四大组件启动流程系列的第一篇：Activity的启动流程
基于 Android 11 源码
Activity 的前世今生不做过多赘述，本系列文章主要面向有一定基础的 Android 开发者，如有疑问，欢迎交流学习。
当我们调用 startActivity 时表示要启动一个新的 Activity 啦，那它怎么把 Activity 启动起来的呢？
熟悉又陌生的 startActivity()
Activity 的启动流程分三部分来看
第一部分：Activity 到 ATMS
第二部分：ATMS 到 ActivityThread
第三部分：ActivityThread 到 Activity 启动
Activity 到 ATMS
app 进程发起 startActivity
startActivity(new Intent(this, MainActivity.class));Activity#startActivity(Intent)
@Override
public void startActivity(Intent intent) {
// 第二个参数为 null
this.startActivity(intent, null);
}调用 startActivity 的重载方法
@Override
public void startActivity(Intent intent, @Nullable Bundle options) {
if (mIntent != null && mIntent.hasExtra(AutofillManager.EXTRA_RESTORE_SESSION_TOKEN)
&& mIntent.hasExtra(AutofillManager.EXTRA_RESTORE_CROSS_ACTIVITY)) {
if (TextUtils.equals(getPackageName(),
intent.resolveActivity(getPackageManager()).getPackageName())) {
// Apply Autofill restore mechanism on the started activity by startActivity()
final IBinder token =
mIntent.getIBinderExtra(AutofillManager.EXTRA_RESTORE_SESSION_TOKEN);
// Remove restore ability from current activity
mIntent.removeExtra(AutofillManager.EXTRA_RESTORE_SESSION_TOKEN);
mIntent.removeExtra(AutofillManager.EXTRA_RESTORE_CROSS_ACTIVITY);
// Put restore token
intent.putExtra(AutofillManager.EXTRA_RESTORE_SESSION_TOKEN, token);
intent.putExtra(AutofillManager.EXTRA_RESTORE_CROSS_ACTIVITY, true);
}
}
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.
// 因为 options == null 所以走到这儿
startActivityForResult(intent, -1);
}
}Activity#startActivityForResult(Intent, int)
public void startActivityForResult(@RequiresPermission Intent intent, int requestCode) {
startActivityForResult(intent, requestCode, null);
}调用 startActivityForResult 的重载方法
public void startActivityForResult(@RequiresPermission Intent intent, int requestCode,
@Nullable Bundle options) {
if (mParent == null) {
options = transferSpringboardActivityOptions(options);
// 关键来了
Instrumentation.ActivityResult ar =
mInstrumentation.execStartActivity(
this, mMainThread.getApplicationThread(), mToken, this,
intent, requestCode, options);
if (ar != null) {
mMainThread.sendActivityResult(
mToken, mEmbeddedID, requestCode, ar.getResultCode(),
ar.getResultData());
}
if (requestCode >= 0) {
// If this start is requesting a result, we can avoid making
// the activity visible until the result is received. Setting
// this code during onCreate(Bundle savedInstanceState) or onResume() will keep the
// activity hidden during this time, to avoid flickering.
// This can only be done when a result is requested because
// that guarantees we will get information back when the
// activity is finished, no matter what happens to it.
mStartedActivity = true;
}
cancelInputsAndStartExitTransition(options);
// TODO Consider clearing/flushing other event sources and events for child windows.
} else {
if (options != null) {
mParent.startActivityFromChild(this, intent, requestCode, options);
} else {
// Note we want to go through this method for compatibility with
// existing applications that may have overridden it.
mParent.startActivityFromChild(this, intent, requestCode);
}
}
}关键调用 mInstrumentation.execStartActivity
app 进程到 ATMS 进程
Instrumentation#execStartActivity
@UnsupportedAppUsage
public ActivityResult execStartActivity(
Context who, IBinder contextThread, IBinder token, Activity target,
Intent intent, int requestCode, Bundle options) {
IApplicationThread whoThread = (IApplicationThread) contextThread;
Uri referrer = target != null ? target.onProvideReferrer() : null;
if (referrer != null) {
intent.putExtra(Intent.EXTRA_REFERRER, referrer);
}
if (mActivityMonitors != null) {
synchronized (mSync) {
final int N = mActivityMonitors.size();
for (int i=0; i<N; i++) {
final ActivityMonitor am = mActivityMonitors.get(i);
ActivityResult result = null;
if (am.ignoreMatchingSpecificIntents()) {
result = am.onStartActivity(intent);
}
if (result != null) {
am.mHits++;
return result;
} else if (am.match(who, null, intent)) {
am.mHits++;
if (am.isBlocking()) {
return requestCode >= 0 ? am.getResult() : null;
}
break;
}
}
}
}
try {
intent.migrateExtraStreamToClipData(who);
intent.prepareToLeaveProcess(who);
int result = ActivityTaskManager.getService().startActivity(whoThread,
who.getBasePackageName(), 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;
}最终通过 ActivityTaskManager.getService() 获取ATMS的服务代理并调用 ATMS.startActivity 方法，这时从 app 的进程进入到 SystemServer 进程，发生了一次跨进程！
ATMS 到 ActivityThread
ActivityTaskManagerService#startActivity
@Override
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());
}ActivityTaskManagerService#startActivityAsUser
@Override
public int startActivityAsUser(IApplicationThread caller, String callingPackage,
String callingFeatureId, Intent intent, String resolvedType, IBinder resultTo,
String resultWho, int requestCode, int startFlags, ProfilerInfo profilerInfo,
Bundle bOptions, int userId) {
return startActivityAsUser(caller, callingPackage, callingFeatureId, intent, resolvedType,
resultTo, resultWho, requestCode, startFlags, profilerInfo, bOptions, userId,
true /*validateIncomingUser*/);
}ActivityTaskManagerService#startActivityAsUser
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);
enforceNotIsolatedCaller("startActivityAsUser");
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();
}最后 return 的对象设置了那么多属性并在最后调用 excute() 执行。
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();先看看 getActivityStartController() 是个啥
ActivityStartController getActivityStartController() {
return mActivityStartController;
}getActivityStartController().obtainStarter(intent, "startActivityAsUser")ActivityStarter obtainStarter(Intent intent, String reason) {
return mFactory.obtain().setIntent(intent).setReason(reason);
}mFactory.obtain() 使用工厂模式从 DefaultFactory 类的缓存池(最大缓存数量为3) 中取出 ActivityStarter 对象
getActivityStartController().obtainStarter(intent, "startActivityAsUser")
.setCaller(caller)setCaller(caller) caller 是 IApplicationThread 类型的对象，等于 ActivityStarter 里缓存了与 app 进程连接的桥梁，之后可以进行跨进程调用 app 进程。
ApplicationThread是ActivityThread内部类，继承自IApplicationThread.Stub，作为服务端接受AMS发出的请求并执行，ApplicationThread是ActivityThread与AMS连接的桥梁。
再看最后的 execute()
return getActivityStartController().obtainStarter(intent, "startActivityAsUser")
.setCaller(caller)
...
.execute();ActivityStarter#execute
int execute() {
try {
...
res = executeRequest(mRequest);
...
return getExternalResult(mRequest.waitResult == null ? res
: waitForResult(res, mLastStartActivityRecord));
} finally {
onExecutionComplete();
}
}ActivityStarter#executeRequest
/**
*执行活动启动请求，开始启动活动的旅程。在这里
*首先执行几个初步检查。正常的活动启动流程将
*通过{@link#startActivityUnchecked}到{@link#startactivitynner}。
*/
private int executeRequest(Request request) {
...
// 在这里创建 ActivityRecord 对象，用于描述 Activity 信息
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);
....
mLastStartActivityResult = startActivityUnchecked(r, sourceRecord, voiceSession,
request.voiceInteractor, startFlags, true /* doResume */, checkedOptions, inTask,
restrictedBgActivity, intentGrants);
....
return mLastStartActivityResult;
}ActivityStarter#startActivityUnchecked
private int startActivityUnchecked(final ActivityRecord r, ActivityRecord sourceRecord,
IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
int startFlags, boolean doResume, ActivityOptions options, Task inTask,
boolean restrictedBgActivity, NeededUriGrants intentGrants) {
int result = START_CANCELED;
final ActivityStack startedActivityStack;
try {
mService.deferWindowLayout();
Trace.traceBegin(Trace.TRACE_TAG_WINDOW_MANAGER, "startActivityInner");
// 关键
result = startActivityInner(r, sourceRecord, voiceSession, voiceInteractor,
startFlags, doResume, options, inTask, restrictedBgActivity, intentGrants);
} finally {
Trace.traceEnd(Trace.TRACE_TAG_WINDOW_MANAGER);
startedActivityStack = handleStartResult(r, result);
mService.continueWindowLayout();
}
postStartActivityProcessing(r, result, startedActivityStack);
return result;
}ActivityStarter#startActivityInner
int startActivityInner(final ActivityRecord r, ActivityRecord sourceRecord,
IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
int startFlags, boolean doResume, ActivityOptions options, Task inTask,
boolean restrictedBgActivity, NeededUriGrants intentGrants) {
....
mRootWindowContainer.resumeFocusedStacksTopActivities(
mTargetStack, mStartActivity, mOptions);
....
return START_SUCCESS;
}调用了RootWindowContainer#resumeFocusedStacksTopActivities
boolean resumeFocusedStacksTopActivities(
ActivityStack targetStack, ActivityRecord target, ActivityOptions targetOptions) {
...
result |= focusedStack.resumeTopActivityUncheckedLocked(target, targetOptions);
...
return result;
}调用 ActivityStack#resumeTopActivityUncheckedLocked
boolean resumeTopActivityUncheckedLocked(ActivityRecord prev, ActivityOptions options) {
...
result = resumeTopActivityInnerLocked(prev, options);
...
return result;
}private boolean resumeTopActivityInnerLocked(ActivityRecord prev, ActivityOptions options) {
...
//关键关键！！！ 传入了 next.app.getThread() 返回的是 IApplication
final ClientTransaction transaction =
ClientTransaction.obtain(next.app.getThread(), next.appToken);
transaction.setLifecycleStateRequest(
ResumeActivityItem.obtain(next.app.getReportedProcState(),
dc.isNextTransitionForward()));
mAtmService.getLifecycleManager().scheduleTransaction(transaction);
...
return true;
}mStackSupervisor.startSpecificActivity(next, true, true);
ActivityStackSupervisor#startSpecificActivity
void startSpecificActivity(ActivityRecord r, boolean andResume, boolean checkConfig) {
...
realStartActivityLocked(r, wpc, andResume, checkConfig);
...
}ActivityStackSupervisor#realStartActivityLocked
boolean realStartActivityLocked(ActivityRecord r, WindowProcessController proc,
boolean andResume, boolean checkConfig) throws RemoteException {
...
// Create activity launch transaction.
// 关键关键！！！ 传入了 proc.getThread() 返回的是 IApplication
// 赋值给 ClientTransaction 的 mClient 对象
final ClientTransaction clientTransaction = ClientTransaction.obtain(
proc.getThread(), r.appToken);
final DisplayContent dc = r.getDisplay().mDisplayContent;
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()));
// Set desired final state.
final ActivityLifecycleItem lifecycleItem;
if (andResume) {
lifecycleItem = ResumeActivityItem.obtain(dc.isNextTransitionForward());
} else {
lifecycleItem = PauseActivityItem.obtain();
}
clientTransaction.setLifecycleStateRequest(lifecycleItem);
// Schedule transaction.
mService.getLifecycleManager().scheduleTransaction(clientTransaction);
...
}通过缓存池获取 ClientTransaction 类型的对象，并通过 mService.getLifecycleManager()调用 scheduleTransaction 方法开始执行任务
mService 是 ActivityTaskManagerService 类型的对象
ActivityTaskManagerService#getLifecycleManager
ClientLifecycleManager getLifecycleManager() {
return mLifecycleManager;
}返回的 ClientLifecycleManager 类型对象，再看它的 scheduleTransaction 做了啥
void scheduleTransaction(ClientTransaction transaction) throws RemoteException {
final IApplicationThread client = transaction.getClient();
transaction.schedule();
if (!(client instanceof Binder)) {
// If client is not an instance of Binder - it's a remote call and at this point it is
// safe to recycle the object. All objects used for local calls will be recycled after
// the transaction is executed on client in ActivityThread.
transaction.recycle();
}
}调用了 ClientTransaction 的 schedule 方法，简单粗暴。。。
要我们回头看 ClientTransaction 的 schedule 是怎么执行的
ATMS 进程切回 app 进程
public void schedule() throws RemoteException {
mClient.scheduleTransaction(this);
}咦 又调用了 mClient 的 scheduleTransaction 。。。 额绕了一圈
又要回头看看 mClient 对象是啥了
在 ClientTransaction 中 mClient 是这样定义的
/** Target client. */
private IApplicationThread mClient;IApplicationThread 对象 是与 app 进程沟通的桥梁！
刚刚在 调用 ActivityStack#resumeTopActivityUncheckedLocked 中将 IApplicationThread 传进 ClientTransaction 里了呢。
所以转了一圈 这个 ClientTransaction 对象会在 IApplicationThread 中调用，
同学们应该了解过 app 进程的 ApplicationThread 是 ActivityThread 内部类，并且实现了 IApplicationThread.Stub 。
mClient.scheduleTransaction(this); 实际上是 ApplicationThread#scheduleTransaction
这时候发生了跨进程：从 SystemServer 进程到 app 进程
ApplicationThread#scheduleTransaction
@Override
public void scheduleTransaction(ClientTransaction transaction) throws RemoteException {
ActivityThread.this.scheduleTransaction(transaction);
}内部调用了 ActivityThread#scheduleTransaction
追代码发现 ActivityThread 并没有 scheduleTransaction 方法的具体实现，这时候需要看看 ActivityThread 的父类有没有
ActivityThread 继承了 ClientTransactionHandler
在 ClientTransactionHandler 中找到了 scheduleTransaction
void scheduleTransaction(ClientTransaction transaction) {
transaction.preExecute(this);
sendMessage(ActivityThread.H.EXECUTE_TRANSACTION, transaction);
}调用 sendMessage 方法并且 what 的值为 ActivityThread.H.EXECUTE_TRANSACTION
ClientTransactionHandler#sendMessage
abstract void sendMessage(int what, Object obj);sendMessage 是抽象方法，找找在 ActivityThread 的具体实现
ActivityThread#sendMessage
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);
}
private void sendMessage(int what, Object obj, int arg1, int arg2, int seq) {
if (DEBUG_MESSAGES) Slog.v(
TAG, "SCHEDULE " + mH.codeToString(what) + " arg1=" + arg1 + " arg2=" + arg2 +
"seq= " + seq);
Message msg = Message.obtain();
msg.what = what;
SomeArgs args = SomeArgs.obtain();
args.arg1 = obj;
args.argi1 = arg1;
args.argi2 = arg2;
args.argi3 = seq;
msg.obj = args;
mH.sendMessage(msg);
}有一系列的重载方法，但最终都是调用 mH.sendMessage
final H mH = new H();H 是 ActivityThread 的内部类，继承了 Handler
class H extends Handler那刚刚 mH.sendMessage 发送的消息 ActivityThread.H.EXECUTE_TRANSACTION 会被 H 来处理
ActivityThread#H#handleMessage
public void handleMessage(Message msg) {
if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what));
switch (msg.what) {
...
case EXECUTE_TRANSACTION:
final ClientTransaction transaction = (ClientTransaction) msg.obj;
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();
}
// TODO(lifecycler): Recycle locally scheduled transactions.
break;
...
}
}调用 TransactionExecutor 的 execute
public void execute(ClientTransaction transaction) {
...
executeCallbacks(transaction);
...
}TransactionExecutor#executeCallbacks
/** Cycle through all states requested by callbacks and execute them at proper times. */
@VisibleForTesting
public void executeCallbacks(ClientTransaction transaction) {
// Callback 在 ActivityStackSupervisor#realStartActivityLocked中被添加 类型为 LaunchActivityItem
final List<ClientTransactionItem> callbacks = transaction.getCallbacks();
if (callbacks == null || callbacks.isEmpty()) {
// No callbacks to execute, return early.
return;
}
final IBinder token = transaction.getActivityToken();
ActivityClientRecord r = mTransactionHandler.getActivityClient(token);
...
final int size = callbacks.size();
for (int i = 0; i < size; ++i) {
final ClientTransactionItem item = callbacks.get(i);
...
item.execute(mTransactionHandler, token, mPendingActions);
item.postExecute(mTransactionHandler, token, mPendingActions);
...
}
}所以在这里执行的是 LaunchActivityItem 的 execute
LaunchActivityItem#execute
@Override
public void execute(ClientTransactionHandler client, IBinder token,
PendingTransactionActions pendingActions) {
Trace.traceBegin(TRACE_TAG_ACTIVITY_MANAGER, "activityStart");
ActivityClientRecord r = new ActivityClientRecord(token, mIntent, mIdent, mInfo,
mOverrideConfig, mCompatInfo, mReferrer, mVoiceInteractor, mState, mPersistentState,
mPendingResults, mPendingNewIntents, mIsForward,
mProfilerInfo, client, mAssistToken, mFixedRotationAdjustments);
client.handleLaunchActivity(r, pendingActions, null /* customIntent */);
Trace.traceEnd(TRACE_TAG_ACTIVITY_MANAGER);
}内部调用 client.handleLaunchActivity ，client 的类型是 ClientTransactionHandler ，刚才梳理过 ActivityThread 继承的就是 ClientTransactionHandler。
ActivityThread 到 Activity 启动
ActivityThread#handleLaunchActivity
public Activity handleLaunchActivity(ActivityClientRecord r,
PendingTransactionActions pendingActions, Intent customIntent) {
...
final Activity a = performLaunchActivity(r, customIntent);
...
return a;
}ActivityThread#performLaunchActivity
/** Core implementation of activity launch. */
private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
...
// app 的 Context 在这儿创建的
ContextImpl appContext = createBaseContextForActivity(r);
Activity activity = null;
try {
// 通过反射方式创建 Activity 对象
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);
}
}
...
// makeApplication 方法内部会判断 app 是否创建了 Application,若已创建则返回，若未创建则新建并返回
Application app = r.packageInfo.makeApplication(false, mInstrumentation);
...
// attach 方法非常关键！！！内部创建了 PhoneWindow 对象
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);
...
// 这里开始 Attach 的生命周期 onCreate
if (r.isPersistable()) {
mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
} else {
mInstrumentation.callActivityOnCreate(activity, r.state);
}
...
}Activity 的启动流程梳理差不多了。涉及的类真多。不过梳理一遍，可以加深对 Activity 的印象，增加记忆力，以后忘的慢 ~~
本文章为转载内容，我们尊重原作者对文章享有的著作权。如有内容错误或侵权问题，欢迎原作者联系我们进行内容更正或删除文章。
