Android系统在启动的过程中，会启动一个应用程序管理服务PackageManagerService，这个服务负责扫描系统中特定的目录，找到里面的应用程序文件，即以Apk为后缀的文件，然后对这些文件进解析，得到应用程序的相关信息，完成应用程序的安装过程，本文将详细分析这个过程。

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        应用程序管理服务PackageManagerService安装应用程序的过程，其实就是解析析应用程序配置文件AndroidManifest.xml的过程，并从里面得到得到应用程序的相关信息，例如得到应用程序的组件Activity、Service、Broadcast Receiver和Content Provider等信息，有了这些信息后，通过ActivityManagerService这个服务，我们就可以在系统中正常地使用这些应用程序了。

        应用程序管理服务PackageManagerService是系统启动的时候由SystemServer组件启动的，启后它就会执行应用程序安装的过程，因此，本文将从SystemServer启动PackageManagerService服务的过程开始分析系统中的应用程序安装的过程。

        应用程序管理服务PackageManagerService从启动到安装应用程序的过程如下图所示：

        下面我们具体分析每一个步骤。

        Step 1. SystemServer.main

        这个函数定义在frameworks/base/services/java/com/android/server/SystemServer.java文件中：

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

        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。

        Step 2. SystemServer.init1

        这个函数是一个JNI方法，实现在 frameworks/base/services/jni/com_android_server_SystemServer.cpp文件中：

namespace android {
    extern "C" int system_init();static void android_server_SystemServer_init1(JNIEnv* env, jobject clazz){    system_init();}/* * JNI registration. */static JNINativeMethod gMethods[] = {    /* name, signature, funcPtr */    { "init1", "([Ljava/lang/String;)V", (void*) android_server_SystemServer_init1 },};int register_android_server_SystemServer(JNIEnv* env){    return jniRegisterNativeMethods(env, "com/android/server/SystemServer",            gMethods, NELEM(gMethods));}}; // namespace android

        这个函数很简单，只是调用了system_init函数来进一步执行操作。

        Step 3. libsystem_server.system_init

        函数system_init实现在libsystem_server库中，源代码位于frameworks/base/cmds/system_server/library/system_init.cpp文件中：

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;}

        这个函数首先会初始化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。关于这个AndroidRuntime实例变量runtime的相关资料，可能参考前面一篇文章 Android应用程序进程启动过程的源代码分析一文。

        Step 4. AndroidRuntime.callStatic

        这个函数定义在frameworks/base/core/jni/AndroidRuntime.cpp文件中：

/** Call a static Java Programming Language function that takes no arguments and returns void.*/status_t AndroidRuntime::callStatic(const char* className, const char* methodName){ JNIEnv* env; jclass clazz; jmethodID methodId; env = getJNIEnv(); if (env == NULL)  return UNKNOWN_ERROR; clazz = findClass(env, className); if (clazz == NULL) {  LOGE("ERROR: could not find class '%s'\n", className);  return UNKNOWN_ERROR; } methodId = env->GetStaticMethodID(clazz, methodName, "()V"); if (methodId == NULL) {  LOGE("ERROR: could not find method %s.%s\n", className, methodName);  return UNKNOWN_ERROR; } env->CallStaticVoidMethod(clazz, methodId); return NO_ERROR;}

        这个函数调用由参数className指定的java类的静态成员函数，这个静态成员函数是由参数methodName指定的。上面传进来的参数className的值为"com/android/server/SystemServer"，而参数methodName的值为"init2"，因此，接下来就会调用SystemServer类的init2函数了。

        Step 5. SystemServer.init2

        这个函数定义在frameworks/base/services/java/com/android/server/SystemServer.java文件中：

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(); }}

        这个函数创建了一个ServerThread线程，PackageManagerService服务就是这个线程中启动的了。这里调用了ServerThread实例thr的start函数之后，下面就会执行这个实例的run函数了。

        Step 6. ServerThread.run

        这个函数定义在frameworks/base/services/java/com/android/server/SystemServer.java文件中：

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);  }  ...... } ......}

        这个函数除了启动PackageManagerService服务之外，还启动了其它很多的服务，例如在前面学习Activity和Service的几篇文章中经常看到的ActivityManagerService服务，有兴趣的读者可以自己研究一下。

        Step 7. PackageManagerService.main

        这个函数定义在frameworks/base/services/java/com/android/server/PackageManagerService.java文件中：

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; } ......}

        这个函数创建了一个PackageManagerService服务实例，然后把这个服务添加到ServiceManager中去，ServiceManager是Android系统Binder进程间通信机制的守护进程，负责管理系统中的Binder对象，具体可以参考 浅谈Service Manager成为Android进程间通信（IPC）机制Binder守护进程之路一文。
        在创建这个PackageManagerService服务实例时，会在PackageManagerService类的构造函数中开始执行安装应用程序的过程：

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);    ......   }  } } ......}

        这里会调用scanDirLI函数来扫描移动设备上的下面这五个目录中的Apk文件：

        /system/framework

        /system/app

        /vendor/app

        /data/app

        /data/app-private

       Step 8. PackageManagerService.scanDirLI
       这个函数定义在frameworks/base/services/java/com/android/server/PackageManagerService.java文件中：

class PackageManagerService extends IPackageManager.Stub { ...... private void scanDirLI(File dir, int flags, int scanMode, long currentTime) {  String[] files = dir.list();  ......  int i;  for (i=0; i<files.length; i++) {   File file = new File(dir, files[i]);   if (!isPackageFilename(files[i])) {    // Ignore entries which are not apk's    continue;   }   PackageParser.Package pkg = scanPackageLI(file,    flags|PackageParser.PARSE_MUST_BE_APK, scanMode, currentTime);   // Don't mess around with apps in system partition.   if (pkg == null && (flags & PackageParser.PARSE_IS_SYSTEM) == 0 &&    mLastScanError == PackageManager.INSTALL_FAILED_INVALID_APK) {     // Delete the apk     Slog.w(TAG, "Cleaning up failed install of " + file);     file.delete();   }  } } ......}

         对于目录中的每一个文件，如果是以后Apk作为后缀名，那么就调用scanPackageLI函数来对它进行解析和安装。

         Step 9. PackageManagerService.scanPackageLI

         这个函数定义在frameworks/base/services/java/com/android/server/PackageManagerService.java文件中：

class PackageManagerService extends IPackageManager.Stub { ...... private PackageParser.Package scanPackageLI(File scanFile,   int parseFlags, int scanMode, long currentTime) {  ......  String scanPath = scanFile.getPath();  parseFlags |= mDefParseFlags;  PackageParser pp = new PackageParser(scanPath);    ......  final PackageParser.Package pkg = pp.parsePackage(scanFile,   scanPath, mMetrics, parseFlags);  ......  return scanPackageLI(pkg, parseFlags, scanMode | SCAN_UPDATE_SIGNATURE, currentTime); } ......}

        这个函数首先会为这个Apk文件创建一个PackageParser实例，接着调用这个实例的parsePackage函数来对这个Apk文件进行解析。这个函数最后还会调用另外一个版本的scanPackageLI函数把来解析后得到的应用程序信息保存在PackageManagerService中。

        Step 10. PackageParser.parsePackage
        这个函数定义在frameworks/base/core/java/android/content/pm/PackageParser.java文件中：

public class PackageParser { ...... public Package parsePackage(File sourceFile, String destCodePath,   DisplayMetrics metrics, int flags) {  ......  mArchiveSourcePath = sourceFile.getPath();  ......  XmlResourceParser parser = null;  AssetManager assmgr = null;  boolean assetError = true;  try {   assmgr = new AssetManager();   int cookie = assmgr.addAssetPath(mArchiveSourcePath);   if(cookie != 0) {    parser = assmgr.openXmlResourceParser(cookie, "AndroidManifest.xml");    assetError = false;   } else {    ......   }  } catch (Exception e) {   ......  }  ......  String[] errorText = new String[1];  Package pkg = null;  Exception errorException = null;  try {   // XXXX todo: need to figure out correct configuration.   Resources res = new Resources(assmgr, metrics, null);   pkg = parsePackage(res, parser, flags, errorText);  } catch (Exception e) {   ......  }  ......  parser.close();  assmgr.close();  // Set code and resource paths  pkg.mPath = destCodePath;  pkg.mScanPath = mArchiveSourcePath;  //pkg.applicationInfo.sourceDir = destCodePath;  //pkg.applicationInfo.publicSourceDir = destRes;  pkg.mSignatures = null;  return pkg; } ......}

        每一个Apk文件都是一个归档文件，它里面包含了Android应用程序的配置文件AndroidManifest.xml，这里主要就是要对这个配置文件就行解析了，从Apk归档文件中得到这个配置文件后，就调用另一外版本的parsePackage函数对这个应用程序进行解析了：

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; } ......}

        这里就是对AndroidManifest.xml文件中的各个标签进行解析了，各个标签的含义可以参考官方文档 http://developer.android.com/guide/topics/manifest/manifest-intro.html，这里我们只简单看一下application标签的解析，这是通过调用parseApplication函数来进行的。

        Step 11. PackageParser.parseApplication
        这个函数定义在frameworks/base/core/java/android/content/pm/PackageParser.java文件中：

public class PackageParser { ...... private boolean parseApplication(Package owner, Resources res,   XmlPullParser parser, AttributeSet attrs, int flags, String[] outError)   throws XmlPullParserException, IOException {  final ApplicationInfo ai = owner.applicationInfo;  final String pkgName = owner.applicationInfo.packageName;  TypedArray sa = res.obtainAttributes(attrs,   com.android.internal.R.styleable.AndroidManifestApplication);  ......  int type;  while ((type=parser.next()) != parser.END_DOCUMENT   && (type != parser.END_TAG || parser.getDepth() > innerDepth)) {    if (type == parser.END_TAG || type == parser.TEXT) {     continue;    }      String tagName = parser.getName();    if (tagName.equals("activity")) {     Activity a = parseActivity(owner, res, parser, attrs, flags, outError, false);     ......     owner.activities.add(a);    } else if (tagName.equals("receiver")) {     Activity a = parseActivity(owner, res, parser, attrs, flags, outError, true);     ......     owner.receivers.add(a);    } else if (tagName.equals("service")) {     Service s = parseService(owner, res, parser, attrs, flags, outError);     ......     owner.services.add(s);    } else if (tagName.equals("provider")) {     Provider p = parseProvider(owner, res, parser, attrs, flags, outError);     ......     owner.providers.add(p);    } else if (tagName.equals("activity-alias")) {     Activity a = parseActivityAlias(owner, res, parser, attrs, flags, outError);     ......     owner.activities.add(a);    } else if (parser.getName().equals("meta-data")) {     ......    } else if (tagName.equals("uses-library")) {     ......    } else if (tagName.equals("uses-package")) {     ......    } else {     ......    }  }  return true; } ......}

        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，各个标签的含义可以参考官方文档 http://developer.android.com/guide/topics/manifest/manifest-intro.html。

        这里解析完成后，一层层返回到Step 9中，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。

        Step 12. PackageManagerService.scanPackageLI

        这个函数定义在frameworks/base/services/java/com/android/server/PackageManagerService.java文件中：

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; } ......}

        这个函数主要就是把前面解析应用程序得到的package、provider、service、receiver和activity等信息保存在PackageManagerService服务中了。

        这样，在Android系统启动的时候安装应用程序的过程就介绍完了，但是，这些应用程序只是相当于在PackageManagerService服务注册好了，如果我们想要在Android桌面上看到这些应用程序，还需要有一个Home应用程序，负责从PackageManagerService服务中把这些安装好的应用程序取出来，并以友好的方式在桌面上展现出来，例如以快捷图标的形式。在Android系统中，负责把系统中已经安装的应用程序在桌面中展现出来的Home应用程序就是Launcher了，在下一篇文章中，我们将介绍Launcher是如何启动的以及它是如何从PackageManagerService服务中把系统中已经安装好的应用程序展现出来的，敬请期待。

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