Repeating Periodic Tasks

Overview

Repeating periodic tasks within an application is a common requirement. This functionality can be used for polling new data from the network, running manual animations, or simply updating the UI. There are several ways to run periodic tasks:

1. Handler - Execute a Runnable task on the UIThread after an optional delay
2. ScheduledThreadPoolExecutor - Execute periodic tasks with a background thread pool
3. WorkManager - The recommended API for deferrable periodic background work that must survive process death or device reboot (data sync, log uploads, etc.). See the WorkManager guide.
4. AlarmManager - Use for exact-time alarms (calendar reminders, alarm clocks) where firing at a precise wall-clock time matters. For deferrable periodic background work, prefer WorkManager.
5. TimerTask - Doesn't run in UIThread and is not reliable. Consensus is to never use TimerTask

Recommended methods are outlined below.

Handler

We can use a Handler to run code on a given thread after a delay or repeat tasks periodically on a thread. This is done by constructing a Handler and then "posting" Runnable code to the event message queue on the thread to be processed.

Note: The no-argument Handler() constructor is deprecated since API 30 (Android 11) because it implicitly picks up the current thread's Looper, which can lead to subtle bugs. New code should pass the Looper explicitly, e.g. new Handler(Looper.getMainLooper()) to post to the main thread, or use a java.util.concurrent.Executor. The examples below use the legacy constructor; update them accordingly for modern projects. See the Handler reference.

Executing Code After Delay

Using a Handler, we can execute arbitrary code a single time after a specified delay:

// We need to use this Handler package
import android.os.Handler;

// Create the Handler object (on the main thread by default)
Handler handler = new Handler();
// Define the code block to be executed
private Runnable runnableCode = new Runnable() {
    @Override
    public void run() {
      // Do something here on the main thread
      Log.d("Handlers", "Called on main thread");
    }
};
// Run the above code block on the main thread after 2 seconds
handler.postDelayed(runnableCode, 2000);

Execute Recurring Code with Specified Interval

Using a similar technique, we can also use a handler to execute a periodic runnable task as demonstrated below:

// We need to use this Handler package
import android.os.Handler;

// Create the Handler object (on the main thread by default)
Handler handler = new Handler();
// Define the code block to be executed
private Runnable runnableCode = new Runnable() {
    @Override
    public void run() {
      // Do something here on the main thread
      Log.d("Handlers", "Called on main thread");
      // Repeat this the same runnable code block again another 2 seconds
      // 'this' is referencing the Runnable object
      handler.postDelayed(this, 2000);
    }
};
// Start the initial runnable task by posting through the handler
handler.post(runnableCode);

We can remove the scheduled execution of a runnable with:

// Removes pending code execution
handler.removeCallbacks(runnableCode);

Note that with a Handler, the Runnable executes in UIThread by default so you can safely update the user interface within the runnable code block. See this handler post and this other handler post for reference.

Refer to our threads and handlers guide for a more advanced breakdown.

ScheduledThreadPoolExecutor

A pool of threads which can schedule commands to execute periodically in the background. Useful when multiple worker threads are needed but generally not needed. See this guide on how they work or this stackoverflow post.

WorkManager

For deferrable periodic background work that should run reliably even if the app exits or the device restarts (data sync, log uploads, scheduled refreshes), use WorkManager. Per the official guidance: "the WorkManager API is the recommended replacement for previous Android background scheduling APIs, including FirebaseJobDispatcher and GcmNetworkManager." Schedule periodic work with PeriodicWorkRequest; the system handles persistence across reboots, Doze, and battery-saving constraints automatically. Note that the minimum periodic interval is 15 minutes.

AlarmManager

AlarmManager should be reserved for user-facing exact-time events such as calendar reminders or alarm clocks where firing at a precise wall-clock time matters. For deferrable periodic background work, prefer WorkManager above — the Android docs explicitly recommend AlarmManager only for use cases that require firing at an exact time. See the AlarmManager section of the services guide for details.

References

• http://www.mopri.de/2010/timertask-bad-do-it-the-android-way-use-a-handler/
• http://stackoverflow.com/questions/18605403/timertask-vs-thread-sleep-vs-handler-postdelayed-most-accurate-to-call-functio
• http://androidtrainningcenter.blogspot.in/2013/12/handler-vs-timer-fixed-period-execution.html
• http://stackoverflow.com/questions/8098806/where-do-i-create-and-use-scheduledthreadpoolexecutor-timertask-or-handler