App啟動流程分析（原始碼版本是8.0）

摘要： 原始碼的程式碼量非常大，分析的過程主要是疏通整個流程，可以忽略部分細節，那麼我們先從點選桌面圖示開始看起,既然是點選肯定要觸發點選事件，我們在在packages/apps/Launcher3/src/com/android/launcher3中.找到launcher.java類，由於And...

原始碼的程式碼量非常大，分析的過程主要是疏通整個流程，可以忽略部分細節，那麼我們先從點選桌面圖示開始看起,既然是點選肯定要觸發點選事件，我們在在packages/apps/Launcher3/src/com/android/launcher3中.找到launcher.java類，由於Android 4.4 開始Launcher預設使用Launcher3，所以我們只看launcher3中的launcher.java就行，點選事件響應的具體程式碼，

/**
 * Launches the intent referred by the clicked shortcut.
 *
 * @param v The view representing the clicked shortcut.
 */
public void onClick(View v) {

....

Object tag = v.getTag();
if (tag instanceof ShortcutInfo) {
onClickAppShortcut(v);
} else if (tag instanceof FolderInfo) {
if (v instanceof FolderIcon) {
onClickFolderIcon(v);
}
} else if ((FeatureFlags.LAUNCHER3_ALL_APPS_PULL_UP && v instanceof PageIndicator) ||
(v == mAllAppsButton && mAllAppsButton != null)) {
onClickAllAppsButton(v);
} else if (tag instanceof AppInfo) {
startAppShortcutOrInfoActivity(v);
} else if (tag instanceof LauncherAppWidgetInfo) {
if (v instanceof PendingAppWidgetHostView) {
onClickPendingWidget((PendingAppWidgetHostView) v);
}
}
}

這段程式碼初步分析是判斷通過什麼方式響應的事件如，快捷方式，資料夾圖示等等。因此我們就看if (tag instanceof ShortcutInfo) 裡面的執行快捷方式的onClickAppShortcut(v);最終會呼叫 startAppShortcutOrInfoActivity(v);

private void startAppShortcutOrInfoActivity(View v) {
ItemInfo item = (ItemInfo) v.getTag();
Intent intent = item.getIntent();
if (intent == null) {
throw new IllegalArgumentException("Input must have a valid intent");
}
boolean success = startActivitySafely(v, intent, item);
getUserEventDispatcher().logAppLaunch(v, intent); // TODO for discovered apps b/35802115

if (success && v instanceof BubbleTextView) {
mWaitingForResume = (BubbleTextView) v;
mWaitingForResume.setStayPressed(true);
}
}

繼續看startActivitySafely(v, intent, item);方法

public boolean startActivitySafely(View v, Intent intent, ItemInfo item) {
if (mIsSafeModeEnabled && !Utilities.isSystemApp(this, intent)) {
Toast.makeText(this, R.string.safemode_shortcut_error, Toast.LENGTH_SHORT).show();
return false;
}
// Only launch using the new animation if the shortcut has not opted out (this is a
// private contract between launcher and may be ignored in the future).
boolean useLaunchAnimation = (v != null) &&
!intent.hasExtra(INTENT_EXTRA_IGNORE_LAUNCH_ANIMATION);
Bundle optsBundle = useLaunchAnimation ? getActivityLaunchOptions(v) : null;

UserHandle user = item == null ? null : item.user;

// Prepare intent
intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
if (v != null) {
intent.setSourceBounds(getViewBounds(v));
}
try {
if (Utilities.ATLEAST_MARSHMALLOW
&& (item instanceof ShortcutInfo)
&& (item.itemType == Favorites.ITEM_TYPE_SHORTCUT
|| item.itemType == Favorites.ITEM_TYPE_DEEP_SHORTCUT)
&& !((ShortcutInfo) item).isPromise()) {
// Shortcuts need some special checks due to legacy reasons.
startShortcutIntentSafely(intent, optsBundle, item);
} else if (user == null || user.equals(Process.myUserHandle())) {
// Could be launching some bookkeeping activity
startActivity(intent, optsBundle);
} else {
LauncherAppsCompat.getInstance(this).startActivityForProfile(
intent.getComponent(), user, intent.getSourceBounds(), optsBundle);
}
return true;
} catch (ActivityNotFoundException|SecurityException e) {
Toast.makeText(this, R.string.activity_not_found, Toast.LENGTH_SHORT).show();
Log.e(TAG, "Unable to launch. tag=" + item + " intent=" + intent, e);
}
return false;
}

這段程式碼中可以看出最後呼叫startActivity(intent, optsBundle);即Activity.java中的方法：

@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);
}
}

上面requestCode引數值為-1表示不需要目標activity結束後返回結果,

*/
public void startActivityForResult(@RequiresPermission Intent intent, int requestCode,
@Nullable Bundle options) {
if (mParent == null) {
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());
}
if (requestCode >= 0) {
// If this start is requesting a result, we can avoid making
// the activity visible until the result is received.Setting
// this code during onCreate(Bundle savedInstanceState) or onResume() will keep the
// activity hidden during this time, to avoid flickering.
// This can only be done when a result is requested because
// that guarantees we will get information back when the
// activity is finished, no matter what happens to it.
mStartedActivity = true;
}

cancelInputsAndStartExitTransition(options);
// TODO Consider clearing/flushing other event sources and events for child windows.
} else {
if (options != null) {
mParent.startActivityFromChild(this, intent, requestCode, options);
} else {
// Note we want to go through this method for compatibility with
// existing applications that may have overridden it.
mParent.startActivityFromChild(this, intent, requestCode);
}
}
}

if mParent為null,則呼叫了方法Instrumentation.execStartActivity，這其中有一些傳入的引數，先來看看這些引數，mMainThread是一個ActivityThread物件，該物件是在當前Activity的attach方法中傳入並賦值給mMainThread，mMainThread.getApplicationThread()返回的是ActivityThread的內部類ApplicationThread物件。

public final class ActivityThread {

final ApplicationThread mAppThread = new ApplicationThread();

//...

private class ApplicationThread extends IApplicationThread.Stub {

//...
}

}

ApplicationThread繼承了IApplicationThread.Stub，那麼這裡使用了Binder的AIDL方式實現程序間通，IApplicationThread是一個AIDl介面，繼承了IInterface介面；Stub是介面IApplicationThread的內部類，繼承了Binder類，實現了IApplicationThread介面。因此ApplicationThread就是一個Binder物件，它重寫了IApplicationThread介面的抽象方法，這些重寫的方法會在後面Binder機制被呼叫，繼續往下看。

public ActivityResult execStartActivity(
Context who, IBinder contextThread, IBinder token, Activity target,
Intent intent, int requestCode, Bundle options) {
IApplicationThread whoThread = (IApplicationThread) contextThread;
Uri referrer = target != null ? target.onProvideReferrer() : null;
if (referrer != null) {
intent.putExtra(Intent.EXTRA_REFERRER, referrer);
}

....

try {
intent.migrateExtraStreamToClipData();
intent.prepareToLeaveProcess(who);


int result = ActivityManager.getService()
.startActivity(whoThread, who.getBasePackageName(), intent,
intent.resolveTypeIfNeeded(who.getContentResolver()),
token, target != null ? target.mEmbeddedID : null,
requestCode, 0, null, options);
checkStartActivityResult(result, intent);
} catch (RemoteException e) {
throw new RuntimeException("Failure from system", e);
}
return null;
}

通過ActivityManager.getService()獲得IActivityManager

/**
 * @hide
 */
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;
}
};

通過ServiceManager.getService(Context.ACTIVITY_SERVICE)獲取一個IBinder物件，獲取到這個IBinder物件後呼叫了IActivityManager.Stub的asInterface方法並傳進去這個IBinder物件獲得了一個IActivityManager物件，呼叫IActivityManager的startActivity()方法，其實就是呼叫AMS的startActivity()方法:

@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());
}

繼續往下看呼叫mActivityStarter.startActivityMayWait

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,
IActivityContainer iContainer, TaskRecord inTask, String reason) {

....

// Save a copy in case ephemeral needs it
final Intent ephemeralIntent = new Intent(intent);
// Don't modify the client's object!
intent = new Intent(intent);


ResolveInfo rInfo = mSupervisor.resolveIntent(intent, resolvedType, userId);
if (rInfo == null) {
UserInfo userInfo = mSupervisor.getUserInfo(userId);
if (userInfo != null && userInfo.isManagedProfile()) {
// Special case for managed profiles, if attempting to launch non-cryto aware
// app in a locked managed profile from an unlocked parent allow it to resolve
// as user will be sent via confirm credentials to unlock the profile.
UserManager userManager = UserManager.get(mService.mContext);
boolean profileLockedAndParentUnlockingOrUnlocked = false;
long token = Binder.clearCallingIdentity();
try {
UserInfo parent = userManager.getProfileParent(userId);
profileLockedAndParentUnlockingOrUnlocked = (parent != null)
&& userManager.isUserUnlockingOrUnlocked(parent.id)
&& !userManager.isUserUnlockingOrUnlocked(userId);
} finally {
Binder.restoreCallingIdentity(token);
}
if (profileLockedAndParentUnlockingOrUnlocked) {
rInfo = mSupervisor.resolveIntent(intent, resolvedType, userId,
PackageManager.MATCH_DIRECT_BOOT_AWARE
| PackageManager.MATCH_DIRECT_BOOT_UNAWARE);
}
}
}
// Collect information about the target of the Intent.
ActivityInfo aInfo = mSupervisor.resolveActivity(intent, rInfo, startFlags, profilerInfo);

ActivityOptions options = ActivityOptions.fromBundle(bOptions);
ActivityStackSupervisor.ActivityContainer container =
(ActivityStackSupervisor.ActivityContainer)iContainer;


....



final ActivityRecord[] outRecord = new ActivityRecord[1];
int res = startActivityLocked(caller, intent, ephemeralIntent, resolvedType,
aInfo, rInfo, voiceSession, voiceInteractor,
resultTo, resultWho, requestCode, callingPid,
callingUid, callingPackage, realCallingPid, realCallingUid, startFlags,
options, ignoreTargetSecurity, componentSpecified, outRecord, container,
inTask, reason);

....


return res;
}
}

這個方法內容比較多，其實就是通過resolveIntent找到匹配的Activity，然後呼叫startActivityLocked啟動Activity，所以我們看看startActivityLocked，這個方法傳入的引數如下IApplicationThread caller：Activity的程序內的ApplicationThread物件

繼續看startActivityLocked方法

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, ActivityStackSupervisor.ActivityContainer container,
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,
container, inTask);

 
.....
 
return mLastStartActivityResult;
}

繼續檢視startActivity方法:

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, ActivityStackSupervisor.ActivityContainer container,
TaskRecord inTask) {

//...

doPendingActivityLaunchesLocked(false);

//...
}

繼續檢視doPendingActivityLaunchesLocked(false)：

final void doPendingActivityLaunchesLocked(boolean doResume) {
...

startActivity(pal.r, pal.sourceRecord, null, null, pal.startFlags, resume, null,
null, null /*outRecords*/);

...
}

依然繼續檢視startActivity方法

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);

...
}

再看看startActivityUnchecked

private int startActivityUnchecked(final ActivityRecord r, ActivityRecord sourceRecord,
IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
int startFlags, boolean doResume, ActivityOptions options, TaskRecord inTask,
ActivityRecord[] outActivity) {

... 

mSupervisor.resumeFocusedStackTopActivityLocked();

...
}

最後呼叫了ActivityStackSupervisor的resumeFocusedStackTopActivityLocked()方法；

boolean resumeFocusedStackTopActivityLocked(
ActivityStack targetStack, ActivityRecord target, ActivityOptions targetOptions) {
if (targetStack != null && isFocusedStack(targetStack)) {
return targetStack.resumeTopActivityUncheckedLocked(target, targetOptions);
}
final ActivityRecord r = mFocusedStack.topRunningActivityLocked();
if (r == null || r.state != RESUMED) {
mFocusedStack.resumeTopActivityUncheckedLocked(null, null);
} else if (r.state == RESUMED) {
// Kick off any lingering app transitions form the MoveTaskToFront operation.
mFocusedStack.executeAppTransition(targetOptions);
}
return false;
}

呼叫ActivityStack的resumeTopActivityUncheckedLocked方法

boolean resumeTopActivityUncheckedLocked(ActivityRecord prev, ActivityOptions options) {
if (mStackSupervisor.inResumeTopActivity) {

...

result = resumeTopActivityInnerLocked(prev, options);

...

}


 private boolean resumeTopActivityInnerLocked(ActivityRecord prev, ActivityOptions options) {

...

mStackSupervisor.startSpecificActivityLocked(next, true, true);

...

 }

追蹤到ActivityStackSupervisor類中

void startSpecificActivityLocked(ActivityRecord r,
boolean andResume, boolean checkConfig) {

...

realStartActivityLocked(r, app, andResume, checkConfig);

...

}




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);

...

 }

通過ProcessRecord物件的成員變數thread，其實就是ApplicationThread類，呼叫ApplicationThread的scheduleLaunchActivity方法；

// we use token to identify this activity without having to send the
// activity itself back to the activity manager. (matters more with ipc)
@Override
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) {

updateProcessState(procState, false);

ActivityClientRecord r = new ActivityClientRecord();

r.token = token;
r.ident = ident;
r.intent = intent;
r.referrer = referrer;
r.voiceInteractor = voiceInteractor;
r.activityInfo = info;
r.compatInfo = compatInfo;
r.state = state;
r.persistentState = persistentState;

r.pendingResults = pendingResults;
r.pendingIntents = pendingNewIntents;

r.startsNotResumed = notResumed;
r.isForward = isForward;

r.profilerInfo = profilerInfo;

r.overrideConfig = overrideConfig;
updatePendingConfiguration(curConfig);

sendMessage(H.LAUNCH_ACTIVITY, r);
}

看最後sendMessage方法，其實是給H Hander傳送LAUNCH_ACTIVITY通知，H Handler是ActivityThread的內部類，我們看看handler是怎麼處理的：

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;

呼叫了ActivityThread的handleLaunchActivity方法：

private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent, String reason) {


...

Activity a = performLaunchActivity(r, customIntent);

if (a != null) {

.....


handleResumeActivity(r.token, false, r.isForward,
!r.activity.mFinished && !r.startsNotResumed, r.lastProcessedSeq, reason);

if (!r.activity.mFinished && r.startsNotResumed) {


....


performPauseActivityIfNeeded(r, reason);

....

try {
ActivityManager.getService()
.finishActivity(r.token, Activity.RESULT_CANCELED, null,
Activity.DONT_FINISH_TASK_WITH_ACTIVITY);
} catch (RemoteException ex) {
throw ex.rethrowFromSystemServer();
}
}
}

通過performLaunchActivity方法啟動Activity；

private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
// System.out.println("##### [" + System.currentTimeMillis() + "] ActivityThread.performLaunchActivity(" + r + ")");

ActivityInfo aInfo = r.activityInfo;
if (r.packageInfo == null) {
r.packageInfo = getPackageInfo(aInfo.applicationInfo, r.compatInfo,
Context.CONTEXT_INCLUDE_CODE);
}

ComponentName component = r.intent.getComponent();
if (component == null) {
component = r.intent.resolveActivity(
mInitialApplication.getPackageManager());
r.intent.setComponent(component);
}

if (r.activityInfo.targetActivity != null) {
component = new ComponentName(r.activityInfo.packageName,
r.activityInfo.targetActivity);
}

ContextImpl appContext = createBaseContextForActivity(r);
Activity activity = null;
try {
java.lang.ClassLoader cl = appContext.getClassLoader();
activity = mInstrumentation.newActivity(
cl, component.getClassName(), r.intent);
StrictMode.incrementExpectedActivityCount(activity.getClass());
r.intent.setExtrasClassLoader(cl);
r.intent.prepareToEnterProcess();
if (r.state != null) {
r.state.setClassLoader(cl);
}
} catch (Exception e) {
if (!mInstrumentation.onException(activity, e)) {
throw new RuntimeException(
"Unable to instantiate activity " + component
+ ": " + e.toString(), e);
}
}

try {
Application app = r.packageInfo.makeApplication(false, mInstrumentation);

if (localLOGV) Slog.v(TAG, "Performing launch of " + r);
if (localLOGV) Slog.v(
TAG, r + ": app=" + app
+ ", appName=" + app.getPackageName()
+ ", pkg=" + r.packageInfo.getPackageName()
+ ", comp=" + r.intent.getComponent().toShortString()
+ ", dir=" + r.packageInfo.getAppDir());

if (activity != null) {
CharSequence title = r.activityInfo.loadLabel(appContext.getPackageManager());
Configuration config = new Configuration(mCompatConfiguration);
if (r.overrideConfig != null) {
config.updateFrom(r.overrideConfig);
}
if (DEBUG_CONFIGURATION) Slog.v(TAG, "Launching activity "
+ r.activityInfo.name + " with config " + config);
Window window = null;
if (r.mPendingRemoveWindow != null && r.mPreserveWindow) {
window = r.mPendingRemoveWindow;
r.mPendingRemoveWindow = null;
r.mPendingRemoveWindowManager = null;
}
appContext.setOuterContext(activity);
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 (customIntent != null) {
activity.mIntent = customIntent;
}
r.lastNonConfigurationInstances = null;
checkAndBlockForNetworkAccess();
activity.mStartedActivity = false;
int theme = r.activityInfo.getThemeResource();
if (theme != 0) {
activity.setTheme(theme);
}

activity.mCalled = false;
if (r.isPersistable()) {
mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
} else {
mInstrumentation.callActivityOnCreate(activity, r.state);
}
if (!activity.mCalled) {
throw new SuperNotCalledException(
"Activity " + r.intent.getComponent().toShortString() +
" did not call through to super.onCreate()");
}
r.activity = activity;
r.stopped = true;
if (!r.activity.mFinished) {
activity.performStart();
r.stopped = false;
}
if (!r.activity.mFinished) {
if (r.isPersistable()) {
if (r.state != null || r.persistentState != null) {
mInstrumentation.callActivityOnRestoreInstanceState(activity, r.state,
r.persistentState);
}
} else if (r.state != null) {
mInstrumentation.callActivityOnRestoreInstanceState(activity, r.state);
}
}
if (!r.activity.mFinished) {
activity.mCalled = false;
if (r.isPersistable()) {
mInstrumentation.callActivityOnPostCreate(activity, r.state,
r.persistentState);
} else {
mInstrumentation.callActivityOnPostCreate(activity, r.state);
}
if (!activity.mCalled) {
throw new SuperNotCalledException(
"Activity " + r.intent.getComponent().toShortString() +
" did not call through to super.onPostCreate()");
}
}
}
r.paused = true;

mActivities.put(r.token, r);

} catch (SuperNotCalledException e) {
throw e;

} catch (Exception e) {
if (!mInstrumentation.onException(activity, e)) {
throw new RuntimeException(
"Unable to start activity " + component
+ ": " + e.toString(), e);
}
}

return activity;
}

performLaunchActivity方法程式碼不算很多可以看出做了一些比較重要的事情

• 建立Activity的上下文環境，即建立了ContextImpl物件；
• 建立Activity的例項：呼叫Instrumentation$newActivity方法，並使用類載入器建立Activity物件；
• 建立Application物件；
• 呼叫Activity$attach方法：初始化Activity類裡的資料；
• 啟動Activity：呼叫Instrumentation$callActivityOnCreate方法；

檢視Activity.attach方法原始碼：

final void attach(Context context, ActivityThread aThread,
Instrumentation instr, IBinder token, int ident,
Application application, Intent intent, ActivityInfo info,
CharSequence title, Activity parent, String id,
NonConfigurationInstances lastNonConfigurationInstances,
Configuration config, String referrer, IVoiceInteractor voiceInteractor,
Window window, ActivityConfigCallback activityConfigCallback) {
attachBaseContext(context);

mFragments.attachHost(null /*parent*/);

mWindow = new PhoneWindow(this, window, activityConfigCallback);
mWindow.setWindowControllerCallback(this);
mWindow.setCallback(this);
mWindow.setOnWindowDismissedCallback(this);
mWindow.getLayoutInflater().setPrivateFactory(this);
if (info.softInputMode != WindowManager.LayoutParams.SOFT_INPUT_STATE_UNSPECIFIED) {
mWindow.setSoftInputMode(info.softInputMode);
}
if (info.uiOptions != 0) {
mWindow.setUiOptions(info.uiOptions);
}
mUiThread = Thread.currentThread();

mMainThread = aThread;
mInstrumentation = instr;
mToken = token;
mIdent = ident;
mApplication = application;
mIntent = intent;
mReferrer = referrer;
mComponent = intent.getComponent();
mActivityInfo = info;
mTitle = title;
mParent = parent;
mEmbeddedID = id;
mLastNonConfigurationInstances = lastNonConfigurationInstances;
if (voiceInteractor != null) {
if (lastNonConfigurationInstances != null) {
mVoiceInteractor = lastNonConfigurationInstances.voiceInteractor;
} else {
mVoiceInteractor = new VoiceInteractor(voiceInteractor, this, this,
Looper.myLooper());
}
}

mWindow.setWindowManager(
(WindowManager)context.getSystemService(Context.WINDOW_SERVICE),
mToken, mComponent.flattenToString(),
(info.flags & ActivityInfo.FLAG_HARDWARE_ACCELERATED) != 0);
if (mParent != null) {
mWindow.setContainer(mParent.getWindow());
}
mWindowManager = mWindow.getWindowManager();
mCurrentConfig = config;

mWindow.setColorMode(info.colorMode);
}

建立了視窗物件PhoneWindow,給視窗物件註冊監聽介面,初始化一些變數的值,通過視窗Window建立WindowManagerImpl的例項並設定給變數mWindowManager。

最後呼叫Instrumentation.callActivityOnCreate方法。

/**
 * Perform calling of an activity's {@link Activity#onCreate}
 * method.The default implementation simply calls through to that method.
 *@param activity The activity being created.
 * @param icicle The previously frozen state (or null) to pass through to
 * @param persistentState The previously persisted state (or null)
 */
public void callActivityOnCreate(Activity activity, Bundle icicle,
PersistableBundle persistentState) {
prePerformCreate(activity);
activity.performCreate(icicle, persistentState);
postPerformCreate(activity);
}

通過activity.performCreate(icicle, persistentState)

final void performCreate(Bundle icicle) {
restoreHasCurrentPermissionRequest(icicle);
onCreate(icicle);
mActivityTransitionState.readState(icicle);
performCreateCommon();
}

即呼叫了Activity的onCreate方法啟動Activity，到此activity就被啟動了。