MyFirebaseInstanceIdService.java

public class MyFirebaseInstanceIDService extends FirebaseInstanceIdService {
    private static final String TAG = "MyFirebaseIIDService";

    /**
     * Called if InstanceID token is updated. This may occur if the security of
     * the previous token had been compromised. Note that this is called when the InstanceID token
     * is initially generated so this is where you would retrieve the token.
     */
    @Override
    public void onTokenRefresh() {
        // Get updated InstanceID token.
        String refreshedToken = FirebaseInstanceId.getInstance().getToken();
        Log.d(TAG, "Refreshed token: " + refreshedToken);

        sendRegistrationToServer(refreshedToken);
    }


    /**
     * Persist token to third-party servers.
     *
     * Modify this method to associate the user's FCM InstanceID token with any server-side account
     * maintained by your application.
     *
     * @param token The new token.
     */
    private void sendRegistrationToServer(String token) {
        Log.d(TAG, "sendRegistrationToServer: sending token to server: " + token);
        DatabaseReference reference = FirebaseDatabase.getInstance().getReference();
        reference.child(getString(R.string.dbnode_users))
                .child(FirebaseAuth.getInstance().getCurrentUser().getUid())
                .child(getString(R.string.field_messaging_token))
                .setValue(token);
    }
}