Does the Android OS release a wakelock if the app or service holding it is killed?

Question

I have question about wakelock. In cases shown below, does android OS release wakelock (PARTIAL_WAKE_LOCK if you need to specify) to prevent wakelock was left a

Answer 1

I would assume (I don't know this for certain) the Android system does not keep wakelocks for killed processes. Most likely when it kills a process with sigkill it also removes any wakelocks held by that process.

Otherwise, as you say, crashes would lead to the phone always being awake, which I have not observed.

Answer 2

WakeLock Implementation Overview

When we use pm.newWakeLock to create a new wakelock, the PowerManager simply creates a new WakeLock object and returns. The WakeLock object is not a binder object, so it cannot be used through multiple processes. However, in that WakeLock object, it contains a Binder object named mToken.

    WakeLock(int flags, String tag) {
        mFlags = flags;
        mTag = tag;
        mToken = new Binder();
    }

So when you call acquire or release on this WakeLock object, it actually passes that token to PowerManagerService.

    private void acquireLocked() {
        if (!mRefCounted || mCount++ == 0) {
            mHandler.removeCallbacks(mReleaser);
            try {
                mService.acquireWakeLock(mToken, mFlags, mTag, mWorkSource);
            } catch (RemoteException e) {
            }
            mHeld = true;
        }
    }

Look at how PowerManagerService works when acquiring or releasing a wakelock will help you answer your question.

void acquireWakeLockInternal(IBinder lock, int flags, String tag, WorkSource ws,
        int uid, int pid) {
    synchronized (mLock) {
        ...
        WakeLock wakeLock;
        int index = findWakeLockIndexLocked(lock);
        if (index >= 0) {
            ...
            // Update existing wake lock.  This shouldn't happen but is harmless.
            ...
        } else {
            wakeLock = new WakeLock(lock, flags, tag, ws, uid, pid);
            try {
                lock.linkToDeath(wakeLock, 0);
            } catch (RemoteException ex) {
                throw new IllegalArgumentException("Wake lock is already dead.");
            }
            notifyWakeLockAcquiredLocked(wakeLock);
            mWakeLocks.add(wakeLock);
        }
        ...
    }
    ...
}

The key statement is the lock.linkToDeath(wakeLock, 0);. That lock is exactly the mToken we mentioned before. This method registers the recipient (the wakeLock) for a notification if this binder goes away. If this binder object unexpectedly goes away (typically because its hosting process has been killed), then the binderDied method will get called on the recipient.

Notice that the WakeLock in PowerManagerService is different from the WakeLock in PowerManager, it is an implementation of IBinder.DeathRecipient. So check out its binderDied method.

    @Override
    public void binderDied() {
        PowerManagerService.this.handleWakeLockDeath(this);
    }

The handleWakeLockDeath will release that wakelock.

private void handleWakeLockDeath(WakeLock wakeLock) {
    synchronized (mLock) {
        ...
        int index = mWakeLocks.indexOf(wakeLock);
        if (index < 0) {
            return;
        }

        mWakeLocks.remove(index);
        notifyWakeLockReleasedLocked(wakeLock);

        applyWakeLockFlagsOnReleaseLocked(wakeLock);
        mDirty |= DIRTY_WAKE_LOCKS;
        updatePowerStateLocked();
    }
}

So I think in both cases in your question, the answer is don't worry. At least in Android 4.2 (where the code comes from), it is true. Moreover, there is a finalize method on the WakeLock class in PowerManager, but this is not the key to your question.