项目有Target30升级到31后项目，Firebase 出现Crash:

Fatal Exception: java.lang.IllegalArgumentException

Targeting S+ (version 31 and above) requires that one of FLAG_IMMUTABLE or FLAG_MUTABLE be specified when creating a PendingIntent. Strongly consider using FLAG_IMMUTABLE, only use FLAG_MUTABLE if some functionality depends on the PendingIntent being mutable, e.g. if it needs to be used with inline replies or bubbles.

出错的代码主要是在创建通知栏消息时抛出的，主要代码如下：

rivate void createNotification() {
    Intent intent = new Intent(this, OtherActivity.class);
    // 这一行报错
    PendingIntent pendingIntent = PendingIntent.getActivity(this, 123, intent, PendingIntent.FLAG_ONE_SHOT);
    NotificationCompat.Builder builder = new NotificationCompat.Builder(this, NOTIFICATION_CHANNEL_ID)
            .setSmallIcon(R.mipmap.ic_launcher)
            .setContentTitle("测试")
            .setContentText("收到一条通知消息")
            .setContentIntent(pendingIntent)
            .setPriority(NotificationCompat.PRIORITY_HIGH)
            .setAutoCancel(true);
    Notification build = builder.build();
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
        NotificationManager notificationManager = (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);
        notificationManager.createNotificationChannel(
                new NotificationChannel(NOTIFICATION_CHANNEL_ID, "测试测试", NotificationManager.IMPORTANCE_HIGH)
        );
    }
    NotificationManagerCompat.from(this).notify(notificationId++, build);
}

查资料后，发现如下几个修复的方法：

1. 将项目的targetSdkVersion由31改为30，也就是退回去，但是Google Play 现在最低是31

2. 如果不想改targetSdkVersion，那就在在创建PendingIntent的时候判断当前系统版本，根据不同系统版本创建带有不同flag的PendingIntent，具体代码实现如下：

   PendingIntent pendingIntent;
   if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.S) {
       pendingIntent = PendingIntent.getActivity(this, 123, intent, PendingIntent.FLAG_IMMUTABLE);
   } else {
       pendingIntent = PendingIntent.getActivity(this, 123, intent, PendingIntent.FLAG_ONE_SHOT);
   }