FirebaseInstanceIDService

// Java 
public class FirebaseInstanceIDService extends FirebaseInstanceIdService {
    private final String TAG = this.getClass().getSimpleName();
    private Context context;

    @Override
    public void onTokenRefresh() {

        String refreshedToken = FirebaseInstanceId.getInstance().getToken();
        sendTokenToServer(refreshedToken);
    }

    private void sendTokenToServer(String token) {
        // kirim token ke backend, etc.
    }
}

// manifest 
        <service android:name=".services.FirebaseInstanceIDService">
            <intent-filter>
                <action android:name="com.google.firebase.INSTANCE_ID_EVENT" />
            </intent-filter>
        </service>