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Android Developing: notifications don't show when app is closed

How can I send scheduled notifications in Android Studio?
I got this task: I want to get notifications at the some chosen time of the day every day. I can easily get them when app is alive, but when its closed notifications don't come up.
I've already tried JobScheduler, AlarmManager and WorkManager and none of these didn't work well.
My project runs at minimum SDK 26 (Android Oreo). Target SDK is 30. Last code version looks like that:
AlertReceiver.java
public class AlertReceiver extends BroadcastReceiver {
#Override
public void onReceive(Context context, Intent intent) {
intent = new Intent(context, NotificationService.class);
context.startService(intent);
}
}
NotificationService.java
public class NotificationService extends Service {
#Override
public int onStartCommand(Intent intent, int flags, int startId) {
showNotifications();
return Service.START_STICKY;
}
private void showNotifications(){
NotificationHelper notificationHelper = new NotificationHelper(getApplicationContext());
NotificationCompat.Builder nb;
nb = notificationHelper.getChannelNotification("Title", "Description");
notificationHelper.getManager().notify(123, nb.build());
}
#Override
public IBinder onBind(Intent intent) {
throw new UnsupportedOperationException("Not yet implemented");
}
}
I schedule the alert like so:
...
Calendar calendar = Calendar.getInstance();
calendar.set(Calendar.HOUR_OF_DAY, hours);
calendar.set(Calendar.MINUTE, minutes);
AlarmManager alarm_manager = (AlarmManager) getSystemService(Context.ALARM_SERVICE);
Intent intent = new Intent(this, AlertReceiver.class);
PendingIntent pending_intent = PendingIntent.getBroadcast(this, 0, intent, 0);
alarm_manager.setExact(AlarmManager.RTC_WAKEUP, calendar.getTimeInMillis(), pending_intent);
```

The problem is that from Android 8 calling startService from the background is not allowed:
In this code:
public class AlertReceiver extends BroadcastReceiver {
#Override
public void onReceive(Context context, Intent intent) {
intent = new Intent(context, NotificationService.class);
context.startService(intent);
}
}
You can change from:
context.startService(intent);
To:
ContextCompat.startForegroundService(getApplicationContext(), intent)
And put this in your Manifest:
<uses-permission android:name="android.permission.FOREGROUND_SERVICE" />
This will at least allow your service to run from the background in your current implementation.
You then have 5 seconds to call startForeground() in your Service and post a Notification that lets the user know that your service is running or it will be terminated.
Also for what you are trying to do, I think you will get a better result with:
setExactAndAllowWhileIdle
Or
setAlarmClock
setExact does not work as the name implies. All the AlarmManager documentation needs to be read carefully and I would suggest studying "doze" in detail before attempting any Service implementations that rely on timing.

This is a notification that I used in a project (sorry for Kotlin language):
private fun createNotification() {
val notification = getServiceNotification("")
notificationManager.notify(NOTIFICATION_ID, notification.build())
}
private fun getServiceNotification(contentText: String): Notification.Builder {
var notification = Notification.Builder(this)
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
createNotificationChannel()
notification = Notification.Builder(this, NOTIFICATION_SERVICE_CHANNEL_ID)
}
val openAppPendingIntent = PendingIntent.getActivity(
this,
0,
Intent(this, MainActivity::class.java),
PendingIntent.FLAG_UPDATE_CURRENT
)
return notification
.setContentTitle("Notification Title")
.setContentText(contentText)
.setSmallIcon(R.drawable.ic_main)
.setContentIntent(openAppPendingIntent)
}
#RequiresApi(Build.VERSION_CODES.O)
private fun createNotificationChannel() {
val notificationChannel = NotificationChannel(
NOTIFICATION_SERVICE_CHANNEL_ID,
NOTIFICATION_SERVICE_CHANNEL_NAME,
NotificationManager.IMPORTANCE_HIGH
).apply {
lightColor = Color.GREEN
lockscreenVisibility = Notification.VISIBILITY_PRIVATE
}
notificationManager.createNotificationChannel(notificationChannel)
}

Handle click events on Notification bar that redirects users to the app, Lollipop

When users click the icon of my app on the Notification bar, users will be redirected to my app.
Can anyone provide sample code? How to subscribe to the click event, and the redirection.
Update
My application might be using some services that cause the display of icon on Notification bar.
My application is calling SetForeground, not getBroadcast().
Update 2
how can I redirect users to the last Activity rather than the hard-code activity? For example, the last Activity might be different when users navigate to different activity.
Notification click event in xamarin forms

It's a sample from my app, it works. I think you can do somthing similar.
public class AlarmReceiver extends BroadcastReceiver {
public final static String NOTIF_TEXT = AlarmSetActivity.class.getPackage() + ".NOTIF_TEXT";
private String notifText;
#Override
public void onReceive(Context context, Intent intent) {
notifText = intent.getExtras().getString(NOTIF_TEXT);
//().getExtras().getString(NOTE_BODY);
Toast.makeText(context, "Notification from " + R.string.app_name,
Toast.LENGTH_LONG).show();
buildNotification(context);
}
private void buildNotification(Context context) {
NotificationManager notificationManager = (NotificationManager) context
.getSystemService(Context.NOTIFICATION_SERVICE);
String channelId = "default_channel_id";
String channelDescription = "Default Channel";
Notification.Builder builder = new Notification.Builder(context);
Intent intent = new Intent(context, **EditorActivity.class**);
PendingIntent pendingIntent = PendingIntent.getActivity(context, 0,
intent, 0);
builder.setSmallIcon(R.mipmap.ic_launcher)
.setContentTitle(context.getString(R.string.notificTitle)).setContentText(notifText)
.setContentInfo(context.getString(R.string.notificInfo)).setTicker(context.getString(R.string.notifTicker))
.setLights(0xFFFF0000, 500, 500)
//.setChannelId(id)
.setContentIntent(pendingIntent).setAutoCancel(true);
Notification notification = builder.build();
//notification.so
notificationManager.notify(2, notification);
}
}
And:
private void setAlarm(Calendar targetCal) {
mTimeTextView.setText(R.string.alarm_on);
mTimeTextView.append(String.valueOf(targetCal.getTime()));
Intent intent = new Intent(getApplicationContext(), AlarmReceiver.class);
intent.putExtra(AlarmReceiver.NOTIF_TEXT,notificationText);
PendingIntent pendingIntent = PendingIntent.getBroadcast(
getApplicationContext(), RQS_TIME, intent, 0);
AlarmManager alarmManager = (AlarmManager) getSystemService(Context.ALARM_SERVICE);
alarmManager.set(AlarmManager.RTC_WAKEUP, targetCal.getTimeInMillis(),
pendingIntent);
}

Android notifications not displaying

I have written a code that is supposed to display notifications. I am choosing hour and minute from TimePicker component (notifications popping up every day at given time), then create an intent for Notification receiver. The database is updated with proper info and everything is being set with the AlarmManager. The request code ("code" variable) is unique for each notification. I am pasting code snippets below:
SettingActivity class:
findViewById(R.id.buttonNotification).setOnClickListener(new View.OnClickListener(){
#Override
public void onClick(View view){
int hour, minute, id;
Calendar calendar = Calendar.getInstance();
PendingIntent pendingIntent;
calendar.set(Calendar.HOUR_OF_DAY,timePicker.getCurrentHour());
calendar.set(Calendar.MINUTE, timePicker.getCurrentMinute());
Intent intent = new Intent(getApplicationContext(),Notifcation_receiver.class);
hour = timePicker.getCurrentHour();
minute = timePicker.getCurrentMinute();
if(extra!=null){
pendingIntent = PendingIntent.getBroadcast(getApplicationContext(),extra.getInt("Code"),intent,PendingIntent.FLAG_UPDATE_CURRENT);
intent.putExtra("Code",extra.getInt("Code"));
db.updateNotification(hour,minute,extra.getInt("ID"));
}
else{
int code= Notification.getID();
intent.putExtra("Code",code);
pendingIntent = PendingIntent.getBroadcast(getApplicationContext(),code,intent,PendingIntent.FLAG_UPDATE_CURRENT);
db.insertNotification(hour,minute);
}
AlarmManager alarmManager = (AlarmManager) getSystemService(ALARM_SERVICE);
alarmManager.setRepeating(AlarmManager.RTC_WAKEUP,calendar.getTimeInMillis(),AlarmManager.INTERVAL_DAY,pendingIntent);
startActivity(new Intent(SettingActivity.this, NotificationActivity.class));
}
});
}
Notification receiver class:
public class Notifcation_receiver extends BroadcastReceiver {
#Override
public void onReceive(Context context, Intent intent) {
int code = intent.getIntExtra("Code",0);
NotificationManager notificationManager = (NotificationManager) context.getSystemService(Context.NOTIFICATION_SERVICE);
Intent repeating_intent = new Intent(context,MainActivity.class);
repeating_intent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
PendingIntent pendingIntent = PendingIntent.getActivity(context,code,repeating_intent,PendingIntent.FLAG_UPDATE_CURRENT);
NotificationCompat.Builder builder = new NotificationCompat.Builder(context)
.setContentIntent(pendingIntent)
.setSmallIcon(android.R.drawable.arrow_up_float)
.setContentTitle("title")
.setContentText("text")
.setAutoCancel(false);
notificationManager.notify(code,builder.build());
}
}
I can't seem to find the problem, causing the lack of planned notifications. Thank You for any help.

The problem was, as Mehmed mentioned, no channel set for the NotificationService.
Logcat log:
"E/NotificationService: No Channel found for pkg=com.example.kuba.quizapp, channelId=null"
It worked on API 22, but for 25 and higher the notification must be build with extra channel info.

scheduled notification on android studio app. alarmManager.set doesn't work

that's my code for scheduled notification for my android app, but it does nothing for some reason. Please tell me where is the problem.
Another question: I also made a button which send a notification - just for learning, and for some reason it works only on my samsung s6. When I run the app on the android studio emulator it gives me an error about notification package. Why is that?
Thank a lot!
public void setAlarm(View view) {
Long alertTime = new GregorianCalendar().getTimeInMillis()+5*1000;
Intent alertIntent = new Intent(this, AlertReceiver.class);
AlarmManager alarmManager = (AlarmManager)
getSystemService(Context.ALARM_SERVICE);
alarmManager.set(AlarmManager.RTC_WAKEUP, alertTime,
PendingIntent.getBroadcast(this, 1, alertIntent, PendingIntent.FLAG_UPDATE_CURRENT));
}
}
public class AlertReceiver extends BroadcastReceiver {
#Override
public void onReceive(Context context, Intent intent) {
createNotification(context, "Time Up", "5 Seconds Has Passed", "Alert");
}
public void createNotification(Context context, String msg, String msgText, String msgAlert) {
PendingIntent noficitIntent = PendingIntent.getActivity(context, 0,
new Intent(context, MainActivity.class), 0);
NotificationCompat.Builder mBuilder = new NotificationCompat.Builder(context)
.setSmallIcon(R.drawable.ic_launcher)
.setContentTitle(msg)
.setTicker(msgAlert)
.setContentText(msgText);
mBuilder.setContentIntent(noficitIntent);
mBuilder.setDefaults(NotificationCompat.DEFAULT_SOUND);
mBuilder.setAutoCancel(true);
NotificationManager mNotificationManager =
(NotificationManager) context.getSystemService(Context.NOTIFICATION_SERVICE);
mNotificationManager.notify(1, mBuilder.build());
}
}

Based on your comments, it seems you just forgot to register your BroadcastReceiver. Per Android docs you will only be able to receive intents on your receiver if you register it first:
You can either dynamically register an instance of this class with Context.registerReceiver() or statically declare an implementation with the tag in your AndroidManifest.xml.
Since you are sending a broadcast directly to your receiver like that:
Intent alertIntent = new Intent(this, AlertReceiver.class);
There is no point in declaring any intent-filter for it, so you just need to add the following line to your AndroidManifest.xml (inside <application> tag):
<receiver android:name="com.your.package.AlertReceiver" />
Hope it helps.

Notifications not showing Android

I am trying to bring up a notification after set period of time using:
scheduleNotification(getNotification("Notification content..") differ);
and the following functions -
private Notification getNotification(String content) {
Notification.Builder builder = new Notification.Builder(this);
builder.setContentTitle("Scheduled Notification");
builder.setContentText(content);
builder.setSmallIcon(R.drawable.icon);
return builder.build();
}
private void scheduleNotification(Notification notification, long delay) {
Intent notificationIntent = new Intent(this, NotificationPublisher.class);
notificationIntent.putExtra(NotificationPublisher.NOTIFICATION_ID, 1);
notificationIntent.putExtra(NotificationPublisher.NOTIFICATION, notification);
PendingIntent pendingIntent = PendingIntent.getBroadcast(this, 0, notificationIntent, PendingIntent.FLAG_UPDATE_CURRENT);
long futureInMillis = SystemClock.elapsedRealtime() + delay;
AlarmManager alarmManager = (AlarmManager)getSystemService(Context.ALARM_SERVICE);
alarmManager.set(AlarmManager.ELAPSED_REALTIME_WAKEUP, futureInMillis, pendingIntent);
}
After logging the delay value, I get 57580 which is roughly 57 seconds, but even after this period of time, I do not get any notification on status-bar.
Please help.

You need a BroadcastReceiver to get notifications from the system.
public static class AlarmReceiver extends BroadcastReceiver {
#Override
public void onReceive(Context context, Intent intent) {
getNotification(String content);
}
}
And remember to declare the class in your AndroidManifest:
<receiver android:name=".AlarmReceiver" />