当系统启动之后，就会注册各种系统服务，如WindowManagerService、ActivityManagerService等，其中就有PackageManagerService。PMS启动之后，就会扫描已安装的apk目录，并解析apk下的androidmanifest.xml文件得到app的信息，而在androidmanifest.xml中又包含了activity、service等组件的注册信息，当PMS解析完成之后，整个app的信息树就被构建出来了。
那么PMS是如何工作的呢，我们来一起分析一下。
PMS的创建
通常情况下，我们可以通过以下方式获取到PackageManager：
PackageManager packageManager = getPackageManager();
点进去可以发现PackageManager也是一个抽象类，而mBase就是Context：
@Override
public PackageManager getPackageManager() {
return mBase.getPackageManager();
}
所以我们直接看Context的实现类ContextImpl：
@Override
public PackageManager getPackageManager() {
if (mPackageManager != null) {
return mPackageManager;
}
IPackageManager pm = ActivityThread.getPackageManager();
if (pm != null) {
// Doesn't matter if we make more than one instance.
return (mPackageManager = new ApplicationPackageManager(this, pm));
}
return null;
}
跳转到ActivityThread：
public static IPackageManager getPackageManager() {
if (sPackageManager != null) {
//Slog.v("PackageManager", "returning cur default = " + sPackageManager);
return sPackageManager;
}
IBinder b = ServiceManager.getService("package");
//Slog.v("PackageManager", "default service binder = " + b);
sPackageManager = IPackageManager.Stub.asInterface(b);
//Slog.v("PackageManager", "default service = " + sPackageManager);
return sPackageManager;
}
可以发现同样和WMS一样都是通过ServiceManager.getService(“package”)来获得IBinder对象，并最终转化为PackageManager。
那么PMS是如何添加到ServiceManager中的呢？
这个时候我们需要去看SystemServer.java，这个类会创建系统的服务：
private PowerManagerService mPowerManagerService;
private ActivityManagerService mActivityManagerService;
private WebViewUpdateService mWebViewUpdateService;
private DisplayManagerService mDisplayManagerService;
private PackageManagerService mPackageManagerService;
mPackageManagerService = PackageManagerService.main(mSystemContext, installer,
mFactoryTestMode != FactoryTest.FACTORY_TEST_OFF, mOnlyCore);
// PackageManagerService.java
public static PackageManagerService main(Context context, Installer installer,
boolean factoryTest, boolean onlyCore) {
// Self-check for initial settings.
PackageManagerServiceCompilerMapping.checkProperties();
PackageManagerService m = new PackageManagerService(context, installer,
factoryTest, onlyCore);
m.enableSystemUserPackages();
// Disable any carrier apps. We do this very early in boot to prevent the apps from being
// disabled after already being started.
CarrierAppUtils.disableCarrierAppsUntilPrivileged(context.getOpPackageName(), m,
UserHandle.USER_SYSTEM);
ServiceManager.addService("package", m);
return m;
}
可以发现，在SystemServer中PMS最终通过PackageManagerService.main函数创建出来，并通过ServiceManager.addService(“package”, m)添加到ServiceManager中。
PMS的工作原理
PMS的主要工作就是解析已安装的apk目录，并解析AndroidManifest.xml文件，最终解析出整个apk的信息树。
首先我们先看PMS的构造函数：
public PackageManagerService(Context context, Installer installer,
boolean factoryTest, boolean onlyCore) {
// 获取/data目录
File dataDir = Environment.getDataDirectory();
// 获取/data/data目录，也就是第三方软件目录
mAppInstallDir = new File(dataDir, "app");
// 获取framework资源、系统app资源
File frameworkDir = new File(Environment.getRootDirectory(), "framework");
final File systemAppDir = new File(Environment.getRootDirectory(), "app");
// 加载framework资源
scanDirTracedLI(frameworkDir, mDefParseFlags
| PackageParser.PARSE_IS_SYSTEM
| PackageParser.PARSE_IS_SYSTEM_DIR
| PackageParser.PARSE_IS_PRIVILEGED,
scanFlags | SCAN_NO_DEX, 0);
// 加载系统应用
scanDirTracedLI(mAppInstallDir, 0, scanFlags | SCAN_REQUIRE_KNOWN, 0);
// 加载第三方应用
scanDirLI(mEphemeralInstallDir, mDefParseFlags
| PackageParser.PARSE_IS_EPHEMERAL,
scanFlags | SCAN_REQUIRE_KNOWN, 0);
}
从上面的代码可以看到，PMS不仅要加载已经安装好的apk，还需要加载framework的相关西苑，当加载好了之后才开始扫描指定目录下的apk文件并解析。
接下来我们继续看scanDirLI：
private void scanDirLI(File dir, final int parseFlags, int scanFlags, long currentTime) {
final File[] files = dir.listFiles();
for (File file : files) {
final boolean isPackage = (isApkFile(file) || file.isDirectory())
&& !PackageInstallerService.isStageName(file.getName());
if (!isPackage) {
// 如果不是package文件，continue
continue;
}
try {
// 解析apk文件
scanPackageTracedLI(file, parseFlags | PackageParser.PARSE_MUST_BE_APK,
scanFlags, currentTime, null);
} catch (PackageManagerException e) {
Slog.w(TAG, "Failed to parse " + file + ": " + e.getMessage());
}
}
}
private PackageParser.Package scanPackageTracedLI(File scanFile, final int parseFlags,
int scanFlags, long currentTime, UserHandle user) throws PackageManagerException {
Trace.traceBegin(TRACE_TAG_PACKAGE_MANAGER, "scanPackage");
try {
return scanPackageLI(scanFile, parseFlags, scanFlags, currentTime, user);
} finally {
Trace.traceEnd(TRACE_TAG_PACKAGE_MANAGER);
}
}
private PackageParser.Package scanPackageLI(File scanFile, int parseFlags, int scanFlags,
long currentTime, UserHandle user) throws PackageManagerException {
// 创建包解析器
PackageParser pp = new PackageParser();
pp.setSeparateProcesses(mSeparateProcesses);
pp.setOnlyCoreApps(mOnlyCore);
pp.setDisplayMetrics(mMetrics);
if ((scanFlags & SCAN_TRUSTED_OVERLAY) != 0) {
parseFlags |= PackageParser.PARSE_TRUSTED_OVERLAY;
}
Trace.traceBegin(TRACE_TAG_PACKAGE_MANAGER, "parsePackage");
final PackageParser.Package pkg;
try {
// 解析apk
pkg = pp.parsePackage(scanFile, parseFlags);
} catch (PackageParserException e) {
throw PackageManagerException.from(e);
} finally {
Trace.traceEnd(TRACE_TAG_PACKAGE_MANAGER);
}
return scanPackageLI(pkg, scanFile, parseFlags, scanFlags, currentTime, user);
}
通过一系列的调用最终发现解析apk是通过PackageParser也就是包解析器，对apk文件进行parsePackage方法的解析：
public Package parsePackage(File packageFile, int flags) throws PackageParserException {
if (packageFile.isDirectory()) {
return parseClusterPackage(packageFile, flags);
} else {
// 解析单个apk
return parseMonolithicPackage(packageFile, flags);
}
}
@Deprecated
public Package parseMonolithicPackage(File apkFile, int flags) throws PackageParserException {
final PackageLite lite = parseMonolithicPackageLite(apkFile, flags);
if (mOnlyCoreApps) {
if (!lite.coreApp) {
throw new PackageParserException(INSTALL_PARSE_FAILED_MANIFEST_MALFORMED,
"Not a coreApp: " + apkFile);
}
}
// 构建AssetManager
final AssetManager assets = new AssetManager();
try {
final Package pkg = parseBaseApk(apkFile, assets, flags);
pkg.setCodePath(apkFile.getAbsolutePath());
pkg.setUse32bitAbi(lite.use32bitAbi);
return pkg;
} finally {
IoUtils.closeQuietly(assets);
}
}
private Package parseBaseApk(File apkFile, AssetManager assets, int flags)
throws PackageParserException {
final String apkPath = apkFile.getAbsolutePath();
// 构建资源
Resources res = null;
XmlResourceParser parser = null;
try {
res = new Resources(assets, mMetrics, null);
assets.setConfiguration(0, 0, null, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
Build.VERSION.RESOURCES_SDK_INT);
// 获取AndroidManifest.xml解析器
parser = assets.openXmlResourceParser(cookie, ANDROID_MANIFEST_FILENAME);
final String[] outError = new String[1];
// 解析
final Package pkg = parseBaseApk(res, parser, flags, outError);
pkg.setVolumeUuid(volumeUuid);
pkg.setApplicationVolumeUuid(volumeUuid);
pkg.setBaseCodePath(apkPath);
pkg.setSignatures(null);
return pkg;
} catch (PackageParserException e) {
throw e;
} catch (Exception e) {
throw new PackageParserException(INSTALL_PARSE_FAILED_UNEXPECTED_EXCEPTION,
"Failed to read manifest from " + apkPath, e);
} finally {
IoUtils.closeQuietly(parser);
}
}
可以发现调用parser = assets.openXmlResourceParser(cookie, ANDROID_MANIFEST_FILENAME)获取AndroidManifest.xml解析器
private static final String ANDROID_MANIFEST_FILENAME = "AndroidManifest.xml";
从这我们可以得出一个结论如果一个apk包没有AndroidManifest.xml文件，那么我们在打开的时候就会报错，包括lib文件。
我们继续追踪，现在到了parseBaseApk方法：
private Package parseBaseApk(Resources res, XmlResourceParser parser, int flags,
String[] outError) throws XmlPullParserException, IOException {
final String splitName;
final String pkgName;
// 解析apk的包名
Pair<String, String> packageSplit = parsePackageSplitNames(parser, parser);
pkgName = packageSplit.first;
splitName = packageSplit.second;
// 构建package对象
final Package pkg = new Package(pkgName);
TypedArray sa = res.obtainAttributes(parser,
com.android.internal.R.styleable.AndroidManifest);
// 获取apk的versioncode、versionname等信息
pkg.mVersionCode = pkg.applicationInfo.versionCode = sa.getInteger(
com.android.internal.R.styleable.AndroidManifest_versionCode, 0);
pkg.baseRevisionCode = sa.getInteger(
com.android.internal.R.styleable.AndroidManifest_revisionCode, 0);
pkg.mVersionName = sa.getNonConfigurationString(
com.android.internal.R.styleable.AndroidManifest_versionName, 0);
if (pkg.mVersionName != null) {
pkg.mVersionName = pkg.mVersionName.intern();
}
pkg.coreApp = parser.getAttributeBooleanValue(null, "coreApp", false);
sa.recycle();
return parseBaseApkCommon(pkg, null, res, parser, flags, outError);
}
上面这个方法我们通过解析AndroidManifest.xml获取到了apk包的相关信息。
最终return调用的parseBaseApkCommon方法才是解析xml文件的函数。
private Package parseBaseApkCommon(Package pkg, Set<String> acceptedTags, Resources res,
XmlResourceParser parser, int flags, String[] outError) throws XmlPullParserException,
IOException {
mParseInstrumentationArgs = null;
mParseActivityArgs = null;
mParseServiceArgs = null;
mParseProviderArgs = null;
parseBaseApplication(pkg, res, parser, flags, outError)；
}
private boolean parseBaseApplication(Package owner, Resources res,
XmlResourceParser parser, int flags, String[] outError)
throws XmlPullParserException, IOException {
if (tagName.equals("activity")) {
Activity a = parseActivity(owner, res, parser, flags, outError, false,
owner.baseHardwareAccelerated);
if (a == null) {
mParseError = PackageManager.INSTALL_PARSE_FAILED_MANIFEST_MALFORMED;
return false;
}
owner.activities.add(a);
} else if (tagName.equals("receiver")) {
Activity a = parseActivity(owner, res, parser, flags, outError, true, false);
if (a == null) {
mParseError = PackageManager.INSTALL_PARSE_FAILED_MANIFEST_MALFORMED;
return false;
}
owner.receivers.add(a);
}
}
由于代码太长，只截取了一点典型代码，通过上面我们终于发现了activity、receiver等标签，可以发现就是普通的xml解析，解析出来之后并返回一个Activity的实例，在加入到activities列表中。
public final ArrayList<Activity> activities = new ArrayList<Activity>(0);
public final ArrayList<Activity> receivers = new ArrayList<Activity>(0);
public final ArrayList<Provider> providers = new ArrayList<Provider>(0);
public final ArrayList<Service> services = new ArrayList<Service>(0);
最终通过解析所有的标签，把所有的节点信息都保存到对应的list中。
到这一步的时候，整个apk的信息树就已经被建立好了，整个apk的应用名、包名、图标、activity、service等信息都存储到了系统中。
