ActivityThread功能
它管理应用进程的主线程的执行(相当于普通Java程序的main入口函数)，并根据AMS的要求（通过IApplicationThread接口，AMS为Client、ActivityThread.ApplicationThread为Server）负责调度和执行activities、broadcasts和其它操作。
在Android系统中，在默认情况下，一个应用程序内的各个组件(如Activity、BroadcastReceiver、Service)都会在同一个进程(Process)里执行，且由此进程的【主线程】负责执行。
在Android系统中，如果有特别指定(通过android:process)，也可以让特定组件在不同的进程中运行。无论组件在哪一个进程中运行，默认情况下，他们都由此进程的【主线程】负责执行。
【主线程】既要处理Activity组件的UI事件，又要处理Service后台服务工作，通常会忙不过来。为了解决此问题，主线程可以创建多个子线程来处理后台服务工作，而本身专心处理UI画面的事件。
【主线程】的主要责任：
• 快速处理UI事件。而且只有它才处理UI事件， 其它线程还不能存取UI画面上的对象(如TextView等)，此时， 主线程就叫做UI线程。基本上，Android希望UI线程能根据用户的要求做出快速响应，如果UI线程花太多时间处理后台的工作，当UI事件发生时，让用户等待时间超过5秒而未处理，Android系统就会给用户显示ANR提示信息。
只有UI线程才能执行View派生类的onDraw()函数。
• 快速处理Broadcast消息。【主线程】除了处理UI事件之外，还要处理Broadcast消息。所以在BroadcastReceiver的onReceive()函数中，不宜占用太长的时间，否则导致【主线程】无法处理其它的Broadcast消息或UI事件。如果占用时间超过10秒， Android系统就会给用户显示ANR提示信息。
注意事项：
• 尽量避免让【主线程】执行耗时的操作，让它能快速处理UI事件和Broadcast消息。
• BroadcastReceiver的子类都是无状态的，即每次启动时，才会创建其对象，然后调用它的onReceive()函数，当执行完onReceive()函数时，就立即删除此对象。由于每次调用其函数时，会重新创建一个新的对象，所以对象里的属性值，是无法让各函数所共享。
一：线程通信、ActivityThread及Thread类是理解Android线程管理的关键。
线程，作为CPU调度资源的基本单位，在Android等针对嵌入式设备的操作系统中，有着非常重要和基础的作用。本小节主要从以下三个方面进行分析：
1.《Android线程管理――线程通信》
2.《Android线程管理――ActivityThread》
3.《Android线程管理――Thread类的内部原理、休眠及唤醒》
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二、ActivityThread的主要工作及实现机制
ActivityThread是Android应用的主线程（UI线程），说起ActivityThread，不得不提到Activity的创建、启动过程以及ActivityManagerService，但本文将仅从线程管理的角度来分析ActivityThread。ActivityManagerService、ActivityStack、ApplicationThread等会在后续文章中详细分析，敬请期待喔~~不过为了说清楚ActivityThread的由来，还是需要简单介绍下。
以下引用自罗升阳大师的博客：《Android应用程序的Activity启动过程简要介绍和学习计划》
Step 1. 无论是通过Launcher来启动Activity，还是通过Activity内部调用startActivity接口来启动新的Activity，都通过Binder进程间通信进入到ActivityManagerService进程中，并且调用ActivityManagerService.startActivity接口；
Step 2. ActivityManagerService调用ActivityStack.startActivityMayWait来做准备要启动的Activity的相关信息；
Step 3. ActivityStack通知ApplicationThread要进行Activity启动调度了，这里的ApplicationThread代表的是调用ActivityManagerService.startActivity接口的进程，对于通过点击应用程序图标的情景来说，这个进程就是Launcher了，而对于通过在Activity内部调用startActivity的情景来说，这个进程就是这个Activity所在的进程了；
Step 4. ApplicationThread不执行真正的启动操作，它通过调用ActivityManagerService.activityPaused接口进入到ActivityManagerService进程中，看看是否需要创建新的进程来启动Activity；
Step 5. 对于通过点击应用程序图标来启动Activity的情景来说，ActivityManagerService在这一步中，会调用startProcessLocked来创建一个新的进程，而对于通过在Activity内部调用startActivity来启动新的Activity来说，这一步是不需要执行的，因为新的Activity就在原来的Activity所在的进程中进行启动；
Step 6. ActivityManagerServic调用ApplicationThread.scheduleLaunchActivity接口，通知相应的进程执行启动Activity的操作；
Step 7. ApplicationThread把这个启动Activity的操作转发给ActivityThread，ActivityThread通过ClassLoader导入相应的Activity类，然后把它启动起来。
大师的这段描述把ActivityManagerService、ActivityStack、ApplicationThread及ActivityThread的调用关系讲的很清楚，本文将从ActivityThread的main()方法开始分析其主要工作及实现机制。
ActivityThread源码来自：https://github.com/android/platform_frameworks_base/blob/master/core/java/android/app/ActivityThread.java
public static void main(String[] args) {Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "ActivityThreadMain");SamplingProfilerIntegration.start();// CloseGuard defaults to true and can be quite spammy. We// disable it here, but selectively enable it later (via// StrictMode) on debug builds, but using DropBox, not logs.CloseGuard.setEnabled(false);Environment.initForCurrentUser();// Set the reporter for event logging in libcoreEventLogger.setReporter(new EventLoggingReporter());AndroidKeyStoreProvider.install();// Make sure TrustedCertificateStore looks in the right place for CA certificatesfinal 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();}if (false) {Looper.myLooper().setMessageLogging(newLogPrinter(Log.DEBUG, "ActivityThread"));}// End of event ActivityThreadMain.Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);Looper.loop();throw new RuntimeException("Main thread loop unexpectedly exited");}上述代码中，红色部分之前的代码主要用于环境初始化、AndroidKeyStoreProvider安装等，这里不做重点说明。红色部分的代码主要分为两个功能块：1）绑定应用进程到ActivityManagerService；2）主线程Handler消息处理。
关于线程通信机制，Handler、MessageQueue、Message及Looper四者的关系请参考上一篇文章《Android线程管理――线程通信》。
2.1 应用进程绑定
main()方法通过thread.attach(false)绑定应用进程。ActivityManagerNative通过getDefault()方法返回ActivityManagerService实例，ActivityManagerService通过attachApplication将ApplicationThread对象绑定到ActivityManagerService，而ApplicationThread作为Binder实现ActivityManagerService对应用进程的通信和控制。
private void attach(boolean system) {sCurrentActivityThread = this;mSystemThread = system;if (!system) {…… RuntimeInit.setApplicationObject(mAppThread.asBinder());final IActivityManager mgr = ActivityManagerNative.getDefault();try {mgr.attachApplication(mAppThread);} catch (RemoteException ex) {// Ignore}…… } else {……}}在ActivityManagerService内部，attachApplication实际是通过调用attachApplicationLocked实现的，这里采用了synchronized关键字保证同步。
@Overridepublic final void attachApplication(IApplicationThread thread) {synchronized (this) {int callingPid = Binder.getCallingPid();final long origId = Binder.clearCallingIdentity();attachApplicationLocked(thread, callingPid);Binder.restoreCallingIdentity(origId);}}attachApplicationLocked的实现较为复杂，其主要功能分为两部分：
thread.bindApplication
mStackSupervisor.attachApplicationLocked(app)
private final boolean attachApplicationLocked(IApplicationThread thread,int pid) {// Find the application record that is being attached... either via// the pid if we are running in multiple processes, or just pull the// next app record if we are emulating process with anonymous threads.ProcessRecord app;if (pid != MY_PID && pid >= 0) {synchronized (mPidsSelfLocked) {app = mPidsSelfLocked.get(pid);}} else {app = null;}// ……try {// ……thread.bindApplication(processName, appInfo, providers, app.instrumentationClass,profilerInfo, app.instrumentationArguments, app.instrumentationWatcher,app.instrumentationUiAutomationConnection, testMode, enableOpenGlTrace,enableTrackAllocation, 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;}}// ……}thread对象其实是ActivityThread里ApplicationThread对象在ActivityManagerService的代理对象，故此执行thread.bindApplication，最终会调用ApplicationThread的bindApplication方法。该bindApplication方法的实质是通过向ActivityThread的消息队列发送BIND_APPLICATION消息，消息的处理调用handleBindApplication方法，handleBindApplication方法比较重要的是会调用如下方法：
mInstrumentation.callApplicationOnCreate(app);
callApplicationOnCreate即调用应用程序Application的onCreate()方法，说明Application的onCreate()方法会比所有activity的onCreate()方法先调用。
mStackSupervisor为ActivityManagerService的成员变量，类型为ActivityStackSupervisor。
/** Run all ActivityStacks through this */
ActivityStackSupervisor mStackSupervisor;
从注释可以看出，mStackSupervisor为Activity堆栈管理辅助类实例。ActivityStackSupervisor的attachApplicationLocked()方法的调用了realStartActivityLocked()方法，在realStartActivityLocked()方法中，会调用scheduleLaunchActivity()方法：
final boolean realStartActivityLocked(ActivityRecord r,ProcessRecord app, boolean andResume, boolean checkConfig)throws RemoteException {//... try {//...app.thread.scheduleLaunchActivity(new Intent(r.intent), r.appToken,System.identityHashCode(r), r.info,new Configuration(mService.mConfiguration),r.compat, r.icicle, results, newIntents, !andResume,mService.isNextTransitionForward(), profileFile, profileFd,profileAutoStop);//...} catch (RemoteException e) {//...}//... return true;}app.thread也是ApplicationThread对象在ActivityManagerService的一个代理对象，最终会调用ApplicationThread的scheduleLaunchActivity方法。
// we use token to identify this activity without having to send the// activity itself back to the activity manager. (matters more with ipc)@Overridepublic 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();……sendMessage(H.LAUNCH_ACTIVITY, r);}同bindApplication()方法，最终是通过向ActivityThread的消息队列发送消息，在ActivityThread完成实际的LAUNCH_ACTIVITY的操作。
public void handleMessage(Message msg) {if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what));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;……}handleLaunchActivity()用于启动Activity。具体的启动流程不在这里详述了，这里重点说明ApplicationThread及ActivityThread的线程通信机制。
2.2 主线程消息处理
在《Android线程管理――线程通信》中谈到了普通线程中Handler、MessageQueue、Message及Looper四者的关系，那么，ActivityThread中的线程通信又有什么不同呢？不同之处主要表现为两点：1）Looper的初始化方式；2）Handler生成。
首先，ActivityThread通过Looper.prepareMainLooper()初始化Looper，为了直观比较ActivityThread与普通线程初始化Looper的区别，把两种初始化方法放在一起：
/** Initialize the current thread as a looper.* This gives you a chance to create handlers that then reference* this looper, before actually starting the loop. Be sure to call* {@link #loop()} after calling this method, and end it by calling* {@link #quit()}.*/public static void prepare() {prepare(true);}private static void prepare(boolean quitAllowed) {if (sThreadLocal.get() != null) {throw new RuntimeException("Only one Looper may be created per thread");}sThreadLocal.set(new Looper(quitAllowed));}/*** Initialize the current thread as a looper, marking it as an* application's main looper. The main looper for your application* is created by the Android environment, so you should never need* to call this function yourself. See also: {@link #prepare()}*/public static void prepareMainLooper() {prepare(false);synchronized (Looper.class) {if (sMainLooper != null) {throw new IllegalStateException("The main Looper has already been prepared.");}sMainLooper = myLooper();}}•普通线程的prepare()方法默认quitAllowed参数为true，表示允许退出，ActivityThread在prepareMainLooper()方法中调用prepare()方法，参数为false，表示主线程不允许退出。
•普通线程只调用prepare()方法，ActivityThread在调用完prepare()方法之后，会通过myLooper()方法将本地线程<ThreadLocal>的Looper对象的引用交给sMainLooper。myLooper()其实就是调用sThreadLocal的get()方法实现的。
/*** Return the Looper object associated with the current thread. Returns* null if the calling thread is not associated with a Looper.*/public static Looper myLooper() {return sThreadLocal.get();}•之所以要通过sMainLooper指向ActivityThread的Looper对象，就是希望通过getMainLooper()方法将主线程的Looper对象开放给其他线程。
/** Returns the application's main looper, which lives in the main thread of the application.*/public static Looper getMainLooper() {synchronized (Looper.class) {return sMainLooper;}}其次，ActivityThread与普通线程的Handler生成方式也不一样。普通线程生成一个与Looper绑定的Handler即可，ActivityThread通过sMainThreadHandler指向getHandler()的返回值，而getHandler()方法返回的其实是一个继承Handler的H对象。。
private class H extends Handler {……}final H mH = new H();final Handler getHandler() {return mH;}真正实现消息机制“通”信的其实是Looper的loop()方法，loop()方法的核心实现如下：
/*** Run the message queue in this thread. Be sure to call* {@link #quit()} to end the loop.*/public static void loop() {final Looper me = myLooper();if (me == null) {throw new RuntimeException("No Looper; Looper.prepare() wasn't called on this thread.");}final MessageQueue queue = me.mQueue;// Make sure the identity of this thread is that of the local process,// and keep track of what that identity token actually is.Binder.clearCallingIdentity();final long ident = Binder.clearCallingIdentity();for (;;) {Message msg = queue.next(); // might blockif (msg == null) {// No message indicates that the message queue is quitting.return;}// This must be in a local variable, in case a UI event sets the loggerPrinter logging = me.mLogging;if (logging != null) {logging.println(">>>>> Dispatching to " + msg.target + " " +msg.callback + ": " + msg.what);}msg.target.dispatchMessage(msg);if (logging != null) {logging.println("<<<<< Finished to " + msg.target + " " + msg.callback);}// Make sure that during the course of dispatching the// identity of the thread wasn't corrupted.final long newIdent = Binder.clearCallingIdentity();if (ident != newIdent) {Log.wtf(TAG, "Thread identity changed from 0x"+ Long.toHexString(ident) + " to 0x"+ Long.toHexString(newIdent) + " while dispatching to "+ msg.target.getClass().getName() + " "+ msg.callback + " what=" + msg.what);}msg.recycle();}}大致流程如下：
•首先通过上述myLooper()方法获取Looper对象，取出Looper持有的MessageQueue；
•然后从MessageQueue取出Message，如果Message为null，说明线程正在退出；
•Message不为空，则调用Message的target handler对该Message进行分发，具体分发、处理流程可参考《Android线程管理――线程通信》；
•消息处理完毕，调用recycle()方法进行回收。
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