android源碼解析之（十二）--)系統啟動並解析Manifest的流程

最近有同學問我關於Manifest何時被系統解析的問題，正好也分析到這一塊了，索性這一章就講解一下android系統何時解析Manifest吧，這裡的Manifest指的是android安裝文件apk中的androidManifest.xml文件是何時被解析的。
大家應該都知道，Android系統啟動之後，我們就可以在一個應用中打開另一個從未打開過的應用，或者是在一個應用中發送廣播，如果另外一個應用設置了這個廣播的接收器，那麼這個應用進程就會被啟動並接收該廣播並作出相應的處理，這樣的例子很多，我們可以猜測到Android系統在啟動的時候就會抓取到了系統中所有安裝的應用信息（應該是解析apk文件的Manifest信息），即在Android系統的啟動過程中就已經解析了系統中安裝應用的androidManifest.xml文件並保存起來了，那麼這個過程具體是如何的呢?

其實android系統啟動過程中解析Manifest的流程是通過PackageManagerService服務來實現的。這裡我們重點分析一下PackageManagerService服務是如何解析Manifest的。

首先看一下在SystemServer進程啟動過程中是如何啟動PackageManagerService服務的：

private void startBootstrapServices() {
        ...
        mPackageManagerService = PackageManagerService.main(mSystemContext, installer,
                mFactoryTestMode != FactoryTest.FACTORY_TEST_OFF, mOnlyCore);
        mFirstBoot = mPackageManagerService.isFirstBoot();
        mPackageManager = mSystemContext.getPackageManager();

        ...
    }

在SystemServer進程啟動過程中會調用SystemServer類的startBootstrapServices方法（主要用於啟動ActivityManagerService服務和PackageManagerService服務），然後會在這個方法中會調用PackageManagerService.main靜態方法，這個方法主要是用來初始化PackageManagerService服務並執行相關邏輯的。下面我來看一下main方法的具體邏輯：

public static PackageManagerService main(Context context, Installer installer,
            boolean factoryTest, boolean onlyCore) {
        PackageManagerService m = new PackageManagerService(context, installer,
                factoryTest, onlyCore);
        ServiceManager.addService("package", m);
        return m;
    }

可以發現main方法的實現邏輯主要是創建了一個PackageManagerService對象，並將這個對象添加到ServierManager中為其他組件提供服務。好吧，看來PackageManagerService的初始化操作主要是在PackageManagerService的構造方法中了，下面我們來看一下其構造方法的實現邏輯：

File dataDir = Environment.getDataDirectory();
            mAppDataDir = new File(dataDir, "data");
            mAppInstallDir = new File(dataDir, "app");
            mAppLib32InstallDir = new File(dataDir, "app-lib");
            mAsecInternalPath = new File(dataDir, "app-asec").getPath();
            mUserAppDataDir = new File(dataDir, "user");
            mDrmAppPrivateInstallDir = new File(dataDir, "app-private");

PackageManagerService的構造方法代碼量比較大，這裡就不貼出所有的代碼了，我們主要和解析Manifest相關的主要代碼，在構造方法中有這樣幾段代碼。可以發現在構造方法中，解析了系統中幾個apk的安裝目錄，這幾個目錄就是系統中安裝apk的目錄，android系統會默認解析這幾個目錄下apk文件，也就是說如果我們android手機在其他的目錄下存在apk文件系統是不會默認解析的，反過來說，如果我們把我們的apk文件移動到這幾個目錄下，那麼重新啟動操作系統，該apk文件就會被系統解析並執行相關的邏輯操作，具體做什麼操作呢？我們看下面的實現。

/ overlay packages if they reside in VENDOR_OVERLAY_DIR.
            File vendorOverlayDir = new File(VENDOR_OVERLAY_DIR);
            scanDirLI(vendorOverlayDir, PackageParser.PARSE_IS_SYSTEM
                    | PackageParser.PARSE_IS_SYSTEM_DIR, scanFlags | SCAN_TRUSTED_OVERLAY, 0);

            // Find base frameworks (resource packages without code).
            scanDirLI(frameworkDir, PackageParser.PARSE_IS_SYSTEM
                    | PackageParser.PARSE_IS_SYSTEM_DIR
                    | PackageParser.PARSE_IS_PRIVILEGED,
                    scanFlags | SCAN_NO_DEX, 0);

            // Collected privileged system packages.
            final File privilegedAppDir = new File(Environment.getRootDirectory(), "priv-app");
            scanDirLI(privilegedAppDir, PackageParser.PARSE_IS_SYSTEM
                    | PackageParser.PARSE_IS_SYSTEM_DIR
                    | PackageParser.PARSE_IS_PRIVILEGED, scanFlags, 0);

            // Collect ordinary system packages.
            final File systemAppDir = new File(Environment.getRootDirectory(), "app");
            scanDirLI(systemAppDir, PackageParser.PARSE_IS_SYSTEM
                    | PackageParser.PARSE_IS_SYSTEM_DIR, scanFlags, 0);

            // Collect all vendor packages.
            File vendorAppDir = new File("/vendor/app");
            try {
                vendorAppDir = vendorAppDir.getCanonicalFile();
            } catch (IOException e) {
                // failed to look up canonical path, continue with original one
            }
            scanDirLI(vendorAppDir, PackageParser.PARSE_IS_SYSTEM
                    | PackageParser.PARSE_IS_SYSTEM_DIR, scanFlags, 0);

            // Collect all OEM packages.
            final File oemAppDir = new File(Environment.getOemDirectory(), "app");
            scanDirLI(oemAppDir, PackageParser.PARSE_IS_SYSTEM
                    | PackageParser.PARSE_IS_SYSTEM_DIR, scanFlags, 0);

在我們剛剛的PackageManagerService.mani方法中，解析完剛剛的幾個系統目錄之後系統會調用scanDirLI方法，那麼這個方法主要是做什麼用的呢？看它的名字應該是遍歷這個系統目錄。好吧，這個方法主要就是用於解析上面幾個目錄下的apk文件的。不信？我們看一下scanDirLI方法的具體實現：

private void scanDirLI(File dir, int parseFlags, int scanFlags, long currentTime) {
        final File[] files = dir.listFiles();
        if (ArrayUtils.isEmpty(files)) {
            Log.d(TAG, "No files in app dir " + dir);
            return;
        }

        if (DEBUG_PACKAGE_SCANNING) {
            Log.d(TAG, "Scanning app dir " + dir + " scanFlags=" + scanFlags
                    + " flags=0x" + Integer.toHexString(parseFlags));
        }

        for (File file : files) {
            final boolean isPackage = (isApkFile(file) || file.isDirectory())
                    && !PackageInstallerService.isStageName(file.getName());
            if (!isPackage) {
                // Ignore entries which are not packages
                continue;
            }
            try {
                scanPackageLI(file, parseFlags | PackageParser.PARSE_MUST_BE_APK,
                        scanFlags, currentTime, null);
            } catch (PackageManagerException e) {
                Slog.w(TAG, "Failed to parse " + file + ": " + e.getMessage());

                // Delete invalid userdata apps
                if ((parseFlags & PackageParser.PARSE_IS_SYSTEM) == 0 &&
                        e.error == PackageManager.INSTALL_FAILED_INVALID_APK) {
                    logCriticalInfo(Log.WARN, "Deleting invalid package at " + file);
                    if (file.isDirectory()) {
                        mInstaller.rmPackageDir(file.getAbsolutePath());
                    } else {
                        file.delete();
                    }
                }
            }
        }
    }

可以放下其首先會遍歷該目錄下的所有文件，並判斷是否是apk文件，如果是apk文件則調用scanPackageLI方法，scanPackageLI方法的名字很明顯，就是用於解析這個apk文件的。

繼續看一下scanPakcageLI方法的實現：

private PackageParser.Package scanPackageLI(File scanFile, int parseFlags, int scanFlags,
            long currentTime, UserHandle user) throws PackageManagerException {
        if (DEBUG_INSTALL) Slog.d(TAG, "Parsing: " + scanFile);
        parseFlags |= mDefParseFlags;
        PackageParser pp = new PackageParser();
        pp.setSeparateProcesses(mSeparateProcesses);
        pp.setOnlyCoreApps(mOnlyCore);
        pp.setDisplayMetrics(mMetrics);

        if ((scanFlags & SCAN_TRUSTED_OVERLAY) != 0) {
            parseFlags |= PackageParser.PARSE_TRUSTED_OVERLAY;
        }

        final PackageParser.Package pkg;
        try {
            pkg = pp.parsePackage(scanFile, parseFlags);
        } catch (PackageParserException e) {
            throw PackageManagerException.from(e);
        }
        ...
}

好吧，這個方法也比較復雜，這裡只是列出重點相關的代碼，我們可以發現在這個方法中創建了一個PackagerParser對象，並調用了parsePackage方法，這個方法其實就是解析Manifest的主要方法，我們可以看一下其具體的實現：

public Package parsePackage(File packageFile, int flags) throws PackageParserException {
        if (packageFile.isDirectory()) {
            return parseClusterPackage(packageFile, flags);
        } else {
            return parseMonolithicPackage(packageFile, flags);
        }
    }

可以發現，若我們解析的File對象是一個文件夾則執行調用parseClusterPackage方法，否則調用執行parseMonolithicPackage方法，很明顯的因為我們這裡解析的是apk文件（在上一方法中我們循環遍歷得到了apk文件，這裡的File對象就代表了一個個的apk文件信息），所以這裡會執行parseMonolithicPackage方法，然後我們來看一下parseMonolithicPackage方法：

public Package parseMonolithicPackage(File apkFile, int flags) throws PackageParserException {
        if (mOnlyCoreApps) {
            final PackageLite lite = parseMonolithicPackageLite(apkFile, flags);
            if (!lite.coreApp) {
                throw new PackageParserException(INSTALL_PARSE_FAILED_MANIFEST_MALFORMED,
                        "Not a coreApp: " + apkFile);
            }
        }

        final AssetManager assets = new AssetManager();
        try {
            final Package pkg = parseBaseApk(apkFile, assets, flags);
            pkg.codePath = apkFile.getAbsolutePath();
            return pkg;
        } finally {
            IoUtils.closeQuietly(assets);
        }
    }

可以看出，這裡又調用了parseBaseApk方法：

private Package parseBaseApk(File apkFile, AssetManager assets, int flags)
            ...
            final Package pkg = parseBaseApk(res, parser, flags, outError);
            ...
    }

可以看出，這個parseBaseApk方法調用了其重載的parseBaseApk方法：

while ((type = parser.next()) != XmlPullParser.END_DOCUMENT
                && (type != XmlPullParser.END_TAG || parser.getDepth() > outerDepth)) {
            if (type == XmlPullParser.END_TAG || type == XmlPullParser.TEXT) {
                continue;
            }

            String tagName = parser.getName();
            if (tagName.equals("application")) {
                if (foundApp) {
                    if (RIGID_PARSER) {
                        outError[0] = " has more than one ";
                        mParseError = PackageManager.INSTALL_PARSE_FAILED_MANIFEST_MALFORMED;
                        return null;
                    } else {
                        Slog.w(TAG, " has more than one ");
                        XmlUtils.skipCurrentTag(parser);
                        continue;
                    }
                }

                foundApp = true;
                if (!parseBaseApplication(pkg, res, parser, attrs, flags, outError)) {
                    return null;
                }
            } else if (tagName.equals("overlay")) {
                pkg.mTrustedOverlay = trustedOverlay;

                sa = res.obtainAttributes(attrs,
                        com.android.internal.R.styleable.AndroidManifestResourceOverlay);
                pkg.mOverlayTarget = sa.getString(
                        com.android.internal.R.styleable.AndroidManifestResourceOverlay_targetPackage);
                pkg.mOverlayPriority = sa.getInt(
                        com.android.internal.R.styleable.AndroidManifestResourceOverlay_priority,
                        -1);
                sa.recycle();

                if (pkg.mOverlayTarget == null) {
                    outError[0] = " does not specify a target package";
                    mParseError = PackageManager.INSTALL_PARSE_FAILED_MANIFEST_MALFORMED;
                    return null;
                }
                if (pkg.mOverlayPriority < 0 || pkg.mOverlayPriority > 9999) {
                    outError[0] = " priority must be between 0 and 9999";
                    mParseError =
                        PackageManager.INSTALL_PARSE_FAILED_MANIFEST_MALFORMED;
                    return null;
                }
                XmlUtils.skipCurrentTag(parser);

            } else if (tagName.equals("key-sets")) {
                if (!parseKeySets(pkg, res, parser, attrs, outError)) {
                    return null;
                }
            } else if (tagName.equals("permission-group")) {
                if (parsePermissionGroup(pkg, flags, res, parser, attrs, outError) == null) {
                    return null;
                }
            } else if (tagName.equals("permission")) {
                if (parsePermission(pkg, res, parser, attrs, outError) == null) {
                    return null;
                }
            } else if (tagName.equals("permission-tree")) {
                if (parsePermissionTree(pkg, res, parser, attrs, outError) == null) {
                    return null;
                }
            } else if (tagName.equals("uses-permission")) {
                if (!parseUsesPermission(pkg, res, parser, attrs)) {
                    return null;
                }
            } else if (tagName.equals("uses-permission-sdk-m")
                    || tagName.equals("uses-permission-sdk-23")) {
                if (!parseUsesPermission(pkg, res, parser, attrs)) {
                    return null;
                }
            } else if (tagName.equals("uses-configuration")) {
                ConfigurationInfo cPref = new ConfigurationInfo();
                sa = res.obtainAttributes(attrs,
                        com.android.internal.R.styleable.AndroidManifestUsesConfiguration);
                cPref.reqTouchScreen = sa.getInt(
                        com.android.internal.R.styleable.AndroidManifestUsesConfiguration_reqTouchScreen,
                        Configuration.TOUCHSCREEN_UNDEFINED);
                cPref.reqKeyboardType = sa.getInt(
                        com.android.internal.R.styleable.AndroidManifestUsesConfiguration_reqKeyboardType,
                        Configuration.KEYBOARD_UNDEFINED);
                if (sa.getBoolean(
                        com.android.internal.R.styleable.AndroidManifestUsesConfiguration_reqHardKeyboard,
                        false)) {
                    cPref.reqInputFeatures |= ConfigurationInfo.INPUT_FEATURE_HARD_KEYBOARD;
                }
                cPref.reqNavigation = sa.getInt(
                        com.android.internal.R.styleable.AndroidManifestUsesConfiguration_reqNavigation,
                        Configuration.NAVIGATION_UNDEFINED);
                if (sa.getBoolean(
                        com.android.internal.R.styleable.AndroidManifestUsesConfiguration_reqFiveWayNav,
                        false)) {
                    cPref.reqInputFeatures |= ConfigurationInfo.INPUT_FEATURE_FIVE_WAY_NAV;
                }
                sa.recycle();
                pkg.configPreferences = ArrayUtils.add(pkg.configPreferences, cPref);

                XmlUtils.skipCurrentTag(parser);

            } else if (tagName.equals("uses-feature")) {
                FeatureInfo fi = parseUsesFeature(res, attrs);
                pkg.reqFeatures = ArrayUtils.add(pkg.reqFeatures, fi);

                if (fi.name == null) {
                    ConfigurationInfo cPref = new ConfigurationInfo();
                    cPref.reqGlEsVersion = fi.reqGlEsVersion;
                    pkg.configPreferences = ArrayUtils.add(pkg.configPreferences, cPref);
                }

                XmlUtils.skipCurrentTag(parser);

            } else if (tagName.equals("feature-group")) {
                FeatureGroupInfo group = new FeatureGroupInfo();
                ArrayList features = null;
                final int innerDepth = parser.getDepth();
                while ((type = parser.next()) != XmlPullParser.END_DOCUMENT
                        && (type != XmlPullParser.END_TAG || parser.getDepth() > innerDepth)) {
                    if (type == XmlPullParser.END_TAG || type == XmlPullParser.TEXT) {
                        continue;
                    }

                    final String innerTagName = parser.getName();
                    if (innerTagName.equals("uses-feature")) {
                        FeatureInfo featureInfo = parseUsesFeature(res, attrs);
                        // FeatureGroups are stricter and mandate that
                        // any  declared are mandatory.
                        featureInfo.flags |= FeatureInfo.FLAG_REQUIRED;
                        features = ArrayUtils.add(features, featureInfo);
                    } else {
                        Slog.w(TAG, "Unknown element under : " + innerTagName +
                                " at " + mArchiveSourcePath + " " +
                                parser.getPositionDescription());
                    }
                    XmlUtils.skipCurrentTag(parser);
                }

                if (features != null) {
                    group.features = new FeatureInfo[features.size()];
                    group.features = features.toArray(group.features);
                }
                pkg.featureGroups = ArrayUtils.add(pkg.featureGroups, group);

            } else if (tagName.equals("uses-sdk")) {
                if (SDK_VERSION > 0) {
                    sa = res.obtainAttributes(attrs,
                            com.android.internal.R.styleable.AndroidManifestUsesSdk);

                    int minVers = 0;
                    String minCode = null;
                    int targetVers = 0;
                    String targetCode = null;

                    TypedValue val = sa.peekValue(
                            com.android.internal.R.styleable.AndroidManifestUsesSdk_minSdkVersion);
                    if (val != null) {
                        if (val.type == TypedValue.TYPE_STRING && val.string != null) {
                            targetCode = minCode = val.string.toString();
                        } else {
                            // If it's not a string, it's an integer.
                            targetVers = minVers = val.data;
                        }
                    }

                    val = sa.peekValue(
                            com.android.internal.R.styleable.AndroidManifestUsesSdk_targetSdkVersion);
                    if (val != null) {
                        if (val.type == TypedValue.TYPE_STRING && val.string != null) {
                            targetCode = minCode = val.string.toString();
                        } else {
                            // If it's not a string, it's an integer.
                            targetVers = val.data;
                        }
                    }

                    sa.recycle();

                    if (minCode != null) {
                        boolean allowedCodename = false;
                        for (String codename : SDK_CODENAMES) {
                            if (minCode.equals(codename)) {
                                allowedCodename = true;
                                break;
                            }
                        }
                        if (!allowedCodename) {
                            if (SDK_CODENAMES.length > 0) {
                                outError[0] = "Requires development platform " + minCode
                                        + " (current platform is any of "
                                        + Arrays.toString(SDK_CODENAMES) + ")";
                            } else {
                                outError[0] = "Requires development platform " + minCode
                                        + " but this is a release platform.";
                            }
                            mParseError = PackageManager.INSTALL_FAILED_OLDER_SDK;
                            return null;
                        }
                    } else if (minVers > SDK_VERSION) {
                        outError[0] = "Requires newer sdk version #" + minVers
                                + " (current version is #" + SDK_VERSION + ")";
                        mParseError = PackageManager.INSTALL_FAILED_OLDER_SDK;
                        return null;
                    }

                    if (targetCode != null) {
                        boolean allowedCodename = false;
                        for (String codename : SDK_CODENAMES) {
                            if (targetCode.equals(codename)) {
                                allowedCodename = true;
                                break;
                            }
                        }
                        if (!allowedCodename) {
                            if (SDK_CODENAMES.length > 0) {
                                outError[0] = "Requires development platform " + targetCode
                                        + " (current platform is any of "
                                        + Arrays.toString(SDK_CODENAMES) + ")";
                            } else {
                                outError[0] = "Requires development platform " + targetCode
                                        + " but this is a release platform.";
                            }
                            mParseError = PackageManager.INSTALL_FAILED_OLDER_SDK;
                            return null;
                        }
                        // If the code matches, it definitely targets this SDK.
                        pkg.applicationInfo.targetSdkVersion
                                = android.os.Build.VERSION_CODES.CUR_DEVELOPMENT;
                    } else {
                        pkg.applicationInfo.targetSdkVersion = targetVers;
                    }
                }

                XmlUtils.skipCurrentTag(parser);

            } else if (tagName.equals("supports-screens")) {
                sa = res.obtainAttributes(attrs,
                        com.android.internal.R.styleable.AndroidManifestSupportsScreens);

                pkg.applicationInfo.requiresSmallestWidthDp = sa.getInteger(
                        com.android.internal.R.styleable.AndroidManifestSupportsScreens_requiresSmallestWidthDp,
                        0);
                pkg.applicationInfo.compatibleWidthLimitDp = sa.getInteger(
                        com.android.internal.R.styleable.AndroidManifestSupportsScreens_compatibleWidthLimitDp,
                        0);
                pkg.applicationInfo.largestWidthLimitDp = sa.getInteger(
                        com.android.internal.R.styleable.AndroidManifestSupportsScreens_largestWidthLimitDp,
                        0);

                // This is a trick to get a boolean and still able to detect
                // if a value was actually set.
                supportsSmallScreens = sa.getInteger(
                        com.android.internal.R.styleable.AndroidManifestSupportsScreens_smallScreens,
                        supportsSmallScreens);
                supportsNormalScreens = sa.getInteger(
                        com.android.internal.R.styleable.AndroidManifestSupportsScreens_normalScreens,
                        supportsNormalScreens);
                supportsLargeScreens = sa.getInteger(
                        com.android.internal.R.styleable.AndroidManifestSupportsScreens_largeScreens,
                        supportsLargeScreens);
                supportsXLargeScreens = sa.getInteger(
                        com.android.internal.R.styleable.AndroidManifestSupportsScreens_xlargeScreens,
                        supportsXLargeScreens);
                resizeable = sa.getInteger(
                        com.android.internal.R.styleable.AndroidManifestSupportsScreens_resizeable,
                        resizeable);
                anyDensity = sa.getInteger(
                        com.android.internal.R.styleable.AndroidManifestSupportsScreens_anyDensity,
                        anyDensity);

                sa.recycle();

                XmlUtils.skipCurrentTag(parser);

            } else if (tagName.equals("protected-broadcast")) {
                sa = res.obtainAttributes(attrs,
                        com.android.internal.R.styleable.AndroidManifestProtectedBroadcast);

                // Note: don't allow this value to be a reference to a resource
                // that may change.
                String name = sa.getNonResourceString(
                        com.android.internal.R.styleable.AndroidManifestProtectedBroadcast_name);

                sa.recycle();

                if (name != null && (flags&PARSE_IS_SYSTEM) != 0) {
                    if (pkg.protectedBroadcasts == null) {
                        pkg.protectedBroadcasts = new ArrayList();
                    }
                    if (!pkg.protectedBroadcasts.contains(name)) {
                        pkg.protectedBroadcasts.add(name.intern());
                    }
                }

                XmlUtils.skipCurrentTag(parser);

            } else if (tagName.equals("instrumentation")) {
                if (parseInstrumentation(pkg, res, parser, attrs, outError) == null) {
                    return null;
                }

            } else if (tagName.equals("original-package")) {
                sa = res.obtainAttributes(attrs,
                        com.android.internal.R.styleable.AndroidManifestOriginalPackage);

                String orig =sa.getNonConfigurationString(
                        com.android.internal.R.styleable.AndroidManifestOriginalPackage_name, 0);
                if (!pkg.packageName.equals(orig)) {
                    if (pkg.mOriginalPackages == null) {
                        pkg.mOriginalPackages = new ArrayList();
                        pkg.mRealPackage = pkg.packageName;
                    }
                    pkg.mOriginalPackages.add(orig);
                }

                sa.recycle();

                XmlUtils.skipCurrentTag(parser);

            } else if (tagName.equals("adopt-permissions")) {
                sa = res.obtainAttributes(attrs,
                        com.android.internal.R.styleable.AndroidManifestOriginalPackage);

                String name = sa.getNonConfigurationString(
                        com.android.internal.R.styleable.AndroidManifestOriginalPackage_name, 0);

                sa.recycle();

                if (name != null) {
                    if (pkg.mAdoptPermissions == null) {
                        pkg.mAdoptPermissions = new ArrayList();
                    }
                    pkg.mAdoptPermissions.add(name);
                }

                XmlUtils.skipCurrentTag(parser);

            } else if (tagName.equals("uses-gl-texture")) {
                // Just skip this tag
                XmlUtils.skipCurrentTag(parser);
                continue;

            } else if (tagName.equals("compatible-screens")) {
                // Just skip this tag
                XmlUtils.skipCurrentTag(parser);
                continue;
            } else if (tagName.equals("supports-input")) {
                XmlUtils.skipCurrentTag(parser);
                continue;

            } else if (tagName.equals("eat-comment")) {
                // Just skip this tag
                XmlUtils.skipCurrentTag(parser);
                continue;

            } else if (RIGID_PARSER) {
                outError[0] = "Bad element under : "
                    + parser.getName();
                mParseError = PackageManager.INSTALL_PARSE_FAILED_MANIFEST_MALFORMED;
                return null;

            } else {
                Slog.w(TAG, "Unknown element under : " + parser.getName()
                        + " at " + mArchiveSourcePath + " "
                        + parser.getPositionDescription());
                XmlUtils.skipCurrentTag(parser);
                continue;
            }
        }

在這個parseBaseApk方法中有一個while循環，該循環主要就是用於解析AndroidManifest.xml文件中的節點信息。在開始解析application節點的時候，同時調用了parseBaseApplication方法，該方法解析了application節點下的activity，service，broadcast，contentprovier等組件的定義信息：

while ((type = parser.next()) != XmlPullParser.END_DOCUMENT
                && (type != XmlPullParser.END_TAG || parser.getDepth() > innerDepth)) {
            if (type == XmlPullParser.END_TAG || type == XmlPullParser.TEXT) {
                continue;
            }

            String tagName = parser.getName();
            if (tagName.equals("activity")) {
                Activity a = parseActivity(owner, res, parser, attrs, flags, outError, false,
                        owner.baseHardwareAccelerated);
                if (a == null) {
                    mParseError = PackageManager.INSTALL_PARSE_FAILED_MANIFEST_MALFORMED;
                    return false;
                }

                owner.activities.add(a);

            } else if (tagName.equals("receiver")) {
                Activity a = parseActivity(owner, res, parser, attrs, flags, outError, true, false);
                if (a == null) {
                    mParseError = PackageManager.INSTALL_PARSE_FAILED_MANIFEST_MALFORMED;
                    return false;
                }

                owner.receivers.add(a);

            } else if (tagName.equals("service")) {
                Service s = parseService(owner, res, parser, attrs, flags, outError);
                if (s == null) {
                    mParseError = PackageManager.INSTALL_PARSE_FAILED_MANIFEST_MALFORMED;
                    return false;
                }

                owner.services.add(s);

            } else if (tagName.equals("provider")) {
                Provider p = parseProvider(owner, res, parser, attrs, flags, outError);
                if (p == null) {
                    mParseError = PackageManager.INSTALL_PARSE_FAILED_MANIFEST_MALFORMED;
                    return false;
                }

                owner.providers.add(p);

            } else if (tagName.equals("activity-alias")) {
                Activity a = parseActivityAlias(owner, res, parser, attrs, flags, outError);
                if (a == null) {
                    mParseError = PackageManager.INSTALL_PARSE_FAILED_MANIFEST_MALFORMED;
                    return false;
                }

                owner.activities.add(a);

            } else if (parser.getName().equals("meta-data")) {
                // note: application meta-data is stored off to the side, so it can
                // remain null in the primary copy (we like to avoid extra copies because
                // it can be large)
                if ((owner.mAppMetaData = parseMetaData(res, parser, attrs, owner.mAppMetaData,
                        outError)) == null) {
                    mParseError = PackageManager.INSTALL_PARSE_FAILED_MANIFEST_MALFORMED;
                    return false;
                }

            } else if (tagName.equals("library")) {
                sa = res.obtainAttributes(attrs,
                        com.android.internal.R.styleable.AndroidManifestLibrary);

                // Note: don't allow this value to be a reference to a resource
                // that may change.
                String lname = sa.getNonResourceString(
                        com.android.internal.R.styleable.AndroidManifestLibrary_name);

                sa.recycle();

                if (lname != null) {
                    lname = lname.intern();
                    if (!ArrayUtils.contains(owner.libraryNames, lname)) {
                        owner.libraryNames = ArrayUtils.add(owner.libraryNames, lname);
                    }
                }

                XmlUtils.skipCurrentTag(parser);

            } else if (tagName.equals("uses-library")) {
                sa = res.obtainAttributes(attrs,
                        com.android.internal.R.styleable.AndroidManifestUsesLibrary);

                // Note: don't allow this value to be a reference to a resource
                // that may change.
                String lname = sa.getNonResourceString(
                        com.android.internal.R.styleable.AndroidManifestUsesLibrary_name);
                boolean req = sa.getBoolean(
                        com.android.internal.R.styleable.AndroidManifestUsesLibrary_required,
                        true);

                sa.recycle();

                if (lname != null) {
                    lname = lname.intern();
                    if (req) {
                        owner.usesLibraries = ArrayUtils.add(owner.usesLibraries, lname);
                    } else {
                        owner.usesOptionalLibraries = ArrayUtils.add(
                                owner.usesOptionalLibraries, lname);
                    }
                }

                XmlUtils.skipCurrentTag(parser);

            } else if (tagName.equals("uses-package")) {
                // Dependencies for app installers; we don't currently try to
                // enforce this.
                XmlUtils.skipCurrentTag(parser);

            } else {
                if (!RIGID_PARSER) {
                    Slog.w(TAG, "Unknown element under : " + tagName
                            + " at " + mArchiveSourcePath + " "
                            + parser.getPositionDescription());
                    XmlUtils.skipCurrentTag(parser);
                    continue;
                } else {
                    outError[0] = "Bad element under : " + tagName;
                    mParseError = PackageManager.INSTALL_PARSE_FAILED_MANIFEST_MALFORMED;
                    return false;
                }
            }
        }

這樣，經過這裡循環遍歷，整個androidManifest的節點信息就被解析並保存在了Package對象中。可以看到我們平時在Manifest中定義的各種節點，其實都是在這裡有所體現。當androidManifest.xml文件被解析完成之後會調用我們剛剛介紹的scanPackageLI的重載方法，將解析完成的Package對象信息保存的Setting對象中，這個對象用於保存app的安裝信息，具體實現是在方法：

private PackageParser.Package scanPackageLI(File scanFile, int parseFlags, int scanFlags,
            long currentTime, UserHandle user) throws PackageManagerException

當解析完成manifest文件之後會調用其重載方法：

// Note that we invoke the following method only if we are about to unpack an application
        PackageParser.Package scannedPkg = scanPackageLI(pkg, parseFlags, scanFlags
                | SCAN_UPDATE_SIGNATURE, currentTime, user);

這樣，解析的manifest文件信息就會被保存到Settings中，並持久化，然後執行安裝apk的操作，我們可以看一下該重載方法的具體實現：

private PackageParser.Package scanPackageLI(PackageParser.Package pkg, int parseFlags,
            int scanFlags, long currentTime, UserHandle user) throws PackageManagerException {
        boolean success = false;
        try {
            final PackageParser.Package res = scanPackageDirtyLI(pkg, parseFlags, scanFlags,
                    currentTime, user);
            success = true;
            return res;
        } finally {
            if (!success && (scanFlags & SCAN_DELETE_DATA_ON_FAILURES) != 0) {
                removeDataDirsLI(pkg.volumeUuid, pkg.packageName);
            }
        }
    }

可以發現其內部調用了scanPackageDirtyLI方法，這個方法就是實際實現持久化manifest信息並安裝APK操作的：

private PackageParser.Package scanPackageDirtyLI(PackageParser.Package pkg, int parseFlags,
            int scanFlags, long currentTime, UserHandle user) throws PackageManagerException {
    ...
    // And now re-install the app.
    ret = createDataDirsLI(pkg.volumeUuid, pkgName, pkg.applicationInfo.uid,
                  pkg.applicationInfo.seinfo);
    ...
}

可以發現其內部調用了createDataDirLI，該方法主要實現安裝apk的操作。

private int createDataDirsLI(String volumeUuid, String packageName, int uid, String seinfo) {
        int[] users = sUserManager.getUserIds();
        int res = mInstaller.install(volumeUuid, packageName, uid, uid, seinfo);
        if (res < 0) {
            return res;
        }
        for (int user : users) {
            if (user != 0) {
                res = mInstaller.createUserData(volumeUuid, packageName,
                        UserHandle.getUid(user, uid), user, seinfo);
                if (res < 0) {
                    return res;
                }
            }
        }
        return res;
    }

查看該方法的實現：

public int install(String uuid, String name, int uid, int gid, String seinfo) {
        StringBuilder builder = new StringBuilder("install");
        builder.append(' ');
        builder.append(escapeNull(uuid));
        builder.append(' ');
        builder.append(name);
        builder.append(' ');
        builder.append(uid);
        builder.append(' ');
        builder.append(gid);
        builder.append(' ');
        builder.append(seinfo != null ? seinfo : "!");
        return mInstaller.execute(builder.toString());
    }

怎麼樣？很熟悉吧，這裡的Installer其實調用的就是我們平時運行android項目很熟悉的install命令，原來android系統安裝apk文件底層都是調用的adb命令。

總結：

android系統啟動之後會解析固定目錄下的apk文件，並執行解析，持久化apk信息，重新安裝等操作；

解析Manifest流程：Zygote進程 –> SystemServer進程 –> PackgeManagerService服務 –> scanDirLI方法 –> scanPackageLI方法 –> PackageParser.parserPackage方法；

解析完成Manifest之後會將apk的Manifest信息保存在Settings對象中並持久化，然後執行重新安裝的操作；