ActivityThread

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

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 libcore  
        EventLogger.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(new
                    LogPrinter(Log.DEBUG, "ActivityThread"));
        }

        Looper.loop();

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

4.2 家族图谱