作為移動終端，電量是一種稀缺資源，需要盡可能的節省。于是，Android系統在空閑時，會主動進入到休眠狀態。?
我們知道整個Android系統中運行著很多個進程，因此必須有一種機制能夠知道每個進程是否正在進行重要的工作，只有這樣Android系統才能對整個終端當前的狀態做出判斷。

顯然我們不能啟動一個進程，去主動監管其它所有進程的工作狀態，這樣CPU開銷太大，反而加劇了電量的消耗。為此Android引入了基于WakeLock的電量管理機制，而PMS就是專門負責管理WakeLock的進程。

個人覺得WakeLock機制的思想，有點類似于早期通信領域局域網中的令牌環機制。當局域網中有設備需要發送數據時，需要申請令牌(Token)，申請到令牌才能發送數據；設備發送完數據后，再釋放掉令牌。

與此相似，Android設備中運行的進程需要使用電量資源時，也需要向PMS申請一個WakeLock；當工作完成后，就釋放掉申請的WakeLock。PMS通過判斷當前是否還有進程持有WakeLock，就能得出系統是否空閑的結論。

從這里也可以看出，當我們寫一個永不停止工作的線程，但不申請WakeLock時，系統仍然可以休眠。在休眠時，CPU不會再耗費資源去調度該線程，于是“永不停止工作”被打上了引號。

接下來，我們就來看看PMS中的WakeLock。

一、創建WakeLock?
我們以RIL.java為例，看看一般情況下，PMS以外的其它進程如何使用WakeLock。?
在RIL.java的構造函數中：

public RIL(Context context, int preferredNetworkType,int cdmaSubscription, Integer instanceId) {.............PowerManager pm = (PowerManager)context.getSystemService(Context.POWER_SERVICE);//獲取WakeLock，第一個參數決定了WakeLock的等級和flagmWakeLock = pm.newWakeLock(PowerManager.PARTIAL_WAKE_LOCK, RILJ_LOG_TAG);//默認WakeLocked會ReferenceCounted，即一次申請對應一次釋放//設為false后，一次釋放就可以對應所有的申請mWakeLock.setReferenceCounted(false);...........//RIL.java中自己維護了WakeLockCountmWakeLockCount = 0;........... }

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從上面的代碼，可以看出調用PowerManager的newWakeLock函數，可以創建出WakeLock。

我們看看newWakeLock函數定義：

public WakeLock newWakeLock(int levelAndFlags, String tag) {//檢查參數有效性，即levelAndFlags必須對應于PowerManager中定義的WakeLock級別和flag，tag不能為空validateWakeLockParameters(levelAndFlags, tag);//此WakeLock為PowerManager定義的內部類return new WakeLock(levelAndFlags, tag, mContext.getOpPackageName()); }

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1、WakeLock Level?
newWakeLock中的第一個參數對應于WakeLock的級別和標志位構成的位圖。?
目前，在PowerManger中一共為WakeLock定義了7種level。

/** * Wake lock level: Ensures that the CPU is running; the screen and keyboard * backlight will be allowed to go off. * * If the user presses the power button, then the screen will be turned off * but the CPU will be kept on until all partial wake locks have been released. * / public static final int PARTIAL_WAKE_LOCK = 0x00000001;/** * Wake lock level: Ensures that the screen is on (but may be dimmed); * the keyboard backlight will be allowed to go off. * * If the user presses the power button, then the SCREEN_DIM_WAKE_LOCK will be * implicitly released by the system, causing both the screen and the CPU to be turned off. */ @Deprecated public static final int SCREEN_DIM_WAKE_LOCK = 0x00000006;/** * Wake lock level: Ensures that the screen is on at full brightness; * the keyboard backlight will be allowed to go off. * *If the user presses the power button, then the SCREEN_BRIGHT_WAKE_LOCK will be * implicitly released by the system, causing both the screen and the CPU to be turned off. */ @Deprecated public static final int SCREEN_BRIGHT_WAKE_LOCK = 0x0000000a;/** * Wake lock level: Ensures that the screen and keyboard backlight are on at * full brightness. * *If the user presses the power button, then the FULL_WAKE_LOCK will be * implicitly released by the system, causing both the screen and the CPU to be turned off. */ @Deprecated public static final int FULL_WAKE_LOCK = 0x0000001a;/** * Wake lock level: Turns the screen off when the proximity sensor activates. * If the proximity sensor detects that an object is nearby, the screen turns off * immediately. Shortly after the object moves away, the screen turns on again. * * A proximity wake lock does not prevent the device from falling asleep * unlike link FULL_WAKE_LOCK, SCREEN_BRIGHT_WAKE_LOCK and SCREEN_DIM_WAKE_LOCK. * If there is no user activity and no other wake locks are held, then the device will fall asleep (and lock) as usual. * However, the device will not fall asleep while the screen has been turned off * by the proximity sensor because it effectively counts as ongoing user activity. * * Cannot be used with ACQUIRE_CAUSES_WAKEUP (WakeLock的flag). */ //例如撥號，打通后接聽電話，屏幕變黑 public static final int PROXIMITY_SCREEN_OFF_WAKE_LOCK = 0x00000020;/** * Wake lock level: Put the screen in a low power state and allow the CPU to suspend * if no other wake locks are held. * * This is used by the dream manager to implement doze mode. It currently * has no effect unless the power manager is in the dozing state. * / public static final int DOZE_WAKE_LOCK = 0x00000040;/** * Wake lock level: Keep the device awake enough to allow drawing to occur. * * This is used by the window manager to allow applications to draw while the * system is dozing. It currently has no effect unless the power manager is in * the dozing state. * / public static final int DRAW_WAKE_LOCK = 0x00000080;

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從上面的代碼注釋可以看出，WakeLock主要用于控制CPU、屏幕和鍵盤三大部分（當然，現在的Anroid中基本沒有鍵盤了）。?
對于PARTIAL_WAKE_LOCK、SCREEN_DIM_WAKE_LOCK、SCREEN_BRIGHT_WAKE_LOCK和FULL_WAKE_LOCK而言，不考慮Power鍵的話，隨著等級的提高，權限也相應增大，即持有高等級的鎖，能夠激活的部分越多；如果考慮Power鍵的話，PARTIAL_WAKE_LOCK可以保證CPU不休眠，反而是權限最大的。

PROXIMITY_SCREEN_OFF_WAKE_LOCK、DOZE_WAKE_LOCK和DRAW_WAKE_LOCK都是和具體場景相關的鎖。

2、WakeLock Flag?
PowerManager定義的WakeLock Flag很多，無法一一列舉，就看一下比較常用的：

/** * Wake lock flag: Turn the screen on when the wake lock is acquired. * * Normally wake locks don't actually wake the device, they just cause * the screen to remain on once it's already on. Think of the video player * application as the normal behavior. Notifications that pop up and want * the device to be on are the exception; use this flag to be like them. * * Cannot be used with PARTIAL_WAKE_LOCK. * / public static final int ACQUIRE_CAUSES_WAKEUP = 0x10000000;/** * Wake lock flag: When this wake lock is released, poke the user activity timer * so the screen stays on for a little longer. * * Will not turn the screen on if it is not already on. * * Cannot be used with PARTIAL_WAKE_LOCK. * / public static final int ON_AFTER_RELEASE = 0x20000000;..................

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總結一下上面的內容，如下所示：?
?
WakeLock Flag一般與WakeLock Level組合使用，使用的時候參照一下注釋即可。

3、WakeLock的構造函數?
最后，我們來看看WakeLock的構造函數：

WakeLock(int flags, String tag, String packageName) {//level and flagmFlags = flags;//創建類對應的打印TagmTag = tag;//創建類的類名 mPackageName = packageName;//創建一個Binder對象//PMS將作為該Binder的客戶端監聽對應進程是否死亡mToken = new Binder();mTraceName = "WakeLock (" + mTag + ")"; }

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WakeLock的構造函數中需要注意的地方是，創建了一個Binder對象。?
回憶一下RIL.java中創建WakeLock的過程，我們就知道這個Binder對象應該是創建在RIL.java所在的Phone進程中。

二、Acquire WakeLock?
從上面的分析，我們知道一個進程創建的WakeLock，實際上表明了該進程執行某個工作時對電量的需求，例如聲明該工作需要保持屏幕處于點亮狀態，或該工作需要CPU處于喚醒態等。?
因此，進程創建了WakeLock后，需要將WakeLock發送到PMS中，讓PMS明白該進程的需求。?
這種將WakeLock通知到PMS的過程，就被稱為acquire WakeLock。

同樣，我們還是以RIL.java中的使用過程舉例：

private void send(RILRequest rr) {Message msg;if (mSocket == null) {rr.onError(RADIO_NOT_AVAILABLE, null);rr.release();return;}msg = mSender.obtainMessage(EVENT_SEND, rr);//重點在這里acquireWakeLock(rr, FOR_WAKELOCK);msg.sendToTarget(); }

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當AP側向modem發送請求時，將要調用RIL.java的send函數。send函數將會發送消息給RILSender，后者利用socket將消息發送給rild進程。?
從上面的代碼可以看出，在發送消息給RILSender之前，調用了acquireWakeLock函數：

private void acquireWakeLock(RILRequest rr, int wakeLockType) {synchronized(rr) {.............switch(wakeLockType) {case FOR_WAKELOCK:synchronized (mWakeLock) {//調用acquire函數mWakeLock.acquire();mWakeLockCount++;mWlSequenceNum++;............}break;.........}rr.mWakeLockType = wakeLockType;} }

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我們跟進一下PowerManager中WakeLock的acquire函數：

public void acquire() {synchronized (mToken) {acquireLocked();} }private void acquireLocked() {//前面已經提過，RIL.java中已經將mRefCounted置為false//如果不將mRefCounted置為false，意味著acquire和release必須一一對應//那么每個WakeLock只能acquire一次if (!mRefCounted || mCount++ == 0) {........try {mService.acquireWakeLock(mToken, mFlags, mTag, mPackageName, mWorkSource,mHistoryTag);} catch (RemoteException e) {throw e.rethrowFromSystemServer();}mHeld = true;} }

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容易看出實際的工作流程將通過Binder通信進入到PMS中：

public void acquireWakeLock(IBinder lock, int flags, String tag, String packageName,WorkSource ws, String historyTag) {//參數和權限檢查............final int uid = Binder.getCallingUid();final int pid = Binder.getCallingPid();final long ident = Binder.clearCallingIdentity();try {acquireWakeLockInternal(lock, flags, tag, packageName, ws, historyTag, uid, pid);} finally {Binder.restoreCallingIdentity(ident);} }private void acquireWakeLockInternal(IBinder lock, int flags, String tag, String packageName,WorkSource ws, String historyTag, int uid, int pid) {synchronized (mLock) {...........//PMS中也定義了WakeLock內部類WakeLock wakeLock;//PMS中維持了一個ArrayList，記錄當前已申請的WakeLock//findWakeLockIndexLocked查找ArrayList，判斷參數對應的WakeLock，是否在之前被申請過int index = findWakeLockIndexLocked(lock);boolean notifyAcquire;if (index >= 0) {//如果index大于0，說明此時Acquire的是一個舊的WakeLock//例如RIL會多次調用send函數，于是除第一次外，都會進入這個分支wakeLock = mWakeLocks.get(index);//這是判斷WakeLock對應的成員變量是否發生改變if (!wakeLock.hasSameProperties(flags, tag, ws, uid, pid)) {// Update existing wake lock. This shouldn't happen but is harmless.notifyWakeLockChangingLocked(wakeLock, flags, tag, packageName,uid, pid, ws, historyTag);//若wakelock屬性發生了變化，更新該屬性wakeLock.updateProperties(flags, tag, packageName, ws, historyTag, uid, pid);}notifyAcquire = false;} else {//創建一個新的WakeLock，例如RIL第一次調用send就會進入該分支wakeLock = new WakeLock(lock, flags, tag, packageName, ws, historyTag, uid, pid);try {//1、監控申請WakeLock的進程是否死亡lock.linkToDeath(wakeLock, 0);} catch (RemoteException ex) {throw new IllegalArgumentException("Wake lock is already dead.");}//添加到wakelock列表mWakeLocks.add(wakeLock);//2、特殊處理PARTIAL_WAKE_LOCK//實際上，根據Doze模式的白名單更新wakelock的disabled變量setWakeLockDisabledStateLocked(wakeLock);notifyAcquire = true;}//3、處理WakeLock對應的Flag//實際上判斷WakeLock是否有ACQUIRE_CAUSES_WAKEUP，在必要時喚醒屏幕applyWakeLockFlagsOnAcquireLocked(wakeLock, uid);mDirty |= DIRTY_WAKE_LOCKS;//更新電源狀態，以后單獨分析updatePowerStateLocked();if (notifyAcquire) {// This needs to be done last so we are sure we have acquired the// kernel wake lock. Otherwise we have a race where the system may// go to sleep between the time we start the accounting in battery// stats and when we actually get around to telling the kernel to// stay awake.//通知wakeLock發生變化 //電量統計服務做相關統計notifyWakeLockAcquiredLocked(wakeLock);}} }

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如上代碼中標注的注釋，acquireWakeLockInternal中有幾處比較重要的地方，我們一起來分析一下。

1、監聽客戶端進程死亡?
上面的代碼中，第一次創建WakeLock后，調用了：

......... lock.linkToDeath(wakeLock, 0); .........

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我們將acquire WakeLock的進程定義為PMS的客戶端進程，那么上面代碼的lock，就是客戶端進程中創建的Binder對象的代理。對于RIL而言，就是存在于Phone進程中的Binder的代理。?
PMS調用Binder代理的linkToDeath，實際上使得PMS成為了對應進程Binder的客戶端。于是，當對應進程死亡后，將通知PMS。?
linkToDeath傳入的必須是繼承IBinder.DeathRecipient的對象，作為進程死亡的”訃告”接收者。

我們看看PMS中WakeLock與此相關的定義：

private final class WakeLock implements IBinder.DeathRecipient {...........@Overridepublic void binderDied() {//發現客戶端進程死亡后，調用PMS的handleWakeLockDeath進行處理，傳入的參數為WakeLock自己PowerManagerService.this.handleWakeLockDeath(this);}....... }

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我們看看PMS的handleWakeLockDeath函數：

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

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跟進removeWakeLockLocked函數：

private void removeWakeLockLocked(WakeLock wakeLock, int index) {mWakeLocks.remove(index);//通知到BatteryStatsServicenotifyWakeLockReleasedLocked(wakeLock);//處理WakeLock對應的flag，與后文applyWakeLockFlagsOnAcquireLocked一起分析//實際上是判斷是否需要立即息屏applyWakeLockFlagsOnReleaseLocked(wakeLock);mDirty |= DIRTY_WAKE_LOCKS;//鎖移除后，還是利用updatePowerStateLocked更新電源狀態updatePowerStateLocked(); }

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2、特殊處理PARTIAL_WAKE_LOCK?
PMS處理第一次創建的WakeLock時，還會調用setWakeLockDisabledStateLocked函數進行處理：

private boolean setWakeLockDisabledStateLocked(WakeLock wakeLock) {//僅會特殊處理PARTIAL_WAKE_LOCK，畢竟PARTIAL_WAKE_LOCK要求按Power鍵后CPU依然可以工作if ((wakeLock.mFlags & PowerManager.WAKE_LOCK_LEVEL_MASK)== PowerManager.PARTIAL_WAKE_LOCK) {boolean disabled = false;//設備處于Doze定義的device idle模式時if (mDeviceIdleMode) {final int appid = UserHandle.getAppId(wakeLock.mOwnerUid);// If we are in idle mode, we will ignore all partial wake locks that are// for application uids that are not whitelisted.//判斷是否為非系統應用 if (appid >= Process.FIRST_APPLICATION_UID &&//白名單searchArrays.binarySearch(mDeviceIdleWhitelist, appid) < 0 &&Arrays.binarySearch(mDeviceIdleTempWhitelist, appid) < 0 &&//判斷進程的類型//ActivityManager中定義的數字最小的為：常駐的操作UI的系統進程//因此大概可理解為：數字越大，對處理事件的時效性要求越低mUidState.get(wakeLock.mOwnerUid,ActivityManager.PROCESS_STATE_CACHED_EMPTY)> ActivityManager.PROCESS_STATE_FOREGROUND_SERVICE) {disabled = true;}}if (wakeLock.mDisabled != disabled) {wakeLock.mDisabled = disabled;return true;}}return false; }

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上面代碼大致的含義就是：?
在Android Doze模式下，當終端處于device Idle Mode時，?
對于一個非系統應用而言，如果該應用不在系統定義的白名單中，?
并且該應用所在進程的類型表明，該進程對事件處理的時效性要求不高，?
那么即使該應用申請了PARTIAL_WAKE_LOCK，也不能阻止系統進入休眠狀態。

有些設備商，為了優化系統的功耗，就修改了這個地方。?
例如，有些系統應用其實也很耗電，因此可以去掉該函數中對非系統應用的限制，對系統應用也進行管控。

3、處理WakeLock對應的Flag?
前面的代碼已經提到，當acquire WakeLock時，將調用applyWakeLockFlagsOnAcquireLocked處理WakeLock對應的flag；?
當由于進程死亡，釋放WakeLock時，會調用applyWakeLockFlagsOnReleaseLocked處理WakeLock對應的flag。?
從函數命名來看，這兩個函數應該有相似的地方。

3.1 applyWakeLockFlagsOnAcquireLocked?
我們先看看applyWakeLockFlagsOnAcquireLocked：

private void applyWakeLockFlagsOnAcquireLocked(WakeLock wakeLock, int uid) {//僅處理ACQUIRE_CAUSES_WAKEUP flag，同時要求WakeLock的level是與screen有關的，//即FULL_WAKE_LOCK、SCREEN_BRIGHT_WAKE_LOCK和SCREEN_DIM_WAKE_LOCKif ((wakeLock.mFlags & PowerManager.ACQUIRE_CAUSES_WAKEUP) != 0&& isScreenLock(wakeLock)) {..............wakeUpNoUpdateLocked(SystemClock.uptimeMillis(), wakeLock.mTag, opUid,opPackageName, opUid);} }private boolean wakeUpNoUpdateLocked(long eventTime, String reason, int reasonUid,String opPackageName, int opUid) {............//不滿足以下條件，沒有喚醒屏幕的必要if (eventTime < mLastSleepTime || mWakefulness == WAKEFULNESS_AWAKE|| !mBootCompleted || !mSystemReady) {return false;}try {mLastWakeTime = eventTime;//修改PMS的一些成員變量，并進行通知//其中主要的是將mDirty變量的DIRTY_WAKEFULNESS位置為了1//PMS根據mDirty的位信息管理電源狀態，同時喚醒屏幕setWakefulnessLocked(WAKEFULNESS_AWAKE, 0);//通知給電源統計服務mNotifier.onWakeUp(reason, reasonUid, opPackageName, opUid);//調用userActivityNoUpdateLocked函數userActivityNoUpdateLocked(eventTime, PowerManager.USER_ACTIVITY_EVENT_OTHER, 0, reasonUid);} .....return true; }

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3.1.1 setWakefulnessLocked?
我們看看喚醒屏幕相關的操作：

private void setWakefulnessLocked(int wakefulness, int reason) {if (mWakefulness != wakefulness) {mWakefulness = wakefulness;mWakefulnessChanging = true;mDirty |= DIRTY_WAKEFULNESS;//定義于frameworks/base/services/core/java/com/android/server/power/Notifier.java中mNotifier.onWakefulnessChangeStarted(wakefulness, reason);} }

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public void onWakefulnessChangeStarted(final int wakefulness, int reason) {final boolean interactive = PowerManagerInternal.isInteractive(wakefulness);.......// Tell the activity manager about changes in wakefulness, not just interactivity.mHandler.post(new Runnable() {@Overridepublic void run() {mActivityManagerInternal.onWakefulnessChanged(wakefulness);}});// Handle any early interactive state changes.// Finish pending incomplete ones from a previous cycle.if (mInteractive != interactive) {// Finish up late behaviors if needed.if (mInteractiveChanging) {handleLateInteractiveChange();}// Start input as soon as we start waking up or going to sleep.mInputManagerInternal.setInteractive(interactive);mInputMethodManagerInternal.setInteractive(interactive);// Notify battery stats.try {mBatteryStats.noteInteractive(interactive);} catch (RemoteException ex) { }// Handle early behaviors.mInteractive = interactive;mInteractiveChangeReason = reason;mInteractiveChanging = true;//重點在這個位置handleEarlyInteractiveChange();} }/** * Handle early interactive state changes such as getting applications or the lock * screen running and ready for the user to see (such as when turning on the screen). */ private void handleEarlyInteractiveChange() {synchronized (mLock) {if (mInteractive) {// Waking up...mHandler.post(new Runnable() {@Overridepublic void run() {EventLog.writeEvent(EventLogTags.POWER_SCREEN_STATE, 1, 0, 0, 0);//mPolicy對應于PhoneWindowManagermPolicy.startedWakingUp();}});// Send interactive broadcast.mPendingInteractiveState = INTERACTIVE_STATE_AWAKE;mPendingWakeUpBroadcast = true;updatePendingBroadcastLocked();} else {// Going to sleep...// Tell the policy that we started going to sleep.final int why = translateOffReason(mInteractiveChangeReason);mHandler.post(new Runnable() {@Overridepublic void run() {mPolicy.startedGoingToSleep(why);}});}} }

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從上面的代碼來看，應該是PhoneWindowManager完成亮屏前的初始化工作，然后回調到PowerManager的wakeUp函數。?
整個過程還是比較復雜的，需要單獨進行分析，此處不做進一步說明。

3.2 applyWakeLockFlagsOnReleaseLocked?
現在我們再看看applyWakeLockFlagsOnReleaseLocked函數：

private void applyWakeLockFlagsOnReleaseLocked(WakeLock wakeLock) {//僅處理ON_AFTER_RELEASE，同樣要求WakeLock的level是與screen有關的//ON_AFTER_RELEASE并不會立即息屏if ((wakeLock.mFlags & PowerManager.ON_AFTER_RELEASE) != 0&& isScreenLock(wakeLock)) {userActivityNoUpdateLocked(SystemClock.uptimeMillis(),PowerManager.USER_ACTIVITY_EVENT_OTHER,PowerManager.USER_ACTIVITY_FLAG_NO_CHANGE_LIGHTS,wakeLock.mOwnerUid);} }

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可以看出applyWakeLockFlagsOnAcquireLocked和applyWakeLockFlagsOnReleaseLocked最后均會調用userActivityNoUpdateLocked函數，只是參數不同。

3.3 userActivityNoUpdateLocked?
我們一起來看一下userActivityNoUpdateLocked：

private boolean userActivityNoUpdateLocked(long eventTime, int event, int flags, int uid) {.............//過時的事件不需要處理if (eventTime < mLastSleepTime || eventTime < mLastWakeTime|| !mBootCompleted || !mSystemReady) {return false;}...........try {if (eventTime > mLastInteractivePowerHintTime) {//調用native加載的動態庫的powerHint函數，具體意義不是很清楚powerHintInternal(POWER_HINT_INTERACTION, 0);mLastInteractivePowerHintTime = eventTime;}//調用BatteryStatsService的noteUserActivity函數，看代碼好像是做一些記錄mNotifier.onUserActivity(event, uid);//根據參數信息修改mDirty的一些變量.............} finally {........} }

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從以上代碼來看，acquire WakeLock將申請信息遞交給PMS統一進行處理。?
PMS根據WakeLock的level和flag，完成修改一些變量、通知BatteryStatsService等工作后，?
最終還是依賴于updatePowerStateLocked函數來進行實際的電源狀態更新操作。

PMS類中有很多***NoUpdateLocked()方法，這些方法都有一些共性，就是僅更新狀態，不負責具體的執行。因為PMS中具體的執行邏輯都是在updatePowerStateLocked方法中。

上述acquire WakeLock主要的工作大致可以總結為下圖：?

三、釋放WakeLock?
當進程完成工作后，需要釋放之前申請的WakeLock。我們同樣以RIL.java中的操作為例：

private void processResponse (Parcel p) {int type;type = p.readInt();if (type == RESPONSE_UNSOLICITED || type == RESPONSE_UNSOLICITED_ACK_EXP) {...........} else if (type == RESPONSE_SOLICITED || type == RESPONSE_SOLICITED_ACK_EXP) {//處理請求對應的回復信息RILRequest rr = processSolicited (p, type);if (rr != null) {if (type == RESPONSE_SOLICITED) {//重點在這里decrementWakeLock(rr);}rr.release();return;}} else if (type == RESPONSE_SOLICITED_ACK) {...........} }

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我們跟進decrementWakeLock函數：

private void decrementWakeLock(RILRequest rr) {synchronized(rr) {switch(rr.mWakeLockType) {case FOR_WAKELOCK:synchronized (mWakeLock) {//前面已經提到過，RIL.java多個請求復用同一個WakeLock//并且利用mWakeLockCount記錄復用的次數//這么設計的目的是：RIL發送請求的數量非常多，復用WakeLock可以避免多次構造釋放//同時減少與PMS之間Binder通信的次數if (mWakeLockCount > 1) {mWakeLockCount--;} else {mWakeLockCount = 0;//所有請求均得到了處理，調用PowerManager中WakeLock的release函數mWakeLock.release();}}break;........}}........ }

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現在我們跟進PowerManager中WakeLock定義的release函數：

/** * Releases the wake lock with flags to modify the release behavior. * * This method releases your claim to the CPU or screen being on. * The screen may turn off shortly after you release the wake lock, or it may * not if there are other wake locks still held. * */ public void release(int flags) {synchronized (mToken) {if (!mRefCounted || --mCount == 0) {mHandler.removeCallbacks(mReleaser);if (mHeld) {.......try {//還是會調用到PMS中的函數mService.releaseWakeLock(mToken, flags);} catch (RemoteException e) {throw e.rethrowFromSystemServer();}mHeld = false;}}....} }

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最后一起來看看PMS中釋放WakeLock的函數：

public void releaseWakeLock(IBinder lock, int flags) {//參數和權限檢查.............final long ident = Binder.clearCallingIdentity();try {releaseWakeLockInternal(lock, flags);} finally {Binder.restoreCallingIdentity(ident);} }private void releaseWakeLockInternal(IBinder lock, int flags) {synchronized (mLock) {//根據Binder代理，從存儲的ArrayList中找到對應WakeLock的序號int index = findWakeLockIndexLocked(lock);...........WakeLock wakeLock = mWakeLocks.get(index);...........//RELEASE_FLAG_WAIT_FOR_NO_PROXIMITY，表示當sensor判斷終端離物體較遠時，//才真正釋放PROXIMITY_SCREEN_OFF_WAKE_LOCK等級的WakeLockif ((flags & PowerManager.RELEASE_FLAG_WAIT_FOR_NO_PROXIMITY) != 0) {mRequestWaitForNegativeProximity = true;}//PMS不再關注客戶端進程是否死亡wakeLock.mLock.unlinkToDeath(wakeLock, 0);removeWakeLockLocked(wakeLock, index);} }private void removeWakeLockLocked(WakeLock wakeLock, int index) {mWakeLocks.remove(index);//通知BatteryStatsServicenotifyWakeLockReleasedLocked(wakeLock);//之前分析過，會做一些記錄信息等applyWakeLockFlagsOnReleaseLocked(wakeLock);mDirty |= DIRTY_WAKE_LOCKS;//依然靠updatePowerStateLocked函數更新終端的電源狀態updatePowerStateLocked(); }

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整個release的過程大致可以總結為下圖：?

四、總結?
通過前面的分析，我們知道了向PMS申請電量的基本用法類似于：

........ //1、創建 PowerManager pm = (PowerManager)context.getSystemService(Context.POWER_SERVICE); mWakeLock = pm.newWakeLock(PowerManager.PARTIAL_WAKE_LOCK, RILJ_LOG_TAG); ...... //2、acquire mWakeLock.acquire(); ......... //3、release mWakeLock.release(); ...........

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當申請發送到PMS后，PMS將針對WakeLock的level和flag信息進行一些處理。?
無論是acquire還是release WakeLock，PMS最終將利用updatePowerStateLocked函數對終端的電源狀態進行調整。?

我們將單獨分析一下PMS核心的updatePowerStateLocked函數。

原文地址：http://blog.csdn.net/gaugamela/article/details/52813400

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