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说说Android应用的persistent属性

说说Android应用的persistent属性

 

侯 亮

 

 

1 启动persistent应用

    在Android系统中,有一种永久性应用。它们对应的AndroidManifest.xml文件里,会将persistent属性设为true,比如:

<applicationandroid:name="PhoneApp"
android:persistent="true"
android:label="@string/dialerIconLabel"
android:icon="@drawable/ic_launcher_phone">


    在系统启动之时,AMS的systemReady()会加载所有persistent为true的应用。

publicvoid systemReady(finalRunnable goingCallback) 
{
    . . . . . .
    . . . . . .
    try{
        List apps = AppGlobals.getPackageManager().
                        getPersistentApplications(STOCK_PM_FLAGS);
        if(apps != null)
        {
            intN = apps.size();
            inti;
             
            for(i=0; i<N; i++) 
            {
                ApplicationInfo info = (ApplicationInfo)apps.get(i);
                if(info != null&&
                        !info.packageName.equals("android"))
                {
                    addAppLocked(info,false);
                }
            }
        }
    }
    catch(RemoteException ex) {
        // pm is in same process, this will never happen.
    }

其中的STOCK_PM_FLAGS的定义如下:

// The flags that are set for all calls we make to the package manager.
staticfinal int STOCK_PM_FLAGS = PackageManager.GET_SHARED_LIBRARY_FILES;


上面代码中的getPersistentApplications()函数的定义如下:

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publicList<ApplicationInfo> getPersistentApplications(intflags)
{
    finalArrayList<ApplicationInfo> finalList = newArrayList<ApplicationInfo>();
 
 
    // reader
    synchronized(mPackages)
    {
        finalIterator<PackageParser.Package> i = mPackages.values().iterator();
        finalintuserId = UserId.getCallingUserId();
        while(i.hasNext())
        {
            finalPackageParser.Package p = i.next();
            if(p.applicationInfo != null
                && (p.applicationInfo.flags & ApplicationInfo.FLAG_PERSISTENT) != 0
                && (!mSafeMode || isSystemApp(p)))
            {
                PackageSetting ps = mSettings.mPackages.get(p.packageName);
                finalList.add(PackageParser.generateApplicationInfo(p, flags,
                        ps != null? ps.getStopped(userId) : false,
                        ps != null? ps.getEnabled(userId) : COMPONENT_ENABLED_STATE_DEFAULT,
                        userId));
            }
        }
    }
 
    returnfinalList;
}


     在PKMS中,有一个记录所有的程序包信息的哈希表(mPackages),每个表项中含有ApplicationInfo信息,该信息的flags(int型)数据中有一个专门的bit用于表示persistent。getPersistentApplications()函数会遍历这张表,找出所有persistent包,并返回ArrayList<ApplicationInfo>。


     从代码里可以看出,带persistent标志的系统应用(即flags中设置了FLAG_SYSTEM)是一定会被选上的,但如果不是系统应用的话,则要进一步判断当前是否处于“安全模式”,一旦处于安全模式,那么就算应用设置了persistent属性,也不会被选中。

     随后systemReady()开始遍历选中的ApplicationInfo,并对包名不为“android”的结点执行addAppLocked()。addAppLocked()的代码如下:

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finalProcessRecord addAppLocked(ApplicationInfo info, booleanisolated)
{
    ProcessRecord app;
    if(!isolated) {
        app = getProcessRecordLocked(info.processName, info.uid);
    }else{
        app = null;
    }
 
    if(app == null) {
        app = newProcessRecordLocked(null, info, null, isolated);
        mProcessNames.put(info.processName, app.uid, app);
        if(isolated) {
            mIsolatedProcesses.put(app.uid, app);
        }
        updateLruProcessLocked(app,true,true);
    }
 
    // This package really, really can not be stopped.
    try{
        AppGlobals.getPackageManager().setPackageStoppedState(
                info.packageName,false, UserId.getUserId(app.uid));
    }catch(RemoteException e) {
    }catch(IllegalArgumentException e) {
        Slog.w(TAG,"Failed trying to unstop package "
                + info.packageName + ": " + e);
    }
 
    if((info.flags&(ApplicationInfo.FLAG_SYSTEM|ApplicationInfo.FLAG_PERSISTENT))
            == (ApplicationInfo.FLAG_SYSTEM|ApplicationInfo.FLAG_PERSISTENT)) {
        app.persistent = true;
        app.maxAdj = ProcessList.PERSISTENT_PROC_ADJ;
    }
    if(app.thread == null&& mPersistentStartingProcesses.indexOf(app) < 0) {
        mPersistentStartingProcesses.add(app);
        startProcessLocked(app,"added application", app.processName);
    }
 
    returnapp;
}


    在AMS中,所谓的“add App”主要是指“添加一个与App进程对应的ProcessRecord节点”。当然,如果该节点已经添加过了,那么是不会重复添加的。在添加节点的动作完成以后,addAppLocked()还会检查App进程是否已经启动好了,如果尚未开始启动,此时就会调用startProcessLocked()启动这个进程。既然addAppLocked()试图确认App“正在正常运作”或者“将被正常启动”,那么其对应的package就不可能处于stopped状态,这就是上面代码调用setPackageStoppedState(...,false,...)的意思。


    现在,我们就清楚了,那些persistent属性为true的应用,基本上都是在系统启动伊始就启动起来的。

    因为启动进程的过程是异步的,所以我们需要一个缓冲列表(即上面代码中的mPersistentStartingProcesses列表)来记录那些“正处于启动状态,而又没有启动完毕的”ProcessRecord结点。一旦目标进程启动完毕后,目标进程会attach系统,于是走到AMS的attachApplicationLocked(),在这个函数里,会把目标进程对应的ProcessRecord结点从mPersistentStartingProcesses缓冲列表里删除。

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privatefinalboolean attachApplicationLocked(IApplicationThread thread, intpid) {
 
        // Find the application record that is being attached...  either via
        // the pid if we are running in multiple processes, or just pull the
        // next app record if we are emulating process with anonymous threads.
        ProcessRecord app;
         
        . . . . . .
        thread.asBinder().linkToDeath(adr,0);
        . . . . . .
        thread.bindApplication(processName, appInfo, providers,
                    app.instrumentationClass, profileFile, profileFd, profileAutoStop,
                    app.instrumentationArguments, app.instrumentationWatcher, testMode,
                    enableOpenGlTrace, isRestrictedBackupMode || !normalMode,
                    app.persistent,
                    newConfiguration(mConfiguration), app.compat,
                    getCommonServicesLocked(),
                    mCoreSettingsObserver.getCoreSettingsLocked());
        . . . . . .
        . . . . . .
        // Remove this record from the list of starting applications.
        mPersistentStartingProcesses.remove(app);
        . . . . . .

2 如何保证应用的持久性(persistent)

    我们知道,persistent一词的意思是“持久”,那么persistent应用的意思又是什么呢?简单地说,这种应用会顽固地运行于系统之中,从系统一启动,一直到系统关机。

    为了保证这种持久性,persistent应用必须能够在异常出现时,自动重新启动。在Android里是这样实现的。每个ActivityThread中会有一个专门和AMS通信的binder实体——final ApplicationThread mAppThread。这个实体在AMS中对应的代理接口为IApplicationThread。

    当AMS执行到attachApplicationLocked()时,会针对目标用户进程的IApplicationThread接口,注册一个binder讣告监听器,一旦日后用户进程意外挂掉,AMS就能在第一时间感知到,并采取相应的措施。如果AMS发现意外挂掉的应用是persistent的,它会尝试重新启动这个应用。

    注册讣告监听器的代码如下:

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AppDeathRecipient adr = newAppDeathRecipient(app, pid, thread);
thread.asBinder().linkToDeath(adr,0);
app.deathRecipient = adr;

其中的thread就是IApplicationThread代理。

    AppDeathRecipient的定义如下:

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privatefinalclass AppDeathRecipient implementsIBinder.DeathRecipient
{
    finalProcessRecord mApp;
    finalintmPid;
    finalIApplicationThread mAppThread;
 
    AppDeathRecipient(ProcessRecord app, intpid,
            IApplicationThread thread)
    {
        if(localLOGV)
            Slog.v(TAG,"New death recipient " + this
                   +" for thread " + thread.asBinder());
        mApp = app;
        mPid = pid;
        mAppThread = thread;
    }
 
    publicvoidbinderDied()
    {
        if(localLOGV)
            Slog.v(TAG,"Death received in " + this
                   +" for thread " + mAppThread.asBinder());
        synchronized(ActivityManagerService.this)
        {
            appDiedLocked(mApp, mPid, mAppThread);
        }
    }
}

当其监听的binder实体死亡时,系统会回调AppDeathRecipient的binderDied()。这个回调函数会辗转重启persistent应用,调用关系如下:

技术分享


    一般情况下,当一个应用进程挂掉后,AMS当然会清理掉其对应的ProcessRecord,这就是cleanUpApplicationRecordLocked()的主要工作。然而,对于persistent应用,cleanUpApplicationRecordLocked()会尝试再次启动对应的应用进程。代码截选如下:

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privatefinalvoid cleanUpApplicationRecordLocked(ProcessRecord app,
                                                  booleanrestarting,
                                                  booleanallowRestart,intindex)
{
    . . . . . .
    . . . . . .
    if(!app.persistent || app.isolated)
    {
        . . . . . .
        mProcessNames.remove(app.processName, app.uid);
        mIsolatedProcesses.remove(app.uid);
        . . . . . .
    }
    elseif(!app.removed)
    {
        if(mPersistentStartingProcesses.indexOf(app) < 0) {
            mPersistentStartingProcesses.add(app);
            restart = true;
        }
    }
    . . . . . .
    . . . . . .
    if(restart && !app.isolated)
    {
        mProcessNames.put(app.processName, app.uid, app);
        startProcessLocked(app,"restart", app.processName);
    }
    elseif(app.pid > 0&& app.pid != MY_PID)
    {
        . . . . . .
    }
    . . . . . .
}

    现在我们可以画一张关于“启动persistent应用”的示意图:

技术分享

 

3 补充知识点


3.1 persistent应用可以在系统未准备好时启动

    在AMS中,有一个isAllowedWhileBooting()函数,其代码如下:

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booleanisAllowedWhileBooting(ApplicationInfo ai)
{
    return(ai.flags & ApplicationInfo.FLAG_PERSISTENT) != 0;
}


    从这个函数可以看到,将persistent属性设为true的应用,是允许在boot的过程中启动的。我们可以查看前文提到的startProcessLocked()函数:

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finalProcessRecord startProcessLocked(String processName,
                                       ApplicationInfo info, booleanknownToBeDead,
                                       intintentFlags,
                                       String hostingType, ComponentName hostingName,
                                       booleanallowWhileBooting,
                                       booleanisolated)
{
    ProcessRecord app;
     
    if(!isolated)
    {
        app = getProcessRecordLocked(processName, info.uid);
    }
    else
    {
        // If this is an isolated process, it can‘t re-use an existing process.
        app = null;
    }
 
    . . . . . .
    . . . . . .
     
    if(!mProcessesReady
        && !isAllowedWhileBooting(info)
        && !allowWhileBooting) {
        if(!mProcessesOnHold.contains(app)) {
            mProcessesOnHold.add(app);
        }
        if(DEBUG_PROCESSES) Slog.v(TAG, "System not ready, putting on hold: " + app);
        returnapp;
    }
 
    startProcessLocked(app, hostingType, hostingNameStr);
    return(app.pid != 0) ? app : null;
}

其中的最后几句可以改写为以下更易理解的形式:

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if (mProcessesReady || isAllowedWhileBooting(info) || allowWhileBooting)
{
    startProcessLocked(app, hostingType, hostingNameStr);
    return(app.pid != 0) ? app : null;
}
else
{
    . . . . . .
    returnapp;
}


    也就是说,当系统已经处于以下几种情况时,多参数的startProcessLocked()会进一步调用另一个只有三个参数的startProcessLocked():
1)系统已经处于ready状态;
2)想要启动persistent应用;
3)参数中明确指定可以在boot过程中启动应用。

    补充说一下,一般情况下,当AMS调用startProcessLocked()时,传入的allowWhileBooting参数都为false。比如说,当系统需要启动“某个content provider或者某个service或者某个特定activity”时,此时传给startProcessLocked()的allowWhileBooting参数是写死为false的。只有一种特殊情况下会在该参数中传入true,那就是当系统发出的广播intent中携带有Intent.FLAG_RECEIVER_BOOT_UPGRADE标记时,此时允许在系统未ready时,启动接受广播的目标进程。

4 结束

    有关Android应用的persistent属性,我们就先说这么多。希望对大家有点儿帮助。

说说Android应用的persistent属性