从安卓应用入口开始
Activity的启动离不开ActivityThread。而ActivityThread的入口在main()方法：
public static void main(String[] args) {
......
Looper.prepareMainLooper();
ActivityThread thread = new ActivityThread();
thread.attach(false);
......
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
} main()方法比较简单，首先准备MainLooper，也就是主线程的looper，然后创建ActivityThread对象，调用其attach方法。在创建ActivityThread对象时其实已经创建了主线程的handler，并绑定至当前的MainLooper。然后调用Looper.loop()。整个应用的循环开始。
接下来追踪ActivityThread的attach方法：
private void attach(boolean system) {
sCurrentActivityThread = this;
mSystemThread = system;
if (!system) {//在main方法调用的attach走的这个if
ViewRootImpl.addFirstDrawHandler(new Runnable() {
@Override
public void run() {
ensureJitEnabled();
}
});
android.ddm.DdmHandleAppName.setAppName("<pre-initialized>",
UserHandle.myUserId());
RuntimeInit.setApplicationObject(mAppThread.asBinder());
final IActivityManager mgr = ActivityManager.getService();//获取AMS的client
try {
mgr.attachApplication(mAppThread);
//mAppThread其实是回调binder，也就是ApplicationThread作为Server端，由AMS回调Activity生命周期的方法。
} catch (RemoteException ex) {
throw ex.rethrowFromSystemServer();
}
......
} else {//特殊情况
android.ddm.DdmHandleAppName.setAppName("system_process",
UserHandle.myUserId());
try {
mInstrumentation = new Instrumentation();
ContextImpl context = ContextImpl.createAppContext(
this, getSystemContext().mPackageInfo);
mInitialApplication = context.mPackageInfo.makeApplication(true, null);
mInitialApplication.onCreate();
} catch (Exception e) {
throw new RuntimeException(
"Unable to instantiate Application():" + e.toString(), e);
}
}
......
} 可以看到主要是获取了AMS的client，然后给AMS设置Activity生命周期的回调。在ApplicationThread中主要是向主线程的handler发送各种Activity生命周期相关的消息。
当启动一个activity时，startActivity方法最终会通过binder调用AMS上的activity启动方法，AMS最终会调用我们传给它的ApplicationThread回调，例如scheduleLaunchActivity方法（在ApplicationThread中具体实现，AMS作为client）。贯穿Activity启动始终的ActivityClientRecord也是在此创建：
@Override
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);
}然后会利用主线程handler处理相关的生命周期方法，例如handlerLaunchActivity：
//对应的代码在ActivityThread的嵌套类H中，也就是主线程handler的定义中
case LAUNCH_ACTIVITY: {
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, "LAUNCH_ACTIVITY");
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
} break;Activity的启动之路
对于应用程序来说，真正的启动工作应该从handleLaunchActivity方法开始。那么这个方法做了哪些事情呢？
private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent, String reason) {
// 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();
Activity a = performLaunchActivity(r, customIntent);//初始化并获得activity对象
if (a != null) {
r.createdConfig = new Configuration(mConfiguration);
reportSizeConfigurations(r);
Bundle oldState = r.state;
//执行resume
handleResumeActivity(r.token, false, r.isForward,
!r.activity.mFinished && !r.startsNotResumed, r.lastProcessedSeq, reason);
......
}这个方法主要对activity进行了初始化工作，后续的一系列关于activity的初始化也由此展开。
performLaunchActivity
private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
......
ContextImpl appContext = createBaseContextForActivity(r);//创建context
Activity activity = null;
try {
java.lang.ClassLoader cl = appContext.getClassLoader();
//利用mInstrumentation创建activity对象
activity = mInstrumentation.newActivity(
cl, component.getClassName(), r.intent);
StrictMode.incrementExpectedActivityCount(activity.getClass());
r.intent.setExtrasClassLoader(cl);
r.intent.prepareToEnterProcess();
if (r.state != null) {
r.state.setClassLoader(cl);
}
} catch (Exception e) {
if (!mInstrumentation.onException(activity, e)) {
throw new RuntimeException(
"Unable to instantiate activity " + component
+ ": " + e.toString(), e);
}
}
try {
//创建application，如果为空就创建，否则直接返回，保证application的唯一。
Application app = r.packageInfo.makeApplication(false, mInstrumentation);
......
if (activity != null) {
......
appContext.setOuterContext(activity);
//调用了activity的attach，在这个里面初始化window和WindowManager。
activity.attach(appContext, this, getInstrumentation(), r.token,
r.ident, app, r.intent, r.activityInfo, title, r.parent,
r.embeddedID, r.lastNonConfigurationInstances, config,
r.referrer, r.voiceInteractor, window, r.configCallback);
.......
if (r.isPersistable()) {//调用activity的onCreate方法
mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
} else {
mInstrumentation.callActivityOnCreate(activity, r.state);
}
......
r.activity = activity;
r.stopped = true;
if (!r.activity.mFinished) {
activity.performStart();//执行onStart()
r.stopped = false;
}
}
......
mActivities.put(r.token, r);//ActivityThread保存了所有的activity
} catch (SuperNotCalledException e) {
throw e;
} catch (Exception e) {
if (!mInstrumentation.onException(activity, e)) {
throw new RuntimeException(
"Unable to start activity " + component
+ ": " + e.toString(), e);
}
}
return activity;
}在这个方法中做了几个比较有分量的事情，我们一步步来分析：
1.初始化context。
2.创建activity对象。
3.创建application。
4.调用Activity的attach方法。初始化context
首先来看一下context是怎么初始化的。
//在performLaunchActivity中调用的创建context方法
private ContextImpl createBaseContextForActivity(ActivityClientRecord r) {
final int displayId;
try {
//r。token是一个IBinder，应该是包含当前activity对应的一些信息以便AMS进行查找。
displayId = ActivityManager.getService().getActivityDisplayId(r.token);//从AMS获取displayId。
} catch (RemoteException e) {
throw e.rethrowFromSystemServer();
}
ContextImpl appContext = ContextImpl.createActivityContext(
this, r.packageInfo, r.activityInfo, r.token, displayId, r.overrideConfig);
......
return appContext;
}
//被createBaseContextForActivity调用的方法
static ContextImpl createActivityContext(ActivityThread mainThread,
LoadedApk packageInfo, ActivityInfo activityInfo, IBinder activityToken, int displayId,
Configuration overrideConfiguration) {
if (packageInfo == null) throw new IllegalArgumentException("packageInfo");
String[] splitDirs = packageInfo.getSplitResDirs();
ClassLoader classLoader = packageInfo.getClassLoader();
......
//创建对象
ContextImpl context = new ContextImpl(null, mainThread, packageInfo, activityInfo.splitName,
activityToken, null, 0, classLoader);
// Clamp display ID to DEFAULT_DISPLAY if it is INVALID_DISPLAY.
displayId = (displayId != Display.INVALID_DISPLAY) ? displayId : Display.DEFAULT_DISPLAY;
final CompatibilityInfo compatInfo = (displayId == Display.DEFAULT_DISPLAY)
? packageInfo.getCompatibilityInfo()
: CompatibilityInfo.DEFAULT_COMPATIBILITY_INFO;
final ResourcesManager resourcesManager = ResourcesManager.getInstance();
// Create the base resources for which all configuration contexts for this Activity
// will be rebased upon.
//做一些初始化设置
context.setResources(resourcesManager.createBaseActivityResources(activityToken,
packageInfo.getResDir(),
splitDirs,
packageInfo.getOverlayDirs(),
packageInfo.getApplicationInfo().sharedLibraryFiles,
displayId,
overrideConfiguration,
compatInfo,
classLoader));
context.mDisplay = resourcesManager.getAdjustedDisplay(displayId,
context.getResources());
return context;
}以上就是context创建相关过程。从代码上来看，这里的context和activity，displayId有比较强的关联。应该是与资源管理相关的context。
初始化activity对象
activity对象的创建是调用的mInstrumentation.newActivity方法。
那么先去看看mInstrumentation是何方神圣。其初始化的位置有两处，一处是在ActivityThread的attach方法中，当参数为true时直接初始化
private void attach(boolean system) {
sCurrentActivityThread = this;
mSystemThread = system;
if (!system) {
......
} else {
// Don't set application object here -- if the system crashes,
// we can't display an alert, we just want to die die die.
android.ddm.DdmHandleAppName.setAppName("system_process",
UserHandle.myUserId());
try {
mInstrumentation = new Instrumentation();//直接新建对象
ContextImpl context = ContextImpl.createAppContext(
this, getSystemContext().mPackageInfo);//顺便初始化AppContext
mInitialApplication = context.mPackageInfo.makeApplication(true, null);
mInitialApplication.onCreate();//顺便初始化Application
} catch (Exception e) {
throw new RuntimeException(
"Unable to instantiate Application():" + e.toString(), e);
}
}
......
}但是这不是我们平常初始化activity的流程，因此要看另一处，ActivityThread的handleBindApplication方法：
private void handleBindApplication(AppBindData data) {
......
final InstrumentationInfo ii;
if (data.instrumentationName != null) {
try {
ii = new ApplicationPackageManager(null, getPackageManager())
.getInstrumentationInfo(data.instrumentationName, 0);
} catch (PackageManager.NameNotFoundException e) {
throw new RuntimeException(
"Unable to find instrumentation info for: " + data.instrumentationName);
}
......
} else {
ii = null;
}
//创建AppContext
final ContextImpl appContext = ContextImpl.createAppContext(this, data.info);
updateLocaleListFromAppContext(appContext,
mResourcesManager.getConfiguration().getLocales());
......
if (ii != null) {
//创建Application
final ApplicationInfo instrApp = new ApplicationInfo();
ii.copyTo(instrApp);
instrApp.initForUser(UserHandle.myUserId());
final LoadedApk pi = getPackageInfo(instrApp, data.compatInfo,
appContext.getClassLoader(), false, true, false);
final ContextImpl instrContext = ContextImpl.createAppContext(this, pi);
try {
final ClassLoader cl = instrContext.getClassLoader();
mInstrumentation = (Instrumentation)
cl.loadClass(data.instrumentationName.getClassName()).newInstance();//新建对象
} catch (Exception e) {
throw new RuntimeException(
"Unable to instantiate instrumentation "
+ data.instrumentationName + ": " + e.toString(), e);
}
......
} else {
mInstrumentation = new Instrumentation();
}
......
} 这个方法是在handler中被调用的，根据前面的分析，有可能是AMS调用的。事实上确实如此。在ActivityThread的main方法中调用了ActivityThread的attach方法，attach方法中会调用AMS的attachApplication方法。这个方法就是告诉AMS我的ActivityThread已经创建好了，然后AMS会回调ActivityThread的bindApplication方法，接着就到了handler发消息，然后就到了我们看到的这个方法。大概做了三件事，初始化context，初始化application，初始化mInstrumentation。
好了，绕了一大圈，下面就要看一下activity到底是怎么被创建的了。经过上面的分析，我们知道这个时候Application和context已经被创建了。下面是mInstrumentation的newActivity方法：
public Activity newActivity(ClassLoader cl, String className,
Intent intent)
throws InstantiationException, IllegalAccessException,
ClassNotFoundException {
return (Activity)cl.loadClass(className).newInstance();
}感觉有点失望，竟然是直接就创建了一个activity。没关系，继续往下看。
创建Application
经过上面的分析，发现在创建mInstrumentation的时候就已经创建了Application，那么这里为什么还要创建呢？是不是创建了多个Application？对比一下两个创建方法就知道了。
public Application makeApplication(boolean forceDefaultAppClass,
Instrumentation instrumentation) {
if (mApplication != null) {
return mApplication;
}
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "makeApplication");
Application app = null;
String appClass = mApplicationInfo.className;
if (forceDefaultAppClass || (appClass == null)) {
appClass = "android.app.Application";
}
try {
java.lang.ClassLoader cl = getClassLoader();
if (!mPackageName.equals("android")) {
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER,
"initializeJavaContextClassLoader");
initializeJavaContextClassLoader();
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
}
ContextImpl appContext = ContextImpl.createAppContext(mActivityThread, this);
app = mActivityThread.mInstrumentation.newApplication(
cl, appClass, appContext);
appContext.setOuterContext(app);
} catch (Exception e) {
if (!mActivityThread.mInstrumentation.onException(app, e)) {
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
throw new RuntimeException(
"Unable to instantiate application " + appClass
+ ": " + e.toString(), e);
}
}
mActivityThread.mAllApplications.add(app);
mApplication = app;
if (instrumentation != null) {
try {
instrumentation.callApplicationOnCreate(app);
} catch (Exception e) {
if (!instrumentation.onException(app, e)) {
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
throw new RuntimeException(
"Unable to create application " + app.getClass().getName()
+ ": " + e.toString(), e);
}
}
}
// Rewrite the R 'constants' for all library apks.
SparseArray<String> packageIdentifiers = getAssets().getAssignedPackageIdentifiers();
final int N = packageIdentifiers.size();
for (int i = 0; i < N; i++) {
final int id = packageIdentifiers.keyAt(i);
if (id == 0x01 || id == 0x7f) {
continue;
}
rewriteRValues(getClassLoader(), packageIdentifiers.valueAt(i), id);
}
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
return app;
}它们调用的都是LoadApk的makeApplication方法，那么看一下在哪里调用的，追踪到最顶部，发现在主线程的handler里面：
......
case LAUNCH_ACTIVITY: {
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, "LAUNCH_ACTIVITY");
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
} break;
......
case BIND_APPLICATION:
Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "bindApplication");
AppBindData data = (AppBindData)msg.obj;
handleBindApplication(data);
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
break;
......
//这是handleBindApplication方法中获取LoadApk的方法。
data.info = getPackageInfoNoCheck(data.appInfo, data.compatInfo);可以发现都是调用的同一个方法，参数是app相关的信息，并且从AMS传递而来。几乎可以确定是同一个对象的makeApplication方法。因此Application只会创建一次。多次调用的原因可能是保证其一定被创建。
调用activity的attach方法
activity的attach方法中主要是创建Window和WindowManager以及绑定与activity相关的一连串变量
handleResumeActivity
handleResumeActivity如下：
final void handleResumeActivity(IBinder token,
boolean clearHide, boolean isForward, boolean reallyResume, int seq, String reason) {
ActivityClientRecord r = mActivities.get(token);
if (!checkAndUpdateLifecycleSeq(seq, r, "resumeActivity")) {
return;
}
// If we are getting ready to gc after going to the background, well
// we are back active so skip it.
unscheduleGcIdler();
mSomeActivitiesChanged = true;
// TODO Push resumeArgs into the activity for consideration
r = performResumeActivity(token, clearHide, reason);//performResumeActivity，执行一些生命周期的方法。最终会调用activity的onResume方法。
if (r != null) {
final Activity a = r.activity;
......
if (r.window == null && !a.mFinished && willBeVisible) {
r.window = r.activity.getWindow();//在Activity的attach方法中初始化完毕。
View decor = r.window.getDecorView();//初始化DecorView
decor.setVisibility(View.INVISIBLE);
ViewManager wm = a.getWindowManager();//在Activity的attach方法中初始化完毕。
WindowManager.LayoutParams l = r.window.getAttributes();
a.mDecor = decor;
l.type = WindowManager.LayoutParams.TYPE_BASE_APPLICATION;
l.softInputMode |= forwardBit;
if (r.mPreserveWindow) {
a.mWindowAdded = true;
r.mPreserveWindow = false;
// Normally the ViewRoot sets up callbacks with the Activity
// in addView->ViewRootImpl#setView. If we are instead reusing
// the decor view we have to notify the view root that the
// callbacks may have changed.
ViewRootImpl impl = decor.getViewRootImpl();
if (impl != null) {
impl.notifyChildRebuilt();
}
if (a.mVisibleFromClient) {
if (!a.mWindowAdded) {
a.mWindowAdded = true;
wm.addView(decor, l);//绑定window和根view，以及建立和RootViewImpl的关系，并开始绘制流程。
}
......
}
}
......
// If the window has already been added, but during resume
// we started another activity, then don't yet make the
// window visible.
} else if (!willBeVisible) {
if (localLOGV) Slog.v(
TAG, "Launch " + r + " mStartedActivity set");
r.hideForNow = true;
}
// Get rid of anything left hanging around.
cleanUpPendingRemoveWindows(r, false /* force */);
......
}从performResumeActivity的代码大致可以看出来其主要做了两件事：
1.调用performResumeActivity方法。
2.初始化window并与activity建立关联。下面先来看一下performResumeActivity做了些什么事情。
public final ActivityClientRecord performResumeActivity(IBinder token,
boolean clearHide, String reason) {
ActivityClientRecord r = mActivities.get(token);
if (localLOGV) Slog.v(TAG, "Performing resume of " + r
+ " finished=" + r.activity.mFinished);
if (r != null && !r.activity.mFinished) {
if (clearHide) {
r.hideForNow = false;
r.activity.mStartedActivity = false;
}
try {
......
r.activity.performResume();//调用activity的performResume方法。
......
} catch (Exception e) {
if (!mInstrumentation.onException(r.activity, e)) {
throw new RuntimeException(
"Unable to resume activity "
+ r.intent.getComponent().toShortString()
+ ": " + e.toString(), e);
}
}
}
return r;
}可以看到主要调用了activity的performResume方法。继续往下看：
final void performResume() {
performRestart();//如果之前app执行了onStop()则执行onRestart(),onStart();
mFragments.execPendingActions();
mLastNonConfigurationInstances = null;
mCalled = false;
mInstrumentation.callActivityOnResume(this);//执行onResume()
......
mCalled = false;
mFragments.dispatchResume();
mFragments.execPendingActions();
......
}由此可见，从上面的handleLaunchActivity的执行顺序来看，也的确是和Activity的启动顺序相符合的：首先执行performLaunchActivity，在这期间创建了Activity，创建了Window，以及Application和context等，然后调用onCreate方法。onCreate方法之后会调用onStart方法，接着执行handleResumeActivity方法，首先会执行onResume等生命周期方法，然后才是创建DecorView，创建ViewRootImpl再执行测量，绘制等工作。
因此我们在onCreate或者onResume中是无法获取控件的宽高度的。 相对于生命周期方法来说，测量和绘制是在最后执行的。
从上面的分析还可以发现一个有趣的地方：
activity的生命周期方法的调用交给了一个特殊的类Instrumentation去处理。 不仅仅是生命周期方法，包括startActivity等方法最终也都是在这个对象中完成的调用。
这种能解耦的方式和代码习惯还是值得我们多多学习。
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