Sometimes you need to serialize or deserialize some fields in a desired format, for example your backend may use the format “YYYY-MM-dd HH:mm” for dates and you want your POJOS to use the DateTime class in Joda Time.

In order to automatically convert these strings into DateTimes object, you can use a custom converter.

/**
 * Gson serialiser/deserialiser for converting Joda {@link DateTime} objects.
 */
public class DateTimeConverter implements JsonSerializer<DateTime>, JsonDeserializer<DateTime> {

    private final DateTimeFormatter dateTimeFormatter;

    @Inject
    public DateTimeConverter() {
        this.dateTimeFormatter = DateTimeFormat.forPattern("YYYY-MM-dd HH:mm");
    }

    @Override
    public JsonElement serialize(DateTime src, Type typeOfSrc, JsonSerializationContext context) {
        return new JsonPrimitive(dateTimeFormatter.print(src));
    }

    @Override
    public DateTime deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context)
            throws JsonParseException {
        
        if (json.getAsString() == null || json.getAsString().isEmpty()) {
            return null;
        }

        return dateTimeFormatter.parseDateTime(json.getAsString());
    }
}

To make Gson use the newly created converter you need to assign it when creating the Gson object:

DateTimeConverter dateTimeConverter = new DateTimeConverter();
Gson gson = new GsonBuilder().registerTypeAdapter(DateTime.class, dateTimeConverter)
        .create();

String s = gson.toJson(DateTime.now());
// this will show the date in the desired format

In order to deserialize the date in that format you only have to define a field in the DateTime format:

public class SomePojo {
    private DateTime someDate;    
}

When Gson encounters a field of type DateTime, it will call your converter in order to deserialize the field.