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
}
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Answer 1

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

@SuppressWarnings("unchecked")
class RatingJsonDeserializer extends JsonDeserializer {

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

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

        return rating;
    }

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

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

        return 0;
    }

    private int getRated(Map 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) 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
}