編輯：關於Android編程
系統在啟動的時候會啟動一個叫做PackageManagerService的服務，顧名思義，這個服務主要管理安裝在設備上的應用程序，其中最為重要的工作就是在在系統啟動之後，PackageManagerService會掃描特定目錄下地以apk為後綴的文件，然後將對應的應用安裝到系統中。注意，這裡的安裝並不是我們平時所說的安裝，它指的的是將存放在磁盤之上的靜態應用程序文件進行解析，並將相關信息注冊到系統中。而具體的解析工作實際就是讀取應用的配置文件manifest.xml，並將文件中配置的組件
（Activity,Service,BroadcastRecevier,ContentProvider），權限等信息注冊到PackageManagerService中。
本篇博客主要介紹PackageManagerService的啟動過程，以及PackageManagerService如何安裝各個應用程序。
和ActivityManagerService，WindowManagerService一樣，PackageManagerService是一個系統級的服務，運行在獨立的進程中，而所有的系統級服務都是由SystemServer啟動的。所以首先來看看SystemServer的啟動過程。
SystemServer組件是由Zygote進程負責啟動的，啟動的時候就會調用它的main函數，這個函數主要調用了JNI方法init1來做一些系統初始化的工作。
public class SystemServer
{
......
native public static void init1(String[] args);
......
public static void main(String[] args) {
......
init1(args);
......
}
......
}
經過一系列調用後轉到system_init方法，這是一個JNI方法
extern "C" status_t system_init()
{
LOGI("Entered system_init()");
sp proc(ProcessState::self());
sp sm = defaultServiceManager();
LOGI("ServiceManager: %p\n", sm.get());
sp 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;
}
在這個方法中，創建了SurfaceFlinger、SensorService、AudioFlinger、MediaPlayerService、CameraService和AudioPolicyService這幾個服務，然後就通過系統全局唯一的AndroidRuntime實例變量runtime的callStatic來調用SystemServer的init2函數了。init2函數很簡單，創建一個線程，而PackageManagerService就是在這個線程中創建的。
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();
}
}
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);
}
......
}
......
}
在這個線程中創建了PackageManagerService,並同時啟動了其main函數。另外在這個線程中還啟動了ActivityManagerService等其他Service
接下來再來看看PackageManagerService啟動之後如何進行應用程序的安裝。
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，創建完成後，就加載到ServiceManager中。接下來看看PackageManagerService的構造函數：
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，在構造函數中，PackageManagerService(PMS)會掃描特定目錄下的APK文件，然後進行相關的加載工作，這些目錄包括：
/system/framework
/system/app
/vendor/app
/data/app
/data/app-private
在每個路徑下，都調用了scanDirLI函數，接下來看看對應的函數做了些什麼。
scanDirLI中又經過多次調用，具體就是掃描對應目錄的文件，如果是apk文件，就找到apk文件中的manifest文件，最後再為每一個apk創建一個PackageParser對象，並將manifest文件傳遞給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;
}
......
}
這裡就是對AndroidManifest.xml文件中的application標簽進行解析了，我們常用到的標簽就有activity、service、receiver和provider，這裡解析完成後，一層層返回，調用另一個版本的scanPackageLI函數把來解析後得到的應用程序信息保存下來。
<code><code><code><code><code><code><code><code><code><code><code><code><code><code><code><code><code><code><code><code><code><code><code>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; packageparser.provider="" p="pkg.providers.get(i);" p.info.processname="fixProcessName(pkg.applicationInfo.processName," new="" ......="" n="pkg.services.size();" for="" i="0;" packageparser.service="" s="pkg.services.get(i);" s.info.processname="fixProcessName(pkg.applicationInfo.processName," r="null;" packageparser.activity="" a="pkg.receivers.get(i);" a.info.processname="fixProcessName(pkg.applicationInfo.processName," return="" pre="">
到這裡整個應用的安裝過程就介紹完了。其實整個過程還是很明確，清晰的。 接下來再來總結一下整個啟動過程：
Zygote—>啟動SystemServer—>啟動ServerThread—>啟動PackageManagerService—>掃描特定目錄下的apk文件，進行加載—>解析APK的manifest文件，將配置信息加載到PackageManagerService中
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