（一）Intent查找与匹配
Android使用Intent进行组件，进程之间的通信和跳转。Intent具有隐式Intent和显式Intent两种，Android系统通过PackageManagerService来进行系统组件的维护。
系统启动之后会注册各种系统服务，其中就包括PackageManagerService。在启动之后，PMS会扫描已安装的apk目录，解析apk包下的AndroidManifest.xml文件得到App的相关信息，而每个AndroidManifest.xml又包含了Activity，Service等组件的注册信息，当PMS扫描并且解析完信息后，就清晰地描绘出了整棵apk的信息树。
PackageManagerService的构造函数大概有700多行，它加载了系统已安装的各类apk，并加载了Framework资源和核心库。加载了资源和核心库之后才开始对扫描的指定目录下的apk文件进行解析。实现这一操作的代码是scanDirLi函数。
private void scanDirLI(File dir, final int parseFlags, int scanFlags, long currentTime) {
final File[] files = dir.listFiles();
if (ArrayUtils.isEmpty(files)) {
Log.d(TAG, "No files in app dir " + dir);
return;
}
if (DEBUG_PACKAGE_SCANNING) {
Log.d(TAG, "Scanning app dir " + dir + " scanFlags=" + scanFlags
+ " flags=0x" + Integer.toHexString(parseFlags));
}
for (File file : files) {
final boolean isPackage = (isApkFile(file) || file.isDirectory())
&& !PackageInstallerService.isStageName(file.getName());
if (!isPackage) {
// Ignore entries which are not packages
continue;
}
try {
scanPackageTracedLI(file, parseFlags | PackageParser.PARSE_MUST_BE_APK,
scanFlags, currentTime, null);
} catch (PackageManagerException e) {
Slog.w(TAG, "Failed to parse " + file + ": " + e.getMessage());
// Delete invalid userdata apps
if ((parseFlags & PackageParser.PARSE_IS_SYSTEM) == 0 &&
e.error == PackageManager.INSTALL_FAILED_INVALID_APK) {
logCriticalInfo(Log.WARN, "Deleting invalid package at " + file);
removeCodePathLI(file);
}
}
}
}其中scanPackageTracedLI实现了包扫描
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 {
if (DEBUG_INSTALL) Slog.d(TAG, "Parsing: " + scanFile);
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 {
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);
}在scanPackageLI方法中，系统会构建一个PackageParser（包解析器），并调用这个包解析器的parsePackage方法进行文件解析。
public Package parsePackage(File packageFile, int flags) throws PackageParserException {
if (packageFile.isDirectory()) {
return parseClusterPackage(packageFile, flags);
} else {
return parseMonolithicPackage(packageFile, flags);
}
}这个方法会根据是否只有一个apk，分别调用不同的方法进行解析。
无论是parseClusterPackage还是parseMonolithicPackage方法，其中都调用到了parseBaseApk方法。这个方法才具体实现了解析一个AndroidManifest.xml文件的流程。
private Package parseBaseApk(Resources res, XmlResourceParser parser, int flags,
String[] outError) throws XmlPullParserException, IOException {
final String splitName;
final String pkgName;
try {
Pair<String, String> packageSplit = parsePackageSplitNames(parser, parser);
pkgName = packageSplit.first;
splitName = packageSplit.second;
if (!TextUtils.isEmpty(splitName)) {
outError[0] = "Expected base APK, but found split " + splitName;
mParseError = PackageManager.INSTALL_PARSE_FAILED_BAD_PACKAGE_NAME;
return null;
}
} catch (PackageParserException e) {
mParseError = PackageManager.INSTALL_PARSE_FAILED_BAD_PACKAGE_NAME;
return null;
}
final Package pkg = new Package(pkgName);
TypedArray sa = res.obtainAttributes(parser,
com.android.internal.R.styleable.AndroidManifest);
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);
}可以清晰地看到它解析了apk的versionName，versionCode,baseVersionCode。那么apk的各个组件是在哪里解析的呢？
打开方法parseBaseApplication，我们的一切疑问都可以得到回答。
private boolean parseBaseApplication(Package owner, Resources res,
XmlResourceParser parser, int flags, String[] outError) {
//代码省略
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);
} else if (tagName.equals("service")) {
Service s = parseService(owner, res, parser, flags, outError);
if (s == null) {
mParseError = PackageManager.INSTALL_PARSE_FAILED_MANIFEST_MALFORMED;
return false;
}
owner.services.add(s);
} else if (tagName.equals("provider")) {
Provider p = parseProvider(owner, res, parser, flags, outError);
if (p == null) {
mParseError = PackageManager.INSTALL_PARSE_FAILED_MANIFEST_MALFORMED;
return false;
}
owner.providers.add(p);
}
// 代码省略
}显而易见，这里系统将应用的各个组件解析出来，并且存入到了各个集合当中。
至此，整个信息树构建完毕，系统已经存储好了这个应用的组件信息。
（二）信息匹配
我们看一下调用intent的具体方法，一般情况下，我们都是使用startActivity这个方法的。我们跟进去看一下。
@Override
public void startActivity(Intent intent, @Nullable Bundle options) {
if (options != null) {
startActivityForResult(intent, -1, options);
} else {
// Note we want to go through this call for compatibility with
// applications that may have overridden the method.
startActivityForResult(intent, -1);
}
}然后我们在看一下startActivityForResult方法。
//省略代码
options = transferSpringboardActivityOptions(options);
Instrumentation.ActivityResult ar =
mInstrumentation.execStartActivity(
this, mMainThread.getApplicationThread(), mToken, this,
intent, requestCode, options);
if (ar != null) {
mMainThread.sendActivityResult(
mToken, mEmbeddedID, requestCode, ar.getResultCode(),
ar.getResultData());
}
//省略代码其中核心代码是execStartActivity这个方法。
这个方法经过一系列调用，最终调用了resolveActivityInfo方法
public ActivityInfo resolveActivityInfo(@NonNull PackageManager pm,
@PackageManager.ComponentInfoFlags int flags) {
ActivityInfo ai = null;
if (mComponent != null) {
try {
ai = pm.getActivityInfo(mComponent, flags);
} catch (PackageManager.NameNotFoundException e) {
// ignore
}
} else {
ResolveInfo info = pm.resolveActivity(
this, PackageManager.MATCH_DEFAULT_ONLY | flags);
if (info != null) {
ai = info.activityInfo;
}
}
return ai;
}这个方法会根据传进来的flags在PMS所存储的组件列表中挑选最合适的系统组件，进行回传。
因此整个流程就是：
在系统启动时，PackageManagerService就会启动，PMS将解析所有已安装的应用信息，构建信息表，当用户通过Intent跳转到某个组件时，会根据Intent中包含的信息到PMS中查找对应的组件列表，最后跳转到目标组件当中。
