Scheduling operations via AlarmManager in Android

Scheduling future operations with AlarmManager in Android

Introduction

Scheduling and repeating alarms are generally used as a local reminder to notify user about some event.

Android provides AlarmManager to create and schedule alarms. AlarmManager class provides access to the system alarm service. This allow you to schedule your application to be run at some point in the future. When an alarm goes off, the Intent that had been registered for it is broadcast by the system, automatically starting the target application if it is not already running.

AlarmManager runs outside the lifetime of your application. Once an alarm is scheduled, it will invoke even when your application is not running or in sleep mode.

An scheduled alarm will execute unless it is stopped explicitly by calling cancel() method, or until device reboots. This means, you need to re-schedule them explicitly when device boot completes.

Alarms offer the following features:

  • Schedule alarms for a set time or interval.
  • Maintained by the OS, not your application, so alarms are triggered even if your application is not running, or the device is asleep.
  • Can be used to trigger periodic tasks (such as an hourly news update), even if your application is not running.
  • Your app does not use resources (such as timers or background services), since the OS manages the scheduling.

Alarms are not the best solution if you need a simple delay while your application is running, for example, a short delay for a UI event. For short delays, it's easier and more efficient to use the postAtTime() and postDelayed() methods of a Handler.

Alarm have three properties, as follows:

  • Alarm type (see in the following list).
  • Trigger time (if the time has already passed, the alarm is triggered immediately).
  • Pending Intent.

A repeating alarm has the same three properties, plus an Interval:

  • Alarm type (see in the following list).
  • Trigger time (if the time has already passed, it triggers immediately).
  • Interval.
  • Pending Intent.

There are four alarm types:

  • RTC (Real Time Clock). The alarm times are referenced to UTC time. This does not wake the device. Fires the pending intent at a specified time. If the device is asleep, it will not be delivered until the next time the device wakes up.
  • RTC_WAKEUP. The alarm times are referenced to UTC time and will wake the device to trigger if it is asleep. Fires the pending intent at a specified time, waking up the device if asleep.
  • ELAPSED_REALTIME. This is based on the time elapsed since the device boot. This does not wake the device. It is better for time interval alarms-such as every 30 minutes. Fires the pending intent after the specified length of time since device boot. If the device is asleep, it fires when the device is next awake.
  • ELAPSED_REALTIME_WAKEUP. This is based on the time elapsed since the device boot. This wakes the device if it is sleeping. Fires the pending intent after the specified length of time since device boot. It wakes up the device if it is asleep.

RTC is most commonly used for setting alarm service in Android.

If you choose a wake-up alarm type, Android will wake the device from sleep but will not keep the device awake for you. You must obtain a WakeLock from PowerManager while doing your background work from a wake-up event. Otherwise, Android is likely to put the device back to sleep quickly, which will halt what you may be doing.

Thera are some methods which are used to schedule alarms. These methods are based on exact, inexact time of execution and repeating or single execution.

  • set(int type, long triggerAtMillis, PendingIntent operation). Schedules the alarm and if there is already an alarm by the intent then previous one will be canceled. It allows executing only one time.
  • setExact(int type, long triggerAtMillis, PendingIntent operation). It behaves same as set() but it does not allow OS to adjust time. The alarm will fire on exact time.
  • setRepeating(int type, long triggerAtMillis, long intervalMillis, PendingIntent operation). It behaves same as set() but repeats for the given interval of time.
  • setInexactRepeating(int type, long triggerAtMillis, long intervalMillis, PendingIntent operation). Same as setRepeating() but the executing time will not be exact.
  • setWindow(int type, long windowStartMillis, long windowLengthMillis, PendingIntent operation). Executes within a given window of time and works same as set(). It can be used for strict ordering of execution in case of multiple alarms.

After android API 19 (KitKat) all the alarms are inexact, this is done to save battery life and disable multiple wakeups. By doing so all the alarms are bundled to its near about time and triggered at once. There are new APIs to support applications which need strict delivery guarantees; see setWindow() and setExact(). Applications whose targetSdkVersion is earlier than API 19 will continue to see the previous behavior in which all alarms are delivered exactly when requested.

One-time alarm

The following recipe will demonstrate how to create alarms with AlarmManager.

Setting an alarm requires a Pending Intent, which Android sends when the alarm is triggered. This intent can point to any system component, such as a BroadcastReceiver or Service, that can be executed when the alarm triggers. Therefore, we need to set up a Broadcast Receiving to capture the alarm intent. Our UI will consist of just a simple button to set the alarm. To start, open the Android Studio and follow these steps:

  1. Add the following <receiver> to the <application> element at the same level as the existing <activity> element:
<receiver android:name=".AlarmBroadcastReceiver">
    <intent-filter>
        <action android:name="me.proft.alarms.ACTION_ALARM" />
    </intent-filter>
</receiver>

Open activity_main.xml and add the following button:

<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:padding="5dp"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <Button
        android:id="@+id/btnAlarm"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Set Alarm"
        android:layout_centerInParent="true"
        android:onClick="setAlarm"/>
</LinearLayout>

Create a new Java class called AlarmBroadcastReceiver using the following code:

public class AlarmBroadcastReceiver extends BroadcastReceiver {
    public static final String ACTION_ALARM = "en.proft.alarms.ACTION_ALARM";
    @Override
    public void onReceive(Context context, Intent intent) {
        if (ACTION_ALARM.equals(intent.getAction())) {
            Toast.makeText(context, ACTION_ALARM, Toast.LENGTH_SHORT).show();
        }
    }
}

Open ActivityMain.java and add the method for the button click:

public void setAlarm(View view) {
    Intent intentToFire = new Intent(getApplicationContext(), AlarmBroadcastReceiver.class);
    intentToFire.setAction(AlarmBroadcastReceiver.ACTION_ALARM);

    // pass something
    // Bundle bundle = new Bundle();
    // bundle.putString("TAG", "FOO");
    // intentToFire.putExtra("BUNDLE", bundle);

    PendingIntent alarmIntent = PendingIntent.getBroadcast(getApplicationContext(), 
        0, intentToFire, 0);
    AlarmManager alarmManager = (AlarmManager)getApplicationContext().
        getSystemService(Context.ALARM_SERVICE);

    Calendar c = Calendar.getInstance();
    c.add(Calendar.MINUTE, 2);
    long afterTwoMinutes = c.getTimeInMillis();

    alarmManager.set(AlarmManager.RTC, afterTwoMinutes, alarmIntent);
}

Creating the alarm is done with this line of code:

alarmManager.set(AlarmManager.ELAPSED_REALTIME, twoMinutes, alarmIntent);

Here's the method signature:

set(AlarmType, Time, PendingIntent);

To set the alarm, we create a Pending Intent with our previously defined alarm action:

public static final String ACTION_ALARM = "en.proft.alarms.ACTION_ALARM";

This is an arbitrary string and could be anything we want, but it needs to be unique, so we prepend our package name. We check for this action in the Broadcast Receiver's onReceive() callback.

If you click the Set Alarm button and wait for two minutes, you will see the Toast when the alarm triggers. If you are too impatient to wait and click the Set Alarm button again before the first alarm is triggered, you wouldn't get two alarms. Instead, the OS will replace the first alarm with the new alarm, since they both use the same Pending Intent. If you need multiple alarms, you need to create different Pending Intents, such as using different Actions.

If you want to cancel the alarm, call the cancel() method by passing the same Pending Intent you have used to create the alarm. If we continue with our recipe, this is how it would look:

alarmManager.cancel(alarmIntent);

Repeating alarm

If you want to create a repeating alarm, use the setRepeating() method. The signature is similar to the set() method, but with an interval. This is shown as follows:

setRepeating(AlarmType, Time (in milliseconds), Interval, PendingIntent);

For the Interval, you can specify the interval time in milliseconds or use one of the predefined AlarmManager constants:

  • INTERVAL_DAY
  • INTERVAL_FIFTEEN_MINUTES
  • INTERVAL_HALF_DAY
  • INTERVAL_HALF_HOUR
  • INTERVAL_HOUR

Let's write a very basic service that will simply display the current time as a Toast every time it is triggered.

public class AlarmService extends Service {
    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        // display the current time
        Calendar now = Calendar.getInstance();
        DateFormat formatter = SimpleDateFormat.getTimeInstance();
        Toast.makeText(this, formatter.format(now.getTime()), Toast.LENGTH_SHORT).show();
        return START_NOT_STICKY;
    }
    @Override
    public IBinder onBind(Intent intent) {
        return null;
    }
}

That service must be registered in the AndroidManifest.xml with a <service> tag. Otherwise, AlarmManager which is external to your application will not be aware of how to trigger it.

<application>
    ...
    <service android:enabled="true" android:name=".AlarmService"/>
</application>

Below is layout to control our AlarmService.

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <Button
        android:id="@+id/start"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Start Periodic Task" />
    <Button
        android:id="@+id/stop"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Cancel Periodic Task" />
</LinearLayout>

Following is the Activity to register/unregister alarm.

public class AlarmActivity extends AppCompatActivity implements View.OnClickListener {
    private PendingIntent alarmIntent;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        // attach the listener to both buttons
        findViewById(R.id.start).setOnClickListener(this);
        findViewById(R.id.stop).setOnClickListener(this);

        // create the launch sender
        Intent launchIntent = new Intent(this, AlarmService.class);
        alarmIntent = PendingIntent.getService(this, 0, launchIntent, 0);
    }

    @Override
    public void onClick(View v) {
        AlarmManager manager = (AlarmManager)getSystemService(Context.ALARM_SERVICE);
        long interval = 60 * 1000; // 1 minute

        switch(v.getId()) {
            case R.id.start:
                Toast.makeText(this, "Scheduled", Toast.LENGTH_SHORT).show();

                Calendar c = Calendar.getInstance();
                c.add(Calendar.SECOND, 10);
                long afterTenSeconds = c.getTimeInMillis();

                manager.setRepeating(AlarmManager.RTC_WAKEUP, afterTenSeconds, interval, alarmIntent);
                break;

            case R.id.stop:
                Toast.makeText(this, "Canceled", Toast.LENGTH_SHORT).show();
                manager.cancel(alarmIntent);
                break;
            default:
            break;
        }
    }
}

As of Android 5.1 (API version 22) there is a minimum period of 1 minute for repeating alarms. If you need to do work within one minute, just set the alarm directly, then set the next one from that alarm's handler, etc. If you need to do work within 5 seconds (for example), post it to a Handler instead of using the AlarmManager.

The sample activity presents two buttons: one to begin firing regular alarms, and the other to cancel them. The operation to trigger is referenced by a PendingIntent, which will be used to both set and cancel the alarms. We create an intent referencing the service directly, and then we wrap that intent inside a PendingIntent obtained with getService().

The alarm in the example is registered to trigger 1 minute after the button is pressed, and then every 1 minute after that. Every 1 minute, a Toast will come onscreen with the current time value, even if the application is no longer running or in front of the user. When the user displays the activity and presses the Stop button, any pending alarms matching our PendingIntent are immediately canceled and will stop the flow of Toasts.

Precision alarm

What if we wanted to schedule an alarm to occur at a specific time? Perhaps exactly at 14:00? Setting AlarmManager with some slightly different parameters could accomplish this.

// get a Calendar and set the time to 14:00:00
Calendar startTime = Calendar.getInstance();
startTime.set(Calendar.HOUR_OF_DAY, 14);
startTime.set(Calendar.MINUTE, 0);

// get a Calendar at the current time
Calendar now = Calendar.getInstance();

if (now.before(startTime)) {
    // it's not 14:00 yet, start today
    time = startTime.getTimeInMillis();
} else {
    // start 14:00 tomorrow
    startTime.add(Calendar.DATE, 1);
    time = startTime.getTimeInMillis();
}

// set the alarm
if (Build.VERSION.SDK_INT < Build.VERSION_CODES.KITKAT) {
    alarmManager.set(AlarmManager.RTC, time, alarmIntent);
} else {
    alarmManager.setExact(AlarmManager.RTC, time, alarmIntent);
}

This example uses an alarm that is referenced to real time. A determination is made whether the next occurrence of 14:00 will be today or tomorrow, and that value is returned as the trigger time for the alarm.

Starting in Android 4.4, the AlarmManager defaults all alarms to be inexact, meaning there is a small window within which they will trigger. Along with this new behavior, the setExact() API method was added to allow developers to declare that the following alarm cannot fall within an inexact window. Prior to 4.4, simply calling set() with the appropriate start time was sufficient.

Re-starting Alarm Service on device reboot

As discussed earlier, once an alarm service is started, it execute until it is explicitly stopped or until device reboots. This means that, if your device is restarted then your alarm is stopped. To avoid such situation, you have to restart your alarm service as soon as device boot completes. Below code snippet will help you to start alarm service once device reboots.

public class DeviceBootReceiver extends BroadcastReceiver {
    @Override
    public void onReceive(Context context, Intent intent) {
        if (intent.getAction().equals("android.intent.action.BOOT_COMPLETED")) {
            Intent alarmIntent = new Intent(context, AlarmReceiver.class);
            PendingIntent pendingIntent = PendingIntent.getBroadcast(context, 0, alarmIntent, 0);

            AlarmManager manager = (AlarmManager) context.
                getSystemService(Context.ALARM_SERVICE);

            int interval = 8000;

            manager.setInexactRepeating(AlarmManager.RTC_WAKEUP, 
                System.currentTimeMillis(), interval, pendingIntent);

            Toast.makeText(context, "Alarm Set", Toast.LENGTH_SHORT).show();
        }
    }
}

To start your alarm on device reboot, you have to register your above declared DeviceBootReciever class in your application manifest. This also need android.permission.RECEIVE_BOOT_COMPLETED.

<application>
    <uses-permission android:name="android.permission.RECEIVE_BOOT_COMPLETED"/>
    ...
    <receiver android:name=".DeviceBootReceiver" android:enabled="false">
        <intent-filter>
            <action android:name="android.intent.action.BOOT_COMPLETED"/>
        </intent-filter>
    </receiver>
</application>

How to check that alarm has been scheduled already or not?

Create an equivalent PendingIntent which you used with setRepeating() with PendingIntent.FLAG_NO_CREATE flag.

Intent intent = new Intent(context, WidgetUpdateReceiver.class); PendingIntent pendingIntent = PendingIntent.getBroadcast(context, 0, intent, PendingIntent.FLAG_NO_CREATE);

if (pendingIntent != null) {
    Log.d(TAG, "Alarm is already active");
}

While using PendingIntent.FLAG_NO_CREATE, if the described PendingIntent does not already exists, it simply return null.

How to get WakeLock

WAKE_LOCK permission is required because the wake lock is being used while processing in onReceive() method present in BroadcastReceiver class.

<uses-permission android:name='android.permission.WAKE_LOCK'/>

<application>
    ...
    <receiver android:name='.AlarmBroadcastReceiver'></receiver>
</application>

Example of WakeLock for BroadcastReceiver.

public class AlarmBroadcastReceiver extends BroadcastReceiver {
    @Override
    public void onReceive(Context context, Intent intent) {
        PowerManager pm = (PowerManager) context.getSystemService(Context.POWER_SERVICE);
        PowerManager.WakeLock wl = pm.newWakeLock(PowerManager.PARTIAL_WAKE_LOCK, "TAG");

        // acquire the lock
        wl.acquire();

        // you can do the processing here.
        // ...

        // release the lock
        wl.release();
    }
}

List all alarms in device

You can know the alarm has been set and when are they going to alarmed and interval. Also how many times this alarm has been invoked.

# from terminal
adb devices

# connect to emulator
adb connect emulator-5554

# list alarms
adb -s emulator-5554 shell dumpsys alarm

Job Scheduler

AlarmManager is a great candidate for scheduling if an application needs to perform a local event at an exact time or inexact interval. Alarm clock or reminder applications are great examples for AlarmManager usage. However, the documentation discourages using AlarmManager for scheduling network-related tasks. For this tasks you can use JobScheduler which is an API for scheduling various types of jobs against the framework that will be executed in your application's own process.

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