android:persistent属性

为什么写这篇文章呢？前段时间在研究telephony时，一直没有在framework下发现对telephony的初始化(PhoneFactory.java中的makeDefaultPhones函数)的调用。结果全局搜索之后发现在application PhoneApp(packages/apps/Phone)中调用了。但是application PhoneApp既没有被Broadcast唤醒，也没有被其他service调用，那么是android是通过什么方式来启动PhoneApp，所以就发现了属性android:persistent。

在AndroidManifest.xml定义中，application有这么一个属性android:persistent，根据字面意思来理解就是说该应用是可持久的，也即是常驻的应用。其实就是这么个理解，被android:persistent修饰的应用会在系统启动之后被AM启动。

AM首先去PM(PackageManagerService)中去查找设置了android:persistent的应用。

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publicvoidsystemReady(finalRunnablegoingCallback){
if(mFactoryTest!=SystemServer.FACTORY_TEST_LOW_LEVEL){
try{
Listapps=AppGlobals.getPackageManager().
getPersistentApplications(STOCK_PM_FLAGS);
if(apps!=null){
intN=apps.size();
inti;
for(i=0;i<N;i++){
ApplicationInfoinfo
=(ApplicationInfo)apps.get(i);
if(info!=null&&
!info.packageName.equals("android")){
addAppLocked(info);
}
}
}
}catch(RemoteExceptionex){
//pmisinsameprocess,thiswillneverhappen.
}
}
}

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. } } }

假如该被android:persistent修饰的应用此时并未运行的话，那么AM将调用startProcessLocked启动该app，关于startProcessLocked不再描述，另外一篇文章《How to start a new process for Android?》中做了详细的介绍。

app的启动过程就是启动app所在的package对应的进程。

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finalProcessRecordaddAppLocked(ApplicationInfoinfo){
ProcessRecordapp=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,"addedapplication",app.processName);
}
returnapp;
}

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; }

下面介绍app所在的package对应的进程启动完成之后，app是如何被create的。

从文章《How to start a new process for Android?》中可知，zygote在创建新的进程均会启动它的mainThread android.app.ActivityThread，因此我们从ActivityThread的main函数中接着分析app的create过程。

在main中有下面这个操作

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thread.attach(false);

thread.attach(false);

在attach过程中，ActivityThread会将对应的application attach到AM中去，交与AM去管理。这里需要注意一个变量

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finalApplicationThreadmAppThread=newApplicationThread();

final ApplicationThread mAppThread = new ApplicationThread();

mAppThread是一个ApplicationThread对象，mAppThread可以看作是当前进程主线程的核心，它负责处理本进程与其他进程(主要是AM)之间的通信，同时通过attachApplication将mAppThread的代理Binder传递给AM。

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privatefinalvoidattach(booleansystem){
sThreadLocal.set(this);
mSystemThread=system;
if(!system){
ViewRoot.addFirstDrawHandler(newRunnable(){
publicvoidrun(){
ensureJitEnabled();
}
});
android.ddm.DdmHandleAppName.setAppName("<pre-initialized>");
RuntimeInit.setApplicationObject(mAppThread.asBinder());
IActivityManagermgr=ActivityManagerNative.getDefault();
try{
mgr.attachApplication(mAppThread);
}catch(RemoteExceptionex){
}
}
}

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) { } } }

上面的attach代码中，我们顺着IPC调用AM的attachApplication过程再往下看。
在该过程中，AM调用到了IPC通信调用mAppThread的bindApplication；

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privatefinalbooleanattachApplicationLocked(IApplicationThreadthread,
intpid){
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(); }

mAppThread的bindApplication再通过消息机制向ActivityThread自身维护的handler发送BIND_APPLICATION消息。下面看看ActivityThread自身维护的handler对消息BIND_APPLICATION的处理，最终会调用到handleBindApplication函数
你会发现在handleBindApplication函数中有这么一句

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mInstrumentation.callApplicationOnCreate(app);

mInstrumentation.callApplicationOnCreate(app);
我们最终在绕了好大一圈之后，调用了app的onCreate函数来启动这个application

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