Using gson dateformat to serialize and deserialize unix-timestamps

I am using Gson to serialize/deserialize my pojos and currently looking for a clean way to tell Gson to parse/output date attributes as unix-timestamps. Here's my attempt:

Gson gson = new GsonBuilder().setDateFormat("U").create();

Comming from PHP where "U" is the dateformat used to serialize/deserialize date as unix-timestamps, when running my attempt code, I am a RuntimeException:

Unknown pattern character 'U'

I am assuming that Gson uses SimpleDateformat under the hood which doesn't define the letter "U".

I could implement a custom DateTypeAdapter but I am looking for a cleaner way to achieve that. Simply changing the DateFormat would be great.

TL;DR: use the following gist UnixTimestampAdapter.java

Creating a custom TypeAdapter (UnixTimestampAdapter) was the way to go.

UnixTimestampAdapter

public class UnixTimestampAdapter extends TypeAdapter<Date> {

    @Override
    public void write(JsonWriter out, Date value) throws IOException {
        if (value == null) {
            out.nullValue();
            return;
        }
        out.value(value.getTime() / 1000);
    }

    @Override
    public Date read(JsonReader in) throws IOException {
        if (in.peek() == JsonToken.NULL) {
            in.nextNull();
            return null;
        }
        return new Date(in.nextLong() * 1000);
    }

}

Now, you have to options (depending on your use case):

1 - If you want apply this serialization on all your date fields then register UnixTimestampAdapter upon creating your Gson instance:

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

2 - Or annotate your date fields with @JsonAdapter (as suggested by @Marcono1234) if you want it applied only to some specific fields.

class Person {
    @JsonAdapter(UnixTimestampAdapter.class)
    private Date birthday;
}