Custom gson deserializer for Date never gets called

Question

As far as I can tell, gson does not automatically serialize and deserialize java.util.Date objects into ISO strings like "yyyy-MM-ddTHH:mm:ssZ" or for example "2014-04-15T18:22:00-05:00". So in order for me to communicate dates properly between my client (using Retrofit with gson) and server, I need to specify the DateFormat to gson. Here is what I've done:

// code defining the creation of a RestAdapter
// ...
new GsonBuilder()
    .setDateFormat("yyyy-MM-dd'T'HH:mm:ssZ")
    .create()

Adding the .setDateFormat line was enough to get gson to properly deserialize timestamp strings into Date objects. However, it did not serialize Date objects into timestamp strings. So I assumed that I would have to create a custom serializer like so:

// code defining the creation of a RestAdapter
// ...
new GsonBuilder()
    .setDateFormat("yyyy-MM-dd'T'HH:mm:ssZ")
    .registerTypeAdapter(Date.class, new DateSerializer())
    .create()

and the DateSerializer class:

class DateSerializer implements JsonSerializer<Date> {
    @Override
    public JsonElement serialize(Date arg0, Type arg1, JsonSerializationContext arg2) {
        SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ssZ", Locale.US);
        return new JsonPrimitive(df.format(arg0));
    }
}

Unfortunately, the serialize function is ignored. Instead gson formats the date as a string like "Tues Mar 15 18:22:00 EST 2014". So to test, I tried replacing the serialize function with:

public JsonElement serialize(Date arg0, Type arg1, JsonSerializationContext arg2) {
    throw new RuntimeException("Serialize function called!");
}

But of course that RuntimeException is never thrown.

Does anyone know why my serialize function is ignored? I think I read somewhere that for some types a registerTypeAdapter will be ignored if there is one defined for a superclass, but since this is java.util.Date I would be confused if that was the problem. I'm probably just doing something stupid, but I probably don't know Date or gson well enough to realize it.

EDIT: Provided more context around the code below:

MyApplication.java

public class MyApplication extends Application {
    public static RestAdapter restAdapter;
    public static The1Api the1Api;

    public static void createRestAdapter(String server_url){
        // enable cookies
        CookieManager cookieManager = new CookieManager();
        cookieManager.setCookiePolicy(CookiePolicy.ACCEPT_ALL);
        CookieHandler.setDefault(cookieManager);

        // create rest adapter
        restAdapter = new RestAdapter.Builder()
            .setEndpoint(server_url)
            .setConverter(new GsonConverter(new GsonBuilder()
                    .setDateFormat("yyyy-MM-dd'T'HH:mm:ssZ")
                    .registerTypeAdapter(Date.class, new DateSerializer())
                    .create()))
            .setLogLevel(LogLevel.FULL)
            .setLog(new ResponseInterceptor())
            .build();

        // create API
        the1Api = restAdapter.create(The1Api.class);
    }
}

The1Api.java

public interface The1Api {
    /* Chat */

    public class PublicMessage {
        String from_user;
        String message;
        Date time;
        Integer microsecond;
        Boolean in_1_percent;
    }
    public class PublicMessageList {
        Integer last_message_id;
        ArrayList<PublicMessage> messages;
    }

    @GET("/chat/get_public_messages/")
    public PublicMessageList getPublicMessages(
            @Query("last_message_id") Integer last_message_id, // optional
            @Query("since") Date since, // optional
            @Query("max") Integer max // optional
            );   

    // ...
}

LoginActivity.java

public class LoginActivity extends Activity {
    // ...

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        Crashlytics.start(this);
        MyApplication.createRestAdapter(getString(R.string.server_url));
        setContentView(R.layout.activity_login);
    }

    @Override
    protected void onPostCreate(Bundle savedInstanceState){
        super.onPostCreate(savedInstanceState);

        Thread thread = new Thread(){
            @Override
            public void run(){
                ArrayList<The1Api.PublicMessage> publicMessages = MyApplication.the1Api.getPublicMessages(null, null, null).messages;
                for (The1Api.PublicMessage m : publicMessages){
                    Log.d("The1", "[" + m.time.toString() + "] " + m.from_user + ": " + m.message);
                }
                // when the following line gets executed, my server receives a request including the date below,
                // but the server does not understand the format of the date because it does not get serialized properly
                MyApplication.the1Api.getPublicMessages(null, new Date(1000000000), null);
            }
        };
        thread.start();
    }

    // ...    
}

DateSerializer.java

class DateSerializer implements JsonSerializer<Date> {
    @Override
    public JsonElement serialize(Date arg0, Type arg1, JsonSerializationContext arg2) {
        SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ssZ", Locale.US);
        return new JsonPrimitive(df.format(arg0));
    }
}

EDIT 2: No solutions as of yet, but as a workaround you can manually convert the Date to some other format before sending it. In the comments Kalel suggested converting it to a String, I converted it to a Long (number of seconds since UNIX epoch).

Answer 1

Are you still able to replicate this issue? I'm seeing Gson serialize and deserialize correctly. It's possible this is a Retrofit issue (I've not used it before), but looking at the docs for GsonConverter I see no reason it would do anything other than delegate to the provided in Gson object.

Here's an SSCCE:

public class GsonDateDemo {
  public static void main(String[] args) {
    Gson gson = new GsonBuilder()
        .setDateFormat("yyyy-MM-dd'T'HH:mm:ssZ")
        .registerTypeAdapter(Date.class, new DateSerializer())
        .create();
    Date d = new Date(1438806977000L);

    System.out.println("Serializing "+d);
    String json = gson.toJson(d);
    System.out.println("JSON: "+json);
    Date d2 = gson.fromJson(json, Date.class);
    System.out.println("Deserialized: "+d2);
    System.out.println("Equal? "+d.equals(d2));
  }

  static class DateSerializer implements JsonSerializer<Date> {
    @Override
    public JsonElement serialize(Date arg0, Type arg1, JsonSerializationContext arg2) {
      System.out.println("Serializer received "+arg0);
      SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ssZ", Locale.US);
      return new JsonPrimitive(df.format(arg0));
    }
  }
}

This outputs:

Calling: local.GsonDateDemo.main[]
Serializing Wed Aug 05 16:36:17 EDT 2015
Serializer received Wed Aug 05 16:36:17 EDT 2015
JSON: "2015-08-05T16:36:17-0400"
Deserialized: Wed Aug 05 16:36:17 EDT 2015
Equal? true

If you can, modify this SSCCE to use Retrofit and see if you can replicate the failure that way.