Schedule alarms (original) (raw)

Alarms (based on the AlarmManagerclass) give you a way to perform time-based operations outside the lifetime of your application. For example, you could use an alarm to initiate a long-running operation, such as starting a service once a day to download a weather forecast.

Alarms have these characteristics:

Set an inexact alarm

When an app sets an inexact alarm, the system delivers the alarm at some point in the future. Inexact alarms provide some guarantees regarding the timing of alarm delivery while respecting battery-saving restrictions such asDoze.

Developers can leverage the following API guarantees to customize the timing of inexact alarm delivery.

Deliver an alarm after a specific time

If your app calls set(),setInexactRepeating(), or setAndAllowWhileIdle(), the alarm never goes off before the supplied trigger time.

On Android 12 (API level 31) and higher, the system invokes the alarm within one hour of the supplied trigger time, unless any battery-saving restrictions are in effect such as battery saver orDoze.

Deliver an alarm during a time window

If your app calls setWindow(), the alarm never goes off before the supplied trigger time. Unless any battery-saving restrictions are in effect, the alarm is delivered within the specified time window, starting from the given trigger time.

If your app targets Android 12 or higher, the system can delay the invocation of a time-windowed inexact alarm by at least 10 minutes. For this reason, windowLengthMillis parameter values under 600000 are clipped to600000.

Deliver a repeating alarm at roughly regular intervals

If your app calls setInexactRepeating(), the system invokes multiple alarms:

  1. The first alarm goes off within the specified time window, starting from the given trigger time.
  2. Subsequent alarms usually go off after the specified time window elapses. The time between two consecutive invocations of the alarm can vary.

Set an exact alarm

The system invokes an exact alarm at a precise moment in the future.

Most apps can schedule tasks and events using inexact alarms to complete several common use cases. If your app's core functionality depends on a precisely-timed alarm—such as for an alarm clock app or a calendar app—then it's OK to use an exact alarm instead.

Use cases that might not require exact alarms

The following list shows common workflows that may not require an exact alarm:

Scheduling timing operations during the lifetime of your app

The Handler class includes several good methods to handle timing operations, such as doing some work every_n_ seconds, while your app is alive:postAtTime()andpostDelayed(). Note that these APIs rely on system uptimeand not real time.

Scheduled background work, such as updating your app and uploading logs

WorkManager provides a way to schedule timing-sensitive periodic work. You can provide a repeat interval and flexInterval (15 minutes minimum) to define granular runtime for the work.

User-specified action that should happen after a specific time (even if system in idle state)

Use an inexact alarm. Specifically, callsetAndAllowWhileIdle().

User-specified action that should happen after a specific time

Use an inexact alarm. Specifically, callset().

User-specified action that can happen within a specified time window

Use an inexact alarm. Specifically, callsetWindow(). Note that, if your app targets Android 12 or higher, the smallest allowed window length is 10 minutes.

Ways to set an exact alarm

Your app can set exact alarms using one of the following methods. These methods are ordered such that the ones closer to the bottom of the list serve more time-critical tasks but demand more system resources.

setExact()

Invoke an alarm at a nearly precise time in the future, as long as other battery-saving measures aren't in effect.

Use this method to set exact alarms, unless your app's work is time-critical for the user.

setExactAndAllowWhileIdle()

Invoke an alarm at a nearly precise time in the future, even if battery-saving measures are in effect.

setAlarmClock()

Invoke an alarm at a precise time in the future. Because these alarms are highly visible to users, the system never adjusts their delivery time. The system identifies these alarms as the most critical ones and leaves low-power modes if necessary to deliver the alarms.

System resource consumption

When the system triggers exact alarms that your app sets, the device consumes a great deal of resources, such as battery life, especially if it's in a power-saving mode. Furthermore, the system cannot easily batch these requests in order to use resources more efficiently.

It's highly recommended that you create an inexact alarm whenever possible. To perform longer work, schedule it usingWorkManager orJobScheduler from your alarm'sBroadcastReceiver. To perform work while the device is in Doze, create an inexact alarm usingsetAndAllowWhileIdle(), and start a job from the alarm.

Declare the appropriate exact alarm permission

If your app targets Android 12 or higher, you must obtain the "Alarms & reminders" special app access. To do so, declare theSCHEDULE_EXACT_ALARMpermission in your app's manifest file, as shown in the following code snippet:

<manifest ...> <application ...> ...

If your app targets Android 13 (API level 33) or higher, you have the option to declare either the SCHEDULE_EXACT_ALARMor the USE_EXACT_ALARMpermission.

<manifest ...> <application ...> ...

While both the SCHEDULE_EXACT_ALARM and the USE_EXACT_ALARM permissions signal the same capabilities, they are granted differently and support different use-cases. Your app should use exact alarms, and declare eitherSCHEDULE_EXACT_ALARM or USE_EXACT_ALARM permission, only if a user-facing function in your app requires precisely-timed actions.

USE_EXACT_ALARM

SCHEDULE_EXACT_ALARM

The SCHEDULE_EXACT_ALARM permission is not pre-granted to fresh installs of apps targeting Android 13 (API level 33) and higher. If a user transfers app data to a device running Android 14 through a backup-and-restore operation, theSCHEDULE_EXACT_ALARM permission will be denied on the new device. However, if an existing app already has this permission, it will be pre-granted when the device upgrades to Android 14.

Note: If the exact alarm is set using anOnAlarmListenerobject, such as with thesetExactAPI, the SCHEDULE_EXACT_ALARM permission isn't required.

Using the SCHEDULE_EXACT_ALARM permission

Unlike USE_EXACT_ALARM, the SCHEDULE_EXACT_ALARM permission must be granted by the user. Both the user and the system can revoke theSCHEDULE_EXACT_ALARM permission.

To check whether the permission is granted to your app, callcanScheduleExactAlarms()before trying to set an exact alarm. When the SCHEDULE_EXACT_ALARM permission is revoked for your app, your app stops, and all future exact alarms are canceled. This also means that the value returned bycanScheduleExactAlarms() stays valid for the entire lifecycle of your app.

When the SCHEDULE_EXACT_ALARMS permission is granted to your app, the system sends it theACTION_SCHEDULE_EXACT_ALARM_PERMISSION_STATE_CHANGEDbroadcast. Your app should implement a broadcast receiver that does the following:

  1. Confirms that your app still has the special app access. To do so, callcanScheduleExactAlarms(). This check protects your app from the case where the user grants your app the permission, then revokes it almost immediately afterward.
  2. Reschedules any exact alarms that your app needs, based on its current state. This logic should be similar to what your app does when it receives theACTION_BOOT_COMPLETEDbroadcast.

Ask users to grant the SCHEDULE_EXACT_ALARM permission

The option is called 'Allow setting alarms and reminders'

Figure 1. "Alarms & reminders" special app access page in system settings, where users can allow your app to set exact alarms.

If necessary, you can send users to the Alarms & reminders screen in system settings, as shown in Figure 1. To do so, complete the following steps:

  1. In your app's UI, explain to the user why your app needs to schedule exact alarms.
  2. Invoke an intent that includes theACTION_REQUEST_SCHEDULE_EXACT_ALARMintent action.

Set a repeating alarm

Repeating alarms allow the system to notify your app on a recurring schedule.

A poorly-designed alarm can cause battery drain and put a significant load on servers. For this reason, on Android 4.4 (API level 19) and higher, all repeating alarms are inexact alarms.

A repeating alarm has the following characteristics:

To cancel a PendingIntent(), passFLAG_NO_CREATEto PendingIntent.getService()to get an instance of the intent (if it exists), then pass that intent toAlarmManager.cancel()

Kotlin

val alarmManager = context.getSystemService(Context.ALARM_SERVICE) as? AlarmManager val pendingIntent = PendingIntent.getService(context, requestId, intent, PendingIntent.FLAG_NO_CREATE) if (pendingIntent != null && alarmManager != null) { alarmManager.cancel(pendingIntent) }

Java

AlarmManager alarmManager = (AlarmManager) context.getSystemService(Context.ALARM_SERVICE); PendingIntent pendingIntent = PendingIntent.getService(context, requestId, intent, PendingIntent.FLAG_NO_CREATE); if (pendingIntent != null && alarmManager != null) { alarmManager.cancel(pendingIntent); }

Choose an alarm type

One of the first considerations in using a repeating alarm is what its type should be.

There are two general clock types for alarms: "elapsed real time" and "real time clock" (RTC). Elapsed real time uses the "time since system boot" as a reference, and real time clock uses UTC (wall clock) time. This means that elapsed real time is suited to setting an alarm based on the passage of time (for example, an alarm that fires every 30 seconds) since it isn't affected by time zone or locale. The real time clock type is better suited for alarms that are dependent on current locale.

Both types have a "wakeup" version, which says to wake up the device's CPU if the screen is off. This ensures that the alarm will fire at the scheduled time. This is useful if your app has a time dependency. For example, if it has a limited window to perform a particular operation. If you don't use the wakeup version of your alarm type, then all the repeating alarms will fire when your device is next awake.

If you simply need your alarm to fire at a particular interval (for example, every half hour), use one of the elapsed real time types. In general, this is the better choice.

If you need your alarm to fire at a particular time of day, then choose one of the clock-based real time clock types. Note, however, that this approach can have some drawbacks. The app may not translate well to other locales, and if the user changes the device's time setting, it could cause unexpected behavior in your app. Using a real time clock alarm type also does not scale well, as discussed above. We recommend that you use an "elapsed real time" alarm if you can.

Here is the list of types:

Examples of elapsed real time alarms

Here are some examples of using ELAPSED_REALTIME_WAKEUP

Wake up the device to fire the alarm in 30 minutes, and every 30 minutes after that:

Kotlin

// Hopefully your alarm will have a lower frequency than this! alarmMgr?.setInexactRepeating( AlarmManager.ELAPSED_REALTIME_WAKEUP, SystemClock.elapsedRealtime() + AlarmManager.INTERVAL_HALF_HOUR, AlarmManager.INTERVAL_HALF_HOUR, alarmIntent )

Java

// Hopefully your alarm will have a lower frequency than this! alarmMgr.setInexactRepeating(AlarmManager.ELAPSED_REALTIME_WAKEUP, SystemClock.elapsedRealtime() + AlarmManager.INTERVAL_HALF_HOUR, AlarmManager.INTERVAL_HALF_HOUR, alarmIntent);

Wake up the device to fire a one-time (non-repeating) alarm in one minute:

Kotlin

private var alarmMgr: AlarmManager? = null private lateinit var alarmIntent: PendingIntent ... alarmMgr = context.getSystemService(Context.ALARM_SERVICE) as AlarmManager alarmIntent = Intent(context, AlarmReceiver::class.java).let { intent -> PendingIntent.getBroadcast(context, 0, intent, 0) }

alarmMgr?.set( AlarmManager.ELAPSED_REALTIME_WAKEUP, SystemClock.elapsedRealtime() + 60 * 1000, alarmIntent )

Java

private AlarmManager alarmMgr; private PendingIntent alarmIntent; ... alarmMgr = (AlarmManager)context.getSystemService(Context.ALARM_SERVICE); Intent intent = new Intent(context, AlarmReceiver.class); alarmIntent = PendingIntent.getBroadcast(context, 0, intent, 0);

alarmMgr.set(AlarmManager.ELAPSED_REALTIME_WAKEUP, SystemClock.elapsedRealtime() + 60 * 1000, alarmIntent);

Examples of real time clock alarms

Here are some examples of usingRTC_WAKEUP.

Wake up the device to fire the alarm at approximately 2:00 p.m., and repeat once a day at the same time:

Kotlin

// Set the alarm to start at approximately 2:00 p.m. val calendar: Calendar = Calendar.getInstance().apply { timeInMillis = System.currentTimeMillis() set(Calendar.HOUR_OF_DAY, 14) }

// With setInexactRepeating(), you have to use one of the AlarmManager interval // constants--in this case, AlarmManager.INTERVAL_DAY. alarmMgr?.setInexactRepeating( AlarmManager.RTC_WAKEUP, calendar.timeInMillis, AlarmManager.INTERVAL_DAY, alarmIntent )

Java

// Set the alarm to start at approximately 2:00 p.m. Calendar calendar = Calendar.getInstance(); calendar.setTimeInMillis(System.currentTimeMillis()); calendar.set(Calendar.HOUR_OF_DAY, 14);

// With setInexactRepeating(), you have to use one of the AlarmManager interval // constants--in this case, AlarmManager.INTERVAL_DAY. alarmMgr.setInexactRepeating(AlarmManager.RTC_WAKEUP, calendar.getTimeInMillis(), AlarmManager.INTERVAL_DAY, alarmIntent);

Wake up the device to fire the alarm at precisely 8:30 a.m., and every 20 minutes thereafter:

Kotlin

private var alarmMgr: AlarmManager? = null private lateinit var alarmIntent: PendingIntent ... alarmMgr = context.getSystemService(Context.ALARM_SERVICE) as AlarmManager alarmIntent = Intent(context, AlarmReceiver::class.java).let { intent -> PendingIntent.getBroadcast(context, 0, intent, 0) }

// Set the alarm to start at 8:30 a.m. val calendar: Calendar = Calendar.getInstance().apply { timeInMillis = System.currentTimeMillis() set(Calendar.HOUR_OF_DAY, 8) set(Calendar.MINUTE, 30) }

// setRepeating() lets you specify a precise custom interval--in this case, // 20 minutes. alarmMgr?.setRepeating( AlarmManager.RTC_WAKEUP, calendar.timeInMillis, 1000 * 60 * 20, alarmIntent )

Java

private AlarmManager alarmMgr; private PendingIntent alarmIntent; ... alarmMgr = (AlarmManager)context.getSystemService(Context.ALARM_SERVICE); Intent intent = new Intent(context, AlarmReceiver.class); alarmIntent = PendingIntent.getBroadcast(context, 0, intent, 0);

// Set the alarm to start at 8:30 a.m. Calendar calendar = Calendar.getInstance(); calendar.setTimeInMillis(System.currentTimeMillis()); calendar.set(Calendar.HOUR_OF_DAY, 8); calendar.set(Calendar.MINUTE, 30);

// setRepeating() lets you specify a precise custom interval--in this case, // 20 minutes. alarmMgr.setRepeating(AlarmManager.RTC_WAKEUP, calendar.getTimeInMillis(), 1000 * 60 * 20, alarmIntent);

Decide how precise your alarm needs to be

As previously described, choosing the alarm type is often the first step in creating an alarm. A further distinction is how precise you need your alarm to be. For most apps,setInexactRepeating()is the right choice. When you use this method, Android synchronizes multiple inexact repeating alarms and fires them at the same time. This reduces the drain on the battery.

Avoid using exact alarms if possible. However, for the rare app that has rigid time requirements, you can set an exact alarm by callingsetRepeating().

With setInexactRepeating(), you can't specify a custom interval the way you can withsetRepeating(). You have to use one of the interval constants, such asINTERVAL_FIFTEEN_MINUTES,INTERVAL_DAY, and so on. See AlarmManagerfor the complete list.

Cancel an alarm

Depending on your app, you may want to include the ability to cancel the alarm. To cancel an alarm, call cancel()on the Alarm Manager, passing in thePendingIntent you no longer want to fire. For example:

Kotlin

// If the alarm has been set, cancel it. alarmMgr?.cancel(alarmIntent)

Java

// If the alarm has been set, cancel it. if (alarmMgr!= null) { alarmMgr.cancel(alarmIntent); }

Start an alarm when the device restarts

By default, all alarms are canceled when a device shuts down. To prevent this from happening, you can design your application to automatically restart a repeating alarm if the user reboots the device. This ensures that the AlarmManager will continue doing its task without the user needing to manually restart the alarm.

Here are the steps:

  1. Set the RECEIVE_BOOT_COMPLETEDpermission in your application's manifest. This allows your app to receive theACTION_BOOT_COMPLETEDthat is broadcast after the system finishes booting (this only works if the app has already been launched by the user at least once):
  2. Implement aBroadcastReceiverto receive the broadcast:

Kotlin

class SampleBootReceiver : BroadcastReceiver() {
override fun onReceive(context: Context, intent: Intent) {
if (intent.action == "android.intent.action.BOOT_COMPLETED") {
// Set the alarm here.
}
}
}

Java

public class SampleBootReceiver extends BroadcastReceiver {
@Override
public void onReceive(Context context, Intent intent) {
if (intent.getAction().equals("android.intent.action.BOOT_COMPLETED")) {
// Set the alarm here.
}
}
} 3. Add the receiver to your app's manifest file with an intent filter that filters on theACTION_BOOT_COMPLETEDaction:





Notice that in the manifest, the boot receiver is set toandroid:enabled="false". This means that the receiver will not be called unless the application explicitly enables it. This prevents the boot receiver from being called unnecessarily. You can enable a receiver (for example, if the user sets an alarm) as follows:

Kotlin

val receiver = ComponentName(context, SampleBootReceiver::class.java)
context.packageManager.setComponentEnabledSetting(
receiver,
PackageManager.COMPONENT_ENABLED_STATE_ENABLED,
PackageManager.DONT_KILL_APP
)

Java

ComponentName receiver = new ComponentName(context, SampleBootReceiver.class);
PackageManager pm = context.getPackageManager();
pm.setComponentEnabledSetting(receiver,
PackageManager.COMPONENT_ENABLED_STATE_ENABLED,
PackageManager.DONT_KILL_APP);
Once you enable the receiver this way, it will stay enabled, even if the user reboots the device. In other words, programmatically enabling the receiver overrides the manifest setting, even across reboots. The receiver will stay enabled until your app disables it. You can disable a receiver (for example, if the user cancels an alarm) as follows:

Kotlin

val receiver = ComponentName(context, SampleBootReceiver::class.java)
context.packageManager.setComponentEnabledSetting(
receiver,
PackageManager.COMPONENT_ENABLED_STATE_DISABLED,
PackageManager.DONT_KILL_APP
)

Java

ComponentName receiver = new ComponentName(context, SampleBootReceiver.class);
PackageManager pm = context.getPackageManager();
pm.setComponentEnabledSetting(receiver,
PackageManager.COMPONENT_ENABLED_STATE_DISABLED,
PackageManager.DONT_KILL_APP);

Invoke alarms while the device is in Doze mode

Devices that run Android 6.0 (API level 23) supportDozemode, which helps extend device battery life. Alarms don't fire when the device is inDoze modeAny scheduled alarms are deferred until the device exits Doze. If you need to complete work even when the device is idle, there are several options available:

Best practices

Every choice you make in designing your repeating alarm can have consequences in how your app uses (or abuses) system resources. For example, imagine a popular app that syncs with a server. If the sync operation is based on clock time and every instance of the app syncs at 11:00 p.m., the load on the server could result in high latency or even "denial of service." Follow these best practices in using alarms: