系统界面 Launcher
在 Android 系统启动以后，系统已经启动了 Zygote，ServiceManager，SystemServer 等系统进程。
ServiceManager进程中完成了Binder初始化；SystemServer进程中 ActivityManagerService，WindowManagerService，PackageManagerService等系统服务在ServiceManager中已经注册；最后启动了Launcher桌面应用。
Launcher作为用户的交互界面，在用户点击 APP 图标的时候提供了打开应用的能力。不同的手机厂商可能会根据Launcher做一些定制，比如miui就是如此，但最终的原理是一致的。
应用安装的时候，通过PackageManagerService解析 apk 的AndroidManifest.xml文件，提取出这个 apk 的信息写入到packages.xml文件中，这些信息包括：权限、应用包名、icon、apk 的安装位置、版本、userID 等等。packages.xml 文件位于系统目录下/data/system/packages.xml。
启动应用流程
当用户点击桌面上的应用图标后，Launcher 会通过 service_manager 向 AMS 服务发出请求，查询对应的 APP 信息，然后由 Zygote 创建目标 APP 进程。
先梳理一下大致的流程：
1.Launcher 通过 Binder 方式沟通 AMS
2.AMS 先检查这个 APP 进程是不是已经创建了
3.如果已经创建，则直接调用realStartActivityLocked直接到第 7 步
4.否则，AMS 接到请求后让 Zygote 通过 fork 创建 APP 进程，完成Application.onCreate、创建应用的上下文和其他各种必要对象，这些对象会在 AMS 中留有备份进行保留。
5.新创建的 APP 进程通过 Binder 发送ATTACH_APPLICATION_TRANSACTION通知 AMS
6.AMS 接到ATTACH_APPLICATION_TRANSACTION后调用realStartActivityLocked
7.设置进程为顶部 Activity，为新进程创建事务发送调度命令H.EXECUTE_TRANSACTION
8.进程处理命令消息时调用 Activity.onCreate 并且初始化应用自己的视图
具体到代码实现中，分为几个步骤，首先启动步骤从 Launcher 开始：
1.检查将要打开的目标 APP 的 Activity 是否存在，如果存在就不需要打开了
Launcher.startActivitySafel->Launcher.startActivity
2.打开目标 Activity
Activity.startActivity
3.通过 ATSM 服务调用该服务提供的 startActivity
Activity.startActivityForResult-Instrumentation.execStartActivity-ActivityTaskManager.getService().startActivity
ActivityManagerNative.getDefault()会返回一个ActivityManagerProxy作为 Launcher 中使用ActivityTaskManager的代理，该代理的 startActivity 会发送START_ACTIVITY_TRANSACTION来通知ActivityTaskManager
完成上述过程后，进程从Launcher切换到system_server中的ActivityManagerService，也就是 AMS。
在 startActivityAsUser 中会先获取用户的 UserID 作为参数然后往下调用 getActivityStartController 中的 starter
startActivity-startActivityAsUser
创建新的 intent 对象，获取 ApplicationPackageManager
ActivityStackSupervisor.startActivityMayWait-resolveActivity
获取 intent 所指向的 Activity 信息，并保存到 Intent 对象。
PackageManagerService.resolveIntent()-queryIntentActivities()
-获取到调用者的进程信息，通过Intent.FLAG_ACTIVITY_FORWARD_RESULT判断是否需要进行startActivityForResult处理。检查调用者是否有权限来调用指定的 Activity
-Activity 有多种启动模式，对 launchMode 的处理，创建 Task 等操作。启动 Activity 所在进程，已存在则直接onResume()，不存在则创建 Activity 并处理是否触发onNewIntent()。
ActivityStackSupervisor.startActivityUncheckedLocked-startActivityLocked
-若找到 resume 状态的 Activity，执行startPausingLocked()暂停该 Activity，同时暂停所有处于后台栈的 Activity，这里一般来说会把桌面，也就是 Launcher 暂停掉。
ActivityStack.resumeTopActivityInnerLocked
-获取要启动的Activity进程信息，若成功，则表示进程已经启动了，通过realStartActivityLocked启动这个 activity；否则，通过 AMS 代理调用 startProcessAsync 去创建进程。前者的条件就是前面所述的目标 APP 已经启动过的情况，后者则是从头开始创建这个 APP 进程。
ActivityStackSupervisor.startSpecificActivity
我们考虑后者的情况，程序将会往下调用startProcessAsync创建新进程：
void startProcessAsync(ActivityRecord activity, boolean knownToBeDead, boolean isTop,String hostingType) {try { ...// 发布消息以启动进程，以避免在持有 ATMS 锁的情况下调用 AMS 可能出现死锁。final Messagem = PooledLambda.obtainMessage(ActivityManagerInternal::startProcess, mAmInternal, activity.processName, activity.info.applicationInfo, knownToBeDead, isTop, hostingType, activity.intent.getComponent()); mH.sendMessage(m); } finally { Trace.traceEnd(TRACE_TAG_WINDOW_MANAGER); } }startProcessAsync 会通过消息的方式让 ATMS 服务在处理该消息时创建对应的进程，调用目标为ActivityManagerInternal::startProcess
而ActivityManagerInternal::startProcess调用ActivityManagerService::startProcessLocked调用ProcessList::startProcessLocked调用ProcessList::startProcess
如果目标进程是 top app，设置 flag 保证启动的最高优先级，并最终在 startProcess 中创建对应的目标进程，也就是 APP 的进程。
在进程创建成功后，将当前进程切换到新进程，并将ActivityThread类加载到新进程，调用ActivityThread.main
1.ActivityThread.main：创建主线程的 Looper 对象，创建ActivityThread对象，ActivityThread.attach()建立 Binder 通道，开启Looper.loop()消息循环
2.ActivityThread.attach：创建ActivityManagerProxy对象，调用基于IActivityManager接口的 Binder 通道ActivityManagerProxy.attachApplication()
public staticvoidmain(String[] args) { ...//初始化当前进程的 Looper 对象 Looper.prepareMainLooper(); ... ActivityThread thread = new ActivityThread();//此处创建Application thread.attach(false, startSeq);if (sMainThreadHandler == null) { sMainThreadHandler = thread.getHandler(); }if (false) { Looper.myLooper().setMessageLogging(newLogPrinter(Log.DEBUG, "ActivityThread")); }// 调用 Looper 的 loop 方法开启无限循环。 Looper.loop();throw new RuntimeException("Main thread loop unexpectedly exited"); }Looper 会持续从消息队列中获取消息，然后处理指定的任务。其中，attach 函数调用时会发送ATTACH_APPLICATION_TRANSACTION通知 system_server 中的服务。
此时，应用的ActivityThread和ApplicationThread已经被创建，并创建了消息循环机制。当调用ActivityThread.attach时，内部会调用ActivityManagerProxy.attachApplication，通过 Binder 来调用 AMS 中的attachApplication函数，此时会把ApplicationThread传递过去。
attachApplication-attachApplicationLocked主要有两个关键函数需要关注：
◆bindApplication
◆ActivityTaskManagerService.LocalService#attachApplication
private booleanattachApplicationLocked(@NonNull IApplicationThread thread,int pid, int callingUid, long startSeq) {synchronized (mProcLock) { app.mState.setCurAdj(ProcessList.INVALID_ADJ); app.mState.setSetAdj(ProcessList.INVALID_ADJ); app.mState.setVerifiedAdj(ProcessList.INVALID_ADJ); mOomAdjuster.setAttachingSchedGroupLSP(app); app.mState.setForcingToImportant(null); updateProcessForegroundLocked(app, false, 0, false); app.mState.setHasShownUi(false); app.mState.setCached(false); app.setDebugging(false); app.setKilledByAm(false); app.setKilled(false); app.setUnlocked(StorageManager.isUserKeyUnlocked(app.userId)); }// 移除进程超时信息 mHandler.removeMessages(PROC_START_TIMEOUT_MSG, app);final ProviderInfoListproviderList = ProviderInfoList.fromList(providers);if (app.getIsolatedEntryPoint() != null) { thread.runIsolatedEntryPoint( app.getIsolatedEntryPoint(), app.getIsolatedEntryPointArgs()); } else if (instr2 != null) { thread.bindApplication(processName, appInfo, app.sdkSandboxClientAppVolumeUuid, app.sdkSandboxClientAppPackage, providerList, instr2.mClass, profilerInfo, instr2.mArguments, instr2.mWatcher, instr2.mUiAutomationConnection, testMode, mBinderTransactionTrackingEnabled, enableTrackAllocation, isRestrictedBackupMode || !normalMode, app.isPersistent(),new Configuration(app.getWindowProcessController().getConfiguration()), app.getCompat(), getCommonServicesLocked(app.isolated), mCoreSettingsObserver.getCoreSettingsLocked(), buildSerial, autofillOptions, contentCaptureOptions, app.getDisabledCompatChanges(), serializedSystemFontMap, app.getStartElapsedTime(), app.getStartUptime()); } else { thread.bindApplication(processName, appInfo, app.sdkSandboxClientAppVolumeUuid, app.sdkSandboxClientAppPackage, providerList, null, profilerInfo, null, null, null, testMode, mBinderTransactionTrackingEnabled, enableTrackAllocation, isRestrictedBackupMode || !normalMode, app.isPersistent(),new Configuration(app.getWindowProcessController().getConfiguration()), app.getCompat(), getCommonServicesLocked(app.isolated), mCoreSettingsObserver.getCoreSettingsLocked(), buildSerial, autofillOptions, contentCaptureOptions, app.getDisabledCompatChanges(), serializedSystemFontMap, app.getStartElapsedTime(), app.getStartUptime()); }if (normalMode) {try {// 最终调用ActivityTaskManagerService.LocalService#attachApplication() didSomething = mAtmInternal.attachApplication(app.getWindowProcessController()); } catch (Exception e) { } }return true;}我们先关注thread.bindApplication，thread 就是刚刚由新进程传过来的。
函数先调用bindApplication向进程发送H.BIND_APPLICATION命令，进程收到该命令后，通过handleBindApplication处理：
private void handleBindApplication(AppBindData data) { // 初始化contextfinal ContextImpl appContext = ContextImpl.createAppContext(this, data.info); // 初始化Instrumentationif (ii != null) { initInstrumentation(ii, data, appContext); } else { mInstrumentation = new Instrumentation(); mInstrumentation.basicInit(this); } Application app; try {// 初始化Application// 调用LoadedApk#makeApplicationInner() app = data.info.makeApplicationInner(data.restrictedBackupMode, null); mInstrumentation.onCreate(data.instrumentationArgs); // 回调Application#onCreate() mInstrumentation.callApplicationOnCreate(app); } finally { }}handleBindApplication初始化context，然后初始化Instrumentation对象，创建Application对象，并调用该对象的onCreate
初始化流程调用链为makeApplication-newApplication:
private void handleBindApplication(AppBindData data) { // 初始化contextfinal ContextImplappContext = ContextImpl.createAppContext(this, data.info); // 初始化Instrumentationif (ii != null) { initInstrumentation(ii, data, appContext); } else { mInstrumentation = new Instrumentation(); mInstrumentation.basicInit(this); } Application app; try {// 初始化Application// 调用LoadedApk#makeApplicationInner() app = data.info.makeApplicationInner(data.restrictedBackupMode, null); mInstrumentation.onCreate(data.instrumentationArgs); // 回调Application#onCreate() mInstrumentation.callApplicationOnCreate(app); } finally { }}public Application makeApplication(boolean forceDefaultAppClass,Instrumentation instrumentation) {if (mApplication != null) { return mApplication; }StringappClass = mApplicationInfo.className; java.lang.ClassLoadercl = getClassLoader();//此时新建一个 Application 的 ContextImpl 对象，ContextImplappContext = ContextImpl.createAppContext(mActivityThread, this);//通过在 handleBindApplication 创建的 mInstrumentation 对象新建一个 Application 对象，同时进行 attach。 app = mActivityThread.mInstrumentation.newApplication(cl, appClass, appContext); appContext.setOuterContext(app);}public Application newApplication(ClassLoader cl, String className, Context context) { return newApplication(cl.loadClass(className), context);}Instrumentation类：static public Application newApplication(Class<?> clazz, Context context) {//实例化 ApplicationApplicationapp = (Application)clazz.newInstance(); // Application 和 context绑定 app.attach(context); return app;}//attach 就是将新建的 ContextImpl 赋值到 mBase，这个 ContextImpl 对象就是所有Application 内 Context 的具体实现，同时赋值一些其他的信息如 mLoadedApk。final void attach(Context context) { mBase = base; mLoadedApk = ContextImpl.getImpl(context).mPackageInfo;}然后是makeApplicationInner的细节
public Application makeApplicationInner(boolean forceDefaultAppClass, Instrumentation instrumentation) {return makeApplicationInner(forceDefaultAppClass, instrumentation, false);}private Application makeApplicationInner(boolean forceDefaultAppClass, Instrumentation instrumentation, boolean allowDuplicateInstances) {if (mApplication != null) {return mApplication; }synchronized (sApplications) {final Applicationcached = sApplications.get(mPackageName);if (cached != null) {if (!allowDuplicateInstances) { mApplication = cached;return cached; } } }Applicationapp =null;final StringmyProcessName = Process.myProcessName();StringappClass = mApplicationInfo.getCustomApplicationClassNameForProcess( myProcessName);try {final java.lang.ClassLoadercl = getClassLoader();if (!mPackageName.equals("android")) { initializeJavaContextClassLoader(); }// 初始化Application的上下文ContextImplappContext = ContextImpl.createAppContext(mActivityThread, this);// 创建Application实例 app = mActivityThread.mInstrumentation.newApplication( cl, appClass, appContext); appContext.setOuterContext(app); } catch (Exception e) { } mActivityThread.mAllApplications.add(app); mApplication = app;if (!allowDuplicateInstances) {synchronized (sApplications) { sApplications.put(mPackageName, app); } }if (instrumentation != null) {try {// 回调Application#onCreate() instrumentation.callApplicationOnCreate(app); } catch (Exception e) { } }return app;}对于新创建的这个进程而言，当callApplicationOnCreate完成调用以后，这个进程的上下文，以及Application对象和Instrumentation对象都完成的创建和初始化。而在进程这波完成上述的初始化过程中，AMS 那边也没闲着，在发送完相应的命令以后，ActivityManagerService#attachApplicationLocked继续往下调用ActivityTaskManagerService.LocalService#attachApplication
public booleanattachApplication(WindowProcessController wpc) throws RemoteException {synchronized (mGlobalLockWithoutBoost) {try {// 调用RootWindowContainer#attachApplication()// 最终调用AttachApplicationHelper#test()return mRootWindowContainer.attachApplication(wpc); } finally { } }}public booleantest(ActivityRecord r) {if (r.finishing || !r.showToCurrentUser() || !r.visibleIgnoringKeyguard || r.app != null || mApp.mUid != r.info.applicationInfo.uid || !mApp.mName.equals(r.processName)) {return false; }try {// 调用ActivityTaskSupervisor#realStartActivityLocked()if (mTaskSupervisor.realStartActivityLocked(r, mApp, mTop == r && r.getTask().canBeResumed(r) /* andResume */,true /* checkConfig */)) { mHasActivityStarted = true; } } catch (RemoteException e) { }return false;}可以注意到，最终这个函数将往下执行ActivityTaskSupervisor#realStartActivityLocked完成最后的步骤。而如果此前不需要创建新进程，那么刚打开 APP 的时候就会从这个地方开始恢复进程的状态了。
booleanrealStartActivityLocked(ActivityRecord r, WindowProcessController proc,boolean andResume, boolean checkConfig) throws RemoteException { // 创建启动Activity的事务 final ClientTransactionclientTransaction = ClientTransaction.obtain(proc.getThread(), r.token);final booleanisTransitionForward = r.isTransitionForward();final IBinderfragmentToken = r.getTaskFragment().getFragmentToken();// 添加callback clientTransaction.addCallback(LaunchActivityItem.obtain(new Intent(r.intent), System.identityHashCode(r), r.info, mergedConfiguration.getGlobalConfiguration(), mergedConfiguration.getOverrideConfiguration(), r.compat, r.getFilteredReferrer(r.launchedFromPackage), task.voiceInteractor, proc.getReportedProcState(), r.getSavedState(), r.getPersistentSavedState(), results, newIntents, r.takeOptions(), isTransitionForward, proc.createProfilerInfoIfNeeded(), r.assistToken, activityClientController, r.shareableActivityToken, r.getLaunchedFromBubble(), fragmentToken));// 设置Activity启动后的生命周期状态final ActivityLifecycleItem lifecycleItem;if (andResume) {// RESUME状态 lifecycleItem = ResumeActivityItem.obtain(isTransitionForward); } else {// PAUSE状态 lifecycleItem = PauseActivityItem.obtain(); }// 设置状态 clientTransaction.setLifecycleStateRequest(lifecycleItem);// 开启事务，最终调用ClientLifecycleManager#scheduleTransaction() mService.getLifecycleManager().scheduleTransaction(clientTransaction); return true;}函数首先创建 Activity 事务，设置对应的 callback ，以及对应的生命周期ActivityLifecycleItem，最终开始调度事务lientLifecycleManager#scheduleTransaction。
// 启动Activity的事务void scheduleTransaction(ClientTransaction transaction) throws RemoteException {final IApplicationThreadclient = transaction.getClient();// 调用ClientTransaction#schedule() transaction.schedule(); }public void schedule() throws RemoteException {// 调用ApplicationThread#scheduleTransaction() mClient.scheduleTransaction(this);}@Overridepublic void scheduleTransaction(ClientTransaction transaction) throws RemoteException {// 最终调用ClientTransactionHandler#scheduleTransaction() ActivityThread.this.scheduleTransaction(transaction);}void scheduleTransaction(ClientTransaction transaction) { transaction.preExecute(this);// 发送消息，在H类中接收消息 sendMessage(ActivityThread.H.EXECUTE_TRANSACTION, transaction);}可以看到，最终由 AMS 向进程发出H.EXECUTE_TRANSACTION命令，这个命令同样会被进程那边接受并处理：
class H extends Handler {public void handleMessage(Message msg) {switch (msg.what) {case EXECUTE_TRANSACTION:final ClientTransactiontransaction = (ClientTransaction) msg.obj;// 执行事务 mTransactionExecutor.execute(transaction);if (isSystem()) { transaction.recycle(); }break; } }}public void execute(ClientTransaction transaction) {// 调用TransactionExecutor#executeCallbacks()executeCallbacks(transaction);// 执行lifecycleStateexecuteLifecycleState(transaction); mPendingActions.clear();}这个函数最终会往下调用ClientTransactionHandler#handleLaunchActivity，最为抽象类的方法，实际调用ActivityThread#handleLaunchActivity：
public Activity handleLaunchActivity(ActivityClientRecord r, PendingTransactionActions pendingActions, Intent customIntent) { // 在创建Activity之前初始化if (ThreadedRenderer.sRendererEnabled && (r.activityInfo.flags & ActivityInfo.FLAG_HARDWARE_ACCELERATED) != 0) { HardwareRenderer.preload(); }// 获取WMS服务，初始化WindowManager WindowManagerGlobal.initialize();// GraphicsEnvironment提示一个activity正在进程上启动 GraphicsEnvironment.hintActivityLaunch();// 启动Activity，调用ActivityThread#performLaunchActivity()final Activitya = performLaunchActivity(r, customIntent); return a;}handleLaunchActivity最终回调目标Activity的onConfigurationChanged，初始化WindowManagerService，调用ActivityThread.performLaunchActivity。
ActivityThread.performLaunchActivity() {//类似 Application 的创建过程，通过 classLoader 加载 activity. activity = mInstrumentation.newActivity(classLoader, component.getClassName(), r.intent);//因为 Activity 有界面，所以其 Context 是 ContextThemeWrapper 类型，但实现类仍是ContextImpl.Context appContext = createBaseContextForActivity(r, activity); activity.attach(context,mInstrumentation,application,...);//与 Window 进行关联//调用 activity 的 onCreate()方法。 mInstrumentation.callActivityOnCreate(activity,...)}public void callActivityOnCreate(Activity activity, Bundle icicle) {prePerformCreate(activity);// 调用Activity#performCreate() activity.performCreate(icicle);postPerformCreate(activity);}callActivityOnCreate中会回调Activity.performCreate，其中调用Activity的onCreate、Activity.setContentView、ActivityThread.performResumeActivity，performResumeActivity最终会回调onResume。
总之，到这里之后，新应用的进程算是创建完成了。
参考文章（见原文末尾）
看雪ID：Tokameine
https://bbs.kanxue.com/user-home-924548.htm
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原文始发于微信公众号（看雪学苑）：Activity 原生启动流程分析
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