1. 确保通知权限已经启用。

if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
        NotificationManager notificationManager = (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);
        NotificationChannel notificationChannel = new NotificationChannel("channel_id", "channel_name", NotificationManager.IMPORTANCE_HIGH);
        notificationManager.createNotificationChannel(notificationChannel);
}

2. 如果您的应用是针对 API 26 或更高版本的，请确保通知渠道与通知中使用的通知 ID 匹配。

if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
        NotificationChannel notificationChannel = new NotificationChannel("channel_id", "channel_name", NotificationManager.IMPORTANCE_HIGH);
        NotificationManager notificationManager = (NotificationManager) getSystemService(Context.NOTIFICATION_SERVICE);
        notificationManager.createNotificationChannel(notificationChannel);
        NotificationCompat.Builder builder = new NotificationCompat.Builder(this, "channel_id")
            .setSmallIcon(R.drawable.notification_icon)
            .setContentTitle("My notification")
            .setContentText("Hello World!")
            .setPriority(NotificationCompat.PRIORITY_HIGH)
            .setAutoCancel(true);
        Notification notification = builder.build();
        notificationManager.notify(1, notification);
 }

3. 有些 ROM（如 MIUI）会自动隐藏通知图标。在这种情况下，您需要通过更改系统设置来使通知图标可见。

Intent intent = new Intent(Settings.ACTION_APPLICATION_DETAILS_SETTINGS);
intent.setData(Uri.fromParts("package", getPackageName(), null));
startActivity(intent);

4. 如果您的应用是针对 API 21 或更高版本的，则可以使用以下代码来设置通知图标。

NotificationCompat.Builder builder = new NotificationCompat.Builder(this, "channel_id")
            .setSmallIcon(R.drawable.notification_icon)
            .setContentTitle("My notification")
            .setContentText("Hello World!")
            .setPriority(NotificationCompat.PRIORITY_HIGH)
            .setAutoCancel(true);
Notification notification = builder.build();
notificationManager.notify(1, notification);