8.2: Scheduling Alarms
- Alarm types
- Alarm best practices
- Scheduling an alarm
- Checking for an existing alarm
- Canceling an alarm
- User-visible alarms ("alarm clocks")
- Related practical
- Learn more
You already know how to use broadcast receivers to make your app respond to system events even when your app isn't running. In this chapter, you'll learn how to use alarms to schedule tasks for specific times, whether or not your app is running at the time the alarm is set to go off. Alarms can either be single use or repeating. For example, you can use a repeating alarm to schedule a download every day at the same time.
To create alarms, you use the
AlarmManager class. Alarms in Android have the following characteristics:
- Alarms let you send intents at set times or intervals. You can use alarms with broadcast receivers to start services and perform other operations.
- Alarms operate outside your app, so you can use them to trigger events or actions even when your app isn't running, and even if the device is asleep.
- When used correctly, alarms can help you minimize your app's resource requirements. For example, you can schedule operations without relying on timers or continuously running background services.
When not to use an alarm:
- For timing events such as ticks and timeouts, and for timed operations that are guaranteed to happen during the lifetime of your app, use the
Thread. This approach gives Android better control over system resources than if you used alarms.
- For server sync operations, use
SyncAdapterwith the Google Cloud Messaging Service.
- For tasks that can wait until conditions are favorable, such as when the device is connected to WiFi and is charging (for example, updating weather information or news stories), you might not want to use alarms. For these tasks on API 21+ devices, consider using
JobScheduler, which you will learn about in an upcoming lesson.
There are two general types of alarms in Android: elapsed real-time alarms and real-time clock (RTC) alarms, and both use
Elapsed real-time alarms
Elapsed real-time alarms use the time, in milliseconds, since the device was booted. Elapsed real-time alarms aren't affected by time zones, so they work well for alarms based on the passage of time. For example, use an elapsed real-time alarm for an alarm that fires every half hour.
The AlarmManager class provides two types of elapsed real-time alarm:
ELAPSED_REALTIME: Fires a
PendingIntentbased on the amount of time since the device was booted, but doesn't wake the device. The elapsed time includes any time during which the device was asleep. All repeating alarms fire when your device is next awake.
ELAPSED_REALTIME_WAKEUP: Fires the
PendingIntentafter the specified length of time has elapsed since device boot, waking the device's CPU if the screen is off. Use this alarm instead of
ELAPSED_REALTIMEif your app has a time dependency, for example if it has a limited window during which to perform an operation.
Real-time clock (RTC) alarms
Real-time clock (RTC) alarms are clock-based alarms that use Coordinated Universal Time (UTC). Only choose an RTC alarm in these types of situations:
- You need your alarm to fire at a particular time of day.
- The alarm time is dependent on current locale.
Apps with clock-based alarms might not work well across locales, because they might fire at the wrong times. And if the user changes the device's time setting, it could cause unexpected behavior in your app.
AlarmManager class provides two types of RTC alarm:
RTC: Fires the pending intent at the specified time but doesn't wake up the device. All repeating alarms fire when your device is next awake.
RTC_WAKEUP: Fires the pending intent at the specified time, waking the device's CPU if the screen is off.
Alarm best practices
Alarms affect 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 connects to the server at the same time, the load on the server could result in delayed response times or even a "denial of service" condition.
To avoid this problem and others, follow these best practices:
- Add randomness (jitter) to network requests that trigger as a result of a repeating alarm. Here's one way to do this:
- Schedule an exact alarm that performs any local work. "Local work" means anything that doesn't contact a server over a network or require data from that server.
- Schedule a separate alarm that contains the network requests, and have this alarm fire after a random period of time. Usually this second alarm is set by whatever component receives the
PendingIntentfrom the first alarm. (You can also set this alarm at the same time as you set the first alarm.)
- Keep your alarm frequency to a minimum.
- Don't wake up the device unnecessarily.
- Use the least precise timing possible to allow the
AlarmManagerto be the most efficient it can be. For example, when you schedule a repeating alarm, use
setRepeating(). For details, see Scheduling a repeating alarm, below.
- Avoid basing your alarm on clock time and use
ELAPSED_REALTIMEfor repeating alarms whenever possible. Repeating alarms that are based on a precise trigger time don't scale well.
Scheduling an alarm
AlarmManager class gives you access to the Android system alarm services.
AlarmManager lets you broadcast an
Intent at a scheduled time, or after a specific interval.
To schedule an alarm:
getSystemService(ALARM_SERVICE)to get an instance of the
- Use one of the
set...()methods available in
AlarmManager(as described below). Which method you use depends on whether the alarm is elapsed real time, or RTC.
AlarmManager.set...() methods include these two arguments:
typeargument, which is how you specify the alarm type:
PendingIntentobject, which is how you specify which task to perform at the given time.
Scheduling a single-use alarm
To schedule a single alarm, use one of the following methods on the
set(): For devices running API 19+, this method schedules a single, inexactly timed alarm, meaning that the system shifts the alarm to minimize wakeups and battery use. For devices running lower API versions, this method schedules an exactly timed alarm.
setWindow(): For devices running API 19+, use this method to set a window of time during which the alarm should be triggered.
setExact(): For devices running API 19+, this method triggers the alarm at an exact time. Use this method only for alarms that must be delivered at an exact time, for example an alarm clock that rings at a requested time. Exact alarms reduce the OS's ability to minimize battery use, so don't use them unnecessarily.
Here's an example of using
set() to schedule a single-use alarm:
alarmMgr.set(AlarmManager.ELAPSED_REALTIME, SystemClock.elapsedRealtime() + 1000*300, alarmIntent);
In this example:
ELAPSED_REALTIME, which means that this is an elapsed real-time alarm. If the device is idle when the alarm is sent, the alarm does not wake the device.
- The alarm is sent 5 minutes (300,000 milliseconds) after the method returns.
PendingIntentbroadcast that contains the action to perform when the alarm is sent.Note: For timing operations like ticks and timeouts, and events that happen more often than once a minute, it's easier and more efficient to use a Handler rather than an alarm.
Doze and App Standby
API 23+ devices sometimes enter Doze or App Standby mode to save power:
- Doze mode is triggered when a user leaves a device unplugged and stationary for a period of time, with the screen off. During short "maintenance windows," the system exits Doze to let apps complete deferred activities, including firing standard alarms, then returns to Doze. Doze mode ends when the user returns to their device.
- App Standby mode is triggered on idle apps that haven't been used recently. App Standby mode ends when the user returns to your app or plugs in the device.
To use alarms with Doze and App Standby:
- If you need an alarm that fires while a device is in Doze or App Standby mode without waiting for a maintenance window, use
setAndAllowWhileIdle()for inexact and
setExactAndAllowWhileIdle()for exact alarms, or set a user-visible alarm (API 21+).
- Some alarms can wait for a maintenance window, or until the device comes out of Doze or App Standby mode. For these alarms, use the standard
setExact()methods to optimize battery life.
Scheduling a repeating alarm
You can also use the
AlarmManager to schedule repeating alarms, using one of the following methods:
setRepeating(): Prior to Android 4.4 (API Level 19), this method creates a repeating, exactly timed alarm. On devices running API 19 and higher,
setRepeating()behaves exactly like
setInexactRepeating(): This method creates a repeating, inexact alarm that allows for batching. When you use
setInexactRepeating(), Android synchronizes repeating alarms from multiple apps and fires them at the same time. This reduces the total number of times the system must wake the device, thus reducing drain on the battery. As of API 19, all repeating alarms are inexact.
To decrease possible battery drain:
- Schedule repeating alarms to be as infrequent as possible.
- Use inexact timing, which allows the system to batch alarms from different apps together.
setInexactRepeating() is an improvement over
setRepeating(), it can still overwhelm a server if every instance of an app hits the server around the same time. Therefore, for network requests, add some randomness to your alarms, as described in Alarm best practices.
If you really need exact repeating alarms on API 19+, set a single-use alarm with
setExact() and set the next alarm once that alarm has triggered. This second alarm is set by whatever component receives the
PendingIntent—usually either a service or a broadcast receiver.
Here's an example of using
setInexactRepeating() to schedule a repeating alarm:
alarmMgr.setInexactRepeating(AlarmManager.RTC_WAKEUP, calendar.getTimeInMillis(), AlarmManager.INTERVAL_FIFTEEN_MINUTES, alarmIntent);
In this example:
RTC_WAKEUP, which means that this is a clock-based alarm that wakes the device when the alarm is sent.
- The first occurrence of the alarm is sent immediately, because
calendar.getTimeInMillis()returns the current time as UTC milliseconds.
After the first occurrence, the alarm is sent approximately every 15 minutes.
If the method were
setInexactRepeating(), and if the device were running an API version lower than 19, the alarm would be sent exactly every 15 minutes.
Possible values for this argument are
PendingIntentthat contains the action to perform when the alarm is sent. This intent typically comes from
Checking for an existing alarm
It's often useful to check whether an alarm is already set. For example, you may want to disable the ability to set another alarm if one already exists.
To check for an existing alarm:
PendingIntentthat contains the same
Intentused to set the alarm, but this time use the
PendingIntentis only created if one with the same
Intentalready exists. Otherwise, the request returns
- Check whether the
- If it's
null, the alarm has not yet been set.
- If it's not
PendingIntentalready exists, meaning that the alarm has been set.
- If it's
For example, the following code returns
true if the alarm contained in
alarmIntent already exists:
boolean alarmExists = (PendingIntent.getBroadcast(this, 0, alarmIntent, PendingIntent.FLAG_NO_CREATE) != null);
Canceling an alarm
To cancel an alarm, use
cancel() and pass in the
PendingIntent. For example:
User-visible alarms ("alarm clocks")
For API 21+ devices, you can set a user-visible alarm clock by calling
setAlarmClock(). Apps can retrieve the next user-visible alarm clock that's set to go off by calling
Alarms clocks set with
setAlarmClock() work even when the device or app is idle (similar to
setExactAndAllowWhileIdle()), which gets you as close to an exact wake up call as possible.
The related exercises and practical documentation is in Android Developer Fundamentals: Practicals.