根据Firebase Cloud Messaging文档-如果Activity位于前台，则将调用onMessageReceived.如果活动"处于后台或关闭状态，则通知消息将显示在通知中心中，用于启动应用程序的活动. 有关更多信息，请检查此链接

As per Firebase Cloud Messaging documentation-If Activity is in foreground then onMessageReceived will get called. If Activity is in background or closed then notification message is shown in the notification center for app launcher activity. For More information Check this link