為什么寫這篇文章呢？前段時間在研究telephony時，一直沒有在framework下發現對telephony的初始化(PhoneFactory.java中的makeDefaultPhones函數)的調用。結果全局搜索之后發現在application PhoneApp(packages/apps/Phone)中調用了。但是application PhoneApp既沒有被Broadcast喚醒，也沒有被其他service調用，那么是android是通過什么方式來啟動PhoneApp，所以就發現了屬性android:persistent。

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??? 在AndroidManifest.xml定義中，application有這么一個屬性android:persistent，根據字面意思來理解就是說該應用是可持久的，也即是常駐的應用。其實就是這么個理解，被android:persistent修飾的應用會在系統啟動之后被AM啟動。

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??? AM首先去PM(PackageManagerService)中去查找設置了android:persistent的應用。

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public?void?systemReady(final?Runnable?goingCallback)?{??

????????????if?(mFactoryTest?!=?SystemServer.FACTORY_TEST_LOW_LEVEL)?{??

????????????????try?{??

????????????????????List?apps?=?AppGlobals.getPackageManager().??

????????????????????????getPersistentApplications(STOCK_PM_FLAGS);??

????????????????????if?(apps?!=?null)?{??

????????????????????????int?N?=?apps.size();??

????????????????????????int?i;??

????????????????????????for?(i=0;?i<N;?i++)?{??

????????????????????????????ApplicationInfo?info??

????????????????????????????????=?(ApplicationInfo)apps.get(i);??

????????????????????????????if?(info?!=?null?&&??

????????????????????????????????????!info.packageName.equals("android"))?{??

????????????????????????????????addAppLocked(info);??

????????????????????????????}??

????????????????????????}??

????????????????????}??

????????????????}?catch?(RemoteException?ex)?{??

????????????????????//?pm?is?in?same?process,?this?will?never?happen. ??

????????????????}??

????????????}?????

}??

public void systemReady(final Runnable goingCallback) {if (mFactoryTest != SystemServer.FACTORY_TEST_LOW_LEVEL) {try {List apps = AppGlobals.getPackageManager().getPersistentApplications(STOCK_PM_FLAGS);if (apps != null) {int N = apps.size();int i;for (i=0; i<N; i++) {ApplicationInfo info= (ApplicationInfo)apps.get(i);if (info != null &&!info.packageName.equals("android")) {addAppLocked(info);}}}} catch (RemoteException ex) {// pm is in same process, this will never happen.}} }

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??? 假如該被android:persistent修飾的應用此時并未運行的話，那么AM將調用startProcessLocked啟動該app，關于startProcessLocked不再描述，另外一篇文章《How to start a new process for Android?》中做了詳細的介紹。

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??? app的啟動過程就是啟動app所在的package對應的進程。

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???

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final?ProcessRecord?addAppLocked(ApplicationInfo?info)?{??

????????ProcessRecord?app?=?getProcessRecordLocked(info.processName,?info.uid);??

????????if?(app?==?null)?{??

????????????app?=?newProcessRecordLocked(null,?info,?null);??

????????????mProcessNames.put(info.processName,?info.uid,?app);??

????????????updateLruProcessLocked(app,?true,?true);??

????????}??

????????if?((info.flags&(ApplicationInfo.FLAG_SYSTEM|ApplicationInfo.FLAG_PERSISTENT))??

????????????????==?(ApplicationInfo.FLAG_SYSTEM|ApplicationInfo.FLAG_PERSISTENT))?{??

????????????app.persistent?=?true;??

????????????app.maxAdj?=?CORE_SERVER_ADJ;??

????????}??

????????if?(app.thread?==?null?&&?mPersistentStartingProcesses.indexOf(app)?<?0)?{??

????????????mPersistentStartingProcesses.add(app);??

????????????startProcessLocked(app,?"added?application",?app.processName);??

????????}??

????????return?app;??

????}??

final ProcessRecord addAppLocked(ApplicationInfo info) {ProcessRecord app = getProcessRecordLocked(info.processName, info.uid);if (app == null) {app = newProcessRecordLocked(null, info, null);mProcessNames.put(info.processName, info.uid, app);updateLruProcessLocked(app, true, true);}if ((info.flags&(ApplicationInfo.FLAG_SYSTEM|ApplicationInfo.FLAG_PERSISTENT))== (ApplicationInfo.FLAG_SYSTEM|ApplicationInfo.FLAG_PERSISTENT)) {app.persistent = true;app.maxAdj = CORE_SERVER_ADJ;}if (app.thread == null && mPersistentStartingProcesses.indexOf(app) < 0) {mPersistentStartingProcesses.add(app);startProcessLocked(app, "added application", app.processName);}return app;}

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??? 下面介紹app所在的package對應的進程啟動完成之后，app是如何被create的。

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??? 從文章《How to start a new process for Android?》中可知，zygote在創建新的進程均會啟動它的mainThread android.app.ActivityThread，因此我們從ActivityThread的main函數中接著分析app的create過程。

??? 在main中有下面這個操作

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thread.attach(false);??

thread.attach(false);

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??? 在attach過程中，ActivityThread會將對應的application attach到AM中去，交與AM去管理。這里需要注意一個變量

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final?ApplicationThread?mAppThread?=?new?ApplicationThread();??

final ApplicationThread mAppThread = new ApplicationThread();

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??? mAppThread是一個ApplicationThread對象，mAppThread可以看作是當前進程主線程的核心，它負責處理本進程與其他進程(主要是AM)之間的通信，同時通過attachApplication將mAppThread的代理Binder傳遞給AM。

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private?final?void?attach(boolean?system)?{??

????????sThreadLocal.set(this);??

????????mSystemThread?=?system;??

????????if?(!system)?{??

????????????ViewRoot.addFirstDrawHandler(new?Runnable()?{??

????????????????public?void?run()?{??

????????????????????ensureJitEnabled();??

????????????????}??

????????????});??

????????????android.ddm.DdmHandleAppName.setAppName("<pre-initialized>");??

????????????RuntimeInit.setApplicationObject(mAppThread.asBinder());??

????????????IActivityManager?mgr?=?ActivityManagerNative.getDefault();??

????????????try?{??

????????????????mgr.attachApplication(mAppThread);??

????????????}?catch?(RemoteException?ex)?{??

????????????}??

????????}??

????}??

private final void attach(boolean system) {sThreadLocal.set(this);mSystemThread = system;if (!system) {ViewRoot.addFirstDrawHandler(new Runnable() {public void run() {ensureJitEnabled();}});android.ddm.DdmHandleAppName.setAppName("<pre-initialized>");RuntimeInit.setApplicationObject(mAppThread.asBinder());IActivityManager mgr = ActivityManagerNative.getDefault();try {mgr.attachApplication(mAppThread);} catch (RemoteException ex) {}}}

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??? 上面的attach代碼中，我們順著IPC調用AM的attachApplication過程再往下看。
??? 在該過程中，AM調用到了IPC通信調用mAppThread的bindApplication；

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private?final?boolean?attachApplicationLocked(IApplicationThread?thread,??

????????????int?pid)?{??

??????

????????????thread.bindApplication(processName,?app.instrumentationInfo?!=?null??

??????????????????????app.instrumentationInfo?:?app.info,?providers,??

????????????????????app.instrumentationClass,?app.instrumentationProfileFile,??

????????????????????app.instrumentationArguments,?app.instrumentationWatcher,?testMode,???

????????????????????isRestrictedBackupMode?||?!normalMode,??

????????????????????mConfiguration,?getCommonServicesLocked());??

????????????updateLruProcessLocked(app,?false,?true);??

????????????app.lastRequestedGc?=?app.lastLowMemory?=?SystemClock.uptimeMillis();??

}??

private final boolean attachApplicationLocked(IApplicationThread thread,int pid) {thread.bindApplication(processName, app.instrumentationInfo != null? app.instrumentationInfo : app.info, providers,app.instrumentationClass, app.instrumentationProfileFile,app.instrumentationArguments, app.instrumentationWatcher, testMode, isRestrictedBackupMode || !normalMode,mConfiguration, getCommonServicesLocked());updateLruProcessLocked(app, false, true);app.lastRequestedGc = app.lastLowMemory = SystemClock.uptimeMillis(); }

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??? mAppThread的bindApplication再通過消息機制向ActivityThread自身維護的handler發送BIND_APPLICATION消息。下面看看ActivityThread自身維護的handler對消息BIND_APPLICATION的處理，最終會調用到handleBindApplication函數
??? 你會發現在handleBindApplication函數中有這么一句

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mInstrumentation.callApplicationOnCreate(app);??

mInstrumentation.callApplicationOnCreate(app); ????
??? 我們最終在繞了好大一圈之后，調用了app的onCreate函數來啟動這個application

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