Android源码解析四大组件系列（四）---Activity启动详细流程

在上一篇博客http://www.jianshu.com/p/98fa08ee1bfb，我们起码知道了Zygote是怎么启动SystemServer进程的，AMS是怎么注册的，启动的，启动之后通过Socket怎样与Zygote进程通信的，而Zygote进程收到AMS的请求之后，是怎么fork进程，将ActivityThread的main方法是执行起来的。这些内容在我的前两篇博客都梳理过，建议阅读一下，在看本文。

一、Activity请求启动

我们以startActivity这种来分析

Intent intent=new Intent(MainActivity.this,TargetActivity.class);
startActivity(intent);

经过startActivity–>startActivityForResult–>execStartActivity,最终会调用Instrumentation的execStartActivity方法。

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

try {
intent.migrateExtraStreamToClipData();
intent.prepareToLeaveProcess(who);
//获取AMS的代理，启动Activity
int result = ActivityManagerNative.getDefault()
.startActivity(whoThread, who.getBasePackageName(), intent,
intent.resolveTypeIfNeeded(who.getContentResolver()),
token, target != null ? target.mEmbeddedID : null,
requestCode, 0, null, options);

//检查Activity是否启动成功，若失败给出是什么原因
checkStartActivityResult(result, intent);
} catch (RemoteException e) {
throw new RuntimeException("Failure from system", e);
}
return null;

}

这些参数的意思是

this，为启动Activity的对象；

contextThread，为Binder对象，是主进程的context对象；

token，也是一个Binder对象，指向了服务端一个ActivityRecord对象；

target，为启动的Activity；

intent，启动的Intent对象；

requestCode，请求码；

options，参数；

先看checkStartActivityResult方法的实现

public static void checkStartActivityResult(int res, Object intent) {
if (res >= ActivityManager.START_SUCCESS) {
return;
}

switch (res) {
case ActivityManager.START_INTENT_NOT_RESOLVED:
case ActivityManager.START_CLASS_NOT_FOUND:
if (intent instanceof Intent && ((Intent)intent).getComponent() != null)
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_CANCELED:
throw new AndroidRuntimeException("Activity could not be started for "
+ intent);
default:
throw new AndroidRuntimeException("Unknown error code "
+ res + " when starting " + intent);
}
}

常见的ActivityNotFoundException，SecurityException等错日志就在这里报出来的。

OK，现在回到execStartActivity，内部调用了ActivityManagerNative.getDefault().startActivity，关于ActivityManagerNative已经说了很多了，相当于AMS服务端存根或者代理，其客户端是ActivityManagerNative的内部类ActivityManagerProxy，这里getDefault获取的就是ActivityManagerProxy对象，通过ActivityManagerProxy对象来使用AMS内部的一些服务，包括单不限于启动Activity。所以通过ActivityManagerProxy对象调用startActivity方法的实质是调用BinderProxy.transact向Binder驱动发送START_ACTIVITY_TRACSACTION命令，binder驱动将处理逻辑从Launcher所在的进程切换到AMS所在的SystemServer进程。OK，我们直接看ActivityManagerService的startActivity方法，不明白，建议看一下Binder的通信原理。

二、AMS处理Activity的启动请求

2.1、AMS处理Activity的启动请求

@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());
}

保持篇幅，以下步骤省略。

ActvityiManagerService.startActivityAsUser()

ActivityStackSupervisor.startActivityMayWait()

ActivityStackSupervisor.startActivityLocked()

ActivityStackSupervisor.startActivityUncheckedLocked()

ActivityStackSupervisor.startActivityLocked()

ActivityStackSupervisor.resumeTopActivitiesLocked()

ActivityStackSupervisor.resumeTopActivityInnerLocked()

在resumeTopActivityInnerLocked的内部有如下代码。

if (mResumedActivity != null) {
if (DEBUG_STATES) Slog.d(TAG_STATES,
"resumeTopActivityLocked: Pausing " + mResumedActivity);
pausing |= startPausingLocked(userLeaving, false, true, dontWaitForPause);
}

这表明在启动一个Activity的时候，前面一个Activity要执行onPause方法，onPause方法是怎么调用的呢？

2.2、AMS调用栈顶Activity的onPause

final boolean startPausingLocked(boolean userLeaving, boolean uiSleeping, boolean resuming,
boolean dontWait) {
......
if (mService.mHasRecents && (next == null || next.noDisplay || next.task != prev.task || uiSleeping)) {
prev.updateThumbnailLocked(screenshotActivities(prev), null);
}
stopFullyDrawnTraceIfNeeded();

mService.updateCpuStats();

if (prev.app != null && prev.app.thread != null) {
if (DEBUG_PAUSE) Slog.v(TAG_PAUSE, "Enqueueing pending pause: " + prev);
try {
//调用ApplicationThread的schedulePauseActivity
prev.app.thread.schedulePauseActivity(prev.appToken, prev.finishing,
userLeaving, prev.configChangeFlags, dontWait);
} catch (Exception e) {
......
}
}

......
}

这里先留一个问题， 我知道prev.app.thread是ApplicationThreadProxy对象，实际是调用了ApplicationThread的schedulePauseActivity方法，但是AMS是怎么获取到prev.app.thread的呢？

public final void schedulePauseActivity(IBinder token, boolean finished,
boolean userLeaving, int configChanges, boolean dontReport) {
sendMessage(
finished ? H.PAUSE_ACTIVITY_FINISHING : H.PAUSE_ACTIVITY,
token,
(userLeaving ? 1 : 0) | (dontReport ? 2 : 0),
configChanges);
}

关键就是最后调用了sendMessage(H.BIND_APPLICATION, data),发送了消息。关于H这个类，其实是个Hander，我觉得它的作用就像是一个交通枢纽。对这个H想更详细了解的，详细戳我。H接收到这个Message之后，调用

handlePauseActivity((IBinder)msg.obj, false, (msg.arg1&1) != 0, msg.arg2,   (msg.arg1&2) != 0);

private void handlePauseActivity(IBinder token, boolean finished,
boolean userLeaving, int configChanges, boolean dontReport) {
ActivityClientRecord r = mActivities.get(token);
.......
r.activity.mConfigChangeFlags |= configChanges;
performPauseActivity(token, finished, r.isPreHoneycomb());
.......

// Tell the activity manager we have paused.
if (!dontReport) {
try {
ActivityManagerNative.getDefault().activityPaused(token);
} catch (RemoteException ex) {
}
}
mSomeActivitiesChanged = true;
}
}

final Bundle performPauseActivity(ActivityClientRecord r, boolean finished,
boolean saveState) {
if (r.paused) {
.......
try {
.......
//调用Instrumentation的callActivityOnPause方法
mInstrumentation.callActivityOnPause(r.activity);
EventLog.writeEvent(LOG_AM_ON_PAUSE_CALLED, UserHandle.myUserId(),
r.activity.getComponentName().getClassName());
if (!r.activity.mCalled) {
throw new SuperNotCalledException(
"Activity " + r.intent.getComponent().toShortString() +
" did not call through to super.onPause()");
}

} catch (SuperNotCalledException e) {
throw e;

} catch (Exception e) {
.......
}
}
r.paused = true;

.......
return !r.activity.mFinished && saveState ? r.state : null;
}

public void callActivityOnPause(Activity activity) {
activity.performPause();
}

final void performPause() {
mDoReportFullyDrawn = false;
mFragments.dispatchPause();
mCalled = false;
//onPause执行
onPause();
mResumed = false;
if (!mCalled && getApplicationInfo().targetSdkVersion
>= android.os.Build.VERSION_CODES.GINGERBREAD) {
throw new SuperNotCalledException(
"Activity " + mComponent.toShortString() +
" did not call through to super.onPause()");
}
mResumed = false;
}

分析到这里 onPause()终于执行了。回到 handlePauseActivity方法中，在这个方法最后调用了ActivityManagerNative.getDefault().activityPaused(token)

2.3、ActivityManagerService的activityPaused方法执行。

@Override
public final void activityPaused(IBinder token) {
final long origId = Binder.clearCallingIdentity();
synchronized(this) {
ActivityStack stack = ActivityRecord.getStackLocked(token);
if (stack != null) {
stack.activityPausedLocked(token, false);
}
}
Binder.restoreCallingIdentity(origId);
}

经过一些方法，最终会调用startSpecificActivityLocked

ActivityStack.activityPausedLocked()

ActivityStack.completePauseLocked()

ActivityStackSupervisor.resumeTopActivitiesLocked()

ActivityStack.resumeTopActivityLocked()

ActivityStack.resumeTopActivityInnerLocked ()

ActivityStack.startSpecificActivityLocked ()

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);

r.task.stack.setLaunchTime(r);

if (app != null && app.thread != null) {
try {
if ((r.info.flags&ActivityInfo.FLAG_MULTIPROCESS) == 0
|| !"android".equals(r.info.packageName)) {
// Don't add this if it is a platform component that is marked
// to run in multiple processes, because this is actually
// part of the framework so doesn't make sense to track as a
// separate apk in the process.
app.addPackage(r.info.packageName, r.info.applicationInfo.versionCode,
mService.mProcessStats);
}
realStartActivityLocked(r, app, andResume, checkConfig);
return;
} catch (RemoteException e) {
Slog.w(TAG, "Exception when starting activity "
+ r.intent.getComponent().flattenToShortString(), e);
}

// If a dead object exception was thrown -- fall through to
// restart the application.
}

mService.startProcessLocked(r.processName, r.info.applicationInfo, true, 0,
"activity", r.intent.getComponent(), false, false, true);
}

如果需要启动的Activity所需要的应用进程是否已经启动，如果没有则执行realStartActivityLocked去启动Activity,否则去启动启动应用进程，相当于一个预启动的过程。

2.4、应用进程创建

关于startProcessLocked怎么执行的，上篇博客分析过，SystemServer进程中Ams通过Socket与Zygote进程进行通信的，Zygote接收到AMS发起的创建进程的请求，会fork出一个子进程，并且反射调用这个进程中ActivityThread的main方法。详情步骤戳我,在应用进程启动之后，ActivityThread的main方法就开始执行了。

三、ActivityThread的main方法执行

在ActivityThread的main方法里调用thread.attach(false);

public static void main(String[] args) {
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");
SamplingProfilerIntegration.start();
......

Process.setArgV0("<pre-initialized>");

Looper.prepareMainLooper();

ActivityThread thread = new ActivityThread();
thread.attach(false);

if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}

if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}

// End of event ActivityThreadMain.
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
Looper.loop();

throw new RuntimeException("Main thread loop unexpectedly exited");
}

对于上面main方法，相信很多人都看多很多遍了，接下来重点分析 thread.attach(false)这行代码。

private void attach(boolean system) {
sCurrentActivityThread = this;
mSystemThread = system;
if (!system) {
.......

//获取ActivityManagerService在客户端的代理
IActivityManager mgr = ActivityManagerNative.getDefault();
try {

//实质是调用ActivityManagerService的attachApplication方法,目的是让ActivityManagerService通过ApplicationThread代理对象控制应用进程
mgr.attachApplication(mAppThread);
} catch (RemoteException ex) {
}
}
.......
}

在调用 mgr.attachApplication()的时候，将mAppThread对象传进去了。

final ApplicationThread mAppThread = new ApplicationThread();

ApplicationThread继承了ApplicationThreadNative， 看一下ApplicationThreadNative里面的方法。



内部调用了ActivityManagerNative.getDefault() .attachApplication，关于ActivityManagerNative已经说了很多了，相当于AMS服务端存根或者代理，其客户端是ActivityManagerNative的内部类ActivityManagerProxy，这里getDefault获取的就是ActivityManagerProxy对象，通过ActivityManagerProxy对象来使用AMS内部的一些服务，包括单不限于启动Activity。所以通过ActivityManagerProxy对象调用attachApplication方法的实质是调用BinderProxy.transact向Binder驱动发送ATTACH_APPLICATION_TRANSACTION命令，binder驱动将处理逻辑从APP应用进程所在的进程切换到AMS所在的SystemServer进程。OK，我们直接看ActivityManagerService的attachApplication方法：

@Override
public final void attachApplication(IApplicationThread thread) {
synchronized (this) {
int callingPid = Binder.getCallingPid();
final long origId = Binder.clearCallingIdentity();
attachApplicationLocked(thread, callingPid);
Binder.restoreCallingIdentity(origId);
}
}

private final boolean attachApplicationLocked(IApplicationThread thread,
int pid) {
ProcessRecord app;
if (pid != MY_PID && pid >= 0) {
synchronized (mPidsSelfLocked) {
//通过进程ID，获取进程的记录信息
app = mPidsSelfLocked.get(pid);
}
} else {
app = null;
}

if (app == null) {
Slog.w(TAG, "No pending application record for pid " + pid
+ " (IApplicationThread " + thread + "); dropping process");
EventLog.writeEvent(EventLogTags.AM_DROP_PROCESS, pid);
if (pid > 0 && pid != MY_PID) {
//如果没有获取到，需要杀死该进程，MY_PID是系统进程
Process.killProcessQuiet(pid);

} else {
......
return false;
}

......
//获取进程名称
final String processName = app.processName;
try {
AppDeathRecipient adr = new AppDeathRecipient(
app, pid, thread);
thread.asBinder().linkToDeath(adr, 0);
app.deathRecipient = adr;
} catch (RemoteException e) {
app.resetPackageList(mProcessStats);
startProcessLocked(app, "link fail", processName);
return false;
}

EventLog.writeEvent(EventLogTags.AM_PROC_BOUND, app.userId, app.pid, app.processName);

//设置进程的OOM，adj等值

//还记得上面留下的那个问题吗，AMS是怎么获取到prev.app.thread的呢？就在这，prev.app是ProcessRecord对象，makeActive内部将thread对象赋值给prev.app.thread。可以自己查阅，这样AMS是可以查询进程记录的，进程记录中就有一个app.thread，若AMS要跟Activity通信直接获取prev.app.thread就可以了。
app.makeActive(thread, mProcessStats);
app.curAdj = app.setAdj = -100;
app.curSchedGroup = app.setSchedGroup = Process.THREAD_GROUP_DEFAULT;
app.forcingToForeground = null;
updateProcessForegroundLocked(app, false, false);
app.hasShownUi = false;
app.debugging = false;
app.cached = false;
app.killedByAm = false;

//将attach之前的超时Meessage移除掉
mHandler.removeMessages(PROC_START_TIMEOUT_MSG, app);

boolean normalMode = mProcessesReady || isAllowedWhileBooting(app.info);
List<ProviderInfo> providers = normalMode ? generateApplicationProvidersLocked(app) : null;

......
try {
......
thread.bindApplication(processName, appInfo, providers, app.instrumentationClass,
profilerInfo, app.instrumentationArguments, app.instrumentationWatcher,
app.instrumentationUiAutomationConnection, testMode, enableOpenGlTrace,
isRestrictedBackupMode || !normalMode, app.persistent,
new Configuration(mConfiguration), app.compat,
getCommonServicesLocked(app.isolated),
mCoreSettingsObserver.getCoreSettingsLocked());
updateLruProcessLocked(app, false, null);
app.lastRequestedGc = app.lastLowMemory = SystemClock.uptimeMillis();
} catch (Exception e) {
// todo: Yikes!  What should we do?  For now we will try to
// start another process, but that could easily get us in
// an infinite loop of restarting processes...
Slog.wtf(TAG, "Exception thrown during bind of " + app, e);

app.resetPackageList(mProcessStats);
app.unlinkDeathRecipient();
startProcessLocked(app, "bind fail", processName);
return false;
}

......

// 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;
}
}

// 执行等待进程而暂时挂起的Service
if (!badApp) {
try {
didSomething |= mServices.attachApplicationLocked(app, processName);
} catch (Exception e) {
Slog.wtf(TAG, "Exception thrown starting services in " + app, e);
badApp = true;
}
}

// 执行等待进程而暂时挂起的广播
if (!badApp && isPendingBroadcastProcessLocked(pid)) {
try {
didSomething |= sendPendingBroadcastsLocked(app);
} catch (Exception e) {
// If the app died trying to launch the receiver we declare it 'bad'
Slog.wtf(TAG, "Exception thrown dispatching broadcasts in " + app, e);
badApp = true;
}
}

......

if (!didSomething) {
//调整进程优先级adj的值
updateOomAdjLocked();
}

return true;
}

发现AMS用IApplicationThread对象thread调用了bindApplication，thread实际上就是ApplicationThreadProxy对象。AMS通过ApplicationThreadProxy对象与ActivityThread通信。所以去ApplicationThread中看bindApplication的真正实现。

public final void bindApplication(String processName, ApplicationInfo appInfo,
List<ProviderInfo> providers, ComponentName instrumentationName,
ProfilerInfo profilerInfo, Bundle instrumentationArgs,
IInstrumentationWatcher instrumentationWatcher,
IUiAutomationConnection instrumentationUiConnection, int debugMode,
boolean enableOpenGlTrace, boolean isRestrictedBackupMode, boolean persistent,
Configuration config, CompatibilityInfo compatInfo, Map<String, IBinder> services,
Bundle coreSettings) {

if (services != null) {
// Setup the service cache in the ServiceManager
ServiceManager.initServiceCache(services);
}

setCoreSettings(coreSettings);
......
AppBindData data = new AppBindData();
data.processName = processName;
data.appInfo = appInfo;
data.providers = providers;
data.instrumentationName = instrumentationName;
data.instrumentationArgs = instrumentationArgs;
data.instrumentationWatcher = instrumentationWatcher;
data.instrumentationUiAutomationConnection = instrumentationUiConnection;
data.debugMode = debugMode;
data.enableOpenGlTrace = enableOpenGlTrace;
data.restrictedBackupMode = isRestrictedBackupMode;
data.persistent = persistent;
data.config = config;
data.compatInfo = compatInfo;
data.initProfilerInfo = profilerInfo;
sendMessage(H.BIND_APPLICATION, data);
}

这里又是用H发送了一个消息，是不是很像一个交通枢纽，做转发用的。详细戳我。

Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER,  "bindApplication");
AppBindData data = (AppBindData)msg.obj;
handleBindApplication(data);
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);

handleBindApplication内部就不分析了，Applcation对象的onCreate方法调用就是在这里面，它内部创建了Applcation对象。

四、Activity周期如何执行

4.1、Activity的onCreate是怎么回调的

回到第二小节的startSpecificActivityLocked方法

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);

r.task.stack.setLaunchTime(r);

if (app != null && app.thread != null) {
try {
if ((r.info.flags&ActivityInfo.FLAG_MULTIPROCESS) == 0
|| !"android".equals(r.info.packageName)) {

app.addPackage(r.info.packageName, r.info.applicationInfo.versionCode,
mService.mProcessStats);
}
realStartActivityLocked(r, app, andResume, checkConfig);
return;
} catch (RemoteException e) {
Slog.w(TAG, "Exception when starting activity "
+ r.intent.getComponent().flattenToShortString(), e);
}

// If a dead object exception was thrown -- fall through to
// restart the application.
}

mService.startProcessLocked(r.processName, r.info.applicationInfo, true, 0,
"activity", r.intent.getComponent(), false, false, true);
}

在预启动之后，一般我们的应用进程就被孵化来了，直接执行realStartActivityLocked去启动Activity。

在realStartActivityLocked内部调用了如下代码，现在你应该知道 app.thread是什么了。

app.thread.scheduleLaunchActivity(new Intent(r.intent), r.appToken,
System.identityHashCode(r), r.info, new Configuration(mService.mConfiguration),
new Configuration(stack.mOverrideConfig), r.compat, r.launchedFromPackage,
task.voiceInteractor, app.repProcState, r.icicle, r.persistentState, results,
newIntents, !andResume, mService.isNextTransitionForward(), profilerInfo);

直接到scheduleLaunchActivity真正实现，它的真正实现是在Applciation中的。

public final void scheduleLaunchActivity(Intent intent, IBinder token, int ident,
ActivityInfo info, Configuration curConfig, Configuration overrideConfig,
CompatibilityInfo compatInfo, String referrer, IVoiceInteractor voiceInteractor,
int procState, Bundle state, PersistableBundle persistentState,
List<ResultInfo> pendingResults, List<ReferrerIntent> pendingNewIntents,
boolean notResumed, boolean isForward, ProfilerInfo profilerInfo) {

updateProcessState(procState, false);

ActivityClientRecord r = new ActivityClientRecord();

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

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

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

r.profilerInfo = profilerInfo;

r.overrideConfig = overrideConfig;
updatePendingConfiguration(curConfig);

sendMessage(H.LAUNCH_ACTIVITY, r);
}

经过一堆的变量赋值之后，又通过H发送了一个消息LAUNCH_ACTIVITY，H在收到这个消息之后，执行下面。

Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityStart");
final ActivityClientRecord r = (ActivityClientRecord) msg.obj;

r.packageInfo = getPackageInfoNoCheck(
r.activityInfo.applicationInfo, r.compatInfo);
handleLaunchActivity(r, null);
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);

private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent) {
// If we are getting ready to gc after going to the background, well
// we are back active so skip it.
unscheduleGcIdler();
mSomeActivitiesChanged = true;

if (r.profilerInfo != null) {
mProfiler.setProfiler(r.profilerInfo);
mProfiler.startProfiling();
}

// Make sure we are running with the most recent config.
handleConfigurationChanged(null, null);

if (localLOGV) Slog.v(
TAG, "Handling launch of " + r);

// Initialize before creating the activity
WindowManagerGlobal.initialize();

//一个 perform开头的方法，启动Activity
Activity a = performLaunchActivity(r, customIntent);

if (a != null) {
r.createdConfig = new Configuration(mConfiguration);
Bundle oldState = r.state;
//Launch
handleResumeActivity(r.token, false, r.isForward,
!r.activity.mFinished && !r.startsNotResumed);

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

try {
r.activity.mCalled = false;

mInstrumentation.callActivityOnPause(r.activity);

if (r.isPreHoneycomb()) {
r.state = oldState;
}
if (!r.activity.mCalled) {
throw new SuperNotCalledException(
"Activity " + r.intent.getComponent().toShortString() +
" did not call through to super.onPause()");
}

} catch (SuperNotCalledException e) {
throw e;

} catch (Exception e) {
.....
}
r.paused = true;
}
.....
}
}

这个方法会将Activity创建出来。

private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {

ActivityInfo aInfo = r.activityInfo;
r.activityInfo.targetActivity);

......
Activity activity = null;
try {
java.lang.ClassLoader cl = r.packageInfo.getClassLoader();

activity = mInstrumentation.newActivity(
cl, component.getClassName(), r.intent);
.......
} catch (Exception e) {
.......
}

Application app = r.packageInfo.makeApplication(false, mInstrumentation);
if (localLOGV) Slog.v(TAG, "Performing launch of " + r);
if (activity != null) {
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);
.......
//Activity的create方法执行
if (r.isPersistable()) {
mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
} else {
mInstrumentation.callActivityOnCreate(activity, r.state);
}
.......

r.activity = activity;
r.stopped = true;
if (!r.activity.mFinished) {
//Activity的start方法执行
activity.performStart();
r.stopped = false;
}
mActivities.put(r.token, r);

return activity;
}

反射创建Activity，为啥要用反射呢，此处搞不清楚。

public Activity newActivity(ClassLoader cl, String className,
Intent intent)
throws InstantiationException, IllegalAccessException,
ClassNotFoundException {
return (Activity)cl.loadClass(className).newInstance();
}

解析来mInstrumentation调用callActivityOnCreate方法。

public void callActivityOnCreate(Activity activity, Bundle icicle,
PersistableBundle persistentState) {
prePerformCreate(activity);
activity.performCreate(icicle, persistentState);
postPerformCreate(activity);
}

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

onCreate方法在此处被调用了。

4.2、Activity的onStart是怎么回调的

在performLaunchActivity中调用了callActivityOnCreate之后，又会调用 activity.performStart()

final void performStart() {
mActivityTransitionState.setEnterActivityOptions(this, getActivityOptions());
mFragments.noteStateNotSaved();
mCalled = false;
mFragments.execPendingActions();
mInstrumentation.callActivityOnStart(this);
if (!mCalled) {
throw new SuperNotCalledException(
"Activity " + mComponent.toShortString() +
" did not call through to super.onStart()");
}
mFragments.dispatchStart();
mFragments.reportLoaderStart();
mActivityTransitionState.enterReady(this);
}

public void callActivityOnStart(Activity activity) {
activity.onStart();
}

到此处start方法又被调用了。

4.3、Activity的onResume是怎么回调的

handleLaunchActivity方法中，在调用performLaunchActivity之后，调用了handleResumeActivity方法。

ActivityThread.handleResumeActivity()
ActivityThread.handleResumeActivity()
Activity.performResume()
Instrumentation.callActivityOnResume();

public void callActivityOnResume(Activity activity) {
activity.mResumed = true;
activity.onResume();

if (mActivityMonitors != null) {
synchronized (mSync) {
final int N = mActivityMonitors.size();
for (int i=0; i<N; i++) {
final ActivityMonitor am = mActivityMonitors.get(i);
am.match(activity, activity, activity.getIntent());
}
}
}
}

到此Activity的onResume方法被调用，onPause方法已经讲解过，关于onStop，onDestory一样的套路分析。

五、总结

关于Activity的启动过程分析，三篇文章都分析完了，现在以一张图总结一下。



启动一个Activity的方式有多种，都是通过向Ams发送startActivity请求

Ams在启动Activity的时候，会判断目标进程有没有被创建，没有创建的话，向zygote进程发送创建进程的请求；

Zygote进程fork出新的子进程，即App进程；

App进程，通过Binder IPC向sytem_server进程发起attachApplication请求；

AMS所在的SystemServer进程在收到请求后，进行一系列准备工作后，再通过binder IPC向App进程发送scheduleLaunchActivity请求；

App进程的binder线程（ApplicationThread）在收到请求后，通过handler向主线程发送LAUNCH_ACTIVITY消息；

主线程在收到Message后，通过发射机制创建目标Activity，并回调Activity.onCreate()等方法。

上图大致说明了一个Activity的启动流程， 重点在于把握Binder的通信原理，接着弄清楚APP进程，SystemServer进程，Zygote进程是怎么互相通信的。seeyou!

参考博客：

http://gityuan.com/2016/03/12/start-activity/