Activity启动流程

首先我们看看Activity的启动流程，Activity的启动流程是

public void startActivity(Intent intent, @Nullable Bundle options) {
        if (options != null) {
            startActivityForResult(intent, -1, options);
        } else {
            // Note we want to go through this call for compatibility with
            // applications that may have overridden the method.
            startActivityForResult(intent, -1);
        }
    }

转移到startActivityForResult

public void startActivityForResult(@RequiresPermission 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());
            } 
        } 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);
            }
        }
    }

可以看到启动是通过Instrumentation的execStartActivity方法实现的。观察Instrumentation类

public ActivityResult execStartActivity(
            Context who, IBinder contextThread, IBinder token, Activity target,
            Intent intent, int requestCode, Bundle options) {
        IApplicationThread whoThread = (IApplicationThread) contextThread;
        Uri referrer = target != null ? target.onProvideReferrer() : null;
        if (referrer != null) {
            intent.putExtra(Intent.EXTRA_REFERRER, referrer);
        }
        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(who);
            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) {
            throw new RuntimeException("Failure from system", e);
        }
        return null;
    }

可以看到Activity的真正启动是通过ActivityManagerNative.getDefault()的startActivity方法来实现的。ActivityManager是一个抽象类，public abstract class ActivityManagerNative extends Binder implements IActivityManager，它继承自Binder并实现了IActivityMnaager接口。ActivityManagerService继承自ActivityManagerNative,所以AMS也是一个Binder,它是IActivityManager的具体实现。由于ActivityManagerNative.getDefault()其实就是一个IActivityManager类型的Binder对象，因此它的具体实现是AMS。

ActivityManagerNative.getDefault()的方法是下面的，它返回一个IActivityManager对象就是AMS

/**
     * Retrieve the system's default/global activity manager.
     */
    static public IActivityManager getDefault() {
        return gDefault.get();
    }

 private static final Singleton<IActivityManager> gDefault = new Singleton<IActivityManager>() {
//初始化AMS这个Binder对象
        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;
        }
    };

Singleton是一个单例类，第一次调用它的get方法时，会通过create方法来初始化AMS这个Binder对象。

public abstract class Singleton<T> {
    private T mInstance;

    protected abstract T create();

    public final T get() {
        synchronized (this) {
            if (mInstance == null) {
//可以看到调用get的时候确实是调用create方法去初始化AMS
                mInstance = create();
            }
            return mInstance;
        }
    }
}

从这边看知道Activity的启动由ActivityManagerNative.getDefault()完成，而getDefault返回放入是IAcvityManager实例实际上就是AMS这个Binder，因此分析启动流程就要去看AMS下的startActivity。最终会调用AMS下的

public final int startActivityAsUser(IApplicationThread caller, String callingPackage,
            Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode,
            int startFlags, ProfilerInfo profilerInfo, Bundle bOptions, int userId) {
        enforceNotIsolatedCaller("startActivity");
        userId = mUserController.handleIncomingUser(Binder.getCallingPid(), Binder.getCallingUid(),
                userId, false, ALLOW_FULL_ONLY, "startActivity", null);
        // TODO: Switch to user app stacks here.
        return mActivityStarter.startActivityMayWait(caller, -1, callingPackage, intent,
                resolvedType, null, null, resultTo, resultWho, requestCode, startFlags,
                profilerInfo, null, null, bOptions, false, userId, null, null);
    }

因此又转移到mActivityStarter的startActivityMayWait，在startActivityMayWait方法中会执行：

final ActivityRecord[] outRecord = new ActivityRecord[1];
            int res = startActivityLocked(caller, intent, ephemeralIntent, resolvedType,
                    aInfo, rInfo, voiceSession, voiceInteractor,
                    resultTo, resultWho, requestCode, callingPid,
                    callingUid, callingPackage, realCallingPid, realCallingUid, startFlags,
                    options, ignoreTargetSecurity, componentSpecified, outRecord, container,
                    inTask);

最终会去调用ActivityStarter的startActivityLocked方法，一路追溯将会到ActivitySTACKsUPERVISOR的realStartActivityLocked方法中，根据名字就可以知道这个才是真正startActivity的地方，里面有一段代码很重要：

app.thread.scheduleLaunchActivity(new Intent(r.intent), r.appToken,
                    System.identityHashCode(r), r.info, new Configuration(mService.mConfiguration),
                    new Configuration(task.mOverrideConfig), r.compat, r.launchedFromPackage,
                    task.voiceInteractor, app.repProcState, r.icicle, r.persistentState, results,
                    newIntents, !andResume, mService.isNextTransitionForward(), profilerInfo);

不用想了就是它完成了startActivity的实现。app.thread是什么东西？他是一个IApplicationThread实例，IApplicationThread是一个接口，它的实现是ApplicationThreadNative，public abstract class ApplicationThreadNative extends Binder
implements IApplicationThread ，但是这货是一个抽象类没法实例化对象，无奈去找其子类，找到了ActivityThread类中的ApplicationThread内部类实现了抽象类ApplicationThreadNative。
首先看看IApplication接口来看看它到底能够干什么？

public interface IApplicationThread extends IInterface {
    void schedulePauseActivity(IBinder token, boolean finished, boolean userLeaving,
            int configChanges, boolean dontReport) throws RemoteException;
    void scheduleStopActivity(IBinder token, boolean showWindow,
            int configChanges) throws RemoteException;
    void scheduleWindowVisibility(IBinder token, boolean showWindow) throws RemoteException;
    void scheduleSleeping(IBinder token, boolean sleeping) throws RemoteException;
    void scheduleResumeActivity(IBinder token, int procState, boolean isForward, Bundle resumeArgs)
            throws RemoteException;
    void scheduleSendResult(IBinder token, List<ResultInfo> results) throws RemoteException;
    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) throws RemoteException;
    void scheduleRelaunchActivity(IBinder token, List<ResultInfo> pendingResults,
            List<ReferrerIntent> pendingNewIntents, int configChanges, boolean notResumed,
            Configuration config, Configuration overrideConfig, boolean preserveWindow)
            throws RemoteException;
    void scheduleNewIntent(List<ReferrerIntent> intent, IBinder token) throws RemoteException;
    void scheduleDestroyActivity(IBinder token, boolean finished,
            int configChanges) throws RemoteException;
    void scheduleReceiver(Intent intent, ActivityInfo info, CompatibilityInfo compatInfo,
            int resultCode, String data, Bundle extras, boolean sync,
            int sendingUser, int processState) throws RemoteException;
    static final int BACKUP_MODE_INCREMENTAL = 0;
    static final int BACKUP_MODE_FULL = 1;
    static final int BACKUP_MODE_RESTORE = 2;
    static final int BACKUP_MODE_RESTORE_FULL = 3;
    void scheduleCreateBackupAgent(ApplicationInfo app, CompatibilityInfo compatInfo,
            int backupMode) throws RemoteException;
    void scheduleDestroyBackupAgent(ApplicationInfo app, CompatibilityInfo compatInfo)
            throws RemoteException;
    void scheduleCreateService(IBinder token, ServiceInfo info,
            CompatibilityInfo compatInfo, int processState) throws RemoteException;
    void scheduleBindService(IBinder token,
            Intent intent, boolean rebind, int processState) throws RemoteException;
    void scheduleUnbindService(IBinder token,
            Intent intent) throws RemoteException;
    void scheduleServiceArgs(IBinder token, boolean taskRemoved, int startId,
            int flags, Intent args) throws RemoteException;
    void scheduleStopService(IBinder token) throws RemoteException;
....

可以知道基本上Activity和Service的启动停止都在这里。所以其实现子类ApplicationThread实现了这些接口方法的具体实现。
因此ApplicationThread就是IApplicationThread的最终实现者。绕来绕去最终还是需要去ApplicationThread中去完成Activity的启动流程。来看看ApplicationThread中的启动Activity实现：

@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);
        }

ActivityClientRecord是记录Activity相关信息的。最终会调用sendMessage方法发送给Handler
看看sendMessage实现：

 private void sendMessage(int what, Object obj) {
        sendMessage(what, obj, 0, 0, false);
    }
private void sendMessage(int what, Object obj, int arg1, int arg2, boolean async) {
        if (DEBUG_MESSAGES) Slog.v(
            TAG, "SCHEDULE " + what + " " + mH.codeToString(what)
            + ": " + arg1 + " / " + obj);
        Message msg = Message.obtain();
        msg.what = what;
        msg.obj = obj;
        msg.arg1 = arg1;
        msg.arg2 = arg2;
        if (async) {
            msg.setAsynchronous(true);
        }
        mH.sendMessage(msg);
    }

上面代码里面有一个mH，它其实是一个Handler，final H mH = new H();看看H类：H extends Handler

private class H extends Handler {
        public static final int LAUNCH_ACTIVITY         = 100;
        public static final int PAUSE_ACTIVITY          = 101;
        public static final int PAUSE_ACTIVITY_FINISHING= 102;
        public static final int STOP_ACTIVITY_SHOW      = 103;
        public static final int STOP_ACTIVITY_HIDE      = 104;
        public static final int SHOW_WINDOW             = 105;
        public static final int HIDE_WINDOW             = 106;
        public static final int RESUME_ACTIVITY         = 107;
        public static final int SEND_RESULT             = 108;
        public static final int DESTROY_ACTIVITY        = 109;
        public static final int BIND_APPLICATION        = 110;
        public static final int EXIT_APPLICATION        = 111;
        public static final int NEW_INTENT              = 112;
        public static final int RECEIVER                = 113;
        public static final int CREATE_SERVICE          = 114;
        public static final int SERVICE_ARGS            = 115;
        public static final int STOP_SERVICE            = 116;

        public static final int CONFIGURATION_CHANGED   = 118;
        public static final int CLEAN_UP_CONTEXT        = 119;
        public static final int GC_WHEN_IDLE            = 120;
        public static final int BIND_SERVICE            = 121;
        public static final int UNBIND_SERVICE          = 122;
        public static final int DUMP_SERVICE            = 123;
        public static final int LOW_MEMORY              = 124;
        public static final int ACTIVITY_CONFIGURATION_CHANGED = 125;
        public static final int RELAUNCH_ACTIVITY       = 126;
        public static final int PROFILER_CONTROL        = 127;
        public static final int CREATE_BACKUP_AGENT     = 128;
        public static final int DESTROY_BACKUP_AGENT    = 129;
        public static final int SUICIDE                 = 130;
        public static final int REMOVE_PROVIDER         = 131;
        public static final int ENABLE_JIT              = 132;
        public static final int DISPATCH_PACKAGE_BROADCAST = 133;
        public static final int SCHEDULE_CRASH          = 134;
        public static final int DUMP_HEAP               = 135;
        public static final int DUMP_ACTIVITY           = 136;
        public static final int SLEEPING                = 137;
        public static final int SET_CORE_SETTINGS       = 138;
        public static final int UPDATE_PACKAGE_COMPATIBILITY_INFO = 139;
        public static final int TRIM_MEMORY             = 140;
        public static final int DUMP_PROVIDER           = 141;
        public static final int UNSTABLE_PROVIDER_DIED  = 142;
        public static final int REQUEST_ASSIST_CONTEXT_EXTRAS = 143;
        public static final int TRANSLUCENT_CONVERSION_COMPLETE = 144;
        public static final int INSTALL_PROVIDER        = 145;
        public static final int ON_NEW_ACTIVITY_OPTIONS = 146;
        public static final int CANCEL_VISIBLE_BEHIND = 147;
        public static final int BACKGROUND_VISIBLE_BEHIND_CHANGED = 148;
        public static final int ENTER_ANIMATION_COMPLETE = 149;
        public static final int START_BINDER_TRACKING = 150;
        public static final int STOP_BINDER_TRACKING_AND_DUMP = 151;
        public static final int MULTI_WINDOW_MODE_CHANGED = 152;
        public static final int PICTURE_IN_PICTURE_MODE_CHANGED = 153;
        public static final int LOCAL_VOICE_INTERACTION_STARTED = 154;

        String codeToString(int code) {
            if (DEBUG_MESSAGES) {
                switch (code) {
                    case LAUNCH_ACTIVITY: return "LAUNCH_ACTIVITY";
                    case PAUSE_ACTIVITY: return "PAUSE_ACTIVITY";
                    case PAUSE_ACTIVITY_FINISHING: return "PAUSE_ACTIVITY_FINISHING";
                    case STOP_ACTIVITY_SHOW: return "STOP_ACTIVITY_SHOW";
                    case STOP_ACTIVITY_HIDE: return "STOP_ACTIVITY_HIDE";
                    case SHOW_WINDOW: return "SHOW_WINDOW";
                    case HIDE_WINDOW: return "HIDE_WINDOW";
                    case RESUME_ACTIVITY: return "RESUME_ACTIVITY";
                    case SEND_RESULT: return "SEND_RESULT";
                    case DESTROY_ACTIVITY: return "DESTROY_ACTIVITY";
                    case BIND_APPLICATION: return "BIND_APPLICATION";
                    case EXIT_APPLICATION: return "EXIT_APPLICATION";
                    case NEW_INTENT: return "NEW_INTENT";
                    case RECEIVER: return "RECEIVER";
                    case CREATE_SERVICE: return "CREATE_SERVICE";
                    case SERVICE_ARGS: return "SERVICE_ARGS";
                    case STOP_SERVICE: return "STOP_SERVICE";
                    case CONFIGURATION_CHANGED: return "CONFIGURATION_CHANGED";
                    case CLEAN_UP_CONTEXT: return "CLEAN_UP_CONTEXT";
                    case GC_WHEN_IDLE: return "GC_WHEN_IDLE";
                    case BIND_SERVICE: return "BIND_SERVICE";
                    case UNBIND_SERVICE: return "UNBIND_SERVICE";
                    case DUMP_SERVICE: return "DUMP_SERVICE";
                    case LOW_MEMORY: return "LOW_MEMORY";
                    case ACTIVITY_CONFIGURATION_CHANGED: return "ACTIVITY_CONFIGURATION_CHANGED";
                    case RELAUNCH_ACTIVITY: return "RELAUNCH_ACTIVITY";
                    case PROFILER_CONTROL: return "PROFILER_CONTROL";
                    case CREATE_BACKUP_AGENT: return "CREATE_BACKUP_AGENT";
                    case DESTROY_BACKUP_AGENT: return "DESTROY_BACKUP_AGENT";
                    case SUICIDE: return "SUICIDE";
                    case REMOVE_PROVIDER: return "REMOVE_PROVIDER";
                    case ENABLE_JIT: return "ENABLE_JIT";
                    case DISPATCH_PACKAGE_BROADCAST: return "DISPATCH_PACKAGE_BROADCAST";
                    case SCHEDULE_CRASH: return "SCHEDULE_CRASH";
                    case DUMP_HEAP: return "DUMP_HEAP";
                    case DUMP_ACTIVITY: return "DUMP_ACTIVITY";
                    case SLEEPING: return "SLEEPING";
                    case SET_CORE_SETTINGS: return "SET_CORE_SETTINGS";
                    case UPDATE_PACKAGE_COMPATIBILITY_INFO: return "UPDATE_PACKAGE_COMPATIBILITY_INFO";
                    case TRIM_MEMORY: return "TRIM_MEMORY";
                    case DUMP_PROVIDER: return "DUMP_PROVIDER";
                    case UNSTABLE_PROVIDER_DIED: return "UNSTABLE_PROVIDER_DIED";
                    case REQUEST_ASSIST_CONTEXT_EXTRAS: return "REQUEST_ASSIST_CONTEXT_EXTRAS";
                    case TRANSLUCENT_CONVERSION_COMPLETE: return "TRANSLUCENT_CONVERSION_COMPLETE";
                    case INSTALL_PROVIDER: return "INSTALL_PROVIDER";
                    case ON_NEW_ACTIVITY_OPTIONS: return "ON_NEW_ACTIVITY_OPTIONS";
                    case CANCEL_VISIBLE_BEHIND: return "CANCEL_VISIBLE_BEHIND";
                    case BACKGROUND_VISIBLE_BEHIND_CHANGED: return "BACKGROUND_VISIBLE_BEHIND_CHANGED";
                    case ENTER_ANIMATION_COMPLETE: return "ENTER_ANIMATION_COMPLETE";
                    case MULTI_WINDOW_MODE_CHANGED: return "MULTI_WINDOW_MODE_CHANGED";
                    case PICTURE_IN_PICTURE_MODE_CHANGED: return "PICTURE_IN_PICTURE_MODE_CHANGED";
                    case LOCAL_VOICE_INTERACTION_STARTED: return "LOCAL_VOICE_INTERACTION_STARTED";
                }
            }
            return Integer.toString(code);
        }
        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, "LAUNCH_ACTIVITY");
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                } break;
                case RELAUNCH_ACTIVITY: {
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityRestart");
                    ActivityClientRecord r = (ActivityClientRecord)msg.obj;
                    handleRelaunchActivity(r);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                } break;
                case PAUSE_ACTIVITY: {
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityPause");
                    SomeArgs args = (SomeArgs) msg.obj;
                    handlePauseActivity((IBinder) args.arg1, false,
                            (args.argi1 & USER_LEAVING) != 0, args.argi2,
                            (args.argi1 & DONT_REPORT) != 0, args.argi3);
                    maybeSnapshot();
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                } break;
                case PAUSE_ACTIVITY_FINISHING: {
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityPause");
                    SomeArgs args = (SomeArgs) msg.obj;
                    handlePauseActivity((IBinder) args.arg1, true, (args.argi1 & USER_LEAVING) != 0,
                            args.argi2, (args.argi1 & DONT_REPORT) != 0, args.argi3);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                } break;
                case STOP_ACTIVITY_SHOW: {
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityStop");
                    SomeArgs args = (SomeArgs) msg.obj;
                    handleStopActivity((IBinder) args.arg1, true, args.argi2, args.argi3);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                } break;
                case STOP_ACTIVITY_HIDE: {
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityStop");
                    SomeArgs args = (SomeArgs) msg.obj;
                    handleStopActivity((IBinder) args.arg1, false, args.argi2, args.argi3);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                } break;
                case SHOW_WINDOW:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityShowWindow");
                    handleWindowVisibility((IBinder)msg.obj, true);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case HIDE_WINDOW:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityHideWindow");
                    handleWindowVisibility((IBinder)msg.obj, false);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case RESUME_ACTIVITY:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityResume");
                    SomeArgs args = (SomeArgs) msg.obj;
                    handleResumeActivity((IBinder) args.arg1, true, args.argi1 != 0, true,
                            args.argi3, "RESUME_ACTIVITY");
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case SEND_RESULT:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityDeliverResult");
                    handleSendResult((ResultData)msg.obj);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case DESTROY_ACTIVITY:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityDestroy");
                    handleDestroyActivity((IBinder)msg.obj, msg.arg1 != 0,
                            msg.arg2, false);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case BIND_APPLICATION:
.....

因此可以知道LaunchActivity的代码是这部分：

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;

进入handleLaunchActivity方法去观察：

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);

        if (a != null) {
            r.createdConfig = new Configuration(mConfiguration);
            reportSizeConfigurations(r);
            Bundle oldState = r.state;
            handleResumeActivity(r.token, false, r.isForward,
                    !r.activity.mFinished && !r.startsNotResumed, r.lastProcessedSeq, reason);

            if (!r.activity.mFinished && r.startsNotResumed) {
                // The activity manager actually wants this one to start out paused, because it
                // needs to be visible but isn't in the foreground. We accomplish this by going
                // through the normal startup (because activities expect to go through onResume()
                // the first time they run, before their window is displayed), and then pausing it.
                // However, in this case we do -not- need to do the full pause cycle (of freezing
                // and such) because the activity manager assumes it can just retain the current
                // state it has.
                performPauseActivityIfNeeded(r, reason);

                // We need to keep around the original state, in case we need to be created again.
                // But we only do this for pre-Honeycomb apps, which always save their state when
                // pausing, so we can not have them save their state when restarting from a paused
                // state. For HC and later, we want to (and can) let the state be saved as the
                // normal part of stopping the activity.
                if (r.isPreHoneycomb()) {
                    r.state = oldState;
                }
            }
        } 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,
                            Activity.DONT_FINISH_TASK_WITH_ACTIVITY);
            } catch (RemoteException ex) {
                throw ex.rethrowFromSystemServer();
            }
        }
    }

可以知道最终是到Activity a = performLaunchActivity(r, customIntent);这里面进行启动，看看performLaunchActivity：

 private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
        // System.out.println("##### [" + System.currentTimeMillis() + "] ActivityThread.performLaunchActivity(" + r + ")");

        ActivityInfo aInfo = r.activityInfo;
        if (r.packageInfo == null) {
            r.packageInfo = getPackageInfo(aInfo.applicationInfo, r.compatInfo,
                    Context.CONTEXT_INCLUDE_CODE);
        }

        ComponentName component = r.intent.getComponent();
        if (component == null) {
            component = r.intent.resolveActivity(
                mInitialApplication.getPackageManager());
            r.intent.setComponent(component);
        }

        if (r.activityInfo.targetActivity != null) {
            component = new ComponentName(r.activityInfo.packageName,
                    r.activityInfo.targetActivity);
        }

        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);
            }
        }

        try {
            Application app = r.packageInfo.makeApplication(false, mInstrumentation);

            if (localLOGV) Slog.v(TAG, "Performing launch of " + r);
            if (localLOGV) Slog.v(
                    TAG, r + ": app=" + app
                    + ", appName=" + app.getPackageName()
                    + ", pkg=" + r.packageInfo.getPackageName()
                    + ", comp=" + r.intent.getComponent().toShortString()
                    + ", dir=" + r.packageInfo.getAppDir());

            if (activity != null) {
                Context appContext = createBaseContextForActivity(r, activity);
                CharSequence title = r.activityInfo.loadLabel(appContext.getPackageManager());
                Configuration config = new Configuration(mCompatConfiguration);
                if (r.overrideConfig != null) {
                    config.updateFrom(r.overrideConfig);
                }
                if (DEBUG_CONFIGURATION) Slog.v(TAG, "Launching activity "
                        + r.activityInfo.name + " with config " + config);
                Window window = null;
                if (r.mPendingRemoveWindow != null && r.mPreserveWindow) {
                    window = r.mPendingRemoveWindow;
                    r.mPendingRemoveWindow = null;
                    r.mPendingRemoveWindowManager = null;
                }
                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);

                if (customIntent != null) {
                    activity.mIntent = customIntent;
                }
                r.lastNonConfigurationInstances = null;
                activity.mStartedActivity = false;
                int theme = r.activityInfo.getThemeResource();
                if (theme != 0) {
                    activity.setTheme(theme);
                }

                activity.mCalled = false;
                if (r.isPersistable()) {
                    mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
                } else {
                    mInstrumentation.callActivityOnCreate(activity, r.state);
                }
                if (!activity.mCalled) {
                    throw new SuperNotCalledException(
                        "Activity " + r.intent.getComponent().toShortString() +
                        " did not call through to super.onCreate()");
                }
                r.activity = activity;
                r.stopped = true;
                if (!r.activity.mFinished) {
                    activity.performStart();
                    r.stopped = false;
                }
                if (!r.activity.mFinished) {
                    if (r.isPersistable()) {
                        if (r.state != null || r.persistentState != null) {
                            mInstrumentation.callActivityOnRestoreInstanceState(activity, r.state,
                                    r.persistentState);
                        }
                    } else if (r.state != null) {
                        mInstrumentation.callActivityOnRestoreInstanceState(activity, r.state);
                    }
                }
                if (!r.activity.mFinished) {
                    activity.mCalled = false;
                    if (r.isPersistable()) {
                        mInstrumentation.callActivityOnPostCreate(activity, r.state,
                                r.persistentState);
                    } else {
                        mInstrumentation.callActivityOnPostCreate(activity, r.state);
                    }
                    if (!activity.mCalled) {
                        throw new SuperNotCalledException(
                            "Activity " + r.intent.getComponent().toShortString() +
                            " did not call through to super.onPostCreate()");
                    }
                }
            }
            r.paused = true;

            mActivities.put(r.token, r);

        } 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;
    }

可以看到performLaunchActivity主要完成了如下几件事情：
1，从ActivityClientRecord中获取待启动的Activity的组件信息

 ActivityInfo aInfo = r.activityInfo;
        if (r.packageInfo == null) {
            r.packageInfo = getPackageInfo(aInfo.applicationInfo, r.compatInfo,
                    Context.CONTEXT_INCLUDE_CODE);
        }

        ComponentName component = r.intent.getComponent();
        if (component == null) {
            component = r.intent.resolveActivity(
                mInitialApplication.getPackageManager());
            r.intent.setComponent(component);
        }

        if (r.activityInfo.targetActivity != null) {
            component = new ComponentName(r.activityInfo.packageName,
                    r.activityInfo.targetActivity);
        }

2,通过Instrumentation的newActivity方法使用类加载器创建Activity对象

java.lang.ClassLoader cl = r.packageInfo.getClassLoader();
            activity = mInstrumentation.newActivity(
                    cl, component.getClassName(), r.intent);

newActivity的代码是：

public Activity newActivity(ClassLoader cl, String className,
            Intent intent)
            throws InstantiationException, IllegalAccessException,
            ClassNotFoundException {
        return (Activity)cl.loadClass(className).newInstance();
    }

就是直接通过类加载器来创建Activity对象
3, 创建Application对象

Application app = r.packageInfo.makeApplication(false, mInstrumentation);

看看makeApplication方法：

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(mActivityThread)
                .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;
    }

4,创建ContextImpl对象并通过Activity的attach方法来完成一些重要数据的初始化

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);

在attch方法中完成了Window的创建并建立了自己和Window之间的关联。
看看attch的方法实现：

final void attach(Context context, ActivityThread aThread,
            Instrumentation instr, IBinder token, int ident,
            Application application, Intent intent, ActivityInfo info,
            CharSequence title, Activity parent, String id,
            NonConfigurationInstances lastNonConfigurationInstances,
            Configuration config, String referrer, IVoiceInteractor voiceInteractor,
            Window window) {
        attachBaseContext(context);

        mFragments.attachHost(null /*parent*/);

        mWindow = new PhoneWindow(this, window);
        mWindow.setWindowControllerCallback(this);
        mWindow.setCallback(this);
        mWindow.setOnWindowDismissedCallback(this);
        mWindow.getLayoutInflater().setPrivateFactory(this);
        if (info.softInputMode != WindowManager.LayoutParams.SOFT_INPUT_STATE_UNSPECIFIED) {
            mWindow.setSoftInputMode(info.softInputMode);
        }
        if (info.uiOptions != 0) {
            mWindow.setUiOptions(info.uiOptions);
        }
        mUiThread = Thread.currentThread();

        mMainThread = aThread;
        mInstrumentation = instr;
        mToken = token;
        mIdent = ident;
        mApplication = application;
        mIntent = intent;
        mReferrer = referrer;
        mComponent = intent.getComponent();
        mActivityInfo = info;
        mTitle = title;
        mParent = parent;
        mEmbeddedID = id;
        mLastNonConfigurationInstances = lastNonConfigurationInstances;
        if (voiceInteractor != null) {
            if (lastNonConfigurationInstances != null) {
                mVoiceInteractor = lastNonConfigurationInstances.voiceInteractor;
            } else {
                mVoiceInteractor = new VoiceInteractor(voiceInteractor, this, this,
                        Looper.myLooper());
            }
        }

        mWindow.setWindowManager(
                (WindowManager)context.getSystemService(Context.WINDOW_SERVICE),
                mToken, mComponent.flattenToString(),
                (info.flags & ActivityInfo.FLAG_HARDWARE_ACCELERATED) != 0);
        if (mParent != null) {
            mWindow.setContainer(mParent.getWindow());
        }
        mWindowManager = mWindow.getWindowManager();
        mCurrentConfig = config;
    }

通过new PhoneWindow()创建Window对象，mWindow.setWindowManager建立与WindowManager的关联，看看setWindowManager

public void setWindowManager(WindowManager wm, IBinder appToken, String appName,
            boolean hardwareAccelerated) {
        mAppToken = appToken;
        mAppName = appName;
        mHardwareAccelerated = hardwareAccelerated
                || SystemProperties.getBoolean(PROPERTY_HARDWARE_UI, false);
        if (wm == null) {
            wm = (WindowManager)mContext.getSystemService(Context.WINDOW_SERVICE);
        }
        mWindowManager = ((WindowManagerImpl)wm).createLocalWindowManager(this);
    }

创建了一个WindowManager，但是传入了Window与Context

public WindowManagerImpl createLocalWindowManager(Window parentWindow) {
        return new WindowManagerImpl(mContext, parentWindow);
    }

5，调用Activity的onCreate()方法

if (r.isPersistable()) {
                    mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
                } else {
                    mInstrumentation.callActivityOnCreate(activity, r.state);
                }