute, I want to instantiate different objects instances (subclasses of ExerciseResponseDTO), so I create this mix in:

@JsonTypeInfo(  
    use = JsonTypeInfo.Id.NAME,  
    include = JsonTypeInfo.As.PROPERTY,  
    property = "exercise_type")  
@JsonSubTypes({  
    @Type(value = ExerciseChoiceResponseDTO.class, name = "CHOICE"),  
    @Type(value = ExerciseQuestionResponseDTO.class, name = "QUESTION")})  
public abstract class ExerciseMixIn  
{}  

public abstract class ExerciseResponseDTO {

    private String id;
    private String statement;
    @JsonProperty(value = "exercise_type") private String exerciseType;
    
    // Getters and setters 
}

public class ExerciseQuestionResponseDTO
    extends ExerciseResponseDTO {}

public class ExerciseChoiceResponseDTO
    extends ExerciseResponseDTO {}

So I create my ObjectMapper as follows

ObjectMapper mapper = new ObjectMapper();
mapper.addMixIn(ExerciseResponseDTO.class, ExerciseMixIn.class);

My test:

ExerciseResponseDTO exercise = mapper.readValue(serviceResponse, ExerciseResponseDTO.class)
Assert.assertTrue(exercise.getClass() == ExerciseQuestionResponseDTO.class); // OK
Assert.assertEquals("decaa828741611e58bcffeff819cdc9f" exercise.getId()); // OK
Assert.assertEquals("question statement", exercise.getStatement()); // OK
Assert.assertEquals("QUESTION", exercise.getExerciseType()); // FAIL. Expected: "QUESTION", actual: null 

The problem is that, for some reason, the exercise_type attribute being used as property on @JsonTypeInfo is being mapped as null. Any idea how i can solve this?

Finally, I've found the solution in the API Doc

> Note on visibility of type identifier: by default, deserialization > (use during reading of JSON) of type identifier is completely handled > by Jackson, and is not passed to deserializers. However, if so > desired, it is possible to define property visible = true in which > case property will be passed as-is to deserializers (and set via > setter or field) on deserialization.

So the solution was simply adding the 'visible' attribute as follows

@JsonTypeInfo(  
    use = JsonTypeInfo.Id.NAME,  
    include = JsonTypeInfo.As.PROPERTY,  
    property = "exercise_type",
    visible = true)  
@JsonSubTypes({  
    @Type(value = ExerciseChoiceResponseDTO.class, name = "CHOICE"),  
    @Type(value = ExerciseQuestionResponseDTO.class, name = "QUESTION")})  
public abstract class ExerciseMixIn  
{}  

Hope this helps someone else.

As per @jscherman answer by setting, 'visible' true in JsonTypeInfo will help in accessing exercise_type as a field.

If you use the same class to serialize also then resulting JSON will have exercise_type appear twice. So it's better to also update include to JsonTypeInfo.As.EXISTING_PROPERTY

And it's also worth looking at all other options for include.