在Android程序開發(fā)時我們會接觸到一些系統(tǒng)為了某些功能而定義的關(guān)鍵屬性，例如在AndroidManifest.xml文件中
經(jīng)?？吹降膒ersistent、process等，下面是自己對persistent關(guān)鍵字的分析，直奔主題。
一、persistent屬性作用
1、定義
該屬性的定義在frameworks/base/core/res/res/values/attrs_manifest.xml中，其定義如下：
<!-- Flag to control special persistent mode of an application. This should
not normally be used by applications; it requires that the system keep
your application running at all times. -->
<attr name="persistent" format="boolean" />
通過官方注釋我知道該屬性用于是否讓你的應(yīng)用一直處于運(yùn)行狀態(tài)（通常說的常駐內(nèi)存）。設(shè)置
該屬性為true的app具有如下特點(diǎn)：
在系統(tǒng)啟動的時候會被系統(tǒng)啟動起來
在該app被強(qiáng)制殺掉后系統(tǒng)會重新啟動該app，這種情況只針對系統(tǒng)內(nèi)置app，第三方安裝的app不會被重啟
2、使用
persistent屬性是用于application標(biāo)簽上的，用法為：
AndroidManifest.xml
<application
android:persistent="true|false"
>
</application>
persistent的值默認(rèn)為false
二、原理分析
通過第一點(diǎn)對persistent的功能說明后我們通過源碼來分析一下它的工作原理
1、persistent屬性的解析
該屬性的解析主要在app被安裝或者系統(tǒng)啟動的時候發(fā)生
解析代碼：
frameworks/base/core/java/com/android/content/pm/PackageParser.java
private boolean parseBaseApplication(Package owner, Resources res,
XmlResourceParser parser, int flags, String[] outError)
throws XmlPullParserException, IOException {
final ApplicationInfo ai = owner.applicationInfo;
//.......................
if ((flags&PARSE_IS_SYSTEM) != 0) {
if (sa.getBoolean(
com.android.internal.R.styleable.AndroidManifestApplication_persistent,
false)) {
ai.flags |= ApplicationInfo.FLAG_PERSISTENT;
}
}
//.............
}
在解析完包信息之后系統(tǒng)會將解析好的所有包信息存放到PKMS中的mPackages的map中,而ApplicationInfo的flag中有一個bit位用于保存該app是否是persistent的。這里只是把保存persistent的flag設(shè)置為FLAG_PERSISTENT。在AndroidManifest設(shè)置了persistent為true的app是否能夠在被異常殺死后能夠得到重啟的權(quán)力需要取決于該app對應(yīng)的ProcessRecord的persistent屬性，該屬性只有在你的app既在AndroidManifest中配置了persistent=“true”，又是系統(tǒng)內(nèi)置app時才會被設(shè)置為true。
2、系統(tǒng)啟動時啟動persistent為true的app
frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java
在系統(tǒng)啟動時ActivityManagerService的systemReady（）方法會將所有在AndroidManifest設(shè)置了persistent為true的app拉起來
public void systemReady(final Runnable goingCallback) {
......
synchronized (this) {
// Only start up encryption-aware persistent apps; once user is
// unlocked we'll come back around and start unaware apps
startPersistentApps(PackageManager.MATCH_DIRECT_BOOT_AWARE);
// Start up initial activity.
mBooting = true;
// Enable home activity for system user, so that the system can always boot
if (UserManager.isSplitSystemUser()) {
ComponentName cName = new ComponentName(mContext, SystemUserHomeActivity.class);
try {
AppGlobals.getPackageManager().setComponentEnabledSetting(cName,
PackageManager.COMPONENT_ENABLED_STATE_ENABLED, 0,
UserHandle.USER_SYSTEM);
} catch (RemoteException e) {
throw e.rethrowAsRuntimeException();
}
}
......
}
systemReady中調(diào)用了startPersistentApps() 方法
private void startPersistentApps(int matchFlags) {
if (mFactoryTest == FactoryTest.FACTORY_TEST_LOW_LEVEL) return;
synchronized (this) {
try {
final List<ApplicationInfo> apps = AppGlobals.getPackageManager()
.getPersistentApplications(STOCK_PM_FLAGS | matchFlags).getList();
for (ApplicationInfo app : apps) {
if (!"android".equals(app.packageName)) {
addAppLocked(app, false, null /* ABI override */);
}
}
} catch (RemoteException ex) {
}
}
}
在startPersistentApps方法中首先是調(diào)用PackageManageServices的getPersistentApplications方法獲取到所有在AndroidManifest設(shè)置了persistent為true的app，然后調(diào)用addAppLocked方法去啟動他們。這樣在AndroidManifest設(shè)置了persistent為true的app就隨著系統(tǒng)的啟動而啟動了。
下面看一下getPersistentApplications方法,該方法調(diào)用了PKMS中的getPersistentApplicationsInternal方法。
private @NonNull List<ApplicationInfo> getPersistentApplicationsInternal(int flags) {
final ArrayList<ApplicationInfo> finalList = new ArrayList<ApplicationInfo>();
// reader
synchronized (mPackages) {
final Iterator<PackageParser.Package> i = mPackages.values().iterator();
final int userId = UserHandle.getCallingUserId();
while (i.hasNext()) {
final PackageParser.Package p = i.next();
if (p.applicationInfo == null) continue;
final boolean matchesUnaware = ((flags & MATCH_DIRECT_BOOT_UNAWARE) != 0)
&& !p.applicationInfo.isDirectBootAware();
final boolean matchesAware = ((flags & MATCH_DIRECT_BOOT_AWARE) != 0)
&& p.applicationInfo.isDirectBootAware();
if ((p.applicationInfo.flags & ApplicationInfo.FLAG_PERSISTENT) != 0
&& (!mSafeMode || isSystemApp(p))
&& (matchesUnaware || matchesAware)) {
PackageSetting ps = mSettings.mPackages.get(p.packageName);
if (ps != null) {
ApplicationInfo ai = PackageParser.generateApplicationInfo(p, flags,
ps.readUserState(userId), userId);
if (ai != null) {
finalList.add(ai);
}
}
}
}
}
return finalList;
}
該方法會遍歷mPackages中的所有app，并找到其中在AndroidManifest設(shè)置了persistent為true的應(yīng)用。從代碼中可以看到，persistent為true并且是系統(tǒng)app的話一定會被選中，但是如果是第三方安裝的應(yīng)用的話只能在非“安全模式”下才會被選中。
之后調(diào)用addAppLocked方法啟動app:
final ProcessRecord addAppLocked(ApplicationInfo info, boolean isolated,
String abiOverride) {
ProcessRecord app;
//傳遞進(jìn)來的isolated=false，所有一定會調(diào)用getProcessRecordLocked方法，但是由于是第一次啟動，所有返回的app = null
if (!isolated) {
app = getProcessRecordLocked(info.processName, info.uid, true);
} else {
app = null;
}
if (app == null) {
//為新的app創(chuàng)建新的ProcessRecord對象
app = newProcessRecordLocked(info, null, isolated, 0);
updateLruProcessLocked(app, false, null);
updateOomAdjLocked();
}
// This package really, really can not be stopped.
try {
//由于是開機(jī)第一次啟動，所以新的app的啟動狀態(tài)是將要被啟動狀態(tài)，所以
//該app的停止?fàn)顟B(tài)stoped被設(shè)置為false
AppGlobals.getPackageManager().setPackageStoppedState(
info.packageName, false, UserHandle.getUserId(app.uid));
} catch (RemoteException e) {
} catch (IllegalArgumentException e) {
Slog.w(TAG, "Failed trying to unstop package "
+ info.packageName + ": " + e);
}
//在這里對persistent的app進(jìn)行過濾，只有既是系統(tǒng)app，persistent為true的app才會在
//異常死亡之后被重啟
if ((info.flags & PERSISTENT_MASK) == PERSISTENT_MASK) {
app.persistent = true;
app.maxAdj = ProcessList.PERSISTENT_PROC_ADJ;
}
//如果該app已經(jīng)啟動了，則不用處理，否則調(diào)用startProcessLocked方法啟動app。
//由于啟動app是異步進(jìn)行的，會將正在啟動而還沒有啟動完成的app添加到
//mPersistentStartingProcesses列表中。當(dāng)啟動完成后 再移除
if (app.thread == null && mPersistentStartingProcesses.indexOf(app) < 0) {
mPersistentStartingProcesses.add(app);
//啟動該app
startProcessLocked(app, "added application", app.processName, abiOverride,
null /* entryPoint */, null /* entryPointArgs */);
}
return app;
}
}
接下來調(diào)用startProcessLocked方法啟動app進(jìn)程，在app啟動完成后會在ActivityThread中調(diào)用AMS的attachApplication，將該app從mPersistentStartingProcesses中移除，并注冊一個死亡訃告監(jiān)聽器AppDeathRecipient，用于在app異常被殺后的處理工作。
private final boolean attachApplicationLocked(IApplicationThread thread,
int pid) {
......
try {
//注冊死亡訃告監(jiān)聽器AppDeathRecipient
AppDeathRecipient adr = new AppDeathRecipient(
app, pid, thread);
thread.asBinder().linkToDeath(adr, 0);
app.deathRecipient = adr;
} catch (RemoteException e) {
app.resetPackageList(mProcessStats);
startProcessLocked(app, "link fail", processName);
return false;
}
......
}
3、app被異常結(jié)束后系統(tǒng)重新啟動persistent為true的app
進(jìn)程啟動時為app注冊了一個死亡訃告，當(dāng)該app被殺掉之后會調(diào)用AppDeathRecipient的binderDied方法，該方法會調(diào)用appDiedLocked方法進(jìn)行善后處理，系統(tǒng)在進(jìn)程死掉之后會對死掉的進(jìn)程進(jìn)行清理和資源回收，但是在這個過程中如果你的app是persistent的話會被重啟：
binderDied
|
|——appDiedLocked
|
|——handleAppDiedLocked
|
|——cleanUpApplicationRecordLocked
在cleanUpApplicationRecordLocked中對persistent為true的app進(jìn)行重啟
private final boolean cleanUpApplicationRecordLocked(ProcessRecord app,
boolean restarting, boolean allowRestart, int index, boolean replacingPid) {
...............
//非persistent的app被殺死后就被清理掉
if (!app.persistent || app.isolated) {
if (DEBUG_PROCESSES || DEBUG_CLEANUP) Slog.v(TAG_CLEANUP,
"Removing non-persistent process during cleanup: " + app);
if (!replacingPid) {
removeProcessNameLocked(app.processName, app.uid, app);
}
if (mHeavyWeightProcess == app) {
mHandler.sendMessage(mHandler.obtainMessage(CANCEL_HEAVY_NOTIFICATION_MSG,
mHeavyWeightProcess.userId, 0));
mHeavyWeightProcess = null;
}
} else if (!app.removed) {
// This app is persistent, so we need to keep its record around.
// If it is not already on the pending app list, add it there
// and start a new process for it.
//該app是persistent的，需要對其進(jìn)行重啟，并把它添加到正在啟動的列表中，并
//設(shè)置restart=true
if (mPersistentStartingProcesses.indexOf(app) < 0) {
mPersistentStartingProcesses.add(app);
restart = true;
}
}
....
//經(jīng)過上面的過濾，會調(diào)用這個分支條件重啟persistent為true的app
if (restart && !app.isolated) {
// We have components that still need to be running in the
// process, so re-launch it.
if (index < 0) {
ProcessList.remove(app.pid);
}
addProcessNameLocked(app);
startProcessLocked(app, "restart", app.processName);
return true;
} else if (app.pid > 0 && app.pid != MY_PID) {
// Goodbye!
boolean removed;
synchronized (mPidsSelfLocked) {
mPidsSelfLocked.remove(app.pid);
mHandler.removeMessages(PROC_START_TIMEOUT_MSG, app);
}
mBatteryStatsService.noteProcessFinish(app.processName, app.info.uid);
if (app.isolated) {
mBatteryStatsService.removeIsolatedUid(app.uid, app.info.uid);
}
app.setPid(0);
}
return false;
}
