（Android 9.0）Activity啟動流程原始碼分析
阿新 • • 發佈：2019-06-21
前言
熟悉Activity的啟動流程和執行原理是一個合格的應用開發人員所應該具備的基本素質，其重要程度就不多做描述了。同時，知識棧應該不斷的更新，最新發布的Android 9.0版本相較於之前的幾個版本也做了許多改動和重構，但是大體流程變化不大。本文基於Android 9.0版本原始碼，從Activity啟動方法startActivity為切入口分析整個流程。
相關類簡介
- Instrumentation
用於實現應用程式測試程式碼的基類。當在開啟儀器的情況下執行時，這個類將在任何應用程式程式碼之前為您例項化，允許您監視系統與應用程式的所有互動。可以通過AndroidManifest.xml的<Instrumentation>標籤描述該類的實現。
- ActivityManager
該類提供與Activity、Service和Process相關的資訊以及互動方法， 可以被看作是ActivityManagerService的輔助類。
- ActivityManagerService
Android中最核心的服務，主要負責系統中四大元件的啟動、切換、排程及應用程式的管理和排程等工作。
- ActivityThread
管理應用程式程序中主執行緒的執行，根據Activity管理者的請求排程和執行activities、broadcasts及其相關的操作。
- ActivityStack
負責單個Activity棧的狀態和管理。
- ActivityStackSupervisor
負責所有Activity棧的管理。內部管理了mHomeStack、mFocusedStack和mLastFocusedStack三個Activity棧。其中，mHomeStack管理的是Launcher相關的Activity棧；mFocusedStack管理的是當前顯示在前臺Activity的Activity棧；mLastFocusedStack管理的是上一次顯示在前臺Activity的Activity棧。
- ClientLifecycleManager
用來管理多個客戶端生命週期執行請求的管理類。
一、發出啟動請求
啟動一個Activity通常都是通過Activity的startActivity方法啟動。
frameworks/base/core/java/android/app/Activity.java
@Override
public void startActivity(Intent intent) {
this.startActivity(intent, null);
}
@Override
public void startActivity(Intent intent, @Nullable Bundle options) {
// Activity中的startActivity也是呼叫startActivityForResult方法來實現的，
// 當startActivityForResult方法的requestCode為-1不返回結果，
// requestCode大於等於零則會回撥Activity.onActivityResult方法返回結果。
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);
}
}
public void startActivityForResult(@RequiresPermission Intent intent, int requestCode) {
startActivityForResult(intent, requestCode, null);
}
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) {
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);
}
}
}
之後Activity的startActivity方法會呼叫startActivityForResult方法來傳遞啟動請求，在startActivityForResult方法中如果是第一次啟動，mParent為空則會去呼叫Instrumentation.execStartActivity方法，否則調動Activity.startActivityFromChild方法。
frameworks/base/core/java/android/app/Instrumentation.java
public ActivityResult execStartActivity(
Context who, IBinder contextThread, IBinder token, Activity target,
Intent intent, int requestCode, Bundle options) {
...
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;
}
二、ActivityManagerService接收並處理啟動請求
在Instrumentation.execStartActivity方法中看到了熟悉的身影ActivityManager，通過ActivityManager.getService()方法可以獲得ActivityManagerService提供的服務，所以直接跳轉到ActivityManagerService.startActivity方法。
frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java
@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) {
return startActivityAsUser(caller, callingPackage, intent, resolvedType, resultTo,
resultWho, requestCode, startFlags, profilerInfo, bOptions, userId,
true /*validateIncomingUser*/);
}
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,
boolean validateIncomingUser) {
enforceNotIsolatedCaller("startActivity");
userId = mActivityStartController.checkTargetUser(userId, validateIncomingUser,
Binder.getCallingPid(), Binder.getCallingUid(), "startActivityAsUser");
// TODO: Switch to user app stacks here.
return mActivityStartController.obtainStarter(intent, "startActivityAsUser")
.setCaller(caller)
.setCallingPackage(callingPackage)
.setResolvedType(resolvedType)
.setResultTo(resultTo)
.setResultWho(resultWho)
.setRequestCode(requestCode)
.setStartFlags(startFlags)
.setProfilerInfo(profilerInfo)
.setActivityOptions(bOptions)
.setMayWait(userId)
.execute();
}
ActivityManagerService.startActivity方法經過多個方法呼叫會去執行startActivityAsUser方法，在startActivityAsUser方法最後會通過ActivityStartController.obtainStarter方法獲得一個包含所有啟動資訊的ActivityStarter物件並呼叫execute方法執行。
frameworks/base/services/core/java/com/android/server/am/ActivityStarter.java
int execute() {
try {
// TODO(b/64750076): Look into passing request directly to these methods to allow
// for transactional diffs and preprocessing.
if (mRequest.mayWait) {
return startActivityMayWait(mRequest.caller, mRequest.callingUid,
mRequest.callingPackage, mRequest.intent, mRequest.resolvedType,
mRequest.voiceSession, mRequest.voiceInteractor, mRequest.resultTo,
mRequest.resultWho, mRequest.requestCode, mRequest.startFlags,
mRequest.profilerInfo, mRequest.waitResult, mRequest.globalConfig,
mRequest.activityOptions, mRequest.ignoreTargetSecurity, mRequest.userId,
mRequest.inTask, mRequest.reason,
mRequest.allowPendingRemoteAnimationRegistryLookup);
} else {
return startActivity(mRequest.caller, mRequest.intent, mRequest.ephemeralIntent,
mRequest.resolvedType, mRequest.activityInfo, mRequest.resolveInfo,
mRequest.voiceSession, mRequest.voiceInteractor, mRequest.resultTo,
mRequest.resultWho, mRequest.requestCode, mRequest.callingPid,
mRequest.callingUid, mRequest.callingPackage, mRequest.realCallingPid,
mRequest.realCallingUid, mRequest.startFlags, mRequest.activityOptions,
mRequest.ignoreTargetSecurity, mRequest.componentSpecified,
mRequest.outActivity, mRequest.inTask, mRequest.reason,
mRequest.allowPendingRemoteAnimationRegistryLookup);
}
} finally {
onExecutionComplete();
}
}
因為在ActivityManagerService.startActivityAsUser中呼叫了ActivityStarter.setMayWait方法，所以這裡 mRequest.mayWait值為true，會去呼叫startActivityMayWait方法。
frameworks/base/services/core/java/com/android/server/am/ActivityStarter.java
private 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, SafeActivityOptions options, boolean ignoreTargetSecurity,
int userId, TaskRecord inTask, String reason,
boolean allowPendingRemoteAnimationRegistryLookup) {
...
int res = startActivity(caller, intent, ephemeralIntent, resolvedType, aInfo, rInfo,
voiceSession, voiceInteractor, resultTo, resultWho, requestCode, callingPid,
callingUid, callingPackage, realCallingPid, realCallingUid, startFlags, options,
ignoreTargetSecurity, componentSpecified, outRecord, inTask, reason,
allowPendingRemoteAnimationRegistryLookup);
...
return res;
}
}
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,
SafeActivityOptions options, boolean ignoreTargetSecurity, boolean componentSpecified,
ActivityRecord[] outActivity, TaskRecord inTask, String reason,
boolean allowPendingRemoteAnimationRegistryLookup) {
...
mLastStartActivityResult = startActivity(caller, intent, ephemeralIntent, resolvedType,
aInfo, rInfo, voiceSession, voiceInteractor, resultTo, resultWho, requestCode,
callingPid, callingUid, callingPackage, realCallingPid, realCallingUid, startFlags,
options, ignoreTargetSecurity, componentSpecified, mLastStartActivityRecord,
inTask, allowPendingRemoteAnimationRegistryLookup);
...
return getExternalResult(mLastStartActivityResult);
}
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,
SafeActivityOptions options,
boolean ignoreTargetSecurity, boolean componentSpecified, ActivityRecord[] outActivity,
TaskRecord inTask, boolean allowPendingRemoteAnimationRegistryLookup) {
...
return startActivity(r, sourceRecord, voiceSession, voiceInteractor, startFlags,
true /* doResume */, checkedOptions, inTask, outActivity);
}
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);
...
postStartActivityProcessing(r, result, mTargetStack);
return result;
}
ActivityStarter.startActivityMayWait方法中呼叫多個startActivity方法後會呼叫到一個比較重要的方法startActivityUnchecked。說它重要是因為這個方法裡會根據啟動標誌位和Activity啟動模式來決定如何啟動一個Activity以及是否要呼叫deliverNewIntent方法通知Activity有一個Intent試圖重新啟動它。具體細節就不在這裡分析了，感興趣的同學自行檢視原始碼體會一下吧。
private int startActivityUnchecked(final ActivityRecord r, ActivityRecord sourceRecord,
IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
int startFlags, boolean doResume, ActivityOptions options, TaskRecord inTask,
ActivityRecord[] outActivity) {
...
ActivityRecord reusedActivity = getReusableIntentActivity();
...
if (reusedActivity != null) {
// When the flags NEW_TASK and CLEAR_TASK are set, then the task gets reused but
// still needs to be a lock task mode violation since the task gets cleared out and
// the device would otherwise leave the locked task.
if (mService.getLockTaskController().isLockTaskModeViolation(reusedActivity.getTask(),
(mLaunchFlags & (FLAG_ACTIVITY_NEW_TASK | FLAG_ACTIVITY_CLEAR_TASK))
== (FLAG_ACTIVITY_NEW_TASK | FLAG_ACTIVITY_CLEAR_TASK))) {
Slog.e(TAG, "startActivityUnchecked: Attempt to violate Lock Task Mode");
return START_RETURN_LOCK_TASK_MODE_VIOLATION;
}
// True if we are clearing top and resetting of a standard (default) launch mode
// ({@code LAUNCH_MULTIPLE}) activity. The existing activity will be finished.
final boolean clearTopAndResetStandardLaunchMode =
(mLaunchFlags & (FLAG_ACTIVITY_CLEAR_TOP | FLAG_ACTIVITY_RESET_TASK_IF_NEEDED))
== (FLAG_ACTIVITY_CLEAR_TOP | FLAG_ACTIVITY_RESET_TASK_IF_NEEDED)
&& mLaunchMode == LAUNCH_MULTIPLE;
// If mStartActivity does not have a task associated with it, associate it with the
// reused activity's task. Do not do so if we're clearing top and resetting for a
// standard launchMode activity.
if (mStartActivity.getTask() == null && !clearTopAndResetStandardLaunchMode) {
mStartActivity.setTask(reusedActivity.getTask());
}
if (reusedActivity.getTask().intent == null) {
// This task was started because of movement of the activity based on affinity...
// Now that we are actually launching it, we can assign the base intent.
reusedActivity.getTask().setIntent(mStartActivity);
}
// This code path leads to delivering a new intent, we want to make sure we schedule it
// as the first operation, in case the activity will be resumed as a result of later
// operations.
if ((mLaunchFlags & FLAG_ACTIVITY_CLEAR_TOP) != 0
|| isDocumentLaunchesIntoExisting(mLaunchFlags)
|| isLaunchModeOneOf(LAUNCH_SINGLE_INSTANCE, LAUNCH_SINGLE_TASK)) {
final TaskRecord task = reusedActivity.getTask();
// In this situation we want to remove all activities from the task up to the one
// being started. In most cases this means we are resetting the task to its initial
// state.
final ActivityRecord top = task.performClearTaskForReuseLocked(mStartActivity,
mLaunchFlags);
// The above code can remove {@code reusedActivity} from the task, leading to the
// the {@code ActivityRecord} removing its reference to the {@code TaskRecord}. The
// task reference is needed in the call below to
// {@link setTargetStackAndMoveToFrontIfNeeded}.
if (reusedActivity.getTask() == null) {
reusedActivity.setTask(task);
}
if (top != null) {
if (top.frontOfTask) {
// Activity aliases may mean we use different intents for the top activity,
// so make sure the task now has the identity of the new intent.
top.getTask().setIntent(mStartActivity);
}
deliverNewIntent(top);
}
}
mSupervisor.sendPowerHintForLaunchStartIfNeeded(false /* forceSend */, reusedActivity);
reusedActivity = setTargetStackAndMoveToFrontIfNeeded(reusedActivity);
final ActivityRecord outResult =
outActivity != null && outActivity.length > 0 ? outActivity[0] : null;
// When there is a reused activity and the current result is a trampoline activity,
// set the reused activity as the result.
if (outResult != null && (outResult.finishing || outResult.noDisplay)) {
outActivity[0] = reusedActivity;
}
if ((mStartFlags & START_FLAG_ONLY_IF_NEEDED) != 0) {
// We don't need to start a new activity, and the client said not to do anything
// if that is the case, so this is it! And for paranoia, make sure we have
// correctly resumed the top activity.
resumeTargetStackIfNeeded();
return START_RETURN_INTENT_TO_CALLER;
}
if (reusedActivity != null) {
setTaskFromIntentActivity(reusedActivity);
if (!mAddingToTask && mReuseTask == null) {
// We didn't do anything... but it was needed (a.k.a., client don't use that
// intent!) And for paranoia, make sure we have correctly resumed the top activity.
resumeTargetStackIfNeeded();
if (outActivity != null && outActivity.length > 0) {
outActivity[0] = reusedActivity;
}
return mMovedToFront ? START_TASK_TO_FRONT : START_DELIVERED_TO_TOP;
}
}
}
if (mStartActivity.packageName == null) {
final ActivityStack sourceStack = mStartActivity.resultTo != null
? mStartActivity.resultTo.getStack() : null;
if (sourceStack != null) {
sourceStack.sendActivityResultLocked(-1 /* callingUid */, mStartActivity.resultTo,
mStartActivity.resultWho, mStartActivity.requestCode, RESULT_CANCELED,
null /* data */);
}
ActivityOptions.abort(mOptions);
return START_CLASS_NOT_FOUND;
}
// If the activity being launched is the same as the one currently at the top, then
// we need to check if it should only be launched once.
final ActivityStack topStack = mSupervisor.mFocusedStack;
final ActivityRecord topFocused = topStack.getTopActivity();
final ActivityRecord top = topStack.topRunningNonDelayedActivityLocked(mNotTop);
final boolean dontStart = top != null && mStartActivity.resultTo == null
&& top.realActivity.equals(mStartActivity.realActivity)
&& top.userId == mStartActivity.userId
&& top.app != null && top.app.thread != null
&& ((mLaunchFlags & FLAG_ACTIVITY_SINGLE_TOP) != 0
|| isLaunchModeOneOf(LAUNCH_SINGLE_TOP, LAUNCH_SINGLE_TASK));
if (dontStart) {
// For paranoia, make sure we have correctly resumed the top activity.
topStack.mLastPausedActivity = null;
if (mDoResume) {
mSupervisor.resumeFocusedStackTopActivityLocked();
}
ActivityOptions.abort(mOptions);
if ((mStartFlags & START_FLAG_ONLY_IF_NEEDED) != 0) {
// We don't need to start a new activity, and the client said not to do
// anything if that is the case, so this is it!
return START_RETURN_INTENT_TO_CALLER;
}
deliverNewIntent(top);
// Don't use mStartActivity.task to show the toast. We're not starting a new activity
// but reusing 'top'. Fields in mStartActivity may not be fully initialized.
mSupervisor.handleNonResizableTaskIfNeeded(top.getTask(), preferredWindowingMode,
preferredLaunchDisplayId, topStack);
return START_DELIVERED_TO_TOP;
}
boolean newTask = false;
final TaskRecord taskToAffiliate = (mLaunchTaskBehind && mSourceRecord != null)
? mSourceRecord.getTask() : null;
// Should this be considered a new task?
int result = START_SUCCESS;
if (mStartActivity.resultTo == null && mInTask == null && !mAddingToTask
&& (mLaunchFlags & FLAG_ACTIVITY_NEW_TASK) != 0) {
newTask = true;
result = setTaskFromReuseOrCreateNewTask(taskToAffiliate, topStack);
} else if (mSourceRecord != null) {
result = setTaskFromSourceRecord();
} else if (mInTask != null) {
result = setTaskFromInTask();
} else {
// This not being started from an existing activity, and not part of a new task...
// just put it in the top task, though these days this case should never happen.
setTaskToCurrentTopOrCreateNewTask();
}
if (result != START_SUCCESS) {
return result;
}
mService.grantUriPermissionFromIntentLocked(mCallingUid, mStartActivity.packageName,
mIntent, mStartActivity.getUriPermissionsLocked(), mStartActivity.userId);
mService.grantEphemeralAccessLocked(mStartActivity.userId, mIntent,
mStartActivity.appInfo.uid, UserHandle.getAppId(mCallingUid));
if (newTask) {
EventLog.writeEvent(EventLogTags.AM_CREATE_TASK, mStartActivity.userId,
mStartActivity.getTask().taskId);
}
ActivityStack.logStartActivity(
EventLogTags.AM_CREATE_ACTIVITY, mStartActivity, mStartActivity.getTask());
mTargetStack.mLastPausedActivity = null;
mSupervisor.sendPowerHintForLaunchStartIfNeeded(false /* forceSend */, mStartActivity);
mTargetStack.startActivityLocked(mStartActivity, topFocused, newTask, mKeepCurTransition,
mOptions);
if (mDoResume) {
final ActivityRecord topTaskActivity =
mStartActivity.getTask().topRunningActivityLocked();
if (!mTargetStack.isFocusable()
|| (topTaskActivity != null && topTaskActivity.mTaskOverlay
&& mStartActivity != topTaskActivity)) {
// If the activity is not focusable, we can't resume it, but still would like to
// make sure it becomes visible as it starts (this will also trigger entry
// animation). An example of this are PIP activities.
// Also, we don't want to resume activities in a task that currently has an overlay
// as the starting activity just needs to be in the visible paused state until the
// over is removed.
mTargetStack.ensureActivitiesVisibleLocked(null, 0, !PRESERVE_WINDOWS);
// Go ahead and tell window manager to execute app transition for this activity
// since the app transition will not be triggered through the resume channel.
mService.mWindowManager.executeAppTransition();
} else {
// If the target stack was not previously focusable (previous top running activity
// on that stack was not visible) then any prior calls to move the stack to the
// will not update the focused stack. If starting the new activity now allows the
// task stack to be focusable, then ensure that we now update the focused stack
// accordingly.
if (mTargetStack.isFocusable() && !mSupervisor.isFocusedStack(mTargetStack)) {
mTargetStack.moveToFront("startActivityUnchecked");
}
mSupervisor.resumeFocusedStackTopActivityLocked(mTargetStack, mStartActivity,
mOptions);
}
} else if (mStartActivity != null) {
mSupervisor.mRecentTasks.add(mStartActivity.getTask());
}
mSupervisor.updateUserStackLocked(mStartActivity.userId, mTargetStack);
mSupervisor.handleNonResizableTaskIfNeeded(mStartActivity.getTask(), preferredWindowingMode,
preferredLaunchDisplayId, mTargetStack);
return START_SUCCESS;
}
無論以何種模式啟動最終都會呼叫ActivityStackSupervisor.resumeFocusedStackTopActivityLocked方法。
frameworks/base/services/core/java/com/android/server/am/ActivityStackSupervisor.java
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.isState(RESUMED)) {
mFocusedStack.resumeTopActivityUncheckedLocked(null, null);
} else if (r.isState(RESUMED)) {
// Kick off any lingering app transitions form the MoveTaskToFront operation.
mFocusedStack.executeAppTransition(targetOptions);
}
...
return false;
}
frameworks/base/services/core/java/com/android/server/am/ActivityStack.java
boolean resumeTopActivityUncheckedLocked(ActivityRecord prev, ActivityOptions options) {
...
result = resumeTopActivityInnerLocked(prev, options);
...
return result;
}
private boolean resumeTopActivityInnerLocked(ActivityRecord prev, ActivityOptions options) {
...
boolean pausing = mStackSupervisor.pauseBackStacks(userLeaving, next, false);
if (mResumedActivity != null) {
pausing |= startPausingLocked(userLeaving, false, next, false);
}
...
mStackSupervisor.startSpecificActivityLocked(next, true, true);
...
return true;
}
在ActivityStack.resumeTopActivityInnerLocked方法中會去判斷是否有Activity處於Resume狀態，如果有的話會先讓這個Activity執行Pausing過程，然後再執行startSpecificActivityLocked方法啟動要啟動Activity。此處分開兩段，先看一下棧頂Activity是如何退出的。
三、棧頂Activity執行onPause方法退出
frameworks/base/services/core/java/com/android/server/am/ActivityStack.java
final boolean startPausingLocked(boolean userLeaving, boolean uiSleeping,
ActivityRecord resuming, boolean pauseImmediately) {
...
if (prev.app != null && prev.app.thread != null) {
try {
EventLogTags.writeAmPauseActivity(prev.userId, System.identityHashCode(prev),
prev.shortComponentName, "userLeaving=" + userLeaving);
mService.updateUsageStats(prev, false);
// Android 9.0在這裡引入了ClientLifecycleManager和
// ClientTransactionHandler來輔助管理Activity生命週期，
// 他會發送EXECUTE_TRANSACTION訊息到ActivityThread.H裡面繼續處理。
mService.getLifecycleManager().scheduleTransaction(prev.app.thread, prev.appToken,
PauseActivityItem.obtain(prev.finishing, userLeaving,
prev.configChangeFlags, pauseImmediately));
} catch (Exception e) {
// Ignore exception, if process died other code will cleanup.
Slog.w(TAG, "Exception thrown during pause", e);
mPausingActivity = null;
mLastPausedActivity = null;
mLastNoHistoryActivity = null;
}
} else {
mPausingActivity = null;
mLastPausedActivity = null;
mLastNoHistoryActivity = null;
}
...
}
在ActivityStack.startPausingLocked方法中通過ClientLifecycleManager的scheduleTransaction方法把PauseActivityItem事件加入到執行計劃中，開始棧頂的pausing過程。
frameworks/base/services/core/java/com/android/server/am/ClientLifecycleManager.java
void scheduleTransaction(ClientTransaction transaction) throws RemoteException {
final IApplicationThread client = transaction.getClient();
transaction.schedule();
...
}
frameworks/base/core/java/android/app/servertransaction/ClientTransaction.java
public void schedule() throws RemoteException {
mClient.scheduleTransaction(this);
}
ClientTransaction.schedule方法的mClient是一個IApplicationThread型別，ActivityThread的內部類ApplicationThread派生這個介面類並實現了對應的方法。所以直接跳轉到ApplicationThread中的scheduleTransaction方法。ActivityThread類中並沒有定義scheduleTransaction方法，所以呼叫的是他父類ClientTransactionHandler的scheduleTransaction方法。
frameworks/base/core/java/android/app/ActivityThread.java
private class ApplicationThread extends IApplicationThread.Stub {
...
@Override
public void scheduleTransaction(ClientTransaction transaction) throws RemoteException {
ActivityThread.this.scheduleTransaction(transaction);
}
}
frameworks/base/core/java/android/app/ClientTransactionHandler.java
void scheduleTransaction(ClientTransaction transaction) {
transaction.preExecute(this);
sendMessage(ActivityThread.H.EXECUTE_TRANSACTION, transaction);
}
在ClientTransactionHandler.scheduleTransaction方法中呼叫了sendMessage方法，這個方法是一個抽象方法，其實現在ClientTransactionHandler派生類的ActivityThread中，ActivityThread.sendMessage方法會把訊息傳送給內部名字叫H的Handler。
frameworks/base/core/java/android/app/ActivityThread.java
void sendMessage(int what, Object obj) {
sendMessage(what, obj, 0, 0, false);
}
private void sendMessage(int what, Object obj, int arg1) {
sendMessage(what, obj, arg1, 0, false);
}
private void sendMessage(int what, Object obj, int arg1, int arg2) {
sendMessage(what, obj, arg1, arg2, 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);
}
public void handleMessage(Message msg) {
...
case EXECUTE_TRANSACTION:
final ClientTransaction transaction = (ClientTransaction) msg.obj;
mTransactionExecutor.execute(transaction);
...
break;
...
}
Handler H的例項接收到EXECUTE_TRANSACTION訊息後呼叫TransactionExecutor.execute方法切換Activity狀態。TransactionExecutor.execute方法裡面先執行Callbacks，然後改變Activity當前的生命週期狀態。此處由於沒有Callback所以直接跳轉executeLifecycleState方法。
frameworks/base/core/java/android/app/servertransaction/TransactionExecutor.java
public void execute(ClientTransaction transaction) {
...
executeCallbacks(transaction);
executeLifecycleState(transaction);
...
}
public void executeCallbacks(ClientTransaction transaction) {
if (callbacks == null) {
// No callbacks to execute, return early.
return;
}
...
for (int i = 0; i < size; ++i) {
final ClientTransactionItem item = callbacks.get(i);
...
item.execute(mTransactionHandler, token, mPendingActions);
item.postExecute(mTransactionHandler, token, mPendingActions);
...
}
}
/** Transition to the final state if requested by the transaction. */
private void executeLifecycleState(ClientTransaction transaction) {
...
// Cycle to the state right before the final requested state.
cycleToPath(r, lifecycleItem.getTargetState(), true /* excludeLastState */);
// Execute the final transition with proper parameters.
lifecycleItem.execute(mTransactionHandler, token, mPendingActions);
lifecycleItem.postExecute(mTransactionHandler, token, mPendingActions);
}
在executeLifecycleState方法裡面，會先去呼叫TransactionExecutor.cycleToPath執行當前生命週期狀態之前的狀態，然後執行ActivityLifecycleItem.execute方法。由於是從ON_RESUME狀態到ON_PAUSE狀態切換，中間沒有其他狀態，cycleToPath這個情況下沒有做什麼實質性的事情，直接執行execute方法。前面在ActivityStack.startPausingLocked方法裡面scheduleTransaction傳遞的是PauseActivityItem物件，所以executeLifecycleState方法裡呼叫的execute方法其實是PauseActivityItem.execute方法。
frameworks/base/core/java/android/app/servertransaction/PauseActivityItem.java
@Override
public void execute(ClientTransactionHandler client, IBinder token,
PendingTransactionActions pendingActions) {
...
client.handlePauseActivity(token, mFinished, mUserLeaving, mConfigChanges, pendingActions,
"PAUSE_ACTIVITY_ITEM");
...
}
在PauseActivityItem.execute方法中呼叫ActivityThread.handlePauseActivity方法，經過一步步呼叫來到performPauseActivity方法，在這個方法中會先去判斷是否需要呼叫callActivityOnSaveInstanceState方法來儲存臨時資料，然後執行Instrumentation.callActivityOnPause方法繼續執行pasue流程。
frameworks/base/core/java/android/app/ActivityThread.java
@Override
public void handlePauseActivity(IBinder token, boolean finished, boolean userLeaving,
int configChanges, PendingTransactionActions pendingActions, String reason) {
...
performPauseActivity(r, finished, reason, pendingActions);
...
}
}
final Bundle performPauseActivity(IBinder token, boolean finished, String reason,
PendingTransactionActions pendingActions) {
ActivityClientRecord r = mActivities.get(token);
return r != null ? performPauseActivity(r, finished, reason, pendingActions) : null;
}
/**
* Pause the activity.
* @return Saved instance state for pre-Honeycomb apps if it was saved, {@code null} otherwise.
*/
private Bundle performPauseActivity(ActivityClientRecord r, boolean finished, String reason,
PendingTransactionActions pendingActions) {
...
if (shouldSaveState) {
callActivityOnSaveInstanceState(r);
}
performPauseActivityIfNeeded(r, reason);
...
return shouldSaveState ? r.state : null;
}
private void performPauseActivityIfNeeded(ActivityClientRecord r, String reason) {
...
try {
r.activity.mCalled = false;
mInstrumentation.callActivityOnPause(r.activity);
...
} catch (SuperNotCalledException e) {
throw e;
} catch (Exception e) {
if (!mInstrumentation.onException(r.activity, e)) {
throw new RuntimeException("Unable to pause activity "
+ safeToComponentShortString(r.intent) + ": " + e.toString(), e);
}
}
r.setState(ON_PAUSE);
}
Instrumentation.callActivityOnPause方法中直接呼叫Activity.performPause，在performPause方法中我們終於看到了熟悉的身影Activity生命週期的onPause方法，至此棧頂Activity的Pausing流程全部完畢。
frameworks/base/core/java/android/app/Instrumentation.java
public void callActivityOnPause(Activity activity) {
activity.performPause();
}
frameworks/base/core/java/android/app/Activity.java
final void performPause() {
...
onPause();
...
}
四、Activity所在的應用程序啟動過程
接下來分析一下應用程序的啟動過程，上面分析到ActivityStackSupervisor.startSpecificActivityLocked方法，在這個方法中會去根據程序和執行緒是否存在判斷App是否已經啟動，如果已經啟動，就會呼叫realStartActivityLocked方法繼續處理。如果沒有啟動則呼叫ActivityManagerService.startProcessLocked方法建立新的程序處理。接下來跟蹤一下一個新的Activity是如何一步步啟動的。
frameworks/base/services/core/java/com/android/server/am/ActivityStackSupervisor.java
void startSpecificActivityLocked(ActivityRecord r,
boolean andResume, boolean checkConfig) {
// Is this activity's application already running?
ProcessRecord app = mService.getProcessRecordLocked(r.processName,
r.info.applicationInfo.uid, true);
...
if (app != null && app.thread != null) {
...
realStartActivityLocked(r, app, andResume, checkConfig);
return;
} catch (RemoteException e) {
Slog.w(TAG, "Exception when starting activity "
+ r.intent.getComponent().flattenToShortString(), e);
}
}
mService.startProcessLocked(r.processName, r.info.applicationInfo, true, 0,
"activity", r.intent.getComponent(), false, false, true);
}
ActivityManagerService.startProcessLocked方法經過多次跳轉最終會通過Process.start方法來為應用建立程序。
frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java
private ProcessStartResult startProcess(String hostingType, String entryPoint,
ProcessRecord app, int uid, int[] gids, int runtimeFlags, int mountExternal,
String seInfo, String requiredAbi, String instructionSet, String invokeWith,
long startTime) {
...
startResult = Process.start(entryPoint,
app.processName, uid, uid, gids, runtimeFlags, mountExternal,
app.info.targetSdkVersion, seInfo, requiredAbi, instructionSet,
app.info.dataDir, invokeWith,
new String[] {PROC_START_SEQ_IDENT + app.startSeq});
...
}
frameworks/base/core/java/android/os/Process.java
public static final ProcessStartResult start(final String processClass,
final String niceName,
int uid, int gid, int[] gids,
int runtimeFlags, int mountExternal,
int targetSdkVersion,
String seInfo,
String abi,
String instructionSet,
String appDataDir,
String invokeWith,
String[] zygoteArgs) {
return zygoteProcess.start(processClass, niceName, uid, gid, gids,
runtimeFlags, mountExternal, targetSdkVersion, seInfo,
abi, instructionSet, appDataDir, invokeWith, zygoteArgs);
}
frameworks/base/core/java/android/os/ZygoteProcess.java
public final Process.ProcessStartResult start(final String processClass,
final String niceName,
int uid, int gid, int[] gids,
int runtimeFlags, int mountExternal,
int targetSdkVersion,
String seInfo,
String abi,
String instructionSet,
String appDataDir,
String invokeWith,
String[] zygoteArgs) {
try {
return startViaZygote(processClass, niceName, uid, gid, gids,
runtimeFlags, mountExternal, targetSdkVersion, seInfo,
abi, instructionSet, appDataDir, invokeWith, false /* startChildZygote */,
zygoteArgs);
} catch (ZygoteStartFailedEx ex) {
Log.e(LOG_TAG,
"Starting VM process through Zygote failed");
throw new RuntimeException(
"Starting VM process through Zygote failed", ex);
}
}
private Process.ProcessStartResult startViaZygote(final String processClass,
final String niceName,
final int uid, final int gid,
final int[] gids,
int runtimeFlags, int mountExternal,
int targetSdkVersion,
String seInfo,
String abi,
String instructionSet,
String appDataDir,
String invokeWith,
boolean startChildZygote,
String[] extraArgs)
throws ZygoteStartFailedEx {
...
synchronized(mLock) {
return zygoteSendArgsAndGetResult(openZygoteSocketIfNeeded(abi), argsForZygote);
}
}
private ZygoteState openZygoteSocketIfNeeded(String abi) throws ZygoteStartFailedEx {
Preconditions.checkState(Thread.holdsLock(mLock), "ZygoteProcess lock not held");
if (primaryZygoteState == null || primaryZygoteState.isClosed()) {
try {
primaryZygoteState = ZygoteState.connect(mSocket);
} catch (IOException ioe) {
throw new ZygoteStartFailedEx("Error connecting to primary zygote", ioe);
}
maybeSetApiBlacklistExemptions(primaryZygoteState, false);
maybeSetHiddenApiAccessLogSampleRate(primaryZygoteState);
}
if (primaryZygoteState.matches(abi)) {
return primaryZygoteState;
}
// The primary zygote didn't match. Try the secondary.
if (secondaryZygoteState == null || secondaryZygoteState.isClosed()) {
try {
secondaryZygoteState = ZygoteState.connect(mSecondarySocket);
} catch (IOException ioe) {
throw new ZygoteStartFailedEx("Error connecting to secondary zygote", ioe);
}
maybeSetApiBlacklistExemptions(secondaryZygoteState, false);
maybeSetHiddenApiAccessLogSampleRate(secondaryZygoteState);
}
if (secondaryZygoteState.matches(abi)) {
return secondaryZygoteState;
}
throw new ZygoteStartFailedEx("Unsupported zygote ABI: " + abi);
}
經過一步步呼叫，可以發現其最終呼叫了Zygote並通過socket通訊的方式讓Zygote程序fork出一個新的程序，並根據傳遞的”android.app.ActivityThread”字串，反射出該物件並執行ActivityThread的main方法對其進行初始化。
五、Activity所在應用主執行緒初始化
在ActivityThread.main方法中對ActivityThread進行了初始化，建立了主執行緒的Looper物件並呼叫Looper.loop()方法啟動Looper，把自定義Handler類H的物件作為主執行緒的handler。接下來跳轉到ActivityThread.attach方法，看都做了什麼。
frameworks/base/core/java/android/app/ActivityThread.java
public static void main(String[] args) {
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");
...
Looper.prepareMainLooper();
// Find the value for {@link #PROC_START_SEQ_IDENT} if provided on the command line.
// It will be in the format "seq=114"
long startSeq = 0;
if (args != null) {
for (int i = args.length - 1; i >= 0; --i) {
if (args[i] != null && args[i].startsWith(PROC_START_SEQ_IDENT)) {
startSeq = Long.parseLong(
args[i].substring(PROC_START_SEQ_IDENT.length()));
}
}
}
ActivityThread thread = new ActivityThread();
thread.attach(false, startSeq);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
...
Looper.loop();
...
}
六、啟動Activity
Activity所在的程序建立完了，主執行緒也初始化了，接下來就該真正的啟動Activity了。在ActivityThread.attach方法中，首先會通過ActivityManagerService為這個應用繫結一個Application，然後新增一個垃圾回收觀察者，每當系統觸發垃圾回收的時候就會在run方法裡面去計算應用使用了多少記憶體，如果超過總量的四分之三就會嘗試釋放記憶體。最後，為根View新增config回撥接收config變化相關的資訊。
frameworks/base/core/java/android/app/ActivityThread.java
private void attach(boolean system, long startSeq) {
sCurrentActivityThread = this;
mSystemThread = system;
if (!system) {
...
final IActivityManager mgr = ActivityManager.getService();
try {
mgr.attachApplication(mAppThread, startSeq);
} catch (RemoteException ex) {
throw ex.rethrowFromSystemServer();
}
// Watch for getting close to heap limit.
BinderInternal.addGcWatcher(new Runnable() {
@Override public void run() {
if (!mSomeActivitiesChanged) {
return;
}
Runtime runtime = Runtime.getRuntime();
long dalvikMax = runtime.maxMemory();
long dalvikUsed = runtime.totalMemory() - runtime.freeMemory();
if (dalvikUsed > ((3*dalvikMax)/4)) {
mSomeActivitiesChanged = false;
try {
mgr.releaseSomeActivities(mAppThread);
} catch (RemoteException e) {
throw e.rethrowFromSystemServer();
}
}
}
});
}
...
ViewRootImpl.addConfigCallback(configChangedCallback);
}
frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java
@Override
public final void attachApplication(IApplicationThread thread, long startSeq) {
synchronized (this) {
int callingPid = Binder.getCallingPid();
final int callingUid = Binder.getCallingUid();
final long origId = Binder.clearCallingIdentity();
attachApplicationLocked(thread, callingPid, callingUid, startSeq);
Binder.restoreCallingIdentity(origId);
}
}
private final boolean attachApplicationLocked(IApplicationThread thread,
int pid, int callingUid, long startSeq) {
...
// See if the top visible activity is waiting to run in this process...
if (normalMode) {
try {
if (mStackSupervisor.attachApplicationLocked(app)) {
didSomething = true;
}
} catch (Exception e) {
Slog.wtf(TAG, "Exception thrown launching activities in " + app, e);
badApp = true;
}
}
...
}
frameworks/base/services/core/java/com/android/server/am/ActivityStackSupervisor.java
boolean attachApplicationLocked(ProcessRecord app) throws RemoteException {
final String processName = app.processName;
boolean didSomething = false;
for (int displayNdx = mActivityDisplays.size() - 1; displayNdx >= 0; --displayNdx) {
final ActivityDisplay display = mActivityDisplays.valueAt(displayNdx);
for (int stackNdx = display.getChildCount() - 1; stackNdx >= 0; --stackNdx) {
final ActivityStack stack = display.getChildAt(stackNdx);
if (!isFocusedStack(stack)) {
continue;
}
stack.getAllRunningVisibleActivitiesLocked(mTmpActivityList);
final ActivityRecord top = stack.topRunningActivityLocked();
final int size = mTmpActivityList.size();
for (int i = 0; i < size; i++) {
final ActivityRecord activity = mTmpActivityList.get(i);
if (activity.app == null && app.uid == activity.info.applicationInfo.uid
&& processName.equals(activity.processName)) {
try {
if (realStartActivityLocked(activity, app,
top == activity /* andResume */, true /* checkConfig */)) {
didSomething = true;
}
} catch (RemoteException e) {
Slog.w(TAG, "Exception in new application when starting activity "
+ top.intent.getComponent().flattenToShortString(), e);
throw e;
}
}
}
}
}
if (!didSomething) {
ensureActivitiesVisibleLocked(null, 0, !PRESERVE_WINDOWS);
}
return didSomething;
}
在ActivityManagerService.attachApplication方法中經過多次跳轉執行到ActivityStackSupervisor.realStartActivityLocked方法。看到這個方法有沒有一點眼熟？沒錯，就是之前分析過程中遇到的如果應用程序已經啟動的情況下去啟動Activity所呼叫的方法，有點繞，自己體會一下。在ActivityStackSupervisor.realStartActivityLocked方法中為ClientTransaction物件新增LaunchActivityItem的callback，然後設定當前的生命週期狀態，最後呼叫ClientLifecycleManager.scheduleTransaction方法執行。
final boolean realStartActivityLocked(ActivityRecord r, ProcessRecord app,
boolean andResume, boolean checkConfig) throws RemoteException {
...
// Create activity launch transaction.
final ClientTransaction clientTransaction = ClientTransaction.obtain(app.thread,
r.appToken);
clientTransaction.addCallback(LaunchActivityItem.obtain(new Intent(r.intent),
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, mService.isNextTransitionForward(),
profilerInfo));
// Set desired final state.
final ActivityLifecycleItem lifecycleItem;
if (andResume) {
lifecycleItem = ResumeActivityItem.obtain(mService.isNextTransitionForward());
} else {
lifecycleItem = PauseActivityItem.obtain();
}
clientTransaction.setLifecycleStateRequest(lifecycleItem);
// Schedule transaction.
mService.getLifecycleManager().scheduleTransaction(clientTransaction);
...
return true;
}
呼叫ClientLifecycleManager.scheduleTransaction方法之後具體是如何執行的前面已經分析過了，這裡就不再分析了。先看一下執行callback後跳轉到LaunchActivityItem.execute方法。
frameworks/base/core/java/android/app/servertransaction/LaunchActivityItem.java
public void execute(ClientTransactionHandler client, IBinder token,
PendingTransactionActions pendingActions) {
...
client.handleLaunchActivity(r, pendingActions, null /* customIntent */);
}
frameworks/base/core/java/android/app/ActivityThread.java
public Activity handleLaunchActivity(ActivityClientRecord r,
PendingTransactionActions pendingActions, Intent customIntent) {
...
final Activity a = performLaunchActivity(r, customIntent);
...
return a;
}
經過上面程式碼一步步的跳轉，執行到ActivityThread.performLaunchActivity方法。在ActivityThread.performLaunchActivity方法中首先對Activity的ComponentName、ContextImpl、Activity以及Application物件進行了初始化並相互關聯，然後設定Activity主題，最後呼叫Instrumentation.callActivityOnCreate方法。
private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
// 初始化ComponentName
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);
}
// 初始化ContextImpl和Activity
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 {
// 初始化Application
Application app = r.packageInfo.makeApplication(false, mInstrumentation);
if (activity != null) {
CharSequence title = r.activityInfo.loadLabel(appContext.getPackageManager());
Configuration config = new Configuration(mCompatConfiguration);
if (r.overrideConfig != null) {
config.updateFrom(r.overrideConfig);
}
Window window = null;
if (r.mPendingRemoveWindow != null && r.mPreserveWindow) {
window = r.mPendingRemoveWindow;
r.mPendingRemoveWindow = null;
r.mPendingRemoveWindowManager = null;
}
// Application、Activity和ContextImpl互相關聯
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;
// 設定Activity的Theme
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.setState(ON_CREATE);
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;
}
從Instrumentation.callActivityOnCreate方法繼續追蹤，跳轉到Activity.performCreate方法，在這裡我們看到了Activity.onCreate方法。
frameworks/base/core/java/android/app/Instrumentation.java
public void callActivityOnCreate(Activity activity, Bundle icicle,
PersistableBundle persistentState) {
...
activity.performCreate(icicle, persistentState);
...
}
frameworks/base/core/java/android/app/Activity.java
final void performCreate(Bundle icicle) {
performCreate(icicle, null);
}
final void performCreate(Bundle icicle, PersistableBundle persistentState) {
...
if (persistentState != null) {
onCreate(icicle, persistentState);
} else {
onCreate(icicle);
}
...
}
至此executeCallbacks執行完畢，開始執行executeLifecycleState方法。先執行cycleToPath方法，生命週期狀態是從ON_CREATE狀態到ON_RESUME狀態，中間有一個ON_START狀態，所以會執行ActivityThread.handleStartActivity方法。
frameworks/base/core/java/android/app/servertransaction/TransactionExecutor.java
private void cycleToPath(ActivityClientRecord r, int finish,
boolean excludeLastState) {
...
final IntArray path = mHelper.getLifecyclePath(start, finish, excludeLastState);
performLifecycleSequence(r, path);
}
/** Transition the client through previously initialized state sequence. */
private void performLifecycleSequence(ActivityClie