這道題在曾經面試「菜鳥網絡」中遇到過，不過當時只問了「Activity 啟動過程」，這里對整個「App 啟動過程」進行完整的源碼分析，希望可以幫助到大家。

源碼分析

1. Launcher 捕獲點擊事件，其過程為 Launcher#onClick -> Launcher#onClickAppShortcut -> Launcher#startAppShortcutOrInfoActivity -> Launcher#startActivitySafely -> Activity#startActivity，其 Launcher3 相關源碼如下所示：

// https://github.com/amirzaidi/Launcher3/blob/f7951c32984036eef2f2130f21abded3ddf6160a/src/com/android/launcher3/Launcher.java#L2249
public void onClick(View v) {
    ...
    Object tag = v.getTag();
    if (tag instanceof ShortcutInfo) {
        onClickAppShortcut(v);
    }
    ...
}

// https://github.com/amirzaidi/Launcher3/blob/f7951c32984036eef2f2130f21abded3ddf6160a/src/com/android/launcher3/Launcher.java#L2412
protected void onClickAppShortcut(final View v) {
    ...
    // Start activities
    startAppShortcutOrInfoActivity(v);
}

// https://github.com/amirzaidi/Launcher3/blob/f7951c32984036eef2f2130f21abded3ddf6160a/src/com/android/launcher3/Launcher.java#L2462
private void startAppShortcutOrInfoActivity(View v) {
    ItemInfo item = (ItemInfo) v.getTag();
    Intent intent;// 應用程序安裝的時候根據 AndroidManifest.xml 由 PackageManagerService 解析并保存的
    if (item instanceof PromiseAppInfo) {
        PromiseAppInfo promiseAppInfo = (PromiseAppInfo) item;
        intent = promiseAppInfo.getMarketIntent();
    } else {
        intent = item.getIntent();
    }
    ...
    boolean success = startActivitySafely(v, intent, item);
    ...
}

// https://github.com/amirzaidi/Launcher3/blob/f7951c32984036eef2f2130f21abded3ddf6160a/src/com/android/launcher3/Launcher.java#L2689
public boolean startActivitySafely(View v, Intent intent, ItemInfo item) {
    ...
    intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
    ...
    startActivity(intent, optsBundle);
    ...
}

2. 以 API 27 源碼為例，說到了 Acitvity#startActivity，我們點擊源碼可以發現調用的是 Activity#startActivityForResult，其中調用到了 Instrumentation#execStartActivity 這個方法，源碼如下所示：

// http://androidxref.com/8.1.0_r33/xref/frameworks/base/core/java/android/app/Activity.java#4800
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);
    }
}

// http://androidxref.com/8.1.0_r33/xref/frameworks/base/core/java/android/app/Activity.java#4482
public void startActivityForResult(@RequiresPermission Intent intent, int requestCode,
        @Nullable Bundle options) {
    ...
    Instrumentation.ActivityResult ar =
        mInstrumentation.execStartActivity(
            this, mMainThread.getApplicationThread(), mToken, this,
            intent, requestCode, options);
    ...
}

3. 在 Instrumentation#execStartActivity 中我們可以發現它調用了 ActivityManager#getService()#startActivity，其 ActivityManager#getService() 是采用單例，返回的是實現 IActivityManager 類型的 Binder 對象，它的具體實現是在 ActivityManagerService 中。

// http://androidxref.com/8.1.0_r33/xref/frameworks/base/core/java/android/app/Instrumentation.java#1578
public ActivityResult execStartActivity(
        Context who, IBinder contextThread, IBinder token, Activity target,
        Intent intent, int requestCode, Bundle options) {
    ...
    try {
        ...
        int result = ActivityManager.getService()
            .startActivity(whoThread, who.getBasePackageName(), intent,
                    intent.resolveTypeIfNeeded(who.getContentResolver()),
                    token, target != null ? target.mEmbeddedID : null,
                    requestCode, 0, null, options);
        ...
    } catch (RemoteException e) {
        ...
    }
    return null;
}

// http://androidxref.com/8.1.0_r33/xref/frameworks/base/core/java/android/app/ActivityManager.java#4216
public static IActivityManager getService() {
    return IActivityManagerSingleton.get();
}
private static final Singleton<IActivityManager> IActivityManagerSingleton =
        new Singleton<IActivityManager>() {
            @Override
            protected IActivityManager create() {
                final IBinder b = ServiceManager.getService(Context.ACTIVITY_SERVICE);
                final IActivityManager am = IActivityManager.Stub.asInterface(b);
                return am;
            }
        };

4. 我們再到 ActivityManagerService#startActivity 查看其源碼，發現其調用了 ActivityManagerService#startActivityAsUser，該方法又調用了 ActivityStarter#startActivityMayWait，源碼如下所示：

// http://androidxref.com/8.1.0_r33/xref/frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java#4516
@Override
public final int startActivity(IApplicationThread caller, String callingPackage,
        Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode,
        int startFlags, ProfilerInfo profilerInfo, Bundle bOptions) {
    return startActivityAsUser(caller, callingPackage, intent, resolvedType, resultTo,
            resultWho, requestCode, startFlags, profilerInfo, bOptions,
            UserHandle.getCallingUserId());
}

@Override
public final int startActivityAsUser(IApplicationThread caller, String callingPackage,
        Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode,
        int startFlags, ProfilerInfo profilerInfo, Bundle bOptions, int userId) {
    enforceNotIsolatedCaller("startActivity");
    userId = mUserController.handleIncomingUser(Binder.getCallingPid(), Binder.getCallingUid(),
            userId, false, ALLOW_FULL_ONLY, "startActivity", null);
    // TODO: Switch to user app stacks here.
    return mActivityStarter.startActivityMayWait(caller, -1, callingPackage, intent,
            resolvedType, null, null, resultTo, resultWho, requestCode, startFlags,
            profilerInfo, null, null, bOptions, false, userId, null, "startActivityAsUser");
}

5. 我們查找到 ActivityStarter#startActivityMayWait，其間調用了 ActivityStarter#startActivityLocked，接著是 ActivityStarter#startActivity，然后是 ActivityStarter#startActivityUnchecked，其調用了 ActivityStackSupervisor#resumeFocusedStackTopActivityLocked，源碼如下所示：

// http://androidxref.com/8.1.0_r33/xref/frameworks/base/services/core/java/com/android/server/am/ActivityStarter.java#673
final int startActivityMayWait(IApplicationThread caller, int callingUid,
        String callingPackage, Intent intent, String resolvedType,
        IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
        IBinder resultTo, String resultWho, int requestCode, int startFlags,
        ProfilerInfo profilerInfo, WaitResult outResult,
        Configuration globalConfig, Bundle bOptions, boolean ignoreTargetSecurity, int userId,
        TaskRecord inTask, String reason) {
    ...
    int res = startActivityLocked(caller, intent, ephemeralIntent, resolvedType,
            aInfo, rInfo, voiceSession, voiceInteractor,
            resultTo, resultWho, requestCode, callingPid,
            callingUid, callingPackage, realCallingPid, realCallingUid, startFlags,
            options, ignoreTargetSecurity, componentSpecified, outRecord, inTask,
            reason);
    ...
}
// http://androidxref.com/8.1.0_r33/xref/frameworks/base/services/core/java/com/android/server/am/ActivityStarter.java#263
int startActivityLocked(IApplicationThread caller, Intent intent, Intent ephemeralIntent,
        String resolvedType, ActivityInfo aInfo, ResolveInfo rInfo,
        IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
        IBinder resultTo, String resultWho, int requestCode, int callingPid, int callingUid,
        String callingPackage, int realCallingPid, int realCallingUid, int startFlags,
        ActivityOptions options, boolean ignoreTargetSecurity, boolean componentSpecified,
        ActivityRecord[] outActivity, TaskRecord inTask, String reason) {
    ...
    mLastStartActivityResult = startActivity(caller, intent, ephemeralIntent, resolvedType,
            aInfo, rInfo, voiceSession, voiceInteractor, resultTo, resultWho, requestCode,
            callingPid, callingUid, callingPackage, realCallingPid, realCallingUid, startFlags,
            options, ignoreTargetSecurity, componentSpecified, mLastStartActivityRecord,
            inTask);
}

// http://androidxref.com/8.1.0_r33/xref/frameworks/base/services/core/java/com/android/server/am/ActivityStarter.java#294
private int startActivity(IApplicationThread caller, Intent intent, Intent ephemeralIntent,
        String resolvedType, ActivityInfo aInfo, ResolveInfo rInfo,
        IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
        IBinder resultTo, String resultWho, int requestCode, int callingPid, int callingUid,
        String callingPackage, int realCallingPid, int realCallingUid, int startFlags,
        ActivityOptions options, boolean ignoreTargetSecurity, boolean componentSpecified,
        ActivityRecord[] outActivity, TaskRecord inTask) {
    ...
    return startActivity(r, sourceRecord, voiceSession, voiceInteractor, startFlags, true,
            options, inTask, outActivity);
}

// http://androidxref.com/8.1.0_r33/xref/frameworks/base/services/core/java/com/android/server/am/ActivityStarter.java#988
private int startActivity(final ActivityRecord r, ActivityRecord sourceRecord,
        IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
        int startFlags, boolean doResume, ActivityOptions options, TaskRecord inTask,
        ActivityRecord[] outActivity) {
    ...
    result = startActivityUnchecked(r, sourceRecord, voiceSession, voiceInteractor,
            startFlags, doResume, options, inTask, outActivity);
    ...
}

// http://androidxref.com/8.1.0_r33/xref/frameworks/base/services/core/java/com/android/server/am/ActivityStarter.java#1015
private int startActivityUnchecked(final ActivityRecord r, ActivityRecord sourceRecord,
        IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
        int startFlags, boolean doResume, ActivityOptions options, TaskRecord inTask,
        ActivityRecord[] outActivity) {
    ...
    mSupervisor.resumeFocusedStackTopActivityLocked(mTargetStack, mStartActivity,
            mOptions);
    ...
}

6. 到 ActivityStackSupervisor#resumeFocusedStackTopActivityLocked 中查看發現其調用了 ActivityStack#resumeTopActivityUncheckedLocked，然后是 ActivityStack#resumeTopActivityInnerLocked，接著變又回到 ActivityStackSupervisor.java，調用了 ActivityStackSupervisor#startSpecificActivityLocked，這個方法中會判斷要啟動 App 的進程是否存在，存在則通知進程啟動 Activity，否則就先將進程創建出來，其源碼如下所示：

// http://androidxref.com/8.1.0_r33/xref/frameworks/base/services/core/java/com/android/server/am/ActivityStackSupervisor.java#2085
boolean resumeFocusedStackTopActivityLocked(
        ActivityStack targetStack, ActivityRecord target, ActivityOptions targetOptions) {
    ...
    return targetStack.resumeTopActivityUncheckedLocked(target, targetOptions);
    ...
}

// http://androidxref.com/8.1.0_r33/xref/frameworks/base/services/core/java/com/android/server/am/ActivityStack.java#2245
boolean resumeTopActivityUncheckedLocked(ActivityRecord prev, ActivityOptions options) {
    ...
    result = resumeTopActivityInnerLocked(prev, options);
    ...
}

http://androidxref.com/8.1.0_r33/xref/frameworks/base/services/core/java/com/android/server/am/ActivityStack.java#2286
private boolean resumeTopActivityInnerLocked(ActivityRecord prev, ActivityOptions options) {
    ...
    mStackSupervisor.startSpecificActivityLocked(next, true, true);
    ...
}

// http://androidxref.com/8.1.0_r33/xref/frameworks/base/services/core/java/com/android/server/am/ActivityStackSupervisor.java#1560
void startSpecificActivityLocked(ActivityRecord r,
        boolean andResume, boolean checkConfig) {
    ...
    if (app != null && app.thread != null) {
        ...
        // 如果進程已存在，則通知進程啟動組件
        realStartActivityLocked(r, app, andResume, checkConfig);
        return;
        ...
    }
    // 否則先將進程創建出來
    mService.startProcessLocked(r.processName, r.info.applicationInfo, true, 0,
            "activity", r.intent.getComponent(), false, false, true);
    ...
}

7. 我們分析進程尚未存在的情況，因為我們后續還會再次遇到 ActivityStackSupervisor#realStartActivityLocked，ActivityStackSupervisor#startSpecificActivityLocked 中創建進程使用到的 mService 為 ActivityManagerService，我們查看 ActivityManagerService#startProcessLocked 的源碼如下所示：

// http://androidxref.com/8.1.0_r33/xref/frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java#3777
private final void startProcessLocked(ProcessRecord app, String hostingType,
        String hostingNameStr, String abiOverride, String entryPoint, String[] entryPointArgs) {
    ...
    if (entryPoint == null) entryPoint = "android.app.ActivityThread";
    startResult = Process.start(entryPoint,
            app.processName, uid, uid, gids, debugFlags, mountExternal,
            app.info.targetSdkVersion, seInfo, requiredAbi, instructionSet,
            app.info.dataDir, invokeWith, entryPointArgs);
    ...
}

8. 發現最終調用的事 Process#start 來啟動進程，進程的入口就是在 android.app.ActivityThread.java 類中的 main() 函數，因此接下來我們從 ActivityThread#main 來分析，其調用了 ActivityThread#attach，其中 ActivityManager.getService() 之前提到過，返回的是一個是實現 IActivityManager 類型的 Binder 對象，它的具體實現是在 ActivityManagerService 中，相關源碼如下所示：

// http://androidxref.com/8.1.0_r33/xref/frameworks/base/core/java/android/app/ActivityThread.java#6459
public static void main(String[] args) {
    ...
    Looper.prepareMainLooper();

    ActivityThread thread = new ActivityThread();
    thread.attach(false);
    ...
    Looper.loop();
    ...
}

// http://androidxref.com/8.1.0_r33/xref/frameworks/base/core/java/android/app/ActivityThread.java#6315
private void attach(boolean system) {
    sCurrentActivityThread = this;
    mSystemThread = system;
    if (!system) {
        ...
        final IActivityManager mgr = ActivityManager.getService();
        try {
            mgr.attachApplication(mAppThread);
        } catch (RemoteException ex) {
            ...
        }
        ...
    }
    ...
}

9. 我們又回到了 ActivityManagerService 中，查看其 attachApplication 函數，發現調用了 thread#bindApplication 和 mStackSupervisor#attachApplicationLocked 我們依次講解這兩個方法要做的事情，源碼如下所示：

// http://androidxref.com/8.1.0_r33/xref/frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java#7215
public final void attachApplication(IApplicationThread thread) {
    ...
    attachApplicationLocked(thread, callingPid);
    ...
}

// http://androidxref.com/8.1.0_r33/xref/frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java#6911
private final boolean attachApplicationLocked(IApplicationThread thread,
        int pid) {
    ....
    thread.bindApplication(processName, appInfo, providers,
            app.instr.mClass,
            profilerInfo, app.instr.mArguments,
            app.instr.mWatcher,
            app.instr.mUiAutomationConnection, testMode,
            mBinderTransactionTrackingEnabled, enableTrackAllocation,
            isRestrictedBackupMode || !normalMode, app.persistent,
            new Configuration(getGlobalConfiguration()), app.compat,
            getCommonServicesLocked(app.isolated),
            mCoreSettingsObserver.getCoreSettingsLocked(),
            buildSerial);
    ...
    if (normalMode) {
        try {
            if (mStackSupervisor.attachApplicationLocked(app)) {
                didSomething = true;
            }
        } catch (Exception e) {
            ...
        }
    }
    ...
}

10. 上面說到的thread#bindApplication 中的這個 thread 是來自于 ActivityThread#mAppThread，其類型是 ApplicationThread，是 ActivityThread 的一個內部類，繼承自 IApplicationThread.Stub，我們來查看 ApplicationThread#bindApplication，發現最后調用了 ActivityThread#sendMessage 方法，它內部調用了 mH.sendMessage 來發送消息，mH 是 ActivityThread 的內部類 H 的一個實例，查看 H#handleMessage 來查看它是怎么處理發送過來的消息，其最終走到了 ActivityThread#handleBindApplication。

在源碼中我們可以發現它先創建 mInstrumentation 對象，調用 data#info#makeApplication 來創建 Application 對象，其對象 data#info 為 LoadedApk 的一個實例，查看 LoadedApk#makeApplication 中代碼可以發現，其調用了 Instrumentation#newApplication 方法，內部靠 Class#newInstance() 完成對 Application 實例化，然后調用 Application#attach(context) 來綁定 Context。

以上創建完 Application 對象后便是調用 Instrumentation#callApplicationOnCreate 走 Application 的 onCreate 生命周期，以上涉及到的全部源碼如下所示：

// http://androidxref.com/8.1.0_r33/xref/frameworks/base/core/java/android/app/ActivityThread.java#899
public final void bindApplication(String processName, ApplicationInfo appInfo,
        List<ProviderInfo> providers, ComponentName instrumentationName,
        ProfilerInfo profilerInfo, Bundle instrumentationArgs,
        IInstrumentationWatcher instrumentationWatcher,
        IUiAutomationConnection instrumentationUiConnection, int debugMode,
        boolean enableBinderTracking, boolean trackAllocation,
        boolean isRestrictedBackupMode, boolean persistent, Configuration config,
        CompatibilityInfo compatInfo, Map services, Bundle coreSettings,
        String buildSerial) {
    ...
    sendMessage(H.BIND_APPLICATION, data);
}

// http://androidxref.com/8.1.0_r33/xref/frameworks/base/core/java/android/app/ActivityThread.java#2593
private void sendMessage(int what, Object obj) {
    sendMessage(what, obj, 0, 0, false);
}

private void sendMessage(int what, Object obj, int arg1, int arg2, boolean async) {
    if (DEBUG_MESSAGES) Slog.v(
        TAG, "SCHEDULE " + what + " " + mH.codeToString(what)
        + ": " + arg1 + " / " + obj);
    Message msg = Message.obtain();
    msg.what = what;
    msg.obj = obj;
    msg.arg1 = arg1;
    msg.arg2 = arg2;
    if (async) {
        msg.setAsynchronous(true);
    }
    mH.sendMessage(msg);
}

// http://androidxref.com/8.1.0_r33/xref/frameworks/base/core/java/android/app/ActivityThread.java#1580
public void handleMessage(Message msg) {
    ...
    switch (msg.what) {
        ...
        case BIND_APPLICATION:
            Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "bindApplication");
            AppBindData data = (AppBindData)msg.obj;
            handleBindApplication(data);
            Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
            break;
        ...
    }
}

// http://androidxref.com/8.1.0_r33/xref/frameworks/base/core/java/android/app/ActivityThread.java#5429
private void handleBindApplication(AppBindData data) {
    ...
    final InstrumentationInfo ii;
    ...
    // 創建 mInstrumentation 實例
    if (ii != null) {
        final ApplicationInfo instrApp = new ApplicationInfo();
        ii.copyTo(instrApp);
        instrApp.initForUser(UserHandle.myUserId());
        final LoadedApk pi = getPackageInfo(instrApp, data.compatInfo,
                appContext.getClassLoader(), false, true, false);
        final ContextImpl instrContext = ContextImpl.createAppContext(this, pi);

        try {
            final ClassLoader cl = instrContext.getClassLoader();
            mInstrumentation = (Instrumentation)
                cl.loadClass(data.instrumentationName.getClassName()).newInstance();
        } catch (Exception e) {
            ...
        }
        ...
    } else {
        mInstrumentation = new Instrumentation();
    }
    ...
    Application app;
    ...
    // 創建 Application 實例
    try {
        ...
        app = data.info.makeApplication(data.restrictedBackupMode, null);
        mInitialApplication = app;
        ...
        try {
            mInstrumentation.callApplicationOnCreate(app);
        } catch (Exception e) {
            ...
        }
    } finally {
        ...
    }
    ...
}

// http://androidxref.com/8.1.0_r33/xref/frameworks/base/core/java/android/app/LoadedApk.java#959
public Application makeApplication(boolean forceDefaultAppClass,
        Instrumentation instrumentation) {
    Application app = null;

    String appClass = mApplicationInfo.className;
    if (forceDefaultAppClass || (appClass == null)) {
        appClass = "android.app.Application";
    }

    try {
        java.lang.ClassLoader cl = getClassLoader();
        ...
        ContextImpl appContext = ContextImpl.createAppContext(mActivityThread, this);
        app = mActivityThread.mInstrumentation.newApplication(
                cl, appClass, appContext);
        appContext.setOuterContext(app);
    } catch (Exception e) {
        ...
    }
    mActivityThread.mAllApplications.add(app);
    mApplication = app;

    if (instrumentation != null) {// 傳入為 null 所以不走
        try {
            instrumentation.callApplicationOnCreate(app);
        } catch (Exception e) {
            ...
        }
    }
    ...
    return app;
}

// http://androidxref.com/8.1.0_r33/xref/frameworks/base/core/java/android/app/Instrumentation.java#1084
public Application newApplication(ClassLoader cl, String className, Context context)
        throws InstantiationException, IllegalAccessException,
        ClassNotFoundException {
    return newApplication(cl.loadClass(className), context);
}

static public Application newApplication(Class<?> clazz, Context context)
        throws InstantiationException, IllegalAccessException,
        ClassNotFoundException {
    Application app = (Application)clazz.newInstance();
    app.attach(context);
    return app;
}

11. 說完了 9 中的 thread#bindApplication，下面我們繼續說 mStackSupervisor#attachApplicationLocked，其 mStackSupervisor 是 ActivityStackSupervisor 的一個實例，我們查看 ActivityStackSupervisor#attachApplicationLocked 方法中發現會調用 ActivityStackSupervisor#realStartActivityLocked，其方法會調用 app#thread#scheduleLaunchActivity，源碼如下所示：

// http://androidxref.com/8.1.0_r33/xref/frameworks/base/services/core/java/com/android/server/am/ActivityStackSupervisor.java#956
boolean attachApplicationLocked(ProcessRecord app) throws RemoteException {
    final String processName = app.processName;
    ...
    if (realStartActivityLocked(activity, app,
            top == activity /* andResume */, true /* checkConfig */)) {
        ...
    }
    ...
}

// http://androidxref.com/8.1.0_r33/xref/frameworks/base/services/core/java/com/android/server/am/ActivityStackSupervisor.java#1313
final boolean realStartActivityLocked(ActivityRecord r, ProcessRecord app,
        boolean andResume, boolean checkConfig) throws RemoteException {
    ...
    app.thread.scheduleLaunchActivity(new Intent(r.intent), r.appToken,
            System.identityHashCode(r), r.info,
            // TODO: Have this take the merged configuration instead of separate global
            // and override configs.
            mergedConfiguration.getGlobalConfiguration(),
            mergedConfiguration.getOverrideConfiguration(), r.compat,
            r.launchedFromPackage, task.voiceInteractor, app.repProcState, r.icicle,
            r.persistentState, results, newIntents, !andResume,
            mService.isNextTransitionForward(), profilerInfo);
    ...
}

12. 上面說到的 app#thread#scheduleLaunchActivity 中的 thread 是前面提到過的 IApplicationThread，它的實現類是 ActivityThread#ApplicationThread ，我們查看 ActivityThread#ApplicationThread#scheduleLaunchActivity 中的代碼發現最終是發送 LAUNCH_ACTIVITY 消息，這步我們在第 10 步中有過分析，我們直接查看其處理消息相關代碼即可，在 H#handleMessage 中，我們可以看到其會接收并處理很多和四大組件相關的操作，我們查看對 LAUNCH_ACTIVITY 的處理，發現對其處理的方法是 ActivityThread#handleLaunchActivity，它調用到了 ActivityThread#performLaunchActivity 方法，其中的實現再次涉及到了 Instrumentation 類，之前是在創建 Application 對象用到了它，如今是創建 Activity 對象又用到了它，其 Instrumentation#newActivity 也是通過 Class.newInstance() 來實例化 Activity，實例化結束后回到 ActivityThread#performLaunchActivity 中來讓 activity 依附到 window 中，然后callActivityOnCreate 走 Activity 的 onCreate 生命周期，涉及到的源碼如下所示：

// http://androidxref.com/8.1.0_r33/xref/frameworks/base/core/java/android/app/ActivityThread.java#756
public final void scheduleLaunchActivity(Intent intent, IBinder token, int ident,
        ActivityInfo info, Configuration curConfig, Configuration overrideConfig,
        CompatibilityInfo compatInfo, String referrer, IVoiceInteractor voiceInteractor,
        int procState, Bundle state, PersistableBundle persistentState,
        List<ResultInfo> pendingResults, List<ReferrerIntent> pendingNewIntents,
        boolean notResumed, boolean isForward, ProfilerInfo profilerInfo) {
    ...
    sendMessage(H.LAUNCH_ACTIVITY, r);
}

// http://androidxref.com/8.1.0_r33/xref/frameworks/base/core/java/android/app/ActivityThread.java#1580
public void handleMessage(Message msg) {
    ...
    switch (msg.what) {
        ...
        case LAUNCH_ACTIVITY: {
            Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityStart");
            final ActivityClientRecord r = (ActivityClientRecord) msg.obj;

            r.packageInfo = getPackageInfoNoCheck(
                    r.activityInfo.applicationInfo, r.compatInfo);
            handleLaunchActivity(r, null, "LAUNCH_ACTIVITY");
            Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
        } break;
        ...
    }
}

// http://androidxref.com/8.1.0_r33/xref/frameworks/base/core/java/android/app/ActivityThread.java#2833
private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent, String reason) {
    ...
    Activity a = performLaunchActivity(r, customIntent);
    ...
}

// http://androidxref.com/8.1.0_r33/xref/frameworks/base/core/java/android/app/ActivityThread.java#2644
private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
    ...
    Activity activity = null;
    try {
        java.lang.ClassLoader cl = appContext.getClassLoader();
        activity = mInstrumentation.newActivity(
                cl, component.getClassName(), r.intent);
        ...
    } catch (Exception e) {
        ...
    }

    try {
        // 返回之前創建過的 application 對象
        Application app = r.packageInfo.makeApplication(false, mInstrumentation);
        ...
        if (activity != null) {
            ...
            // attach 到 window 上
            activity.attach(appContext, this, getInstrumentation(), r.token,
                    r.ident, app, r.intent, r.activityInfo, title, r.parent,
                    r.embeddedID, r.lastNonConfigurationInstances, config,
                    r.referrer, r.voiceInteractor, window, r.configCallback);
            ...
            if (r.isPersistable()) {
                mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
            } else {
                mInstrumentation.callActivityOnCreate(activity, r.state);
            }
            ...
        }
    } catch (Exception e) {
        ...
    }
    return activity;
}

// http://androidxref.com/8.1.0_r33/xref/frameworks/base/core/java/android/app/Instrumentation.java#1143
public Activity newActivity(ClassLoader cl, String className,
        Intent intent)
        throws InstantiationException, IllegalAccessException,
        ClassNotFoundException {
    return (Activity)cl.loadClass(className).newInstance();
}

public Activity newActivity(Class<?> clazz, Context context,
        IBinder token, Application application, Intent intent, ActivityInfo info,
        CharSequence title, Activity parent, String id,
        Object lastNonConfigurationInstance) throws InstantiationException,
        IllegalAccessException {
    Activity activity = (Activity)clazz.newInstance();
    ActivityThread aThread = null;
    activity.attach(context, aThread, this, token, 0 /* ident */, application, intent,
            info, title, parent, id,
            (Activity.NonConfigurationInstances)lastNonConfigurationInstance,
            new Configuration(), null /* referrer */, null /* voiceInteractor */,
            null /* window */, null /* activityConfigCallback */);
    return activity;
}

到此為止，一個 App 的啟動過程已分析結束，最后獻上啟動涉及到的類的流程圖：

App 的啟動流程圖

結語

我正在打造一個幫助 Android 開發者們拿到更好 offer 的面試庫————安卓 offer 收割基，歡迎 star，覺得不錯的可以持續關注，有興趣的可以一起加入進來和我一同打造。