Mr_and_Mrs_D Mr_and_Mrs_D - 5 months ago 17
Android Question

WakefulIntentService implementation clarifications

Commonsware's WakefulIntentService works beautifully but there are some things I do not quite get. Below is the core of the service - a stripped down version of the source :

class WIS extends IntentService {

private static final String NAME = WIS.class.getName() + ".Lock";
private static volatile WakeLock lockStatic = null;

synchronized private static PowerManager.WakeLock getLock(Context context) {
if (lockStatic == null) {
PowerManager mgr = (PowerManager) context
.getSystemService(Context.POWER_SERVICE);
lockStatic = mgr.newWakeLock(PowerManager.PARTIAL_WAKE_LOCK, NAME);
lockStatic.setReferenceCounted(true);
}
return (lockStatic);
}

public static void startWIS(Context ctxt, Intent i) {
getLock(ctxt.getApplicationContext()).acquire();
ctxt.startService(i);
}

public WIS(String name) {
super(name);
setIntentRedelivery(true);
}

@Override
public int onStartCommand(Intent intent, int flags, int startId) {
PowerManager.WakeLock lock = getLock(this.getApplicationContext());
if (!lock.isHeld() || (flags & START_FLAG_REDELIVERY) != 0) { // ?
lock.acquire();
}
super.onStartCommand(intent, flags, startId);
return (START_REDELIVER_INTENT);
}

@Override
protected void onHandleIntent(Intent intent) {
try {
// do your thing
} finally {
PowerManager.WakeLock lock = getLock(this.getApplicationContext());
if (lock.isHeld()) lock.release();
}
}
}


Questions


  • What happens if the process is killed just after the
    onReceive()
    of our alarm receiver returns ? That is if service
    onCreate()
    (if the service is not already instantiated) or
    onStartCommand()
    never run. AFAIK a process killed takes its locks with it. Or is this an impossible scenario ?

  • In view of the previous should
    (flags & START_FLAG_RETRY)
    be added ?

  • Why the
    if (!lock.isHeld())
    check ?

  • Why is
    this.getApplicationContext()
    needed ? is not
    this
    enough ?


Answer

AFAIK a process killed takes its locks with it.

Correct.

Or is this an impossible scenario ?

It's fairly unlikely, but certainly not impossible.

In view of the previous should (flags & START_FLAG_RETRY) be added ?

That should be covered by START_FLAG_REDELIVERY. AFAIK, with START_REDELIVER_INTENT, there is no RETRY without REDELIVERY. If you have evidence to the contrary, I'd love to see it.

Why the if (!lock.isHeld()) check ?

Calling release() on a WakeLock that is not held results in an exception. This is just a safety blanket to ensure we don't wind up throwing an unnecessary exception. In theory, it should never be needed; in theory, I should have hair.

Why is this.getApplicationContext() needed ? is not this enough ?

We create a WakeLock, which we hold in a static data member. Probably the getSystemService() call does not wind up putting the Context that called it inside the PowerManager. And, even if it did, probably the Context would not be passed to the resulting WakeLock instance. However, to be safe, by using getApplicationContext(), we obtain the WakeLock in a fashion that ensures that the only Context we could possibly "leak" is the singleton application context, which, as a singleton, is effectively pre-leaked. :-)