Jackson deserialization of type with different objects

Question

I have a result from a web service that returns either a boolean value or a singleton map, e.g.

Boolean result:

{
    id: 24428,
    rated: false
}

Map result:

{
    id: 78,
    rated: {
        value: 10
    }
}

Individually I can map both of these easily, but how do I do it generically?

Basically I want to map it to a class like:

public class Rating {
    private int id;
    private int rated;
    ...
    public void setRated(?) {
        // if value == false, set rated = -1;
        // else decode "value" as rated
    }
}

All of the polymorphic examples use @JsonTypeInfo to map based on a property in the data, but I don't have that option in this case.

---

EDIT
The updated section of code:

@JsonProperty("rated")
public void setRating(JsonNode ratedNode) {
    JsonNode valueNode = ratedNode.get("value");
    // if the node doesn't exist then it's the boolean value
    if (valueNode == null) {
        // Use a default value
        this.rating = -1;
    } else {
        // Convert the value to an integer
        this.rating = valueNode.asInt();
    }
}

Answer 1

No no no. You do NOT have to write a custom deserializer. Just use "untyped" mapping first:

public class Response {
  public long id;
  public Object rated;
}
// OR
public class Response {
  public long id;
  public JsonNode rated;
}
Response r = mapper.readValue(source, Response.class);

which gives value of Boolean or java.util.Map for "rated" (with first approach); or a JsonNode in second case.

From that, you can either access data as is, or, perhaps more interestingly, convert to actual value:

if (r.rated instanced Boolean) {
    // handle that
} else {
    ActualRated actual = mapper.convertValue(r.rated, ActualRated.class);
}
// or, if you used JsonNode, use "mapper.treeToValue(ActualRated.class)

There are other kinds of approaches too -- using creator "ActualRated(boolean)", to let instance constructed either from POJO, or from scalar. But I think above should work.

Answer 2

You have to write your own deserializer. It could look like this:

@SuppressWarnings("unchecked")
class RatingJsonDeserializer extends JsonDeserializer<Rating> {

    @Override
    public Rating deserialize(JsonParser jp, DeserializationContext ctxt) throws IOException, JsonProcessingException {
        Map<String, Object> map = jp.readValueAs(Map.class);

        Rating rating = new Rating();
        rating.setId(getInt(map, "id"));
        rating.setRated(getRated(map));

        return rating;
    }

    private int getInt(Map<String, Object> map, String propertyName) {
        Object object = map.get(propertyName);

        if (object instanceof Number) {
            return ((Number) object).intValue();
        }

        return 0;
    }

    private int getRated(Map<String, Object> map) {
        Object object = map.get("rated");
        if (object instanceof Boolean) {
            if (((Boolean) object).booleanValue()) {
                return 0; // or throw exception
            }

            return -1;
        }

        if (object instanceof Map) {
            return getInt(((Map<String, Object>) object), "value");
        }

        return 0;
    }
}

Now you have to tell Jackson to use this deserializer for Rating class:

@JsonDeserialize(using = RatingJsonDeserializer.class)
class Rating {
...
}

Simple usage:

ObjectMapper objectMapper = new ObjectMapper();
System.out.println(objectMapper.readValue(json, Rating.class));

Above program prints:

Rating [id=78, rated=10]

for JSON:

{
    "id": 78,
    "rated": {
        "value": 10
    }
}

and prints:

Rating [id=78, rated=-1]

for JSON:

{
    "id": 78,
    "rated": false
}