How to show a notification everyday at a certain time even when the app is closed?

This is updated solution, and it works Android Oreo

Step 1: Create a Method in your MainActivity and use AlarmManager to set alarm at a specified time.

public void myAlarm() {
  
    Calendar calendar = Calendar.getInstance();
    calendar.set(Calendar.HOUR_OF_DAY, 21);
    calendar.set(Calendar.MINUTE, 47);
    calendar.set(Calendar.SECOND, 0);
    
    if (calendar.getTime().compareTo(new Date()) < 0) 
        calendar.add(Calendar.DAY_OF_MONTH, 1);

    Intent intent = new Intent(getApplicationContext(), NotificationReceiver.class);
    PendingIntent pendingIntent = PendingIntent.getBroadcast(getApplicationContext(), 0, intent, PendingIntent.FLAG_UPDATE_CURRENT);
    AlarmManager alarmManager = (AlarmManager) getSystemService(ALARM_SERVICE);
    
    if (alarmManager != null) {
        alarmManager.setRepeating(AlarmManager.RTC_WAKEUP, calendar.getTimeInMillis(), AlarmManager.INTERVAL_DAY, pendingIntent);

  }
  
}

I'm setting my alarm at 09:47 PM

Step 2: Create BroadcastReceiver to listen when the alarm happens

public class NotificationReceiver extends BroadcastReceiver {

@Override
public void onReceive(Context context, Intent intent) {

    NotificationHelper notificationHelper = new NotificationHelper(context);
    notificationHelper.createNotification();

   }
}

I'm creating this class named NotificationReceiver and extends BroadcastReceiver, in onReceive there is Class named NotificationHelper, don't confuse I will explain this Class for next steps.

Step 3: Create the Notification class

class NotificationHelper {

private Context mContext;
private static final String NOTIFICATION_CHANNEL_ID = "10001";

NotificationHelper(Context context) {
    mContext = context;
}

void createNotification()
{
   
    Intent intent = new Intent(mContext , NotificationActivity.class);
   
    intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);

    PendingIntent resultPendingIntent = PendingIntent.getActivity(mContext,
            0 /* Request code */, intent,
            PendingIntent.FLAG_UPDATE_CURRENT);


    NotificationCompat.Builder mBuilder = new NotificationCompat.Builder(mContext, NOTIFICATION_CHANNEL_ID);
    mBuilder.setSmallIcon(R.mipmap.ic_launcher);
    mBuilder.setContentTitle("Title")
            .setContentText("Content")
            .setAutoCancel(false)
            .setSound(Settings.System.DEFAULT_NOTIFICATION_URI)
            .setContentIntent(resultPendingIntent);

    NotificationManager mNotificationManager = (NotificationManager) mContext.getSystemService(Context.NOTIFICATION_SERVICE);

    if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.O)
    {
        int importance = NotificationManager.IMPORTANCE_HIGH;
        NotificationChannel notificationChannel = new NotificationChannel(NOTIFICATION_CHANNEL_ID, "NOTIFICATION_CHANNEL_NAME", importance);
        notificationChannel.enableLights(true);
        notificationChannel.setLightColor(Color.RED);
        notificationChannel.enableVibration(true);
        notificationChannel.setVibrationPattern(new long[]{100, 200, 300, 400, 500, 400, 300, 200, 400});
        assert mNotificationManager != null;
        mBuilder.setChannelId(NOTIFICATION_CHANNEL_ID);
        mNotificationManager.createNotificationChannel(notificationChannel);
    }
    assert mNotificationManager != null;
    mNotificationManager.notify(0 /* Request Code */, mBuilder.build());
       }
     }

This class handles the notification

Step 4: Come back to Step 2: and call the Notification Class

 NotificationHelper notificationHelper = new NotificationHelper(context);
 notificationHelper.createNotification();

Registering a BroadcastReceiver Go to your Androidmanifest file and register your broadcast receiver

<receiver
    android:name=".NotificationReceiver"></receiver>

For more info refer this guide from google

I hope it helps you.

If I understood you correctly, I believe that you need setup a recurring alarm using AlarmManager. You also need to setup starting alarm service on device reboot. You can write a method that does what you want so it get executed when the alarm runs e.g. show notification. The following links should help you:

• Android Fundamentals: Scheduling Recurring Tasks
• Repeat Alarm Example In Android Using AlarmManager