它的实现代码在executePostCommitSteps(commitRequest)中，看一下它的代码：
/*** On successful install, executes remaining steps after commit completes and the package lock* is released. These are typically more expensive or require calls to installd, which often* locks on {@link #mLock}.*/private void executePostCommitSteps(CommitRequest commitRequest) {final ArraySet<IncrementalStorage> incrementalStorages = new ArraySet<>();for (ReconciledPackage reconciledPkg : commitRequest.reconciledPackages.values()) {final boolean instantApp = ((reconciledPkg.scanResult.request.scanFlags& PackageManagerService.SCAN_AS_INSTANT_APP) != 0);final AndroidPackage pkg = reconciledPkg.pkgSetting.pkg;final String packageName = pkg.getPackageName();final String codePath = pkg.getPath();final boolean onIncremental = mIncrementalManager != null&& isIncrementalPath(codePath);if (onIncremental) {IncrementalStorage storage = mIncrementalManager.openStorage(codePath);if (storage == null) {throw new IllegalArgumentException("Install: null storage for incremental package " + packageName);}incrementalStorages.add(storage);}prepareAppDataAfterInstallLIF(pkg);if (reconciledPkg.prepareResult.clearCodeCache) {clearAppDataLIF(pkg, UserHandle.USER_ALL, FLAG_STORAGE_DE | FLAG_STORAGE_CE| FLAG_STORAGE_EXTERNAL | Installer.FLAG_CLEAR_CODE_CACHE_ONLY);}if (reconciledPkg.prepareResult.replace) {mDexManager.notifyPackageUpdated(pkg.getPackageName(),pkg.getBaseApkPath(), pkg.getSplitCodePaths());}// Prepare the application profiles for the new code paths.// This needs to be done before invoking dexopt so that any install-time profile// can be used for optimizations.mArtManagerService.prepareAppProfiles(pkg,resolveUserIds(reconciledPkg.installArgs.user.getIdentifier()),/* updateReferenceProfileContent= */ true);// Compute the compilation reason from the installation scenario.final int compilationReason = mDexManager.getCompilationReasonForInstallScenario(reconciledPkg.installArgs.mInstallScenario);// Construct the DexoptOptions early to see if we should skip running dexopt.//// Do not run PackageDexOptimizer through the local performDexOpt// method because `pkg` may not be in `mPackages` yet.//// Also, don't fail application installs if the dexopt step fails.final boolean isBackupOrRestore =reconciledPkg.installArgs.installReason == INSTALL_REASON_DEVICE_RESTORE|| reconciledPkg.installArgs.installReason == INSTALL_REASON_DEVICE_SETUP;final int dexoptFlags = DexoptOptions.DEXOPT_BOOT_COMPLETE| DexoptOptions.DEXOPT_INSTALL_WITH_DEX_METADATA_FILE| (isBackupOrRestore ? DexoptOptions.DEXOPT_FOR_RESTORE : 0);DexoptOptions dexoptOptions =new DexoptOptions(packageName, compilationReason, dexoptFlags);// Check whether we need to dexopt the app.//// NOTE: it is IMPORTANT to call dexopt:// - after doRename which will sync the package data from AndroidPackage and// its corresponding ApplicationInfo.// - after installNewPackageLIF or replacePackageLIF which will update result with the// uid of the application (pkg.applicationInfo.uid).// This update happens in place!//// We only need to dexopt if the package meets ALL of the following conditions:// 1) it is not an instant app or if it is then dexopt is enabled via gservices.// 2) it is not debuggable.// 3) it is not on Incremental File System.//// Note that we do not dexopt instant apps by default. dexopt can take some time to// complete, so we skip this step during installation. Instead, we'll take extra time// the first time the instant app starts. It's preferred to do it this way to provide// continuous progress to the useur instead of mysteriously blocking somewhere in the// middle of running an instant app. The default behaviour can be overridden// via gservices.//// Furthermore, dexopt may be skipped, depending on the install scenario and current// state of the device.//// TODO(b/174695087): instantApp and onIncremental should be removed and their install// path moved to SCENARIO_FAST.final boolean performDexopt =(!instantApp || Global.getInt(mContext.getContentResolver(),Global.INSTANT_APP_DEXOPT_ENABLED, 0) != 0)&& !pkg.isDebuggable()&& (!onIncremental)&& dexoptOptions.isCompilationEnabled();if (performDexopt) {// Compile the layout resources.if (SystemProperties.getBoolean(PRECOMPILE_LAYOUTS, false)) {Trace.traceBegin(TRACE_TAG_PACKAGE_MANAGER, "compileLayouts");mViewCompiler.compileLayouts(pkg);Trace.traceEnd(TRACE_TAG_PACKAGE_MANAGER);}Trace.traceBegin(TRACE_TAG_PACKAGE_MANAGER, "dexopt");ScanResult result = reconciledPkg.scanResult;// This mirrors logic from commitReconciledScanResultLocked, where the library files// needed for dexopt are assigned.// TODO: Fix this to have 1 mutable PackageSetting for scan/install. If the previous// setting needs to be passed to have a comparison, hide it behind an immutable// interface. There's no good reason to have 3 different ways to access the real// PackageSetting object, only one of which is actually correct.PackageSetting realPkgSetting = result.existingSettingCopied? result.request.pkgSetting : result.pkgSetting;if (realPkgSetting == null) {realPkgSetting = reconciledPkg.pkgSetting;}// Unfortunately, the updated system app flag is only tracked on this PackageSettingboolean isUpdatedSystemApp = reconciledPkg.pkgSetting.getPkgState().isUpdatedSystemApp();realPkgSetting.getPkgState().setUpdatedSystemApp(isUpdatedSystemApp);mPackageDexOptimizer.performDexOpt(pkg, realPkgSetting,null /* instructionSets */,getOrCreateCompilerPackageStats(pkg),mDexManager.getPackageUseInfoOrDefault(packageName),dexoptOptions);Trace.traceEnd(TRACE_TAG_PACKAGE_MANAGER);}// Notify BackgroundDexOptService that the package has been changed.// If this is an update of a package which used to fail to compile,// BackgroundDexOptService will remove it from its denylist.// TODO: Layering violationBackgroundDexOptService.notifyPackageChanged(packageName);notifyPackageChangeObserversOnUpdate(reconciledPkg);}waitForNativeBinariesExtraction(incrementalStorages);}
首先对ReconciledPackage对象循环，得到AndroidPackage对象pkg，包名、文件路径。
接着调用prepareAppDataAfterInstallLIF(pkg)，它主要用来创建应用需要使用的文件或文件夹。应用需要使用的文件或文件夹和参数uuid（存储位置相关）、userId和包名相关。如果uuid为null，userId为0，则文件夹为"/data/data/“（版本遗留） + 包名 或 “/data/user/0/” + 包名，及其目录下面的"cache"和"code_cache"目录；”/data/user_de/0/" + 包名，及其目录下面的"cache"和"code_cache"目录，如果"dalvik.vm.usejitprofiles"属性值为true的情况下（和JIT编译有关），还会创建 “/data/misc/profiles/cur/0” + 包名 目录 和 “/data/misc/profiles/ref/” + 包名 目录。
如果reconciledPkg.prepareResult.clearCodeCache为true，代表需要清除代码缓存，所以调用带有Installer.FLAG_CLEAR_CODE_CACHE_ONLY标识的clearAppDataLIF()方法。带这个标识的方法，就是清除这个应用数据文件夹下面的"code_cache"目录及其中文件。
reconciledPkg.prepareResult.replace为true，代表是替换升级。mDexManager.notifyPackageUpdated()通知mDexManager应用包发生了更新，它会更新其中应用的安装位置，去除使用该应用的其他APP。
mArtManagerService.prepareAppProfiles()是和使用profile优化应用有关，它的参数updateReferenceProfileContent为true，代表它需要将dex元数据文件（“.dm"结尾文件）更新到它自己引用的profile内容中，这块需要使用Profman程序。主APK文件相关的profile文件名为"primary.prof”。
接着往下就是和执行Dex优化相关的，就是将相关的Dex指令转化成对应的机器指令。
在属性变量PRECOMPILE_LAYOUTS值为true，调用mViewCompiler.compileLayouts(pkg)。它是用来将布局资源文件转化成dex文件。生成的dex文件为 包的数据目录 + “/code_cache/compiled_view.dex”。包的数据目录和uuid、userId、ce标识相关，如果uuid为null，userId为0，ce标识存在，则生成的文件位置为"/data/user/0/" + packageName + “/code_cache/compiled_view.dex”。它是对应用的一种优化。
可以看到执行dexopt需要满足几个条件：不是instantApp，除非设置了Global.INSTANT_APP_DEXOPT_ENABLED对应的值不为0；包不是debuggable；不是在增量文件系统上。
接着，BackgroundDexOptService.notifyPackageChanged(packageName) 是从BackgroundDexOptService中将应用包从之前编译失败的名单中去除。
notifyPackageChangeObserversOnUpdate(reconciledPkg) 是通知注册在PackageManagerService对象成员mPackageChangeObservers集合中的每一个元素，调用它的onPackageChanged(event)方法。
最后waitForNativeBinariesExtraction(incrementalStorages) 是处理增量更新的。
总结
该阶段主要就是创建应用需要使用的目录，在更新应用安装情况下，会将代码缓存目录"code_cache"目录及其中文件清除。如果有dex元数据文件(.dm文件)会将它更新到应用的profile中去，如果满足dex优化条件，会执行Dexopt。