Android系統啟動時的apk安裝過程

1.前言

在Android系統啟動的過程中，系統會通過PackageManagerService來掃描特定的目錄，以便可以對保存在裡面的apk進行安裝。PackageManagerService主要做兩件事情：
- 解析這個apk的配置文件AndroidManifest.xml以便獲得它的安裝信息
- 為這個apk分配Linux用戶ID和用戶組ID以便apk在系統中可以獲取到合適的運行權限
下面我們來開始分析PackageManagerService是如何安裝apk並執行上面兩件事情的

2.源碼解析

在前面分析Zygote的時候，有分析到PackageManagerService是由PackageManagerService.main來啟動的，因此我們一開始就來分析PackageManagerService.main

2.1 PackageManagerService.main

    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;
    }

這裡一開始先把PackageManagerService服務new一個出來，在啟動的時候會調用他的構造函數，構造函數裡面會對系統中特定的目錄以便對保存在裡面的apk進行安裝。之後將它注冊到ServiceManager中，這樣就其他組件就可以通過ServiceManager來獲得他的訪問接口了。
來看看PackageManagerService的構造函數

2.2 PackageManagerService

    public PackageManagerService(Context context, Installer installer,
            boolean factoryTest, boolean onlyCore) {
        EventLog.writeEvent(EventLogTags.BOOT_PROGRESS_PMS_START,
                SystemClock.uptimeMillis());

        if (mSdkVersion <= 0) {
            Slog.w(TAG, "**** ro.build.version.sdk not set!");
        }

        mContext = context;
        mFactoryTest = factoryTest;
        mOnlyCore = onlyCore;
        mLazyDexOpt = "eng".equals(SystemProperties.get("ro.build.type"));
        mMetrics = new DisplayMetrics();
        mSettings = new Settings(mPackages);
        mSettings.addSharedUserLPw("android.uid.system", Process.SYSTEM_UID,
                ApplicationInfo.FLAG_SYSTEM, ApplicationInfo.PRIVATE_FLAG_PRIVILEGED);
        mSettings.addSharedUserLPw("android.uid.phone", RADIO_UID,
                ApplicationInfo.FLAG_SYSTEM, ApplicationInfo.PRIVATE_FLAG_PRIVILEGED);
        mSettings.addSharedUserLPw("android.uid.log", LOG_UID,
                ApplicationInfo.FLAG_SYSTEM, ApplicationInfo.PRIVATE_FLAG_PRIVILEGED);
        mSettings.addSharedUserLPw("android.uid.nfc", NFC_UID,
                ApplicationInfo.FLAG_SYSTEM, ApplicationInfo.PRIVATE_FLAG_PRIVILEGED);
        mSettings.addSharedUserLPw("android.uid.bluetooth", BLUETOOTH_UID,
                ApplicationInfo.FLAG_SYSTEM, ApplicationInfo.PRIVATE_FLAG_PRIVILEGED);
        mSettings.addSharedUserLPw("android.uid.shell", SHELL_UID,
                ApplicationInfo.FLAG_SYSTEM, ApplicationInfo.PRIVATE_FLAG_PRIVILEGED);

        // TODO: add a property to control this?
        long dexOptLRUThresholdInMinutes;
        if (mLazyDexOpt) {
            dexOptLRUThresholdInMinutes = 30; // only last 30 minutes of apps for eng builds.
        } else {
            dexOptLRUThresholdInMinutes = 7 * 24 * 60; // apps used in the 7 days for users.
        }
        mDexOptLRUThresholdInMills = dexOptLRUThresholdInMinutes * 60 * 1000;

        String separateProcesses = SystemProperties.get("debug.separate_processes");
        if (separateProcesses != null && separateProcesses.length() > 0) {
            if ("*".equals(separateProcesses)) {
                mDefParseFlags = PackageParser.PARSE_IGNORE_PROCESSES;
                mSeparateProcesses = null;
                Slog.w(TAG, "Running with debug.separate_processes: * (ALL)");
            } else {
                mDefParseFlags = 0;
                mSeparateProcesses = separateProcesses.split(",");
                Slog.w(TAG, "Running with debug.separate_processes: "
                        + separateProcesses);
            }
        } else {
            mDefParseFlags = 0;
            mSeparateProcesses = null;
        }

        mInstaller = installer;
        mPackageDexOptimizer = new PackageDexOptimizer(this);
        mMoveCallbacks = new MoveCallbacks(FgThread.get().getLooper());

        mOnPermissionChangeListeners = new OnPermissionChangeListeners(
                FgThread.get().getLooper());

        getDefaultDisplayMetrics(context, mMetrics);

        SystemConfig systemConfig = SystemConfig.getInstance();
        mGlobalGids = systemConfig.getGlobalGids();
        mSystemPermissions = systemConfig.getSystemPermissions();
        mAvailableFeatures = systemConfig.getAvailableFeatures();

        synchronized (mInstallLock) {
        // writer
        synchronized (mPackages) {
            mHandlerThread = new ServiceThread(TAG,
                    Process.THREAD_PRIORITY_BACKGROUND, true /*allowIo*/);
            mHandlerThread.start();
            mHandler = new PackageHandler(mHandlerThread.getLooper());
            Watchdog.getInstance().addThread(mHandler, WATCHDOG_TIMEOUT);

            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");

            sUserManager = new UserManagerService(context, this,
                    mInstallLock, mPackages);

            // Propagate permission configuration in to package manager.
            ArrayMap permConfig
                    = systemConfig.getPermissions();
            for (int i=0; i libConfig = systemConfig.getSharedLibraries();
            for (int i=0; i alreadyDexOpted = new ArraySet();

            /**
             * Add everything in the in the boot class path to the
             * list of process files because dexopt will have been run
             * if necessary during zygote startup.
             */
            final String bootClassPath = System.getenv("BOOTCLASSPATH");
            final String systemServerClassPath = System.getenv("SYSTEMSERVERCLASSPATH");

            if (bootClassPath != null) {
                String[] bootClassPathElements = splitString(bootClassPath, ':');
                for (String element : bootClassPathElements) {
                    alreadyDexOpted.add(element);
                }
            } else {
                Slog.w(TAG, "No BOOTCLASSPATH found!");
            }

            if (systemServerClassPath != null) {
                String[] systemServerClassPathElements = splitString(systemServerClassPath, ':');
                for (String element : systemServerClassPathElements) {
                    alreadyDexOpted.add(element);
                }
            } else {
                Slog.w(TAG, "No SYSTEMSERVERCLASSPATH found!");
            }

            final List allInstructionSets = InstructionSets.getAllInstructionSets();
            final String[] dexCodeInstructionSets =
                    getDexCodeInstructionSets(
                            allInstructionSets.toArray(new String[allInstructionSets.size()]));

            /**
             * Ensure all external libraries have had dexopt run on them.
             */
            if (mSharedLibraries.size() > 0) {
                // NOTE: For now, we're compiling these system "shared libraries"
                // (and framework jars) into all available architectures. It's possible
                // to compile them only when we come across an app that uses them (there's
                // already logic for that in scanPackageLI) but that adds some complexity.
                for (String dexCodeInstructionSet : dexCodeInstructionSets) {
                    for (SharedLibraryEntry libEntry : mSharedLibraries.values()) {
                        final String lib = libEntry.path;
                        if (lib == null) {
                            continue;
                        }

                        try {
                            int dexoptNeeded = DexFile.getDexOptNeeded(lib, null, dexCodeInstructionSet, false);
                            if (dexoptNeeded != DexFile.NO_DEXOPT_NEEDED) {
                                alreadyDexOpted.add(lib);
                                mInstaller.dexopt(lib, Process.SYSTEM_UID, true, dexCodeInstructionSet, dexoptNeeded, false);
                            }
                        } catch (FileNotFoundException e) {
                            Slog.w(TAG, "Library not found: " + lib);
                        } catch (IOException e) {
                            Slog.w(TAG, "Cannot dexopt " + lib + "; is it an APK or JAR? "
                                    + e.getMessage());
                        }
                    }
                }
            }

            File frameworkDir = new File(Environment.getRootDirectory(), "framework");

            // Gross hack for now: we know this file doesn't contain any
            // code, so don't dexopt it to avoid the resulting log spew.
            alreadyDexOpted.add(frameworkDir.getPath() + "/framework-res.apk");

            // Gross hack for now: we know this file is only part of
            // the boot class path for art, so don't dexopt it to
            // avoid the resulting log spew.
            alreadyDexOpted.add(frameworkDir.getPath() + "/core-libart.jar");

            /**
             * There are a number of commands implemented in Java, which
             * we currently need to do the dexopt on so that they can be
             * run from a non-root shell.
             */
            String[] frameworkFiles = frameworkDir.list();
            if (frameworkFiles != null) {
                // TODO: We could compile these only for the most preferred ABI. We should
                // first double check that the dex files for these commands are not referenced
                // by other system apps.
                for (String dexCodeInstructionSet : dexCodeInstructionSets) {
                    for (int i=0; i<frameworkfiles.length; i++)="" {="" file="" libpath="new" file(frameworkdir,="" frameworkfiles[i]);="" string="" path="libPath.getPath();" skip="" the="" if="" we="" already="" did="" it.="" (alreadydexopted.contains(path))="" continue;="" }="" it="" is="" not="" a="" type="" want="" to="" dexopt.="" (!path.endswith(".apk")="" &&="" !path.endswith(".jar"))="" try="" int="" dexoptneeded="DexFile.getDexOptNeeded(path," null,="" dexcodeinstructionset,="" false);="" (dexoptneeded="" !="DexFile.NO_DEXOPT_NEEDED)" minstaller.dexopt(path,="" process.system_uid,="" true,="" dexoptneeded,="" catch="" (filenotfoundexception="" e)="" slog.w(tag,="" "jar="" found:="" "="" +="" path);="" (ioexception="" "exception="" reading="" jar:="" path,="" e);="" final="" versioninfo="" ver="mSettings.getInternalVersion();" misupgrade="!Build.FINGERPRINT.equals(ver.fingerprint);" when="" upgrading="" from="" pre-m,="" promote="" system="" app="" permissions="" install="" runtime="" mpromotesystemapps="mIsUpgrade" ver.sdkversion="" 

這段代碼很長，不過其實其邏輯也很是很簡單的。其中有個類型為Settings的成員變量mSettings，這是用來管理apk安裝信息的，假如Apk安裝了，那麼Settings就會記錄下他的信息，下次再碰到要安裝這個apk的時候直接調用之前保存的信息就可以了。這個操作是由Settings.readLPw操作來完成的。
接著使用scanDirLI來安裝保存在/system/framework、/system/app、/vendor/app、/data/app、/data/app-private中的apk。
安裝完之後就調用updatePermissionsLPw來申請為了特定的資源訪問權限的apk分配相應的linux用戶組ID。
最後調用Settings.readLPw將之前獲得的權限和apk安裝信息保存到本地的一個配置文件中，以便下次在安裝這些apk的時候，就可以將需要的apk信息很快速的恢復回來。
接著我們來一一分析這幾個函數

2.3 Settings.readLPw

    boolean readLPw(PackageManagerService service, List users, int sdkVersion,
            boolean onlyCore) {
        FileInputStream str = null;
        if (mBackupSettingsFilename.exists()) {
            try {
                str = new FileInputStream(mBackupSettingsFilename);

                }
            } catch (java.io.IOException e) {
                // We'll try for the normal settings file.
            }
        }

        mPendingPackages.clear();
        mPastSignatures.clear();
        mKeySetRefs.clear();

        try {
            if (str == null) {
                if (!mSettingsFilename.exists()) {

                    return false;
                }
                str = new FileInputStream(mSettingsFilename);
            }
            XmlPullParser parser = Xml.newPullParser();
            parser.setInput(str, StandardCharsets.UTF_8.name());

            int type;


            int outerDepth = parser.getDepth();
            while ((type = parser.next()) != XmlPullParser.END_DOCUMENT
                    && (type != XmlPullParser.END_TAG || parser.getDepth() > outerDepth)) {


                String tagName = parser.getName();
                if (tagName.equals("package")) {
                    readPackageLPw(parser);
                }
                ...
                } else if (tagName.equals("shared-user")) {
                    readSharedUserLPw(parser);
                } 
            }

            str.close();

        } catch (XmlPullParserException e) {
           ...
        } catch (java.io.IOException e) {
           ...
        }
        return true;
    }

這裡是主要是針對packages.xml和packages-backup.xml，第一個是保存的上一次的apk安裝信息，第二個是備份前一個的文件。通過XmlPullParser將安裝信息寫入這個兩個xml文件，具體什麼信息就不關心了，主要看看用戶ID相關的信息就好了。通過調用readPackageLPw來獲取用戶ID。接著調用readSharedUserLPw來獲取上一次安裝這個apk時分配過來的用戶ID。

2.3 PackageManagerService.scanDirLI

    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();
                    }
                }
            }
        }
    }

先循環檢測參數dir所描述的一個目錄中的每一個文件。檢測到以”.apk”結尾的文件後，就調用scanPackageLI來對他們進行解析，解析完後就會返回一個Package對象pkg。
如果scanPackageLI發現他檢測的apk不是一個真正的apk的話，就會刪除它。

2.3 PackageManagerService.updatePermissionsLPw

private void updatePermissionsLPw(String changingPkg, PackageParser.Package pkgInfo,
            int flags) {
        final String volumeUuid = (pkgInfo != null) ? getVolumeUuidForPackage(pkgInfo) : null;
        updatePermissionsLPw(changingPkg, pkgInfo, volumeUuid, flags);
    }

    private void updatePermissionsLPw(String changingPkg,
            PackageParser.Package pkgInfo, String replaceVolumeUuid, int flags) {
        // Make sure there are no dangling permission trees.
        Iterator it = mSettings.mPermissionTrees.values().iterator();
        while (it.hasNext()) {
            final BasePermission bp = it.next();
            if (bp.packageSetting == null) {
                // We may not yet have parsed the package, so just see if
                // we still know about its settings.
                bp.packageSetting = mSettings.mPackages.get(bp.sourcePackage);
            }
            if (bp.packageSetting == null) {
                Slog.w(TAG, "Removing dangling permission tree: " + bp.name
                        + " from package " + bp.sourcePackage);
                it.remove();
            } else if (changingPkg != null && changingPkg.equals(bp.sourcePackage)) {
                if (pkgInfo == null || !hasPermission(pkgInfo, bp.name)) {
                    Slog.i(TAG, "Removing old permission tree: " + bp.name
                            + " from package " + bp.sourcePackage);
                    flags |= UPDATE_PERMISSIONS_ALL;
                    it.remove();
                }
            }
        }

        // Make sure all dynamic permissions have been assigned to a package,
        // and make sure there are no dangling permissions.
        it = mSettings.mPermissions.values().iterator();
        while (it.hasNext()) {
            final BasePermission bp = it.next();
            if (bp.type == BasePermission.TYPE_DYNAMIC) {
                if (DEBUG_SETTINGS) Log.v(TAG, "Dynamic permission: name="
                        + bp.name + " pkg=" + bp.sourcePackage
                        + " info=" + bp.pendingInfo);
                if (bp.packageSetting == null && bp.pendingInfo != null) {
                    final BasePermission tree = findPermissionTreeLP(bp.name);
                    if (tree != null && tree.perm != null) {
                        bp.packageSetting = tree.packageSetting;
                        bp.perm = new PackageParser.Permission(tree.perm.owner,
                                new PermissionInfo(bp.pendingInfo));
                        bp.perm.info.packageName = tree.perm.info.packageName;
                        bp.perm.info.name = bp.name;
                        bp.uid = tree.uid;
                    }
                }
            }
            if (bp.packageSetting == null) {
                // We may not yet have parsed the package, so just see if
                // we still know about its settings.
                bp.packageSetting = mSettings.mPackages.get(bp.sourcePackage);
            }
            if (bp.packageSetting == null) {
                Slog.w(TAG, "Removing dangling permission: " + bp.name
                        + " from package " + bp.sourcePackage);
                it.remove();
            } else if (changingPkg != null && changingPkg.equals(bp.sourcePackage)) {
                if (pkgInfo == null || !hasPermission(pkgInfo, bp.name)) {
                    Slog.i(TAG, "Removing old permission: " + bp.name
                            + " from package " + bp.sourcePackage);
                    flags |= UPDATE_PERMISSIONS_ALL;
                    it.remove();
                }
            }
        }

        // Now update the permissions for all packages, in particular
        // replace the granted permissions of the system packages.
        if ((flags&UPDATE_PERMISSIONS_ALL) != 0) {
            for (PackageParser.Package pkg : mPackages.values()) {
                if (pkg != pkgInfo) {
                    // Only replace for packages on requested volume
                    final String volumeUuid = getVolumeUuidForPackage(pkg);
                    final boolean replace = ((flags & UPDATE_PERMISSIONS_REPLACE_ALL) != 0)
                            && Objects.equals(replaceVolumeUuid, volumeUuid);
                    grantPermissionsLPw(pkg, replace, changingPkg);
                }
            }
        }

        if (pkgInfo != null) {
            // Only replace for packages on requested volume
            final String volumeUuid = getVolumeUuidForPackage(pkgInfo);
            final boolean replace = ((flags & UPDATE_PERMISSIONS_REPLACE_PKG) != 0)
                    && Objects.equals(replaceVolumeUuid, volumeUuid);
            grantPermissionsLPw(pkgInfo, replace, changingPkg);
        }
    }