一、前言

???????前段時間在看當下主流的安卓插件化技術原理的時候，發現目前插件化技術對于四大組件的處理基本都是通過代理來實現的（動態注冊的廣播接收器除外）。簡單說就是需要先在主包里預埋一個注冊在AndroidManifest.xml中的組件，再由這個組件來連接SystemServer和插件中的業務邏輯。

???????這里難免就會有一些疑問，系統從AndroidManifest.xml文件解析出來的四大組件的信息存放在哪里？能否不利用預埋+代理的技術，直接通過修改中間的存儲容器來實現四大組件的插件化？
???????要想解答這些疑惑，我們就得知道系統是如何從AndroidManifest.xml中解析出四大組件的？以及解析后的組件信息又是如何保存的？

???????系統中負責解析AndroidManifest.xml文件的系統服務是PackageManagerService（以下簡稱PMS）。PMS中關于AndroidManifest.xml文件的處理邏輯入口主要有兩處，一個是SystemServer進程啟動時，另一個則是應用安裝/更新時。兩個入口對于xml的解析流程大體上是一致的，本篇文章只分析SystemServer進程啟動時，PMS對AndroidManifest.xml的解析流程。

二、相關知識

2.1 方法后綴LI、LIF、LPr、LPw的含義

??????PMS中有很多方法的名字后綴都是LI/LIF/LPr/LPw，因此我們需要先簡單了解下這些后綴的含義是什么。這些后綴涉及到PMS兩個很重要的鎖mPackages和mInstallLock以及一個mFrozenPackages變量。相關后綴的含義見下表。詳見：https://blog.csdn.net/u013553529/article/details/61962439。

三、調用流程

SystemServer的啟動屬于開機流程中的一步，入口如下：

• SystemServer.main()

/*** The main entry point from zygote.* zygote進程會調用該方法*/public static void main(String[] args) {new SystemServer().run();}

??????main()方法中new了一個SystemServer對象，并調用了它的run()方法

• SystemServer.run()

private void run() {try {// 省略部分代碼，只列出關鍵部分的代碼// The system server should never make non-oneway callsBinder.setWarnOnBlocking(true);// Initialize native services.System.loadLibrary("android_servers");// Create the system service manager.// SystemServiceManager是整個Service的大管家mSystemServiceManager = new SystemServiceManager(mSystemContext);mSystemServiceManager.setStartInfo(mRuntimeRestart,mRuntimeStartElapsedTime, mRuntimeStartUptime);LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);} finally {t.traceEnd(); // InitBeforeStartServices}// Start services.try {t.traceBegin("StartServices");// start critical services, include PMS/AMSstartBootstrapServices(t);// start essential servicesstartCoreServices(t);// start other servicestartOtherServices(t);} catch (Throwable ex) {Slog.e("System", "******************************************");Slog.e("System", "************ Failure starting system services", ex);throw ex;} finally {t.traceEnd(); // StartServices}}

??????run()方法中主要做了一些系統啟動時的初始化工作，其中很關鍵的一步就是啟動各項service服務。PMS就是在startBootstrapServices()中初始化的。

• SystemServer.startBootstrapServices(@NonNull TimingsTraceAndSlog t)

/*** Starts the small tangle of critical services that are needed to get the system off the* ground. These services have complex mutual dependencies which is why we initialize them all* in one place here. Unless your service is also entwined in these dependencies, it should be* initialized in one of the other functions.*/private void startBootstrapServices(@NonNull TimingsTraceAndSlog t) {// 省略部分代碼t.traceBegin("StartPackageManagerService");try {Watchdog.getInstance().pauseWatchingCurrentThread("packagemanagermain");mPackageManagerService = PackageManagerService.main(mSystemContext, installer,mFactoryTestMode != FactoryTest.FACTORY_TEST_OFF, mOnlyCore);} finally {Watchdog.getInstance().resumeWatchingCurrentThread("packagemanagermain");}// Now that the package manager has started, register the dex load reporter to capture any// dex files loaded by system server.// These dex files will be optimized by the BackgroundDexOptService.SystemServerDexLoadReporter.configureSystemServerDexReporter(mPackageManagerService);mFirstBoot = mPackageManagerService.isFirstBoot();mPackageManager = mSystemContext.getPackageManager();// 省略部分代碼}

??????startBootstrapServices()方法中調用了PMS的main()方法來初始化PMS服務

• PackageManagerService.main()

public static PackageManagerService main(Context context, Installer installer,boolean factoryTest, boolean onlyCore) {// 省略部分代碼// new一個Java層的PMS對象PackageManagerService m = new PackageManagerService(injector, onlyCore, factoryTest);// 省略部分代碼// 將new出來的PMS對象注冊到ServiceManager中，之后我們便能夠通過Context對象的getPackageManager()來獲取PM對象了，這部分源碼在Context.getPackageManager()/ContextImpl.getPackageManager()中m.installWhitelistedSystemPackages();ServiceManager.addService("package", m);final PackageManagerNative pmn = m.new PackageManagerNative();ServiceManager.addService("package_native", pmn);return m;}

??????這里我們主要關注PMS的實例化過程，PMS的構造函數如下：

• PackageManagerService.PackageManagerService()

/** Directory where installed applications are stored */private static final File sAppInstallDir =new File(Environment.getDataDirectory(), "app");public PackageManagerService(Injector injector, boolean onlyCore, boolean factoryTest) {// 省略部分代碼if (!mOnlyCore) {EventLog.writeEvent(EventLogTags.BOOT_PROGRESS_PMS_DATA_SCAN_START,SystemClock.uptimeMillis());scanDirTracedLI(sAppInstallDir, 0, scanFlags | SCAN_REQUIRE_KNOWN, 0,packageParser, executorService);}// 省略部分代碼}

??????PMS的構造函數中通過scanDirTracedLI()來掃描已安裝的三方App apk信息，路徑為/data/app/

• PackageManagerService.scanDirTracedLI()

private void scanDirTracedLI(File scanDir, final int parseFlags, int scanFlags,long currentTime, PackageParser2 packageParser, ExecutorService executorService) {Trace.traceBegin(TRACE_TAG_PACKAGE_MANAGER, "scanDir [" + scanDir.getAbsolutePath() + "]");try {scanDirLI(scanDir, parseFlags, scanFlags, currentTime, packageParser, executorService);} finally {Trace.traceEnd(TRACE_TAG_PACKAGE_MANAGER);}}

• PackageManagerService.scanDirLI()

private void scanDirLI(File scanDir, int parseFlags, int scanFlags, long currentTime,PackageParser2 packageParser, ExecutorService executorService) {ParallelPackageParser parallelPackageParser =new ParallelPackageParser(packageParser, executorService);// Submit files for parsing in parallelint fileCount = 0;for (File file : files) {final boolean isPackage = (isApkFile(file) || file.isDirectory())&& !PackageInstallerService.isStageName(file.getName());if (!isPackage) {// Ignore entries which are not packagescontinue;}// 繼續往下調用了parallelPackageParser.submit()parallelPackageParser.submit(file, parseFlags);fileCount++;}}

• ParallelPackageParser.submit(File scanFile, int parseFlags)

/*** Submits the file for parsing* @param scanFile file to scan* @param parseFlags parse flags*/public void submit(File scanFile, int parseFlags) {mExecutorService.submit(() -> {ParseResult pr = new ParseResult();Trace.traceBegin(TRACE_TAG_PACKAGE_MANAGER, "parallel parsePackage [" + scanFile + "]");try {pr.scanFile = scanFile;// 繼續往下調用了parserPackage()pr.parsedPackage = parsePackage(scanFile, parseFlags);} catch (Throwable e) {pr.throwable = e;} finally {Trace.traceEnd(TRACE_TAG_PACKAGE_MANAGER);}try {mQueue.put(pr);} catch (InterruptedException e) {Thread.currentThread().interrupt();// Propagate result to callers of take().// This is helpful to prevent main thread from getting stuck waiting on// ParallelPackageParser to finish in case of interruptionmInterruptedInThread = Thread.currentThread().getName();}});}

• ParallelPackageParser.parsePackage(File scanFile, int parseFlags)

protected ParsedPackage parsePackage(File scanFile, int parseFlags)throws PackageParser.PackageParserException {return mPackageParser.parsePackage(scanFile, parseFlags, true);}

• PackageParser2.parsePackage(File packageFile, int flags, boolean useCaches)

/*** TODO(b/135203078): Document new package parsing*/@AnyThreadpublic ParsedPackage parsePackage(File packageFile, int flags, boolean useCaches)throws PackageParserException {// 有緩存的話優先使用緩存if (useCaches && mCacher != null) {ParsedPackage parsed = mCacher.getCachedResult(packageFile, flags);if (parsed != null) {return parsed;}}long parseTime = LOG_PARSE_TIMINGS ? SystemClock.uptimeMillis() : 0;ParseInput input = mSharedResult.get().reset();// 繼續往下調用了parsePackage()ParseResult<ParsingPackage> result = parsingUtils.parsePackage(input, packageFile, flags);// 省略部分代碼return parsed;}

• ParsingPackageUtils.parsePackage(ParseInput input, File packageFile, int flags)

public ParseResult<ParsingPackage> parsePackage(ParseInput input, File packageFile,int flags)throws PackageParserException {if (packageFile.isDirectory()) {// 如果給定的路徑是文件夾，則循環解析其中的所有apk文件return parseClusterPackage(input, packageFile, flags);} else {// 如果給定的路徑是文件，則調用parseMonolithicPackage()解析文件return parseMonolithicPackage(input, packageFile, flags);} }

注： Android5.0引入了Split APK機制，這是為了解決65536上限以及APK安裝包越來越大等問題。Split APK機制可以將一個APK，拆分成多個獨立APK。
在引入了Split APK機制后，APK有兩種分類：

Single APK：安裝文件為一個完整的APK，即base APK。Android稱其為Monolithic。

Mutiple APK：安裝文件在一個文件目錄中，其內部有多個被拆分的APK，這些APK由一個 base APK和一個或多個split APK組成。Android稱其為Cluster。

??????我們直接看解析Single APK文件的邏輯

• ParsingPackageUtils.parseMonolithicPackage(ParseInput input, File apkFile, int flags)

/*** Parse the given APK file, treating it as as a single monolithic package.* <p>* Note that this <em>does not</em> perform signature verification; that* must be done separately in {@link #getSigningDetails(ParsingPackageRead, boolean)}.*/private ParseResult<ParsingPackage> parseMonolithicPackage(ParseInput input, File apkFile,int flags) throws PackageParserException {// 省略部分代碼final SplitAssetLoader assetLoader = new DefaultSplitAssetLoader(lite, flags);try {// 真正開始解析apk信息ParseResult<ParsingPackage> result = parseBaseApk(input,apkFile,apkFile.getCanonicalPath(),assetLoader.getBaseAssetManager(), flags);if (result.isError()) {return input.error(result);}return input.success(result.getResult().setUse32BitAbi(lite.use32bitAbi));} catch (IOException e) {return input.error(INSTALL_PARSE_FAILED_UNEXPECTED_EXCEPTION,"Failed to get path: " + apkFile, e);} finally {IoUtils.closeQuietly(assetLoader);}}

• parsingPackageUtils.parseBaseApk(ParseInput input, File apkFile, String codePath, AssetManager assets, int flags)

private ParseResult<ParsingPackage> parseBaseApk(ParseInput input, File apkFile,String codePath, AssetManager assets, int flags) {final String apkPath = apkFile.getAbsolutePath();// 省略部分代碼// 打開apk的AndroidManifest.xml文件try (XmlResourceParser parser = assets.openXmlResourceParser(cookie,PackageParser.ANDROID_MANIFEST_FILENAME)) {final Resources res = new Resources(assets, mDisplayMetrics, null);// 開始解析AndroidManifest.xml的信息ParseResult<ParsingPackage> result = parseBaseApk(input, apkPath, codePath, res,parser, flags);// 省略部分代碼ApkAssets apkAssets = assets.getApkAssets()[0];if (apkAssets.definesOverlayable()) {SparseArray<String> packageNames = assets.getAssignedPackageIdentifiers();int size = packageNames.size();for (int index = 0; index < size; index++) {String packageName = packageNames.get(index);Map<String, String> overlayableToActor = assets.getOverlayableMap(packageName);if (overlayableToActor != null && !overlayableToActor.isEmpty()) {for (String overlayable : overlayableToActor.keySet()) {pkg.addOverlayable(overlayable, overlayableToActor.get(overlayable));}}}}pkg.setVolumeUuid(volumeUuid);if ((flags & PackageParser.PARSE_COLLECT_CERTIFICATES) != 0) {pkg.setSigningDetails(getSigningDetails(pkg, false));} else {pkg.setSigningDetails(SigningDetails.UNKNOWN);}return input.success(pkg);} catch (Exception e) {return input.error(INSTALL_PARSE_FAILED_UNEXPECTED_EXCEPTION,"Failed to read manifest from " + apkPath, e);}}

• parsingPackageUtils.parseBaseApk(ParseInput input, String apkPath, String codePath, Resources res, XmlResourceParser parser, int flags)

/*** Parse the manifest of a <em>base APK</em>. When adding new features you* need to consider whether they should be supported by split APKs and child* packages.** @param apkPath The package apk file path* @param res The resources from which to resolve values* @param parser The manifest parser* @param flags Flags how to parse* @return Parsed package or null on error.*/private ParseResult<ParsingPackage> parseBaseApk(ParseInput input, String apkPath,String codePath, Resources res, XmlResourceParser parser, int flags)throws XmlPullParserException, IOException, PackageParserException {// 省略部分代碼final TypedArray manifestArray = res.obtainAttributes(parser, R.styleable.AndroidManifest);try {final boolean isCoreApp =parser.getAttributeBooleanValue(null, "coreApp", false);final ParsingPackage pkg = mCallback.startParsingPackage(pkgName, apkPath, codePath, manifestArray, isCoreApp);// 繼續往下調用了parseBaseApkTags()final ParseResult<ParsingPackage> result =parseBaseApkTags(input, pkg, manifestArray, res, parser, flags);if (result.isError()) {return result;}return input.success(pkg);} finally {manifestArray.recycle();}}

• parsingPackageUtils.parseBaseApkTags(ParseInput input, ParsingPackage pkg, TypedArray sa, Resources res, XmlResourceParser parser, int flags)

private ParseResult<ParsingPackage> parseBaseApkTags(ParseInput input, ParsingPackage pkg,TypedArray sa, Resources res, XmlResourceParser parser, int flags)throws XmlPullParserException, IOException {//省略部分代碼while ((type = parser.next()) != XmlPullParser.END_DOCUMENT&& (type != XmlPullParser.END_TAG|| parser.getDepth() > depth)) {if (type != XmlPullParser.START_TAG) {continue;}String tagName = parser.getName();final ParseResult result;if (PackageParser.TAG_APPLICATION.equals(tagName)) {// 找到<applicaton>標簽，四大組件的聲明都在里面if (foundApp) {if (PackageParser.RIGID_PARSER) {result = input.error("<manifest> has more than one <application>");} else {Slog.w(TAG, "<manifest> has more than one <application>");result = input.success(null);}} else {foundApp = true;// 關鍵在這里result = parseBaseApplication(input, pkg, res, parser, flags);}} else {// 解析除<application>外的其他標簽result = parseBaseApkTag(tagName, input, pkg, res, parser, flags);}if (result.isError()) {return input.error(result);}}// 省略部分代碼return input.success(pkg);}

• parsingPackageUtils.parseBaseApplication(ParseInput input, ParsingPackage pkg, Resources res, XmlResourceParser parser, int flags)

private ParseResult<ParsingPackage> parseBaseApplication(ParseInput input,ParsingPackage pkg, Resources res, XmlResourceParser parser, int flags)throws XmlPullParserException, IOException {// 省略部分代碼boolean hasActivityOrder = false;boolean hasReceiverOrder = false;boolean hasServiceOrder = false;final int depth = parser.getDepth();int type;while ((type = parser.next()) != XmlPullParser.END_DOCUMENT&& (type != XmlPullParser.END_TAG|| parser.getDepth() > depth)) {if (type != XmlPullParser.START_TAG) {continue;}final ParseResult result;String tagName = parser.getName();boolean isActivity = false;switch (tagName) {case "activity":isActivity = true;// fall-throughcase "receiver":ParseResult<ParsedActivity> activityResult =ParsedActivityUtils.parseActivityOrReceiver(mSeparateProcesses, pkg,res, parser, flags, PackageParser.sUseRoundIcon, input);if (activityResult.isSuccess()) {ParsedActivity activity = activityResult.getResult();if (isActivity) {hasActivityOrder |= (activity.getOrder() != 0);pkg.addActivity(activity);} else {hasReceiverOrder |= (activity.getOrder() != 0);pkg.addReceiver(activity);}}result = activityResult;break;case "service":ParseResult<ParsedService> serviceResult =ParsedServiceUtils.parseService(mSeparateProcesses, pkg, res, parser,flags, PackageParser.sUseRoundIcon, input);if (serviceResult.isSuccess()) {ParsedService service = serviceResult.getResult();hasServiceOrder |= (service.getOrder() != 0);pkg.addService(service);}result = serviceResult;break;case "provider":// Provider的解析ParseResult<ParsedProvider> providerResult =ParsedProviderUtils.parseProvider(mSeparateProcesses, pkg, res, parser,flags, PackageParser.sUseRoundIcon, input);if (providerResult.isSuccess()) {// 注冊provider信息pkg.addProvider(providerResult.getResult());}result = providerResult;break;case "activity-alias":activityResult = ParsedActivityUtils.parseActivityAlias(pkg, res,parser, PackageParser.sUseRoundIcon, input);if (activityResult.isSuccess()) {ParsedActivity activity = activityResult.getResult();hasActivityOrder |= (activity.getOrder() != 0);pkg.addActivity(activity);}result = activityResult;break;default:result = parseBaseAppChildTag(input, tagName, pkg, res, parser, flags);break;}if (result.isError()) {return input.error(result);}}if (TextUtils.isEmpty(pkg.getStaticSharedLibName())) {// Add a hidden app detail activity to normal apps which forwards user to App Details// page.// 如果App沒有入口Activity，就將其入口設置為App詳情頁ParseResult<ParsedActivity> a = generateAppDetailsHiddenActivity(input, pkg);if (a.isError()) {return input.error(a);}pkg.addActivity(a.getResult());}return input.success(pkg);}

??????parseBaseApplication()方法中PMS根據tag來區分四大組件，并調用相應的方法做進一步的解析。最終PMS會通過ParsingPackage的addXXX()方法將解析到的組件信息添加到對應的List中。ParsingPackage是一個接口，對應的實現類為ParsingPackageImpl

• ParsingPackageImpl.addXXX()

@NonNullprotected List<ParsedActivity> activities = emptyList();@NonNullprotected List<ParsedActivity> receivers = emptyList();@NonNullprotected List<ParsedService> services = emptyList();@NonNullprotected List<ParsedProvider> providers = emptyList();@Overridepublic ParsingPackageImpl addActivity(ParsedActivity parsedActivity) {this.activities = CollectionUtils.add(this.activities, parsedActivity);addMimeGroupsFromComponent(parsedActivity);return this;}@Overridepublic ParsingPackageImpl addReceiver(ParsedActivity parsedReceiver) {this.receivers = CollectionUtils.add(this.receivers, parsedReceiver);addMimeGroupsFromComponent(parsedReceiver);return this;}@Overridepublic ParsingPackageImpl addService(ParsedService parsedService) {this.services = CollectionUtils.add(this.services, parsedService);addMimeGroupsFromComponent(parsedService);return this;}@Overridepublic ParsingPackageImpl addProvider(ParsedProvider parsedProvider) {this.providers = CollectionUtils.add(this.providers, parsedProvider);addMimeGroupsFromComponent(parsedProvider);return this;}

??????至此，四大組件的信息就都被解析并且保存下來了。

三、小結

??????整個流程看下來我們會發現，PMS對四大組件信息的處理采用的是實時解析AndroidManifest.xml文件的方式，中間并沒有多余的轉存邏輯。因此，想要通過修改中間容器的方式來實現四大組件的插件化看起來是不太可行的。只能老老實實走預埋+代理的邏輯。
??????但是，實時解析的方式雖然安全準確，但是速度慢呀。當你手機里安裝的App越來越多，設備的開機速度就會變得越來越慢。因此，新版本上安卓系統為了提升設備的啟動速度，在PMS解析AndroidManifest.xml文件的流程中引入了相應的緩存邏輯來優化解析速度。

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