通过调用FirebaseInstanceID.getToken()检查app / main活动启动时的当前令牌：

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        ...
        String token = FirebaseInstanceId.getInstance().getToken();
        if (token != null) {
            sendRegistrationToServer(refreshedToken);
        }
    }
    ...

您在此代码段中调用的sendRegistrationToServer()方法是您将注册令牌发送到您自己的服务器(或Firebase数据库之类的无服务器解决方案)的方法.

通过继承FirebaseInstanceIdService并覆盖onTokenRefresh()来监视令牌的更改：

@Override
public void onTokenRefresh() {
    String refreshedToken = FirebaseInstanceId.getInstance().getToken();

    // If you want to send messages to this application instance or
    // manage this apps subscriptions on the server side,send the
    // Instance ID token to your app server.
    sendRegistrationToServer(refreshedToken);
}

请参阅此处的文档：https://firebase.google.com/docs/cloud-messaging/android/client#sample-register