(*￣(oo)￣) 这篇文章是按照自己看源码的思路写的一篇文章，主要是给自己看的，其中还有很多不足，写的也不是太好，如果您发现了哪里我理解的不对请告诉我，我好补缺补漏。
致敬！
一名爬坑的应届生
一般而言通过调用activity.onCreate()方法即可创建一个活动运行实例，众所周知，在java中，main函数为我们提供了入口，而在activity的源码中并没有找到类似与main函数的函数，android入口的main函数在ActivityThread当中，下面就深入学习一下关于ActivityThread。ActivityThread类即代表Application主线程。
ActivityThread的main函数介绍：
public static void main(String[] args) {
SamplingProfilerIntegration.start();
CloseGuard.setEnabled(false);
Environment.initForCurrentUser();
// Set the reporter for event logging in libcore
EventLogger.setReporter(new EventLoggingReporter());
Security.addProvider(new AndroidKeyStoreProvider());
// Make sure TrustedCertificateStore looks in the right place for CA certificates
final File configDir = Environment.getUserConfigDirectory(UserHandle.myUserId());
TrustedCertificateStore.setDefaultUserDirectory(configDir);
Process.setArgV0("<pre-initialized>");
Looper.prepareMainLooper();
ActivityThread thread = new ActivityThread();
thread.attach(false);
if (sMainThreadHandler == null) {
sMainThreadHandler = thread.getHandler();
}
AsyncTask.init();
if (false) {
Looper.myLooper().setMessageLogging(new
LogPrinter(Log.DEBUG, "ActivityThread"));
}
Looper.loop();
throw new RuntimeException("Main thread loop unexpectedly exited");
}
在其中有两句话可以证明handler在UI线程之所以不需要初始化looper的原因：
Looper.prepareMainLooper();
Looper.loop();
因为ActivityThread已经帮我们初始化过了。
在main函数中主要进行的是初始化的操作，初始化过程包括looper，运行环境，logger等一系列东东，在其中有个重要的方法 thread.attach(false)：
private void attach(boolean system) {
sCurrentActivityThread = this;
mSystemThread = system;
if (!system) {
ViewRootImpl.addFirstDrawHandler(new Runnable() {
@Override
public void run() {
//检查jit是否能用
ensureJitEnabled();
}
});
android.ddm.DdmHandleAppName.setAppName("<pre-initialized>",
UserHandle.myUserId());
RuntimeInit.setApplicationObject(mAppThread.asBinder());
//获得IActivityManager的一个实例，IActivityManager其实算是一个binder对象，负责跟底层沟通
final IActivityManager mgr = ActivityManagerNative.getDefault();
try {
mgr.attachApplication(mAppThread);
} catch (RemoteException ex) {
// Ignore
}
// 添加GC监察者
BinderInternal.addGcWatcher(new Runnable() {
@Override public void run() {
if (!mSomeActivitiesChanged) {
return;
}
Runtime runtime = Runtime.getRuntime();
long dalvikMax = runtime.maxMemory();
long dalvikUsed = runtime.totalMemory() - runtime.freeMemory();
if (dalvikUsed > ((3*dalvikMax)/4)) {
if (DEBUG_MEMORY_TRIM) Slog.d(TAG, "Dalvik max=" + (dalvikMax/1024)
+ " total=" + (runtime.totalMemory()/1024)
+ " used=" + (dalvikUsed/1024));
mSomeActivitiesChanged = false;
try {
mgr.releaseSomeActivities(mAppThread);
} catch (RemoteException e) {
}
}
}
});
} 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();
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);
}
}
// add dropbox logging to libcore
DropBox.setReporter(new DropBoxReporter());
ViewRootImpl.addConfigCallback(new ComponentCallbacks2() {
@Override
public void onConfigurationChanged(Configuration newConfig) {
synchronized (mResourcesManager) {
// We need to apply this change to the resources
// immediately, because upon returning the view
// hierarchy will be informed about it.
if (mResourcesManager.applyConfigurationToResourcesLocked(newConfig, null)) {
// This actually changed the resources! Tell
// everyone about it.
if (mPendingConfiguration == null ||
mPendingConfiguration.isOtherSeqNewer(newConfig)) {
mPendingConfiguration = newConfig;
sendMessage(H.CONFIGURATION_CHANGED, newConfig);
}
}
}
}
@Override
public void onLowMemory() {
}
@Override
public void onTrimMemory(int level) {
}
});
}
attach方法中会通过传入的布尔值来判断执行相关方法，其中在main方中传入false，因此对于当前情况进行研究：
ViewRootImpl.addFirstDrawHandler(new Runnable() {
@Override
public void run() {
ensureJitEnabled();
}
});
android.ddm.DdmHandleAppName.setAppName("<pre-initialized>",
UserHandle.myUserId());
RuntimeInit.setApplicationObject(mAppThread.asBinder());
final IActivityManager mgr = ActivityManagerNative.getDefault();
try {
mgr.attachApplication(mAppThread);
} catch (RemoteException ex) {
// Ignore
}
// Watch for getting close to heap limit.
BinderInternal.addGcWatcher(new Runnable() {
@Override public void run() {
if (!mSomeActivitiesChanged) {
return;
}
Runtime runtime = Runtime.getRuntime();
long dalvikMax = runtime.maxMemory();
long dalvikUsed = runtime.totalMemory() - runtime.freeMemory();
if (dalvikUsed > ((3*dalvikMax)/4)) {
if (DEBUG_MEMORY_TRIM) Slog.d(TAG, "Dalvik max=" + (dalvikMax/1024)
+ " total=" + (runtime.totalMemory()/1024)
+ " used=" + (dalvikUsed/1024));
mSomeActivitiesChanged = false;
try {
mgr.releaseSomeActivities(mAppThread);
} catch (RemoteException e) {
}
}
}
});
- 在代码中，首先调用ViewRootImpl的addFirstDrawHandler，推测是为了对view进行初始化操作而进行添加的一个处理的handler对象。
- IActivityManager的获取，通过ActivityManagerNative.getDefault()，在源码可以知道IActivityManager实现了Binder接口，Binder接口的作用就是为了跟底层进行交互。又将其通过mgr.attachApplication(mAppThread)关联到apllicationThread类中。
static public IActivityManager getDefault() {
return gDefault.get();
}
private static final Singleton<IActivityManager> gDefault = new Singleton<IActivityManager>() {
protected IActivityManager create() {
IBinder b = ServiceManager.getService("activity");
if (false) {
Log.v("ActivityManager", "default service binder = " + b);
}
IActivityManager am = asInterface(b);
if (false) {
Log.v("ActivityManager", "default service = " + am);
}
return am;
}
};
static public IActivityManager asInterface(IBinder obj) {
if (obj == null) {
return null;
}
IActivityManager in =
(IActivityManager)obj.queryLocalInterface(descriptor);
if (in != null) {
return in;
}
return new ActivityManagerProxy(obj);
}
- ActivityManagerNative.getDefault()获取到的是ActivityManagerNative内部的一个代理类 ActivityManagerProxy，这就是客户端跟底层交互的一个非常重要的类，ActivityThread通过attach获取到，然后将applciationThread将其关联，把activity相关信息存储在applciationThread里面，apllicationThread的类为activity的各种状态做了相对应的准备工作.
主要研究一下scheduleLaunchActivity方法：
public final void scheduleLaunchActivity(Intent intent, IBinder token, int ident,
ActivityInfo info, Configuration curConfig, CompatibilityInfo compatInfo,
IVoiceInteractor voiceInteractor, int procState, Bundle state,
PersistableBundle persistentState, List<ResultInfo> pendingResults,
List<Intent> pendingNewIntents, boolean notResumed, boolean isForward,
ProfilerInfo profilerInfo) {
updateProcessState(procState, false);
ActivityClientRecord r = new ActivityClientRecord();
r.token = token;
r.ident = ident;
r.intent = intent;
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;
updatePendingConfiguration(curConfig);
sendMessage(H.LAUNCH_ACTIVITY, r);
}
名字中更可以体会到，这是一个计划执行加载活动的方法，ActivityClientRecord用来记录一系列关于activity的相关变量的信息，并将ActivityClientRecord对象通过handler发送。在ActivityThread中有一个handler对象H,接收上述的apllicationThread发来的相关信息，并且执行对应的操作，主要看下handleMessage中对于加载Activity相对应的方法：
switch (msg.what) {
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);
Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
} break;
以下代码省略
当前handler(H)又调用了handleLaunchActivity来进行事件的处理操作，查看handleLaunchActivity方法：
private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent) {
unscheduleGcIdler();
mSomeActivitiesChanged = true;
if (r.profilerInfo != null) {
mProfiler.setProfiler(r.profilerInfo);
mProfiler.startProfiling();
}
Activity a = performLaunchActivity(r, customIntent);
if (a != null) {
r.createdConfig = new Configuration(mConfiguration);
Bundle oldState = r.state;
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) {
if (!mInstrumentation.onException(r.activity, e)) {
throw new RuntimeException(
"Unable to pause activity "
+ r.intent.getComponent().toShortString()
+ ": " + e.toString(), e);
}
}
r.paused = true;
}
} else {
// If there was an error, for any reason, tell the activity
// manager to stop us.
try {
ActivityManagerNative.getDefault()
.finishActivity(r.token, Activity.RESULT_CANCELED, null, false);
} catch (RemoteException ex) {
// Ignore
}
}
其中通过performLaunchActivity获取所需要的activity对象，查看performLaunchActivity中的主要代码：
Activity activity = null;
try {
java.lang.ClassLoader cl = r.packageInfo.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);
}
}
所对应的activity通过mInstrumentation来进行加载，关于mInstrumentation是一个Instrumentation类，这个类的作用是相当于ActivityThread与Actvity沟通的桥梁，对应的activity通过mInstrumentation.newActivity()方法来获取。newActivity中通过反射来实例化activity，最终返回这个对象，最后我们回溯到handleLaunchActivity。到此，attach方法中的 mgr.attachApplication(mAppThread)方法完成。其主要的操作就是将ApllicationThread绑定到ActvityManager当中。在ApllicationThread中定义了处理actvity周期的相关方法，并且通过H(handler)来进行消息的分发，最后调用Instrumentation方法进行对应activity的周期方法的调用操作。最终attach方法完成。
<h3>startActivity</h3>
一般对于启动一个Activity我们都采用startActivity(),以这个为入口我们看看其如何跟ActivityThread进行关联通信的。所有的startActivity()最终都会调用到如下的代码中：
public void startActivityForResult(Intent intent, int requestCode, @Nullable Bundle options) {
if (mParent == null) {
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;
}
final View decor = mWindow != null ? mWindow.peekDecorView() : null;
if (decor != null) {
decor.cancelPendingInputEvents();
}
// 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);
}
}
if (options != null && !isTopOfTask()) {
mActivityTransitionState.startExitOutTransition(this, options);
}
}
对于mParent变量的作用不是特别清楚，但是看到一个方法mInstrumentation.execStartActivity()方法来进行活动的启动，查看代码：
public ActivityResult execStartActivity(
Context who, IBinder contextThread, IBinder token, Activity target,
Intent intent, int requestCode, Bundle options) {
IApplicationThread whoThread = (IApplicationThread) contextThread;
if (mActivityMonitors != null) {
synchronized (mSync) {
final int N = mActivityMonitors.size();
for (int i=0; i<N; i++) {
final ActivityMonitor am = mActivityMonitors.get(i);
if (am.match(who, null, intent)) {
am.mHits++;
if (am.isBlocking()) {
return requestCode >= 0 ? am.getResult() : null;
}
break;
}
}
}
}
try {
intent.migrateExtraStreamToClipData();
intent.prepareToLeaveProcess();
int result = ActivityManagerNative.getDefault()
.startActivity(whoThread, who.getBasePackageName(), intent,
intent.resolveTypeIfNeeded(who.getContentResolver()),
token, target != null ? target.mEmbeddedID : null,
requestCode, 0, null, options);
checkStartActivityResult(result, intent);
} catch (RemoteException e) {
}
return null;
}
传进来的IBinder对象是一个ApplicationThread，在上述分析main方法的时候，可以了解到ApplicationThread储存了activity的相关启动过得信息，并且通过ActivityManagerNative.getDefault().startActivity()进行activity的启动，ActivityManagerNative又会交给ActivityManagerNative的内部类ActivityManagerProxy通过Binder机制来进行通知底层启动activity，然后回调返回一个 Instrumentation.ActivityResult。获取完 Instrumentation.ActivityResult后，如果他不为空，则交还给ActivityThread，调用其sendActivityResult()：
public final void sendActivityResult(
IBinder token, String id, int requestCode,
int resultCode, Intent data) {
if (DEBUG_RESULTS) Slog.v(TAG, "sendActivityResult: id=" + id
+ " req=" + requestCode + " res=" + resultCode + " data=" + data);
ArrayList<ResultInfo> list = new ArrayList<ResultInfo>();
list.add(new ResultInfo(id, requestCode, resultCode, data));
mAppThread.scheduleSendResult(token, list);
}
再调用mAppThread.scheduleSendResult(token, list),mAppthread是当时分析的ApplicationThread：
public final void scheduleSendResult(IBinder token, List<ResultInfo> results) {
ResultData res = new ResultData();
res.token = token;
res.results = results;
sendMessage(H.SEND_RESULT, res);
}
}
又通过handler对象进行传递消息，最终调用handleMessage中的 handleSendResult((ResultData)msg.obj)方法进行处理：
private void handleSendResult(ResultData res) {
ActivityClientRecord r = mActivities.get(res.token);
if (DEBUG_RESULTS) Slog.v(TAG, "Handling send result to " + r);
if (r != null) {
final boolean resumed = !r.paused;
if (!r.activity.mFinished && r.activity.mDecor != null
&& r.hideForNow && resumed) {
// We had hidden the activity because it started another
// one... we have gotten a result back and we are not
// paused, so make sure our window is visible.
updateVisibility(r, true);
}
if (resumed) {
try {
// Now we are idle.
r.activity.mCalled = false;
r.activity.mTemporaryPause = true;
mInstrumentation.callActivityOnPause(r.activity);
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) {
if (!mInstrumentation.onException(r.activity, e)) {
throw new RuntimeException(
"Unable to pause activity "
+ r.intent.getComponent().toShortString()
+ ": " + e.toString(), e);
}
}
}
deliverResults(r, res.results);
if (resumed) {
r.activity.performResume();
r.activity.mTemporaryPause = false;
}
}
}
最终完成一个activity的实例化操作，显示在前台页面当中。
附：
startActivity图解：
startActivity
