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