1. 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。

1.1 Thread与SurfaceView

View组件由UI线程(主线程)所执行。如果需要迅速更新UI画面或UI画图需要较长时间，则需要使用SurfaceView。它可由后台线程(background thread)来执行，而View只能由UI(主)线程执行。SurfaceView内有高效的rendering机制，可以让后台线程快速刷新Surface的内容。

View ---> UI(主)线程

SurfaceView ---> 后台线程

2. Android应用程序主线程stack

在一个只有Activity派生类的应用程序中，它包含如下线程:

main线程stack如下：

  at android.os.MessageQueue.nativePollOnce(Native Method)	at android.os.MessageQueue.next(MessageQueue.java:118)	at android.os.Looper.loop(Looper.java:118)	at android.app.ActivityThread.main(ActivityThread.java:4424)	// Java main入口函数at java.lang.reflect.Method.invokeNative(Native Method)	at java.lang.reflect.Method.invoke(Method.java:511)	at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:784)	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:551)	at dalvik.system.NativeStart.main(Native Method)

JDWP线程stack如下：

  at org.apache.harmony.dalvik.ddmc.DdmVmInternal.getStackTraceById(Native Method)	at android.ddm.DdmHandleThread.handleSTKL(DdmHandleThread.java:131)	at android.ddm.DdmHandleThread.handleChunk(DdmHandleThread.java:77)	at org.apache.harmony.dalvik.ddmc.DdmServer.dispatch(DdmServer.java:171)	at dalvik.system.NativeStart.run(Native Method)

3. IApplicationThread关系图

4. ActivityThread类

ActivityThread类即代表Application主线程。

4.1 类中关键信息

/*** This manages the execution of the main thread in an* application process, scheduling and executing activities,* broadcasts, and other operations on it as the activity* manager requests.** {@hide}*/
public final class ActivityThread {static ContextImpl mSystemContext = null;static IPackageManager sPackageManager;// 创建ApplicationThread实例，以接收AMS指令并执行final ApplicationThread mAppThread = new ApplicationThread();final Looper mLooper = Looper.myLooper();final H mH = new H();final HashMap<IBinder, ActivityClientRecord> mActivities= new HashMap<IBinder, ActivityClientRecord>();// List of new activities (via ActivityRecord.nextIdle) that should// be reported when next we idle.ActivityClientRecord mNewActivities = null;// Number of activities that are currently visible on-screen.int mNumVisibleActivities = 0;final HashMap<IBinder, Service> mServices= new HashMap<IBinder, Service>();Application mInitialApplication;final ArrayList<Application> mAllApplications= new ArrayList<Application>();static final ThreadLocal<ActivityThread> sThreadLocal = new ThreadLocal<ActivityThread>();Instrumentation mInstrumentation;static Handler sMainThreadHandler;  // set once in main()static final class ActivityClientRecord {IBinder token;int ident;Intent intent;Bundle state;Activity activity;Window window;Activity parent;String embeddedID;Activity.NonConfigurationInstances lastNonConfigurationInstances;boolean paused;boolean stopped;boolean hideForNow;Configuration newConfig;Configuration createdConfig;ActivityClientRecord nextIdle;String profileFile;ParcelFileDescriptor profileFd;boolean autoStopProfiler;ActivityInfo activityInfo;CompatibilityInfo compatInfo;LoadedApk packageInfo; //包信息，通过调用ActivityThread.getPapckageInfo而获得List<ResultInfo> pendingResults;List<Intent> pendingIntents;boolean startsNotResumed;boolean isForward;int pendingConfigChanges;boolean onlyLocalRequest;View mPendingRemoveWindow;WindowManager mPendingRemoveWindowManager;...}private class ApplicationThread extends ApplicationThreadNative {private void updatePendingConfiguration(Configuration config) {synchronized (mPackages) {if (mPendingConfiguration == null ||mPendingConfiguration.isOtherSeqNewer(config)) {mPendingConfiguration = config;}}}public final void schedulePauseActivity(IBinder token, boolean finished,boolean userLeaving, int configChanges) {queueOrSendMessage(finished ? H.PAUSE_ACTIVITY_FINISHING : H.PAUSE_ACTIVITY,token,(userLeaving ? 1 : 0),configChanges);}// we use token to identify this activity without having to send the// activity itself back to the activity manager. (matters more with ipc)public final void scheduleLaunchActivity(Intent intent, IBinder token, int ident,ActivityInfo info, Configuration curConfig, CompatibilityInfo compatInfo,Bundle state, List<ResultInfo> pendingResults,List<Intent> pendingNewIntents, boolean notResumed, boolean isForward,String profileName, ParcelFileDescriptor profileFd, boolean autoStopProfiler) {ActivityClientRecord r = new ActivityClientRecord();r.token = token;r.ident = ident;r.intent = intent;r.activityInfo = info;r.compatInfo = compatInfo;r.state = state;r.pendingResults = pendingResults;r.pendingIntents = pendingNewIntents;r.startsNotResumed = notResumed;r.isForward = isForward;r.profileFile = profileName;r.profileFd = profileFd;r.autoStopProfiler = autoStopProfiler;updatePendingConfiguration(curConfig);queueOrSendMessage(H.LAUNCH_ACTIVITY, r);}...}private class H extends Handler {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");ActivityClientRecord r = (ActivityClientRecord)msg.obj;r.packageInfo = getPackageInfoNoCheck(r.activityInfo.applicationInfo, r.compatInfo);handleLaunchActivity(r, null);Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);} break;...}if (DEBUG_MESSAGES) Slog.v(TAG, "<<< done: " + codeToString(msg.what));}...}public static ActivityThread currentActivityThread() {return sThreadLocal.get();}public static void main(String[] args) {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());Process.setArgV0("<pre-initialized>");Looper.prepareMainLooper();// 创建ActivityThread实例ActivityThread thread = new ActivityThread();thread.attach(false);if (sMainThreadHandler == null) {sMainThreadHandler = thread.getHandler();}AsyncTask.init();if (false) {Looper.myLooper().setMessageLogging(newLogPrinter(Log.DEBUG, "ActivityThread"));}Looper.loop();throw new RuntimeException("Main thread loop unexpectedly exited");}
}