FirebaseMessagingService.java

public class FirebaseMessagingService extends com.google.firebase.messaging.FirebaseMessagingService {

    @Override
    public void onMessageReceived(RemoteMessage remoteMessage) {
        super.onMessageReceived(remoteMessage);
        try {
            if (remoteMessage.getData().size() > 0) {
                // do stuff.....
            }

            if (remoteMessage.getNotification() != null) {
                Timber.d("Message Notification Body: %s", remoteMessage.getNotification().getBody());
            }
        } catch (Exception exception) {
            Timber.e(exception);
        }

    }
}

Não esquece de colocar no Manifest

         <service android:name=".services.FirebaseMessagingService">
            <intent-filter>
                <action android:name="com.google.firebase.MESSAGING_EVENT" />
            </intent-filter>
        </service>
        <service android:name=".services.FirebaseMessagingHelper">
            <intent-filter>
                <action android:name="com.google.firebase.INSTANCE_ID_EVENT" />
            </intent-filter>
        </service>