How to serialize Date to long using gson?

Question

I recently switched some of our serialization from Jackson to Gson. Found out that Jackson serializes dates to longs.

But, Gson serializes Dates to strings by default.

How do I serialize dates to longs when using Gson? Thanks.

Answer 1

First type adapter does the deserialization and the second one the serialization.

Gson gson = new GsonBuilder()
        .registerTypeAdapter(Date.class, (JsonDeserializer<Date>) (json, typeOfT, context) -> new Date(json.getAsJsonPrimitive().getAsLong()))
        .registerTypeAdapter(Date.class, (JsonSerializer<Date>) (date, type, jsonSerializationContext) -> new JsonPrimitive(date.getTime()))
        .create();

Usage:

String jsonString = gson.toJson(objectWithDate1);
ClassWithDate objectWithDate2 = gson.fromJson(jsonString, ClassWithDate.class);
assert objectWithDate1.equals(objectWithDate2);

Answer 2

You can do both direction with one type adapter:

public class DateLongFormatTypeAdapter extends TypeAdapter<Date> {

    @Override
    public void write(JsonWriter out, Date value) throws IOException {
        if(value != null) out.value(value.getTime());
        else out.nullValue();
    }

    @Override
    public Date read(JsonReader in) throws IOException {
        return new Date(in.nextLong());
    }

}

Gson builder:

Gson gson = new GsonBuilder()
        .registerTypeAdapter(Date.class, new DateLongFormatTypeAdapter())
        .create();