把targetSdkVersion改成26后通知栏不显示。

对于旧的代码，要修改两个地方

1. 创建通知渠道
2. 改用NotificationCompat

示例：

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
            String channelId = "chat";
            String channelName = "聊天消息";
            int importance = NotificationManager.IMPORTANCE_HIGH;
            createNotificationChannel(channelId, channelName, importance);

            channelId = "subscribe";
            channelName = "订阅消息";
            importance = NotificationManager.IMPORTANCE_DEFAULT;
            createNotificationChannel(channelId, channelName, importance);
        }
    }

    @TargetApi(Build.VERSION_CODES.O)
    private void createNotificationChannel(String channelId, String channelName, int importance) {
        NotificationChannel channel = new NotificationChannel(channelId, channelName, importance);
        NotificationManager notificationManager = (NotificationManager) getSystemService(
                NOTIFICATION_SERVICE);
        notificationManager.createNotificationChannel(channel);
    }

    public void sendChatMsg(View view) {
        NotificationManager manager = (NotificationManager) getSystemService(NOTIFICATION_SERVICE);
        Notification notification = new NotificationCompat.Builder(this, "chat")
                .setContentTitle("收到一条聊天消息")
                .setContentText("今天中午吃什么？")
                .setWhen(System.currentTimeMillis())
                .setSmallIcon(R.mipmap.ic_launcher)
                .setLargeIcon(BitmapFactory.decodeResource(getResources(), R.mipmap.ic_launcher))
                .setAutoCancel(true)
                .build();
        manager.notify(1, notification);
    }

    public void sendSubscribeMsg(View view) {
        NotificationManager manager = (NotificationManager) getSystemService(NOTIFICATION_SERVICE);
        Notification notification = new NotificationCompat.Builder(this, "subscribe")
                .setContentTitle("收到一条订阅消息")
                .setContentText("地铁沿线30万商铺抢购中！")
                .setWhen(System.currentTimeMillis())
                .setSmallIcon(R.mipmap.ic_launcher)
                .setLargeIcon(BitmapFactory.decodeResource(getResources(), R.mipmap.ic_launcher))
                .setAutoCancel(true)
                .build();
        manager.notify(2, notification);
    }
}

源码：https://gitee.com/obarong/TestNotification-android

参考

Android通知栏微技巧，8.0系统中通知栏的适配 - 郭霖的专栏 - CSDN博客
https://blog.csdn.net/guolin_blog/article/details/79854070

android 实现Service上传并在通知栏显示进度条 - 简书
http://08643.cn/p/2d0a93d1e197