Firebase Messaging Service新令牌

Question

public class MyFirebaseInstanceService extends FirebaseMessagingService
{
    @Override
    public void onNewToken(String s) {
        super.onNewToken(s);

        Log.d("DTAG","onNewToken: "+s);
    }
}

在MainClass中：

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    FirebaseInstanceId.getInstance().getInstanceId().addOnSuccessListener( MainActivity.this,  new OnSuccessListener<InstanceIdResult>() {
        @Override
        public void onSuccess(InstanceIdResult instanceIdResult) {
            String newToken = instanceIdResult.getToken();
            Log.d("DTAG","Token Main: "+newToken);

        }
    });
}

第一次运行我的应用时，我得到的输出是：

D/DTAG: Token Main: token_a
D/DTAG: onNewToken: token_b

我得到的秒时间：

D/DTAG: Token Main: token_a

请帮助我得到这个...

 1. If I run for the first time, from where is gets token in MainClass (token_a)
 2. In case the token is renewed (onNewToken) which is token_b, why I still keep seeing token_a when launching the app in second time and what happened to token_b?