了解Activity啟動過程，從startActivity到ATMS的高效協作
作者：Reathin
在Android的不同版本中，啟動流程可能會有所不同，并且可能涉及到更多的細節和組件。此外，如果啟動的是一個根Activity（例如從Launcher啟動），那么流程中可能還包括應用進程的創建等步驟。
在Android系統中，啟動一個Activity無論是從應用內部啟動Activity，還是通過桌面程序（Launcher）啟動，都需要通過調用startActivity方法來發起啟動請求。
啟動請求的發起
- 「應用內部啟動Activity」：
當應用內部需要啟動一個新的Activity時，開發者會調用startActivity方法，并傳遞一個包含目標Activity信息的Intent對象。
這個Intent對象可以指定要啟動的Activity的類名、傳遞的數據、附加的extras等。
調用startActivity后，系統會開始處理啟動請求，并按照Activity啟動流程進行后續操作。
- 「Launcher啟動Activity」：
Launcher是Android系統的桌面程序，它負責顯示已安裝的應用程序圖標，并提供用戶與應用程序交互的入口。
當用戶從Launcher點擊一個應用程序圖標時，Launcher會創建一個新的Intent，用于啟動該應用程序的主Activity（通常是根Activity）。
然后，Launcher調用startActivity方法，并將該Intent傳遞給系統，以啟動目標Activity。
在兩種情況下，啟動請求的發起都是通過調用startActivity方法實現的。這個方法會觸發一系列的操作，包括將啟動請求傳遞給ActivityTaskManagerService（ATMS），進行線程切換和消息處理，以及最終完成Activity的初始化和顯示。無論是startActivity還是startActivityForResult最終都是調用startActivityForResult。
圖片
public class Activity extends ContextThemeWrapper
implements LayoutInflater.Factory2,
Window.Callback, KeyEvent.Callback,
OnCreateContextMenuListener, ComponentCallbacks2,
Window.OnWindowDismissedCallback,
AutofillManager.AutofillClient, ContentCaptureManager.ContentCaptureClient {
@Override
public void startActivity(Intent intent, @Nullable Bundle options) {
//...
if (options != null) {
startActivityForResult(intent, -1, options);
} else {
startActivityForResult(intent, -1);
}
}
public void startActivityForResult(@RequiresPermission Intent intent, int requestCode,
@Nullable Bundle options) {
// mParent 是Activity類型，是當前Activity的父類
if (mParent == null) {
options = transferSpringboardActivityOptions(options);
// 調用Instrumentation.execStartActivity啟動activity
Instrumentation.ActivityResult ar =
mInstrumentation.execStartActivity(
this, mMainThread.getApplicationThread(), mToken, this,
intent, requestCode, options);
//...
} else {
//...
}
}
}startActivityForResult中調用Instrumentation.execStartActivity方法。Activity中的mInstrumentation是在attach()方法中初始化，由ActivityThread傳入，其作用是通過遠程服務調用啟動activity，連接ActivityThread與activity，處理activity生命周期回調。
// Instrumentation主要用來監控應用程序和系統的交互，比如調用ATMS啟動activity，回調生命周期
public class Instrumentation {
public ActivityResult execStartActivity(
Context who, IBinder contextThread, IBinder token, Activity target,
Intent intent, int requestCode, Bundle options) {
//...
try {
//...
// 通過ATMS遠程調用startActivity
int result = ActivityTaskManager.getService().startActivity(whoThread,
who.getOpPackageName(), who.getAttributionTag(), 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;
}
/**
* 根據result判斷當前能否啟動activity，不能則拋出異常
*/
public static void checkStartActivityResult(int res, Object intent) {
if (!ActivityManager.isStartResultFatalError(res)) {
return;
}
switch (res) {
case ActivityManager.START_INTENT_NOT_RESOLVED:
case ActivityManager.START_CLASS_NOT_FOUND:
if (intent instanceof Intent && ((Intent)intent).getComponent() != null)
// 沒有在manifest中聲明
throw new ActivityNotFoundException(
"Unable to find explicit activity class "
+ ((Intent)intent).getComponent().toShortString()
+ "; have you declared this activity in your AndroidManifest.xml?");
throw new ActivityNotFoundException(
"No Activity found to handle " + intent);
case ActivityManager.START_PERMISSION_DENIED:
throw new SecurityException("Not allowed to start activity "
+ intent);
case ActivityManager.START_FORWARD_AND_REQUEST_CONFLICT:
throw new AndroidRuntimeException(
"FORWARD_RESULT_FLAG used while also requesting a result");
case ActivityManager.START_NOT_ACTIVITY:
throw new IllegalArgumentException(
"PendingIntent is not an activity");
case ActivityManager.START_NOT_VOICE_COMPATIBLE:
throw new SecurityException(
"Starting under voice control not allowed for: " + intent);
case ActivityManager.START_VOICE_NOT_ACTIVE_SESSION:
throw new IllegalStateException(
"Session calling startVoiceActivity does not match active session");
case ActivityManager.START_VOICE_HIDDEN_SESSION:
throw new IllegalStateException(
"Cannot start voice activity on a hidden session");
case ActivityManager.START_ASSISTANT_NOT_ACTIVE_SESSION:
throw new IllegalStateException(
"Session calling startAssistantActivity does not match active session");
case ActivityManager.START_ASSISTANT_HIDDEN_SESSION:
throw new IllegalStateException(
"Cannot start assistant activity on a hidden session");
case ActivityManager.START_CANCELED:
throw new AndroidRuntimeException("Activity could not be started for "
+ intent);
default:
throw new AndroidRuntimeException("Unknown error code "
+ res + " when starting " + intent);
}
}
}@SystemService(Context.ACTIVITY_TASK_SERVICE)
public class ActivityTaskManager {
/**
* IActivityTaskManager是一個Binder，用于和system_server進程中的ActivityTaskManagerService通信
*/
public static IActivityTaskManager getService() {
return IActivityTaskManagerSingleton.get();
}
private static final Singleton<IActivityTaskManager> IActivityTaskManagerSingleton =
new Singleton<IActivityTaskManager>() {
@Override
protected IActivityTaskManager create() {
final IBinder b = ServiceManager.getService(Context.ACTIVITY_TASK_SERVICE);
return IActivityTaskManager.Stub.asInterface(b);
}
};
}當請求到達ATMS時，ATMS會首先檢查該請求是否合法，包括檢查Intent的有效性、權限等。一旦請求被驗證為有效，ATMS會進一步處理這個請求。
處理過程中，ATMS會根據當前系統的狀態和任務棧的情況來決定如何響應這個啟動請求。例如，它可能會決定創建一個新的Activity實例，或者將已存在的Activity實例帶到前臺。ATMS還會與ActivityManagerService（AMS）進行交互，以協調應用程序組件的生命周期管理。AMS負責跟蹤和管理這些組件的生命周期，確保它們按照預期的方式運行。
Activity的初始化與生命周期管理
當Activity啟動請求到達ActivityTaskManagerService（ATMS）并被驗證為有效后，ATMS會通知相應的應用進程進行Activity的初始化。
// /frameworks/base/services/core/java/com/android/server/wm/ActivityTaskManagerService.java
public class ActivityTaskManagerService extends IActivityTaskManager.Stub {
public final int startActivity(IApplicationThread caller, String callingPackage,
String callingFeatureId, Intent intent, String resolvedType, IBinder resultTo,
String resultWho, int requestCode, int startFlags, ProfilerInfo profilerInfo,
Bundle bOptions) {
return startActivityAsUser(caller, callingPackage, callingFeatureId, intent, resolvedType,
resultTo, resultWho, requestCode, startFlags, profilerInfo, bOptions,
UserHandle.getCallingUserId());
}
private int startActivityAsUser(IApplicationThread caller, String callingPackage,
@Nullable String callingFeatureId, Intent intent, String resolvedType,
IBinder resultTo, String resultWho, int requestCode, int startFlags,
ProfilerInfo profilerInfo, Bundle bOptions, int userId, boolean validateIncomingUser) {
assertPackageMatchesCallingUid(callingPackage);
// 判斷調用者進程是否被隔離
enforceNotIsolatedCaller("startActivityAsUser");
// 檢查調用者權限
userId = getActivityStartController().checkTargetUser(userId, validateIncomingUser,
Binder.getCallingPid(), Binder.getCallingUid(), "startActivityAsUser");
return getActivityStartController().obtainStarter(intent, "startActivityAsUser")
.setCaller(caller)
.setCallingPackage(callingPackage)
.setCallingFeatureId(callingFeatureId)
.setResolvedType(resolvedType)
.setResultTo(resultTo)
.setResultWho(resultWho)
.setRequestCode(requestCode)
.setStartFlags(startFlags)
.setProfilerInfo(profilerInfo)
.setActivityOptions(bOptions)
.setUserId(userId)
.execute();
}
}ATMS通過一系列方法最終調到startActivityAsUser方法，先檢查調用者權限，再通過getActivityStartController().obtainStarter創建ActivityStarter類，把參數設置到ActivityStarter.Request類中，最后執行ActivityStarter.execute()方法。
// /frameworks/base/services/core/java/com/android/server/wm/ActivityStarter.java
class ActivityStarter {
int execute() {
try {
//...
int res;
synchronized (mService.mGlobalLock) {
//...
res = executeRequest(mRequest);
//...
}
} finally {
onExecutionComplete();
}
}
private int executeRequest(Request request) {
// 判斷啟動的理由不為空
if (TextUtils.isEmpty(request.reason)) {
throw new IllegalArgumentException("Need to specify a reason.");
}
// 獲取調用的進程
WindowProcessController callerApp = null;
if (caller != null) {
callerApp = mService.getProcessController(caller);
if (callerApp != null) {
// 獲取調用進程的pid和uid并賦值
callingPid = callerApp.getPid();
callingUid = callerApp.mInfo.uid;
} else {
err = ActivityManager.START_PERMISSION_DENIED;
}
}
final int userId = aInfo != null && aInfo.applicationInfo != null
? UserHandle.getUserId(aInfo.applicationInfo.uid) : 0;
ActivityRecord sourceRecord = null;
ActivityRecord resultRecord = null;
if (resultTo != null) {
// 獲取調用者所在的ActivityRecord
sourceRecord = mRootWindowContainer.isInAnyStack(resultTo);
if (sourceRecord != null) {
if (requestCode >= 0 && !sourceRecord.finishing) {
//requestCode = -1 則不進入
resultRecord = sourceRecord;
}
}
}
final int launchFlags = intent.getFlags();
if ((launchFlags & Intent.FLAG_ACTIVITY_FORWARD_RESULT) != 0 && sourceRecord != null) {
// activity執行結果的返回由源Activity轉換到新Activity, 不需要返回結果則不會進入該分支
}
if (err == ActivityManager.START_SUCCESS && intent.getComponent() == null) {
// 從Intent中無法找到相應的Component
err = ActivityManager.START_INTENT_NOT_RESOLVED;
}
if (err == ActivityManager.START_SUCCESS && aInfo == null) {
// 從Intent中無法找到相應的ActivityInfo
err = ActivityManager.START_CLASS_NOT_FOUND;
}
//執行后resultStack = null
final ActivityStack resultStack = resultRecord == null
? null : resultRecord.getRootTask();
//權限檢查
if (mService.mController != null) {
try {
Intent watchIntent = intent.cloneFilter();
abort |= !mService.mController.activityStarting(watchIntent,
aInfo.applicationInfo.packageName);
} catch (RemoteException e) {
mService.mController = null;
}
}
if (abort) {
//權限檢查不滿足,才進入該分支則直接返回；
return START_ABORTED;
if (aInfo != null) {
if (mService.getPackageManagerInternalLocked().isPermissionsReviewRequired(
aInfo.packageName, userId)) {
// 向PKMS獲取啟動Activity的ResolveInfo
rInfo = mSupervisor.resolveIntent(intent, resolvedType, userId, 0,
computeResolveFilterUid(
callingUid, realCallingUid, request.filterCallingUid));
// 向PKMS獲取啟動Activity的ActivityInfo
aInfo = mSupervisor.resolveActivity(intent, rInfo, startFlags,
null /*profilerInfo*/);
}
}
// 創建即將要啟動的Activity的描述類ActivityRecord
final ActivityRecord r = new ActivityRecord(mService, callerApp, callingPid, callingUid,
callingPackage, callingFeatureId, intent, resolvedType, aInfo,
mService.getGlobalConfiguration(), resultRecord, resultWho, requestCode,
request.componentSpecified, voiceSession != null, mSupervisor, checkedOptions,
sourceRecord);
mLastStartActivityRecord = r;
// 調用 startActivityUnchecked
mLastStartActivityResult = startActivityUnchecked(r, sourceRecord, voiceSession,
request.voiceInteractor, startFlags, true /* doResume */, checkedOptions, inTask,
restrictedBgActivity, intentGrants);
if (request.outActivity != null) {
request.outActivity[0] = mLastStartActivityRecord;
}
return mLastStartActivityResult;
}
}在ActivityStarter中調用executeRequest方法，先做一系列檢查，包括進程檢查、intent檢查、權限檢查、向PKMS獲取啟動Activity的ActivityInfo等信息，調用startActivityUnchecked方法開始對要啟動的activity進行任務棧管理。
private int startActivityUnchecked(final ActivityRecord r, ActivityRecord sourceRecord,
IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
int startFlags, boolean doResume, ActivityOptions options, Task inTask,
boolean restrictedBgActivity, NeededUriGrants intentGrants) {
try {
result = startActivityInner(r, sourceRecord, voiceSession, voiceInteractor,
startFlags, doResume, options, inTask, restrictedBgActivity, intentGrants);
} finally {
//...
}
//...
return result;
}
ActivityRecord mStartActivity;
private ActivityStack mSourceStack;
private ActivityStack mTargetStack;
private Task mTargetTask;
// 主要處理棧管理相關的邏輯
int startActivityInner(final ActivityRecord r, ActivityRecord sourceRecord,
IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
int startFlags, boolean doResume, ActivityOptions options, Task inTask,
boolean restrictedBgActivity, NeededUriGrants intentGrants) {
// 初始化啟動Activity的各種配置，在初始化前會重置各種配置再進行配置，
// 這些配置包括：ActivityRecord、Intent、Task和LaunchFlags（啟動的FLAG）等等
setInitialState(r, options, inTask, doResume, startFlags, sourceRecord, voiceSession,
voiceInteractor, restrictedBgActivity);
// 給不同的啟動模式計算出mLaunchFlags
computeLaunchingTaskFlags();
// 主要作用是設置ActivityStack
computeSourceStack();
// 將mLaunchFlags設置給Intent
mIntent.setFlags(mLaunchFlags);
// 確定是否應將新活動插入現有任務。如果不是，則返回null，
// 或者返回帶有應將新活動添加到其中的任務的ActivityRecord。
final Task reusedTask = getReusableTask();
// 如果reusedTask為null，則計算是否存在可以使用的任務棧
final Task targetTask = reusedTask != null ? reusedTask : computeTargetTask();
final boolean newTask = targetTask == null; // 啟動Activity是否需要新創建棧
mTargetTask = targetTask;
computeLaunchParams(r, sourceRecord, targetTask);
// 檢查是否允許在給定任務或新任務上啟動活動。
int startResult = isAllowedToStart(r, newTask, targetTask);
if (startResult != START_SUCCESS) {
return startResult;
}
final ActivityStack topStack = mRootWindowContainer.getTopDisplayFocusedStack();
if (topStack != null) {
// 檢查正在啟動的活動是否與當前位于頂部的活動相同，并且應該只啟動一次
startResult = deliverToCurrentTopIfNeeded(topStack, intentGrants);
if (startResult != START_SUCCESS) {
return startResult;
}
}
if (mTargetStack == null) {
// 復用或者創建堆棧
mTargetStack = getLaunchStack(mStartActivity, mLaunchFlags, targetTask, mOptions);
}
if (newTask) {
// 新建一個task
final Task taskToAffiliate = (mLaunchTaskBehind && mSourceRecord != null)
? mSourceRecord.getTask() : null;
setNewTask(taskToAffiliate);
if (mService.getLockTaskController().isLockTaskModeViolation(
mStartActivity.getTask())) {
Slog.e(TAG, "Attempted Lock Task Mode violation mStartActivity=" + mStartActivity);
return START_RETURN_LOCK_TASK_MODE_VIOLATION;
}
} else if (mAddingToTask) {
// 復用之前的task
addOrReparentStartingActivity(targetTask, "adding to task");
}
// 檢查是否需要觸發過渡動畫和開始窗口
mTargetStack.startActivityLocked(mStartActivity,
topStack != null ? topStack.getTopNonFinishingActivity() : null, newTask,
mKeepCurTransition, mOptions);
if (mDoResume) {
// 調用RootWindowContainer的resumeFocusedStacksTopActivities方法
mRootWindowContainer.resumeFocusedStacksTopActivities(
mTargetStack, mStartActivity, mOptions);
}
return START_SUCCESS;
}
private void setInitialState(ActivityRecord r, ActivityOptions options, Task inTask,
boolean doResume, int startFlags, ActivityRecord sourceRecord,
IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
boolean restrictedBgActivity) {
reset(false /* clearRequest */);
mStartActivity = r;
mIntent = r.intent;
mSourceRecord = sourceRecord;
mLaunchMode = r.launchMode;
// 啟動Flags
mLaunchFlags = adjustLaunchFlagsToDocumentMode(
r, LAUNCH_SINGLE_INSTANCE == mLaunchMode,
LAUNCH_SINGLE_TASK == mLaunchMode, mIntent.getFlags());
mInTask = inTask;
// ...
}
private void computeLaunchingTaskFlags() {
if (mInTask == null) {
if (mSourceRecord == null) {
if ((mLaunchFlags & FLAG_ACTIVITY_NEW_TASK) == 0 && mInTask == null) {
mLaunchFlags |= FLAG_ACTIVITY_NEW_TASK;
}
} else if (mSourceRecord.launchMode == LAUNCH_SINGLE_INSTANCE) {
mLaunchFlags |= FLAG_ACTIVITY_NEW_TASK;
} else if (isLaunchModeOneOf(LAUNCH_SINGLE_INSTANCE, LAUNCH_SINGLE_TASK)) {
mLaunchFlags |= FLAG_ACTIVITY_NEW_TASK;
}
}
}
// 設置ActivityStack
private void computeSourceStack() {
if (mSourceRecord == null) {
mSourceStack = null;
return;
}
if (!mSourceRecord.finishing) {
mSourceStack = mSourceRecord.getRootTask();
return;
}
if ((mLaunchFlags & FLAG_ACTIVITY_NEW_TASK) == 0) {
mLaunchFlags |= FLAG_ACTIVITY_NEW_TASK;
mNewTaskInfo = mSourceRecord.info;
final Task sourceTask = mSourceRecord.getTask();
mNewTaskIntent = sourceTask != null ? sourceTask.intent : null;
}
mSourceRecord = null;
mSourceStack = null;
}
private Task getReusableTask() {
// If a target task is specified, try to reuse that one
if (mOptions != null && mOptions.getLaunchTaskId() != INVALID_TASK_ID) {
Task launchTask = mRootWindowContainer.anyTaskForId(mOptions.getLaunchTaskId());
if (launchTask != null) {
return launchTask;
}
return null;
}
//標志位，如果為true，說明要放入已經存在的棧，
// 可以看出，如果是設置了FLAG_ACTIVITY_NEW_TASK 而沒有設置 FLAG_ACTIVITY_MULTIPLE_TASK，
// 或者設置了singleTask以及singleInstance
boolean putIntoExistingTask = ((mLaunchFlags & FLAG_ACTIVITY_NEW_TASK) != 0 &&
(mLaunchFlags & FLAG_ACTIVITY_MULTIPLE_TASK) == 0)
|| isLaunchModeOneOf(LAUNCH_SINGLE_INSTANCE, LAUNCH_SINGLE_TASK);
// 重新檢驗
putIntoExistingTask &= mInTask == null && mStartActivity.resultTo == null;
ActivityRecord intentActivity = null;
if (putIntoExistingTask) {
if (LAUNCH_SINGLE_INSTANCE == mLaunchMode) {
//如果是 singleInstance，那么就找看看之前存在的該實例，找不到就為null
intentActivity = mRootWindowContainer.findActivity(mIntent, mStartActivity.info,
mStartActivity.isActivityTypeHome());
} else if ((mLaunchFlags & FLAG_ACTIVITY_LAUNCH_ADJACENT) != 0) {
// For the launch adjacent case we only want to put the activity in an existing
// task if the activity already exists in the history.
intentActivity = mRootWindowContainer.findActivity(mIntent, mStartActivity.info,
!(LAUNCH_SINGLE_TASK == mLaunchMode));
} else {
// Otherwise find the best task to put the activity in.
intentActivity =
mRootWindowContainer.findTask(mStartActivity, mPreferredTaskDisplayArea);
}
}
if (intentActivity != null
&& (mStartActivity.isActivityTypeHome() || intentActivity.isActivityTypeHome())
&& intentActivity.getDisplayArea() != mPreferredTaskDisplayArea) {
// Do not reuse home activity on other display areas.
intentActivity = null;
}
return intentActivity != null ? intentActivity.getTask() : null;
}
// 計算啟動的Activity的棧
private Task computeTargetTask() {
if (mStartActivity.resultTo == null && mInTask == null && !mAddingToTask
&& (mLaunchFlags & FLAG_ACTIVITY_NEW_TASK) != 0) {
// 返回null，應該新創建一個Task，而不是使用現有的Task
return null;
} else if (mSourceRecord != null) {
// 使用源Activity的task
return mSourceRecord.getTask();
} else if (mInTask != null) {
// 使用啟動時傳遞的task
return mInTask;
} else {
// 理論上的可能，不可能走到這里
final ActivityStack stack = getLaunchStack(mStartActivity, mLaunchFlags,
null /* task */, mOptions);
final ActivityRecord top = stack.getTopNonFinishingActivity();
if (top != null) {
return top.getTask();
} else {
// Remove the stack if no activity in the stack.
stack.removeIfPossible();
}
}
return null;
}在startActivityInner方法中，根據啟動模式和flag等條件判斷要啟動的activity的ActivityRecord是加入現有的Task棧中或創建新的Task棧。在為Activity準備好Task棧后，調用RootWindowContainer.resumeFocusedStacksTopActivities方法。
class RootWindowContainer extends WindowContainer<DisplayContent>
implements DisplayManager.DisplayListener {
boolean resumeFocusedStacksTopActivities(
ActivityStack targetStack, ActivityRecord target, ActivityOptions targetOptions) {
//...
boolean result = false;
if (targetStack != null && (targetStack.isTopStackInDisplayArea()
|| getTopDisplayFocusedStack() == targetStack)) {
// 調用ActivityStack.resumeTopActivityUncheckedLocked
result = targetStack.resumeTopActivityUncheckedLocked(target, targetOptions);
}
//...
return result;
}
}
class ActivityStack extends Task {
boolean resumeTopActivityUncheckedLocked(ActivityRecord prev, ActivityOptions options) {
if (mInResumeTopActivity) {
// Don't even start recurscheduleTransactionsing.
return false;
}
boolean result = false;
try {
mInResumeTopActivity = true;
// 繼續調用resumeTopActivityInnerLocked
result = resumeTopActivityInnerLocked(prev, options);
final ActivityRecord next = topRunningActivity(true /* focusableOnly */);
if (next == null || !next.canTurnScreenOn()) {
checkReadyForSleep();
}
} finally {
mInResumeTopActivity = false;
}
return result;
}
private boolean resumeTopActivityInnerLocked(ActivityRecord prev, ActivityOptions options) {
// Find the next top-most activity to resume in this stack that is not finishing and is
// focusable. If it is not focusable, we will fall into the case below to resume the
// top activity in the next focusable task.
// 在當前Task棧中找到最上層正在運行的activity，如果這個activity沒有獲取焦點，那這個activity將會被重新啟動
ActivityRecord next = topRunningActivity(true /* focusableOnly */);
final boolean hasRunningActivity = next != null;
if (next.attachedToProcess()) {
...
} else {
...
// 調用StackSupervisor.startSpecificActivity
mStackSupervisor.startSpecificActivity(next, true, true);
}
return true;
}
}ActivityStack用于單個活動棧的管理，最終調到ActivityStackSupervisor.startSpecificActivity()。
public class ActivityStackSupervisor implements RecentTasks.Callbacks {
// 檢查啟動Activity所在進程是否有啟動，沒有則先啟動進程
void startSpecificActivity(ActivityRecord r, boolean andResume, boolean checkConfig) {
// 根據processName和Uid查找啟動Activity的所在進程
final WindowProcessController wpc =
mService.getProcessController(r.processName, r.info.applicationInfo.uid);
boolean knownToBeDead = false;
if (wpc != null && wpc.hasThread()) {
// 進程已經存在，則直接啟動Activity
try {
// 啟動Activity ,并返回
realStartActivityLocked(r, wpc, andResume, checkConfig);
return;
} catch (RemoteException e) {
...
}
knownToBeDead = true;
}
// 所在進程沒創建則調用ATMS.startProcessAsync創建進程并啟動Activity
mService.startProcessAsync(r, knownToBeDead, isTop, isTop ? "top-activity" : "activity");
}
// 啟動Activity的進程存在，則執行此方法
boolean realStartActivityLocked(ActivityRecord r, WindowProcessController proc,
boolean andResume, boolean checkConfig) throws RemoteException {
...
// 創建活動啟動事務
// proc.getThread()是一個IApplicationThread對象，可以通過ClientTransaction.getClient()獲取
final ClientTransaction clientTransaction = ClientTransaction.obtain(
proc.getThread(), r.appToken);
// 為事務設置Callback，為LaunchActivityItem，在客戶端時會被調用
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, proc.getReportedProcState(),
r.getSavedState(), r.getPersistentSavedState(), results, newIntents,
dc.isNextTransitionForward(), proc.createProfilerInfoIfNeeded(),
r.assistToken, r.createFixedRotationAdjustmentsIfNeeded()));
// 設置所需的最終狀態
final ActivityLifecycleItem lifecycleItem;
if (andResume) {
lifecycleItem = ResumeActivityItem.obtain(dc.isNextTransitionForward());
} else {
lifecycleItem = PauseActivityItem.obtain();
}
clientTransaction.setLifecycleStateRequest(lifecycleItem);
// 執行事件，調用ClientLifecycleManager.scheduleTransaction
mService.getLifecycleManager().scheduleTransaction(clientTransaction);
...
return true;
}
}在ActivityStackSupervisor中，先檢查要啟動activity的進程是否存在，存在則調用realStartActivityLocked方法，通過ClientTransaction事務回調ApplicationThread.scheduleTransaction方法；進程不存在則創建進程。
Activity的顯示
// 主要是處理AMS端的請求
private class ApplicationThread extends IApplicationThread.Stub {
@Override
public void scheduleTransaction(ClientTransaction transaction) throws RemoteException {
ActivityThread.this.scheduleTransaction(transaction);
}
}
// ActivityThread的父類
public abstract class ClientTransactionHandler {
void scheduleTransaction(ClientTransaction transaction) {
transaction.preExecute(this);
// 發送EXECUTE_TRANSACTION消息
sendMessage(ActivityThread.H.EXECUTE_TRANSACTION, transaction);
}
}
// 它管理應用程序進程中主線程的執行，根據活動管理器的請求，在其上調度和執行活動、廣播和其他操作。
public final class ActivityThread extends ClientTransactionHandler {
class H extends Handler {
public void handleMessage(Message msg) {
switch (msg.what) {
case EXECUTE_TRANSACTION:
final ClientTransaction transaction = (ClientTransaction) msg.obj;
// 調用TransactionExecutor.execute去處理ATMS階段傳過來的ClientTransaction
mTransactionExecutor.execute(transaction);
if (isSystem()) {
// Client transactions inside system process are recycled on the client side
// instead of ClientLifecycleManager to avoid being cleared before this
// message is handled.
transaction.recycle();
}
break;
}
}
}
}
public class TransactionExecutor {
public void execute(ClientTransaction transaction) {
//...
// 調用傳過來的ClientTransaction事務的Callback
executeCallbacks(transaction);
executeLifecycleState(transaction);
}
public void executeCallbacks(ClientTransaction transaction) {
final List<ClientTransactionItem> callbacks = transaction.getCallbacks();
...
final int size = callbacks.size();
for (int i = 0; i < size; ++i) {
final ClientTransactionItem item = callbacks.get(i);
...
// 調用LaunchActivityItem.execute
item.execute(mTransactionHandler, token, mPendingActions);
item.postExecute(mTransactionHandler, token, mPendingActions);
...
}
}
}
public class LaunchActivityItem extends ClientTransactionItem {
public void execute(ClientTransactionHandler client, IBinder token,
PendingTransactionActions pendingActions) {
ActivityClientRecord r = new ActivityClientRecord(token, mIntent, mIdent, mInfo,
mOverrideConfig, mCompatInfo, mReferrer, mVoiceInteractor, mState, mPersistentState,
mPendingResults, mPendingNewIntents, mIsForward,
mProfilerInfo, client, mAssistToken, mFixedRotationAdjustments);
// 調用ActivityThread.handleLaunchActivity
client.handleLaunchActivity(r, pendingActions, null /* customIntent */);
}
}ApplicationThread最后調用在ATMS階段設置的ClientTransaction的CallBack的execute方法，即LaunchActivityItem.execute方法，此時創建一個ActivityClientRecord對象，然后通過ActivityThread.handleLaunchActivity開啟真正的Actvity啟動。
public final class ActivityThread extends ClientTransactionHandler {
// ActivityThread啟動Activity的過程
@Override
public Activity handleLaunchActivity(ActivityClientRecord r,
PendingTransactionActions pendingActions, Intent customIntent) {
// ...
WindowManagerGlobal.initialize();
// 啟動Activity
final Activity a = performLaunchActivity(r, customIntent);
if (a != null) {
...
} else {
// 啟動失敗，調用ATMS停止Activity啟動
try {
ActivityTaskManager.getService()
.finishActivity(r.token, Activity.RESULT_CANCELED, null,
Activity.DONT_FINISH_TASK_WITH_ACTIVITY);
} catch (RemoteException ex) {
throw ex.rethrowFromSystemServer();
}
}
return a;
}
private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
// 獲取ActivityInfo類
ActivityInfo aInfo = r.activityInfo;
if (r.packageInfo == null) {
// 獲取APK文件的描述類LoadedApk
r.packageInfo = getPackageInfo(aInfo.applicationInfo, r.compatInfo,
Context.CONTEXT_INCLUDE_CODE);
}
// 啟動的Activity的ComponentName類
ComponentName component = r.intent.getComponent();
// 創建要啟動Activity的上下文環境
ContextImpl appContext = createBaseContextForActivity(r);
Activity activity = null;
try {
java.lang.ClassLoader cl = appContext.getClassLoader();
// 用類加載器來創建該Activity的實例
activity = mInstrumentation.newActivity(
cl, component.getClassName(), r.intent);
// ...
} catch (Exception e) {
// ...
}
try {
// 創建Application
Application app = r.packageInfo.makeApplication(false, mInstrumentation);
if (activity != null) {
// 初始化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,
r.assistToken);
...
// 調用Instrumentation的callActivityOnCreate方法來啟動Activity
if (r.isPersistable()) {
mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
} else {
mInstrumentation.callActivityOnCreate(activity, r.state);
}
...
}
...
} catch (SuperNotCalledException e) {
throw e;
} catch (Exception e) {
...
}
return activity;
}
public class Instrumentation {
public void callActivityOnCreate(Activity activity, Bundle icicle) {
prePerformCreate(activity);
// 調用Activity的performCreate
activity.performCreate(icicle);
postPerformCreate(activity);
}
}
public class Activity extends ContextThemeWrapper
implements LayoutInflater.Factory2,
Window.Callback, KeyEvent.Callback,
OnCreateContextMenuListener, ComponentCallbacks2,
Window.OnWindowDismissedCallback,
AutofillManager.AutofillClient, ContentCaptureManager.ContentCaptureClient {
final void performCreate(Bundle icicle) {
performCreate(icicle, null);
}
@UnsupportedAppUsage
final void performCreate(Bundle icicle, PersistableBundle persistentState) {
...
// 調用onCreate方法
if (persistentState != null) {
onCreate(icicle, persistentState);
} else {
onCreate(icicle);
}
...
}
}當初始化完成后，Activity的界面會被繪制并顯示到屏幕上。此時，用戶可以與Activity進行交互。
總結
- 「啟動請求的發起」：
無論是從應用內部啟動Activity，還是通過桌面程序（Launcher）啟動，都需要通過調用startActivity方法來發起啟動請求。
這個請求包含了要啟動的Activity的信息，通常通過Intent對象來傳遞。
- 「請求到達ActivityTaskManagerService（ATMS）」：
當啟動請求被發起后，它會首先到達ActivityTaskManagerService（ATMS）。
ATMS是負責管理Activity生命周期和任務棧的系統服務。
「線程切換與消息處理」：
在ATMS處理啟動請求的過程中，可能涉及到線程切換和消息處理。
例如，將請求從應用線程切換到系統服務線程，或者通過消息隊列來處理請求。
「Activity的初始化與生命周期管理」：
一旦ATMS決定了要啟動哪個Activity，它會通知相應的應用進程進行Activity的初始化。
這包括創建Activity的實例、加載布局、初始化組件等。
同時，Activity的生命周期方法（如onCreate、onStart、onResume等）也會被調用，以確保Activity的正確初始化和狀態管理。
「Activity的顯示」：
當初始化完成后，Activity的界面會被繪制并顯示到屏幕上。
此時，用戶可以與Activity進行交互。
在Android的不同版本中，啟動流程可能會有所不同，并且可能涉及到更多的細節和組件。此外，如果啟動的是一個根Activity（例如從Launcher啟動），那么流程中可能還包括應用進程的創建等步驟。Activity的啟動流程是一個涉及多個組件和服務的復雜過程，它確保了Android應用程序能夠正確地創建、初始化和顯示Activity，從而為用戶提供流暢和一致的體驗。
責任編輯：武曉燕
來源：
沐雨花飛蝶
