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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