在AndroidManifest.xml文件中将应用程序的目标API级别升级为31及以上，并添加以下代码示例：

if (Build.VERSION.SDK_INT >= 31) {
  Intent alarmIntent = new Intent(this, MyAlarmReceiver.class);      
  PendingIntent pendingIntent = 
            PendingIntent.getBroadcast(this, 0, alarmIntent, PendingIntent.FLAG_UPDATE_CURRENT);        
  AlarmManager alarmManager = getSystemService(AlarmManager.class);
  AlarmManager.AlarmClockInfo alarmClockInfo = 
            new AlarmManager.AlarmClockInfo(System.currentTimeMillis() + 10000, pendingIntent);
            
  alarmManager.setAlarmClock(alarmClockInfo, pendingIntent);
} else {
  Intent intent = new Intent(this, MyAlarmReceiver.class);
  PendingIntent pendingIntent = PendingIntent.getBroadcast(this, 0, intent, PendingIntent.FLAG_UPDATE_CURRENT);
  AlarmManager alarmManager = (AlarmManager) getSystemService(Context.ALARM_SERVICE);
  
  if(Build.VERSION.SDK_INT>=23)
    alarmManager.setExactAndAllowWhileIdle(AlarmManager.RTC_WAKEUP, System.currentTimeMillis() + 10000, pendingIntent);
  else if(Build.VERSION.SDK_INT>=19)
    alarmManager.setExact(AlarmManager.RTC_WAKEUP, System.currentTimeMillis() + 10000, pendingIntent);
  else
    alarmManager.set(AlarmManager.RTC_WAKEUP, System.currentTimeMillis() + 10000, pendingIntent);
}

在代码示例中，我们使用了if语句，如果设备的API级别大于或等于31，则使用setAlarmClock()方法设置闹钟；否则，使用setExactAndAllowWhileIdle()，setExact()或set()方法设置闹钟。这样可以确保应用程序在Android 12上运行时能够正确地设置闹钟。