以下源码基于android-11.0.0_r1
启动一个Activity，通常有两种情况：第一种是不同进程的的根activity，比如laucnher启动app；第二种是同进程内部启动activity。这两种情况的启动流程类似，大致分为以下三个步骤：
- 调用进程的activity收集好信息后，向system_server进程的ActivityTaskManagerSrvice服务发起请求。
- ATMS向PKMS寻找启动的activity的信息和进程信息，如果启动的activity没有被创建，则创建新进程，之后管理activity栈，并回调启动activity所在进程的ApplicationThread类。
- ApplicationThread通过调用ActivityThread来反射创建并启动Activity。
以下就逐一讲解这三大过程：
一、从startActivity到ATMS
下图是调用进程向system_server进程发起请求的过程：
无论是startActivity还是startActivityForResult最终都是调用startActivityForResult
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 {
// 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,
@Nullable Bundle options) {
// mParent 是Activity类型，是当前Activity的父类
if (mParent == null) {
options = transferSpringboardActivityOptions(options);
// 调用Instrumentation.execStartActivity启动activity
Instrumentation.ActivityResult ar =
mInstrumentation.execStartActivity(
this, mMainThread.getApplicationThread(), mToken, this,
intent, requestCode, options);
...
} else {
...
}
}
}
startActivityForResult中继续调用Instrumentation.execStartActivity方法。Activity中的mInstrumentation是在attach()方法中初始化，由ActivityThread传入，其作用一是通过远程服务调用启动activity；二是连接ActivityThread与activity，处理activity生命周期回调。
// Instrumentation主要用来监控应用程序和系统的交互，比如调用ATMS启动activity，回调生命周期
public class Instrumentation {
public ActivityResult execStartActivity(
Context who, IBinder contextThread, IBinder token, Activity target,
Intent intent, int requestCode, Bundle options) {
...
try {
...
// 通过ATMS远程调用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;
}
/**
* 根据result判断当前能否启动activity，不能则抛出异常
*/
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)
// 没有在manifest中声明
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是一个Binder，用于和system_server进程中的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);
}
};
}
ActivityTaskManager.getService()获取ATMS的代理类，通过Binder跨进程通信，向ATMS发起startActivity请求。- 向ATMS发起启动activity请求，获得启动结果result，根据result判断能否启动activity，不能则抛出异常，比如activity未在manifest中声明。
二、ATMS到ApplicationThread
下图为ATMS到ApplicationThread时序图：
// /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);
// 判断调用者进程是否被隔离
enforceNotIsolatedCaller("startActivityAsUser");
// 检查调用者权限
userId = getActivityStartController().checkTargetUser(userId, validateIncomingUser,
Binder.getCallingPid(), Binder.getCallingUid(), "startActivityAsUser");
// TODO: Switch to user app stacks here.
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通过一系列方法调用最终调到startActivityAsUser方法，该方法先检查调用者权限，再通过getActivityStartController().obtainStarter创建ActivityStarter类，把参数设置到ActivityStarter.Request类中，最后执行ActivityStarter. execute()方法。
// /frameworks/base/services/core/java/com/android/server/wm/ActivityStarter.java
/**
* Controller for interpreting how and then launching an activity.
*
* This class collects all the logic for determining how an intent and flags should be turned into
* an activity and associated task and stack.
*/
class ActivityStarter {
/**
* Resolve necessary information according the request parameters provided earlier, and execute
* the request which begin the journey of starting an activity.
* @return The starter result.
*/
int execute() {
try {
...
int res;
synchronized (mService.mGlobalLock) {
...
res = executeRequest(mRequest);
...
}
} finally {
onExecutionComplete();
}
}
/**
* Executing activity start request and starts the journey of starting an activity. Here
* begins with performing several preliminary checks. The normally activity launch flow will
* go through {@link #startActivityUnchecked} to {@link #startActivityInner}.
*/
private int executeRequest(Request request) {
// 判断启动的理由不为空
if (TextUtils.isEmpty(request.reason)) {
throw new IllegalArgumentException("Need to specify a reason.");
}
// 获取调用的进程
WindowProcessController callerApp = null;
if (caller != null) {
callerApp = mService.getProcessController(caller);
if (callerApp != null) {
// 获取调用进程的pid和uid并赋值
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) {
// 获取调用者所在的ActivityRecord
sourceRecord = mRootWindowContainer.isInAnyStack(resultTo);
if (sourceRecord != null) {
if (requestCode >= 0 && !sourceRecord.finishing) {
//requestCode = -1 则不进入
resultRecord = sourceRecord;
}
}
}
final int launchFlags = intent.getFlags();
if ((launchFlags & Intent.FLAG_ACTIVITY_FORWARD_RESULT) != 0 && sourceRecord != null) {
... // activity执行结果的返回由源Activity转换到新Activity, 不需要返回结果则不会进入该分支
}
if (err == ActivityManager.START_SUCCESS && intent.getComponent() == null) {
// 从Intent中无法找到相应的Component
err = ActivityManager.START_INTENT_NOT_RESOLVED;
}
if (err == ActivityManager.START_SUCCESS && aInfo == null) {
// 从Intent中无法找到相应的ActivityInfo
err = ActivityManager.START_CLASS_NOT_FOUND;
}
...
//执行后resultStack = null
final ActivityStack resultStack = resultRecord == null
? null : resultRecord.getRootTask();
... //权限检查
// ActivityController不为空的情况，比如monkey测试过程
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) {
... //权限检查不满足,才进入该分支则直接返回；
return START_ABORTED;
if (aInfo != null) {
if (mService.getPackageManagerInternalLocked().isPermissionsReviewRequired(
aInfo.packageName, userId)) {
...
// 向PKMS获取启动Activity的ResolveInfo
rInfo = mSupervisor.resolveIntent(intent, resolvedType, userId, 0,
computeResolveFilterUid(
callingUid, realCallingUid, request.filterCallingUid));
// 向PKMS获取启动Activity的ActivityInfo
aInfo = mSupervisor.resolveActivity(intent, rInfo, startFlags,
null /*profilerInfo*/);
}
}
...
// 创建即将要启动的Activity的描述类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;
...
// 调用 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中通过调用executeRequest方法，先做一系列检查，包括进程检查、intent检查、权限检查、向PKMS获取启动Activity的ActivityInfo等信息，然后调用startActivityUnchecked方法开始对要启动的activity做任务栈管理。
/**
* Start an activity while most of preliminary checks has been done and caller has been
* confirmed that holds necessary permissions to do so.
* Here also ensures that the starting activity is removed if the start wasn't successful.
*/
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;
// 主要处理栈管理相关的逻辑
int startActivityInner(final ActivityRecord r, ActivityRecord sourceRecord,
IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
int startFlags, boolean doResume, ActivityOptions options, Task inTask,
boolean restrictedBgActivity, NeededUriGrants intentGrants) {
// 初始化启动Activity的各种配置，在初始化前会重置各种配置再进行配置，
// 这些配置包括：ActivityRecord、Intent、Task和LaunchFlags（启动的FLAG）等等
setInitialState(r, options, inTask, doResume, startFlags, sourceRecord, voiceSession,
voiceInteractor, restrictedBgActivity);
// 给不同的启动模式计算出mLaunchFlags
computeLaunchingTaskFlags();
// 主要作用是设置ActivityStack
computeSourceStack();
// 将mLaunchFlags设置给Intent
mIntent.setFlags(mLaunchFlags);
// 确定是否应将新活动插入现有任务。如果不是，则返回null，
// 或者返回带有应将新活动添加到其中的任务的ActivityRecord。
final Task reusedTask = getReusableTask();
//...
// 如果reusedTask为null，则计算是否存在可以使用的任务栈
final Task targetTask = reusedTask != null ? reusedTask : computeTargetTask();
final boolean newTask = targetTask == null; // 启动Activity是否需要新创建栈
mTargetTask = targetTask;
computeLaunchParams(r, sourceRecord, targetTask);
// 检查是否允许在给定任务或新任务上启动活动。
int startResult = isAllowedToStart(r, newTask, targetTask);
if (startResult != START_SUCCESS) {
return startResult;
}
final ActivityStack topStack = mRootWindowContainer.getTopDisplayFocusedStack();
if (topStack != null) {
// 检查正在启动的活动是否与当前位于顶部的活动相同，并且应该只启动一次
startResult = deliverToCurrentTopIfNeeded(topStack, intentGrants);
if (startResult != START_SUCCESS) {
return startResult;
}
}
if (mTargetStack == null) {
// 复用或者创建堆栈
mTargetStack = getLaunchStack(mStartActivity, mLaunchFlags, targetTask, mOptions);
}
if (newTask) {
// 新建一个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) {
// 复用之前的task
addOrReparentStartingActivity(targetTask, "adding to task");
}
...
// 检查是否需要触发过渡动画和开始窗口
mTargetStack.startActivityLocked(mStartActivity,
topStack != null ? topStack.getTopNonFinishingActivity() : null, newTask,
mKeepCurTransition, mOptions);
if (mDoResume) {
//...
// 调用RootWindowContainer的resumeFocusedStacksTopActivities方法
mRootWindowContainer.resumeFocusedStacksTopActivities(
mTargetStack, mStartActivity, mOptions);
}
// ...
return START_SUCCESS;
}
// 初始化状态
private void setInitialState(ActivityRecord r, ActivityOptions options, Task inTask,
boolean doResume, int startFlags, ActivityRecord sourceRecord,
IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
boolean restrictedBgActivity) {
reset(false /* clearRequest */); //重置状态
mStartActivity = r; // 被启动的Activity
mIntent = r.intent; //启动的intent
mSourceRecord = sourceRecord; //发起启动的Activity
mLaunchMode = r.launchMode; // 启动模式
// 启动Flags
mLaunchFlags = adjustLaunchFlagsToDocumentMode(
r, LAUNCH_SINGLE_INSTANCE == mLaunchMode,
LAUNCH_SINGLE_TASK == mLaunchMode, mIntent.getFlags());
mInTask = inTask;
// ...
}
// 根据不同启动模式计算出不同的mLaunchFlags
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;
}
}
}
// 设置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;
}
//标志位，如果为true，说明要放入已经存在的栈，
// 可以看出，如果是设置了FLAG_ACTIVITY_NEW_TASK 而没有设置 FLAG_ACTIVITY_MULTIPLE_TASK，
// 或者设置了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);
// 重新检验
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该实例，找不到就为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;
}
// 计算启动的Activity的栈
private Task computeTargetTask() {
if (mStartActivity.resultTo == null && mInTask == null && !mAddingToTask
&& (mLaunchFlags & FLAG_ACTIVITY_NEW_TASK) != 0) {
// 返回null，应该新创建一个Task，而不是使用现有的Task
return null;
} else if (mSourceRecord != null) {
// 使用源Activity的task
return mSourceRecord.getTask();
} else if (mInTask != null) {
// 使用启动时传递的task
return mInTask;
} else {
// 理论上的可能，不可能走到这里
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，根据启动模式计算出flag，再根据flag等条件判断要启动的activity的ActivityRecord是加入现有的Task栈中，还是创建新的Task栈。在未Activity准备好Task栈后，调用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)) {
// 调用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;
// 继续调用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.
// 在当前Task栈中找到最上层正在运行的activity，如果这个activity没有获取焦点，那这个activity将会被重新启动
ActivityRecord next = topRunningActivity(true /* focusableOnly */);
final boolean hasRunningActivity = next != null;
if (next.attachedToProcess()) {
...
} else {
...
// 调用StackSupervisor.startSpecificActivity
mStackSupervisor.startSpecificActivity(next, true, true);
}
return true;
}
}
ActivityStack的作用的用于单个活动栈的管理。最终调到ActivityStackSupervisor . startSpecificActivity。
public class ActivityStackSupervisor implements RecentTasks.Callbacks {
// 检查启动Activity所在进程是否有启动，没有则先启动进程
void startSpecificActivity(ActivityRecord r, boolean andResume, boolean checkConfig) {
// 根据processName和Uid查找启动Activity的所在进程
final WindowProcessController wpc =
mService.getProcessController(r.processName, r.info.applicationInfo.uid);
boolean knownToBeDead = false;
if (wpc != null && wpc.hasThread()) {
// 进程已经存在，则直接启动Activity
try {
// 启动Activity ,并返回
realStartActivityLocked(r, wpc, andResume, checkConfig);
return;
} catch (RemoteException e) {
...
}
knownToBeDead = true;
}
// 所在进程没创建则调用ATMS.startProcessAsync创建进程并启动Activity
mService.startProcessAsync(r, knownToBeDead, isTop, isTop ? "top-activity" : "activity");
}
// 启动Activity的进程存在，则执行此方法
boolean realStartActivityLocked(ActivityRecord r, WindowProcessController proc,
boolean andResume, boolean checkConfig) throws RemoteException {
...
// 创建活动启动事务
// proc.getThread()是一个IApplicationThread对象，可以通过ClientTransaction.getClient()获取
final ClientTransaction clientTransaction = ClientTransaction.obtain(
proc.getThread(), r.appToken);
// 为事务设置Callback，为LaunchActivityItem，在客户端时会被调用
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()));
// 设置所需的最终状态
final ActivityLifecycleItem lifecycleItem;
if (andResume) {
lifecycleItem = ResumeActivityItem.obtain(dc.isNextTransitionForward());
} else {
lifecycleItem = PauseActivityItem.obtain();
}
clientTransaction.setLifecycleStateRequest(lifecycleItem);
// 执行事件，调用ClientLifecycleManager.scheduleTransaction
mService.getLifecycleManager().scheduleTransaction(clientTransaction);
...
return true;
}
}
在ActivityStackSupervisor中，先检查要启动activity的进程是否存在，如果存在则调用realStartActivityLocked方法，通过ClientTransaction事务回调ApplicationThread. scheduleTransaction方法；如果进程不存在，则创建进程。
ATMS小结
- 在
ActivityStarter中检查调用进程，intent，权限等检查，通过PKMS获取ResolveInfo，ActivityInfo信息，符合条件的则往下走。
- 在
ActivityStarter中根据启动模式计算flag，设置启动activity的Task栈。
- 在
ActivityStackSupervisor检查要启动的activity的进程是否存在，存在则向客户端进程ApplicationThread回调启动Activity；否则创建进程。
三、ApplicationThread到启动Activity
下图为ActivityThread启动Activity过程的时序图：
// 主要是处理AMS端的请求
private class ApplicationThread extends IApplicationThread.Stub {
@Override
public void scheduleTransaction(ClientTransaction transaction) throws RemoteException {
ActivityThread.this.scheduleTransaction(transaction);
}
}
// ActivityThread的父类
public abstract class ClientTransactionHandler {
void scheduleTransaction(ClientTransaction transaction) {
transaction.preExecute(this);
// 发送EXECUTE_TRANSACTION消息
sendMessage(ActivityThread.H.EXECUTE_TRANSACTION, transaction);
}
}
// 它管理应用程序进程中主线程的执行，根据活动管理器的请求，在其上调度和执行活动、广播和其他操作。
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;
// 调用TransactionExecutor.execute去处理ATMS阶段传过来的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) {
//...
// 调用传过来的ClientTransaction事务的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);
...
// 调用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);
// 调用ActivityThread.handleLaunchActivity
client.handleLaunchActivity(r, pendingActions, null /* customIntent */);
}
}
ApplicationThread类绕了一大圈，最后调用在ATMS阶段设置的ClientTransaction的CallBack的execute方法，也就是LaunchActivityItem.execute方法，此方法创建一个ActivityClientRecord对象，然后调用ActivityThread. handleLaunchActivity开启真正的Actvity启动。
真正启动activity：
public final class ActivityThread extends ClientTransactionHandler {
// ActivityThread启动Activity的过程
@Override
public Activity handleLaunchActivity(ActivityClientRecord r,
PendingTransactionActions pendingActions, Intent customIntent) {
// ...
WindowManagerGlobal.initialize();
// 启动Activity
final Activity a = performLaunchActivity(r, customIntent);
if (a != null) {
...
} else {
// 启动失败，调用ATMS停止Activity启动
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) {
// 获取ActivityInfo类
ActivityInfo aInfo = r.activityInfo;
if (r.packageInfo == null) {
// 获取APK文件的描述类LoadedApk
r.packageInfo = getPackageInfo(aInfo.applicationInfo, r.compatInfo,
Context.CONTEXT_INCLUDE_CODE);
}
// 启动的Activity的ComponentName类
ComponentName component = r.intent.getComponent();
// 创建要启动Activity的上下文环境
ContextImpl appContext = createBaseContextForActivity(r);
Activity activity = null;
try {
java.lang.ClassLoader cl = appContext.getClassLoader();
// 用类加载器来创建该Activity的实例
activity = mInstrumentation.newActivity(
cl, component.getClassName(), r.intent);
// ...
} catch (Exception e) {
// ...
}
try {
// 创建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);
...
// 调用Instrumentation的callActivityOnCreate方法来启动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);
// 调用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) {
...
// 调用onCreate方法
if (persistentState != null) {
onCreate(icicle, persistentState);
} else {
onCreate(icicle);
}
...
}
}
小结：
- 在
ActivityThread.mH中处理EXECUTE_TRANSACTION消息，处理从system_server进程传过来的ClientTransaction，然后回调ClientTransactionItem的execute()方法，而这个ClientTransactionItem就是在ActivityStakSupervisor类的realStartActivityLocked()方法中设置的其子类LaunchActivityItem。
- 调用
LaunchActivityItem的execute()方法，创建了一个ActivityClientRecord，对应system_server进程中的ActivityRecord，其作用是封装保存一个activity的相关信息，并传递到ActivityThread中。
- 继续回到
ActivityThread中，通过一系列调用，最终来到performLaunchActivity方法：
- 为ActivityInfo设置packageInfo，是一个LoadedApk类，用来保存当前应用的一些信息，如包名，资源目录等；
- 创建要启动activity的上下文环境；
- 通过
Instrumentation实例化activity；
- 创建Application；
- 调用
activity.attach()方法，初始化activity；
- 调用
Instrumentation.callActivityOnCreate()方法启动activity，并在activity的performCreate()方法中回调生命周期onCreate();
最后附一张完整流程图：
