@JsonTypeInfo(use = JsonTypeInfo.Id.CLASS, include = JsonTypeInfo.As.PROPERTY, property = "@class")
public class CountableSet<T> implements Serializable {

public class StringLongMapDeserializer extends JsonDeserializer<Map<String, Long>> {
    @Override
    public Map<String, Long> deserialize(JsonParser jsonParser, DeserializationContext deserializationContext) throws IOException, JsonProcessingException {
        ObjectMapper mapper = new ObjectMapper();
        JavaType type = mapper.getTypeFactory().constructParametricType(Map.class, String.class, Long.class);
        return mapper.readValue(jsonParser, type);
    }
}