当应用程序处于前台状态时，不会自行生成通知.您需要编写一些其他代码.收到消息后，将调用onMessageReceived()方法，您可以在其中生成通知.这是代码:

When app is in foreground, notifications are not generated themselves. You need to write some additional code. When message is received onMessageReceived() method is called where you can generate the notification. Here is the code:

public class MyFirebaseMessagingService extends FirebaseMessagingService {
    @Override
    public void onMessageReceived(RemoteMessage remoteMessage) {
        super.onMessageReceived(remoteMessage);
        Log.d("msg", "onMessageReceived: " + remoteMessage.getData().get("message"));
        Intent intent = new Intent(this, MainActivity.class);
        intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
        PendingIntent pendingIntent = PendingIntent.getActivity(this, 0, intent, PendingIntent.FLAG_ONE_SHOT);
        String channelId = "Default";
        NotificationCompat.Builder builder = new  NotificationCompat.Builder(this, channelId)
                .setSmallIcon(R.mipmap.ic_launcher)
                .setContentTitle(remoteMessage.getNotification().getTitle())
                .setContentText(remoteMessage.getNotification().getBody()).setAutoCancel(true).setContentIntent(pendingIntent);;
        NotificationManager manager = (NotificationManager) getSystemService(NOTIFICATION_SERVICE);
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
            NotificationChannel channel = new NotificationChannel(channelId, "Default channel", NotificationManager.IMPORTANCE_DEFAULT);
            manager.createNotificationChannel(channel);
        }
        manager.notify(0, builder.build());
    }
}