系统在启动的时候会启动一个叫做PackageManagerService的服务，顾名思义，这个服务主要管理安装在设备上的应用程序，其中最为重要的工作就是在在系统启动之后，PackageManagerService会扫描特定目录下地以apk为后缀的文件，然后将对应的应用安装到系统中。注意，这里的安装并不是我们平时所说的安装，它指的的是将存放在磁盘之上的静态应用程序文件进行解析，并将相关信息注册到系统中。而具体的解析工作实际就是读取应用的配置文件manifest.xml，并将文件中配置的组件
（Activity,Service,BroadcastRecevier,ContentProvider），权限等信息注册到PackageManagerService中。
本篇博客主要介绍PackageManagerService的启动过程，以及PackageManagerService如何安装各个应用程序。

1.PackageManagerService启动过程

和ActivityManagerService，WindowManagerService一样，PackageManagerService是一个系统级的服务，运行在独立的进程中，而所有的系统级服务都是由SystemServer启动的。所以首先来看看SystemServer的启动过程。

1) SystemServer启动：

SystemServer组件是由Zygote进程负责启动的，启动的时候就会调用它的main函数，这个函数主要调用了JNI方法init1来做一些系统初始化的工作。

public class SystemServer
{
    ......
    native public static void init1(String[] args);
    ......
    public static void main(String[] args) {
        ......
        init1(args);
        ......
    }
    ......
}

2)SystemServer.system_init

经过一系列调用后转到system_init方法，这是一个JNI方法

extern "C" status_t system_init()
{
    LOGI("Entered system_init()");
    sp<ProcessState> proc(ProcessState::self());
    sp<IServiceManager> sm = defaultServiceManager();
    LOGI("ServiceManager: %p\n", sm.get());
    sp<GrimReaper> grim = new GrimReaper();
    sm->asBinder()->linkToDeath(grim, grim.get(), 0);
    char propBuf[PROPERTY_VALUE_MAX];
    property_get("system_init.startsurfaceflinger", propBuf, "1");
    if (strcmp(propBuf, "1") == 0) {
        // Start the SurfaceFlinger
        SurfaceFlinger::instantiate();
    }

    // Start the sensor service
    SensorService::instantiate();

    // On the simulator, audioflinger et al don't get started the
    // same way as on the device, and we need to start them here
    if (!proc->supportsProcesses()) {

        // Start the AudioFlinger
        AudioFlinger::instantiate();

        // Start the media playback service
        MediaPlayerService::instantiate();

        // Start the camera service
        CameraService::instantiate();

        // Start the audio policy service
        AudioPolicyService::instantiate();
    }

    // And now start the Android runtime.  We have to do this bit
    // of nastiness because the Android runtime initialization requires
    // some of the core system services to already be started.
    // All other servers should just start the Android runtime at
    // the beginning of their processes's main(), before calling
    // the init function.
    LOGI("System server: starting Android runtime.\n");

    AndroidRuntime* runtime = AndroidRuntime::getRuntime();

    LOGI("System server: starting Android services.\n");
    runtime->callStatic("com/android/server/SystemServer", "init2");

    // If running in our own process, just go into the thread
    // pool.  Otherwise, call the initialization finished
    // func to let this process continue its initilization.
    if (proc->supportsProcesses()) {
        LOGI("System server: entering thread pool.\n");
        ProcessState::self()->startThreadPool();
        IPCThreadState::self()->joinThreadPool();
        LOGI("System server: exiting thread pool.\n");
    }

    return NO_ERROR;
}

在这个方法中，创建了SurfaceFlinger、SensorService、AudioFlinger、MediaPlayerService、CameraService和AudioPolicyService这几个服务，然后就通过系统全局唯一的AndroidRuntime实例变量runtime的callStatic来调用SystemServer的init2函数了。init2函数很简单，创建一个线程，而PackageManagerService就是在这个线程中创建的。

public class SystemServer
{
    ......
    public static final void init2() {
        Slog.i(TAG, "Entered the Android system server!");
        Thread thr = new ServerThread();
        thr.setName("android.server.ServerThread");
        thr.start();
    }
}

3)ServerThread.run

class ServerThread extends Thread {
    ......
    @Override
    public void run() {
        ......
        IPackageManager pm = null;
        ......
        // Critical services...
        try {
            ......
            Slog.i(TAG, "Package Manager");
            pm = PackageManagerService.main(context,
                        factoryTest != SystemServer.FACTORY_TEST_OFF);
            ......
        } catch (RuntimeException e) {
            Slog.e("System", "Failure starting core service", e);
        }
        ......
    }
    ......
}

在这个线程中创建了PackageManagerService,并同时启动了其main函数。另外在这个线程中还启动了ActivityManagerService等其他Service

2.应用安装

接下来再来看看PackageManagerService启动之后如何进行应用程序的安装。

1）PackageManagerService.main

class PackageManagerService extends IPackageManager.Stub {
    ......
    public static final IPackageManager main(Context context, boolean factoryTest) {
        PackageManagerService m = new PackageManagerService(context, factoryTest);
        ServiceManager.addService("package", m);
        return m;
    }
    ......
}

可以看到，创建完成后，就加载到ServiceManager中。接下来看看PackageManagerService的构造函数：

class PackageManagerService extends IPackageManager.Stub {
    ......

    public PackageManagerService(Context context, boolean factoryTest) {
        ......

        synchronized (mInstallLock) {
            synchronized (mPackages) {
                ......

                File dataDir = Environment.getDataDirectory();
                mAppDataDir = new File(dataDir, "data");
                mSecureAppDataDir = new File(dataDir, "secure/data");
                mDrmAppPrivateInstallDir = new File(dataDir, "app-private");

                ......

                mFrameworkDir = new File(Environment.getRootDirectory(), "framework");
                mDalvikCacheDir = new File(dataDir, "dalvik-cache");

                ......

                // Find base frameworks (resource packages without code).
                mFrameworkInstallObserver = new AppDirObserver(
                mFrameworkDir.getPath(), OBSERVER_EVENTS, true);
                mFrameworkInstallObserver.startWatching();
                scanDirLI(mFrameworkDir, PackageParser.PARSE_IS_SYSTEM
                    | PackageParser.PARSE_IS_SYSTEM_DIR,
                    scanMode | SCAN_NO_DEX, 0);

                // Collect all system packages.
                mSystemAppDir = new File(Environment.getRootDirectory(), "app");
                mSystemInstallObserver = new AppDirObserver(
                    mSystemAppDir.getPath(), OBSERVER_EVENTS, true);
                mSystemInstallObserver.startWatching();
                scanDirLI(mSystemAppDir, PackageParser.PARSE_IS_SYSTEM
                    | PackageParser.PARSE_IS_SYSTEM_DIR, scanMode, 0);

                // Collect all vendor packages.
                mVendorAppDir = new File("/vendor/app");
                mVendorInstallObserver = new AppDirObserver(
                    mVendorAppDir.getPath(), OBSERVER_EVENTS, true);
                mVendorInstallObserver.startWatching();
                scanDirLI(mVendorAppDir, PackageParser.PARSE_IS_SYSTEM
                    | PackageParser.PARSE_IS_SYSTEM_DIR, scanMode, 0);


                mAppInstallObserver = new AppDirObserver(
                    mAppInstallDir.getPath(), OBSERVER_EVENTS, false);
                mAppInstallObserver.startWatching();
                scanDirLI(mAppInstallDir, 0, scanMode, 0);

                mDrmAppInstallObserver = new AppDirObserver(
                    mDrmAppPrivateInstallDir.getPath(), OBSERVER_EVENTS, false);
                mDrmAppInstallObserver.startWatching();
                scanDirLI(mDrmAppPrivateInstallDir, PackageParser.PARSE_FORWARD_LOCK,
                    scanMode, 0);

                ......
            }
        }
    }

    ......
}

可以看到，在构造函数中，PackageManagerService(PMS)会扫描特定目录下的APK文件，然后进行相关的加载工作，这些目录包括：

/system/framework
/system/app
/vendor/app
/data/app
/data/app-private

在每个路径下，都调用了scanDirLI函数，接下来看看对应的函数做了些什么。

2）PackageParser.parsePackage

scanDirLI中又经过多次调用，具体就是扫描对应目录的文件，如果是apk文件，就找到apk文件中的manifest文件，最后再为每一个apk创建一个PackageParser对象，并将manifest文件传递给PackageParser.parsePackage。

public class PackageParser {
    ......

    private Package parsePackage(
            Resources res, XmlResourceParser parser, int flags, String[] outError)
            throws XmlPullParserException, IOException {
        ......

        String pkgName = parsePackageName(parser, attrs, flags, outError);

        ......

        final Package pkg = new Package(pkgName);

        ......

        int type;

        ......

        TypedArray sa = res.obtainAttributes(attrs,
            com.android.internal.R.styleable.AndroidManifest);

        ......

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

                String tagName = parser.getName();
                if (tagName.equals("application")) {
                    ......

                    if (!parseApplication(pkg, res, parser, attrs, flags, outError)) {
                        return null;
                    }
                } else if (tagName.equals("permission-group")) {
                    ......
                } else if (tagName.equals("permission")) {
                    ......
                } else if (tagName.equals("permission-tree")) {
                    ......
                } else if (tagName.equals("uses-permission")) {
                    ......
                } else if (tagName.equals("uses-configuration")) {
                    ......
                } else if (tagName.equals("uses-feature")) {
                    ......
                } else if (tagName.equals("uses-sdk")) {
                    ......
                } else if (tagName.equals("supports-screens")) {
                    ......
                } else if (tagName.equals("protected-broadcast")) {
                    ......
                } else if (tagName.equals("instrumentation")) {
                    ......
                } else if (tagName.equals("original-package")) {
                    ......
                } else if (tagName.equals("adopt-permissions")) {
                    ......
                } else if (tagName.equals("uses-gl-texture")) {
                    ......
                } else if (tagName.equals("compatible-screens")) {
                    ......
                } else if (tagName.equals("eat-comment")) {
                    ......
                } else if (RIGID_PARSER) {
                    ......
                } else {
                    ......
                }
        }

        ......

        return pkg;
    }

    ......
        private Package parsePackage(
            Resources res, XmlResourceParser parser, int flags, String[] outError)
            throws XmlPullParserException, IOException {
        ......

        String pkgName = parsePackageName(parser, attrs, flags, outError);

        ......

        final Package pkg = new Package(pkgName);

        ......

        int type;

        ......

        TypedArray sa = res.obtainAttributes(attrs,
            com.android.internal.R.styleable.AndroidManifest);

        ......

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

                String tagName = parser.getName();
                if (tagName.equals("application")) {
                    ......

                    if (!parseApplication(pkg, res, parser, attrs, flags, outError)) {
                        return null;
                    }
                } else if (tagName.equals("permission-group")) {
                    ......
                } else if (tagName.equals("permission")) {
                    ......
                } else if (tagName.equals("permission-tree")) {
                    ......
                } else if (tagName.equals("uses-permission")) {
                    ......
                } else if (tagName.equals("uses-configuration")) {
                    ......
                } else if (tagName.equals("uses-feature")) {
                    ......
                } else if (tagName.equals("uses-sdk")) {
                    ......
                } else if (tagName.equals("supports-screens")) {
                    ......
                } else if (tagName.equals("protected-broadcast")) {
                    ......
                } else if (tagName.equals("instrumentation")) {
                    ......
                } else if (tagName.equals("original-package")) {
                    ......
                } else if (tagName.equals("adopt-permissions")) {
                    ......
                } else if (tagName.equals("uses-gl-texture")) {
                    ......
                } else if (tagName.equals("compatible-screens")) {
                    ......
                } else if (tagName.equals("eat-comment")) {
                    ......
                } else if (RIGID_PARSER) {
                    ......
                } else {
                    ......
                }
        }

        ......

        return pkg;
    }

    ......
}

这里就是对AndroidManifest.xml文件中的application标签进行解析了，我们常用到的标签就有activity、service、receiver和provider，这里解析完成后，一层层返回，调用另一个版本的scanPackageLI函数把来解析后得到的应用程序信息保存下来。

class PackageManagerService extends IPackageManager.Stub {
    ......

    // Keys are String (package name), values are Package.  This also serves
    // as the lock for the global state.  Methods that must be called with
    // this lock held have the prefix "LP".
    final HashMap<String, PackageParser.Package> mPackages =
        new HashMap<String, PackageParser.Package>();

    ......

    // All available activities, for your resolving pleasure.
    final ActivityIntentResolver mActivities =
    new ActivityIntentResolver();

    // All available receivers, for your resolving pleasure.
    final ActivityIntentResolver mReceivers =
        new ActivityIntentResolver();

    // All available services, for your resolving pleasure.
    final ServiceIntentResolver mServices = new ServiceIntentResolver();

    // Keys are String (provider class name), values are Provider.
    final HashMap<ComponentName, PackageParser.Provider> mProvidersByComponent =
        new HashMap<ComponentName, PackageParser.Provider>();

    ......

    private PackageParser.Package scanPackageLI(PackageParser.Package pkg,
            int parseFlags, int scanMode, long currentTime) {
        ......

        synchronized (mPackages) {
            ......

            // Add the new setting to mPackages
            mPackages.put(pkg.applicationInfo.packageName, pkg);

            ......

            int N = pkg.providers.size();
            int i;
            for (i=0; i<N; i++) {
                PackageParser.Provider p = pkg.providers.get(i);
                p.info.processName = fixProcessName(pkg.applicationInfo.processName,
                    p.info.processName, pkg.applicationInfo.uid);
                mProvidersByComponent.put(new ComponentName(p.info.packageName,
                    p.info.name), p);

                ......
            }

            N = pkg.services.size();
            for (i=0; i<N; i++) {
                PackageParser.Service s = pkg.services.get(i);
                s.info.processName = fixProcessName(pkg.applicationInfo.processName,
                    s.info.processName, pkg.applicationInfo.uid);
                mServices.addService(s);

                ......
            }

            N = pkg.receivers.size();
            r = null;
            for (i=0; i<N; i++) {
                PackageParser.Activity a = pkg.receivers.get(i);
                a.info.processName = fixProcessName(pkg.applicationInfo.processName,
                    a.info.processName, pkg.applicationInfo.uid);
                mReceivers.addActivity(a, "receiver");

                ......
            }

            N = pkg.activities.size();
            for (i=0; i<N; i++) {
                PackageParser.Activity a = pkg.activities.get(i);
                a.info.processName = fixProcessName(pkg.applicationInfo.processName,
                    a.info.processName, pkg.applicationInfo.uid);
                mActivities.addActivity(a, "activity");

                ......
            }

            ......
        }

        ......

        return pkg;
    }

    ......
}

到这里整个应用的安装过程就介绍完了。其实整个过程还是很明确，清晰的。
接下来再来总结一下整个启动过程：

Zygote—>启动SystemServer—>启动ServerThread—>启动PackageManagerService—>扫描特定目录下的apk文件，进行加载—>解析APK的manifest文件，将配置信息加载到PackageManagerService中