Question

I’ve got following classes:

public class Container {
    private String name;
    private Data data;
}

public class Data {
    private Long id;
}

When I serialize Container class using Jackson I get

{"name":"Some name","data":{"id":1}}

But I need the result to be:

{"name":"Some name","id":1}

Is it possible (without adding Container.getDataId() method)? If so, how to do it?

---

update

I’ve tried to create custom JsonSerializer<Data> but the result was same as before

public class JsonDataSerializer extends JsonSerializer<Data> {

    private static Logger logger = Logger.getLogger(JsonDataSerializer.class);

    @Override
    public void serialize(Data value, JsonGenerator jgen, 
            SerializerProvider provider)
            throws IOException,JsonProcessingException {

        Long id = (value.getId() == null) ? 0l : value.getId();
        jgen.writeStartObject();
        jgen.writeNumberField("id", id);
        jgen.writeEndObject();
        logger.debug("Data id " + id + " serialized to JSON.");       
    }
}

I’ve also tried to add @JsonSerialize annotation above Data class, then above getter in Container class. As I mentioned before without any success. My serializer is used, logger logs message.

---

update 2

When I remove writeStartObject() and writeEndObject() then no JSON is returnesd, only HTTP Status 500 error and no exception is thrown except of what I found in debug output.

DEBUG: org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerExceptionResolver - Resolving exception from handler [com.example.DataController@16be8a0]: org.springframework.http.converter.HttpMessageNotWritableException: Could not write JSON: Can not write a field name, expecting a value; nested exception is org.codehaus.jackson.JsonGenerationException: Can not write a field name, expecting a value
DEBUG: org.springframework.web.servlet.mvc.annotation.ResponseStatusExceptionResolver - Resolving exception from handler [com.example.DataController@16be8a0]: org.springframework.http.converter.HttpMessageNotWritableException: Could not write JSON: Can not write a field name, expecting a value; nested exception is org.codehaus.jackson.JsonGenerationException: Can not write a field name, expecting a value
DEBUG: org.springframework.web.servlet.mvc.support.DefaultHandlerExceptionResolver - Resolving exception from handler [com.example.DataController@16be8a0]: org.springframework.http.converter.HttpMessageNotWritableException: Could not write JSON: Can not write a field name, expecting a value; nested exception is org.codehaus.jackson.JsonGenerationException: Can not write a field name, expecting a value

Answer 1

Jackson 1.9 has introduced JsonUnwrapped annotation which does what you are looking for.