Why is JsonHttpContent's output empty?

I am using Google Http Client library (1.20) on Google App Engine (1.9.30) to submit a POST request to Google Cloud Messaging (GCM) servers. Here's the code:

public static HttpRequestFactory getGcmRequestFactory() {
    if (null == gcmFactory) {
        gcmFactory = (new UrlFetchTransport())
                .createRequestFactory(new HttpRequestInitializer() {
                    @Override
                    public void initialize(HttpRequest request) throws IOException {
                        request.getHeaders().setAuthorization(
                                "key=" + Config.get(Config.Keys.GCM_SERVER_API_KEY).orNull());
                        request.getHeaders().setContentType("application/json");
                        request.getHeaders().setAcceptEncoding(null);
                    }
                });
    }
    return gcmFactory;
}

public static JsonFactory getJsonFactory() {
    return jacksonFactory;
}

public static String sendGcmMessage(GcmDownstreamDto message) {
    HttpRequestFactory factory = getGcmRequestFactory();
    JsonHttpContent content = new JsonHttpContent(getJsonFactory(), message);
    String response = EMPTY;
    try {
        HttpRequest req = factory.buildPostRequest(gcmDownstreamUrl, content);
        log.info("req headers = " + req.getHeaders());
        System.out.print("req content = ");
        content.writeTo(System.out); // prints out "{}"
        System.out.println(EMPTY);
        HttpResponse res = req.execute(); // IOException here
        response = IOUtils.toString(res.getContent());
    } catch (IOException e) {
        log.log(Level.WARNING, "IOException...", e);
    }
    return response;
}

Now the content.writeTo() always prints out empty JSON. Why is that? What am I doing wrong? The GcmDownstreamDto class (using Lombok to generate getters and setters):

@Data
@Accessors(chain = true)
public class GcmDownstreamDto {

    private String to;

    private Object data;

    private List<String> registration_ids;

    private GcmNotificationDto notification;

    public GcmDownstreamDto addRegistrationId(String regId) {
        if (null == this.registration_ids) {
            this.registration_ids = new ArrayList<>();
        }
        if (isNotBlank(regId)) {
            this.registration_ids.add(regId);
        }
        return this;
    }
}

The immediate goal is to generate the same response as (from Checking the validity of an API key):

api_key=YOUR_API_KEY

curl --header "Authorization: key=$api_key" \
       --header Content-Type:"application/json" \
       https://gcm-http.googleapis.com/gcm/send \
       -d "{\"registration_ids\":[\"ABC\"]}"

{"multicast_id":6782339717028231855,"success":0,"failure":1,
"canonical_ids":0,"results":[{"error":"InvalidRegistration"}]}

I've already tested using curl so I know the API key is valid, I just want to do the same thing in Java code to build up my base classes.

sendGcmMessage() is being invoked as follows:

@Test
public void testGcmDownstreamMessage() {
    GcmDownstreamDto message = new GcmDownstreamDto().addRegistrationId("ABC");
    System.out.println("message = " + message);
    String response = NetCall.sendGcmMessage(message);
    System.out.println("Response: " + response);
}

All help appreciated.

Found out the problem: it's the way JacksonFactory().createJsonGenerator().searialize() works (I was expecting it to serialize the way ObjectMapper serializes). This is the code for JsonHttpContent.writeTo() (from JsonHttpContent.java in google-http-java-client):

public void writeTo(OutputStream out) throws IOException {
    JsonGenerator generator = jsonFactory.createJsonGenerator(out, getCharset());
    generator.serialize(data);
    generator.flush();
}

The Jackson JsonGenerator expects a key-value pairing (represented in Java as Map) which is not obvious from the constructor signature of the JsonHttpContent constructor: JsonHttpContent(JsonFactory, Object).

So if instead of passing a GcmDownstreamDto (as defined in the question, which is what would have worked with an ObjectMapper), I were to do the following:

Map<String, Object> map = new HashMap<>();
List<String> idList = Arrays.asList("ABC");
map.put("registration_ids", idList);

everything works as expected and the output is:

{"registration_ids":["ABC"]}

So just remember to pass the JsonHttpContent(JsonFactory, Object) constructor a Map<String, Object> as the second parameter, and everything will work as you would expect it to.