App的启动过程

流程概述
- Launcher应用
- 总结

参考资料

https://blog.csdn.net/zzw0221/article/details/106716620

源码文件

\packages\apps\launcher3\src\com\android\launcher3\Launcher.java
\packages\apps\launcher3\src\com\android\launcher3\touch\ItemClickHandler.java

流程概述

当点击桌面的应用图标，就会启动一个应用。从桌面上点击app图标到app启动，以下是这部分的代码执行流程。
安卓设备的桌面本身就是一个app，我们一般称之为Launcher。不过由于 Android 是开源的，很多手机厂商都做了相关的修改，甚至重新做一个新的启动页。
当我们点击桌面的应用图标开始，就是Launcher应用中的一次点击事件。

Launcher应用

在Launcher.java中的createShortcut(ViewGroup parent, WorkspaceItemInfo info)方法中设置的app图标点击事件，点击事件具体调用是在ItemClickHandler.java中。

1. Launcher.java

public class Launcher extends BaseDraggingActivity implements LauncherExterns,
        LauncherModel.Callbacks, LauncherProviderChangeListener, UserEventDelegate,
        InvariantDeviceProfile.OnIDPChangeListener {
    
    View createShortcut(WorkspaceItemInfo info) {
        return createShortcut((ViewGroup) mWorkspace.getChildAt(mWorkspace.getCurrentPage()), info);
    }
    
    public View createShortcut(ViewGroup parent, WorkspaceItemInfo info) {
        BubbleTextView favorite = (BubbleTextView) LayoutInflater.from(parent.getContext())
                .inflate(R.layout.app_icon, parent, false);
        favorite.applyFromWorkspaceItem(info);
        favorite.setOnClickListener(ItemClickHandler.INSTANCE);
        favorite.setOnFocusChangeListener(mFocusHandler);
        return favorite;
    }
            
}

2. ItemClickHandler.java

public static final OnClickListener INSTANCE = getInstance(null);
    //....
    
    public static final OnClickListener getInstance(String sourceContainer) {
        return v -> onClick(v, sourceContainer);
    }

    private static void onClick(View v, String sourceContainer) {
        if (TestProtocol.sDebugTracing) {
            android.util.Log.d(TestProtocol.NO_START_TAG,
                    "onClick 1");
        }
        // Make sure that rogue clicks don't get through while allapps is launching, or after the
        // view has detached (it's possible for this to happen if the view is removed mid touch).
        if (v.getWindowToken() == null) {
            if (TestProtocol.sDebugTracing) {
                android.util.Log.d(TestProtocol.NO_START_TAG,
                        "onClick 2");
            }
            return;
        }

        Launcher launcher = Launcher.getLauncher(v.getContext());
        if (!launcher.getWorkspace().isFinishedSwitchingState()) {
            if (TestProtocol.sDebugTracing) {
                android.util.Log.d(TestProtocol.NO_START_TAG,
                        "onClick 3");
            }
            return;
        }

        Object tag = v.getTag();
        if (tag instanceof WorkspaceItemInfo) {
            onClickAppShortcut(v, (WorkspaceItemInfo) tag, launcher, sourceContainer);
        } else if (tag instanceof FolderInfo) {
            if (v instanceof FolderIcon) {
                onClickFolderIcon(v);
            }
        } else if (tag instanceof AppInfo) {
            if (TestProtocol.sDebugTracing) {
                android.util.Log.d(TestProtocol.NO_START_TAG,
                        "onClick 4");
            }
            //关键代码；
            startAppShortcutOrInfoActivity(v, (AppInfo) tag, launcher,
                    sourceContainer == null ? CONTAINER_ALL_APPS: sourceContainer);
        } else if (tag instanceof LauncherAppWidgetInfo) {
            if (v instanceof PendingAppWidgetHostView) {
                onClickPendingWidget((PendingAppWidgetHostView) v, launcher);
            }
        }
    }
    
    private static void startAppShortcutOrInfoActivity(View v, ItemInfo item, Launcher launcher,
            @Nullable String sourceContainer) {
        if (TestProtocol.sDebugTracing) {
            android.util.Log.d(TestProtocol.NO_START_TAG,
                    "startAppShortcutOrInfoActivity");
        }
        Intent intent;
        if (item instanceof PromiseAppInfo) {
            PromiseAppInfo promiseAppInfo = (PromiseAppInfo) item;
            intent = promiseAppInfo.getMarketIntent(launcher);
        } else {
            intent = item.getIntent();
        }
        if (intent == null) {
            throw new IllegalArgumentException("Input must have a valid intent");
        }
        if (item instanceof WorkspaceItemInfo) {
            WorkspaceItemInfo si = (WorkspaceItemInfo) item;
            if (si.hasStatusFlag(WorkspaceItemInfo.FLAG_SUPPORTS_WEB_UI)
                    && Intent.ACTION_VIEW.equals(intent.getAction())) {
                // make a copy of the intent that has the package set to null
                // we do this because the platform sometimes disables instant
                // apps temporarily (triggered by the user) and fallbacks to the
                // web ui. This only works though if the package isn't set
                intent = new Intent(intent);
                intent.setPackage(null);
            }
        }
        if (v != null && launcher.getAppTransitionManager().supportsAdaptiveIconAnimation()) {
            // Preload the icon to reduce latency b/w swapping the floating view with the original.
            FloatingIconView.fetchIcon(launcher, v, item, true /* isOpening */);
        }
        //关键代码；
        launcher.startActivitySafely(v, intent, item, sourceContainer);
    }
    //...
}

在这里，执行的是tag instanceof AppInfo这个条件，执行的是startAppShortcutOrInfoActivity(v, (AppInfo) tag, launcher,sourceContainer == null ? CONTAINER_ALL_APPS: sourceContainer)方法。然后又是调用了launcher.startActivitySafely(v, intent, item, sourceContainer)，代码执行又回到了Luancher.java中了。

3. Launcher.java

public class Launcher extends BaseDraggingActivity implements LauncherExterns,
        LauncherModel.Callbacks, LauncherProviderChangeListener, UserEventDelegate,
        InvariantDeviceProfile.OnIDPChangeListener {
    //...
    f (TestProtocol.sDebugTracing) {
            android.util.Log.d(TestProtocol.NO_START_TAG,
                    "startActivitySafely outer");
        }

        if (!hasBeenResumed()) {
            // Workaround an issue where the WM launch animation is clobbered when finishing the
            // recents animation into launcher. Defer launching the activity until Launcher is
            // next resumed.
            addOnResumeCallback(() -> startActivitySafely(v, intent, item, sourceContainer));
            UiFactory.clearSwipeSharedState(true /* finishAnimation */);
            return true;
        }
        //关键代码；
        boolean success = super.startActivitySafely(v, intent, item, sourceContainer);
        if (success && v instanceof BubbleTextView) {
            // This is set to the view that launched the activity that navigated the user away
            // from launcher. Since there is no callback for when the activity has finished
            // launching, enable the press state and keep this reference to reset the press
            // state when we return to launcher.
            BubbleTextView btv = (BubbleTextView) v;
            btv.setStayPressed(true);
            addOnResumeCallback(btv);
        }
        return success;
    //...        
}

调用了super.startActivitySafely(v, intent, item, sourceContainer)，是调用了父类的方法，Launcher的父类是BaseDraggingActivity。

4. BaseDraggingActivity

public abstract class BaseDraggingActivity extends BaseActivity
        implements WallpaperColorInfo.OnChangeListener {
    //...
    public boolean startActivitySafely(View v, Intent intent, @Nullable ItemInfo item,
            @Nullable String sourceContainer) {
        if (TestProtocol.sDebugTracing) {
            android.util.Log.d(TestProtocol.NO_START_TAG,
                    "startActivitySafely 1");
        }
        if (mIsSafeModeEnabled && !Utilities.isSystemApp(this, intent)) {
            Toast.makeText(this, R.string.safemode_shortcut_error, Toast.LENGTH_SHORT).show();
            return false;
        }

        Bundle optsBundle = (v != null) ? getActivityLaunchOptionsAsBundle(v) : null;
        UserHandle user = item == null ? null : item.user;

        // Prepare intent
        //注释（1）;
        intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
        if (v != null) {
            intent.setSourceBounds(getViewBounds(v));
        }
        try {
            boolean isShortcut = (item instanceof WorkspaceItemInfo)
                    && (item.itemType == Favorites.ITEM_TYPE_SHORTCUT
                    || item.itemType == Favorites.ITEM_TYPE_DEEP_SHORTCUT)
                    && !((WorkspaceItemInfo) item).isPromise();
            if (isShortcut) {
                // Shortcuts need some special checks due to legacy reasons.
                startShortcutIntentSafely(intent, optsBundle, item, sourceContainer);
            } else if (user == null || user.equals(Process.myUserHandle())) {
                // Could be launching some bookkeeping activity
                if (TestProtocol.sDebugTracing) {
                    android.util.Log.d(TestProtocol.NO_START_TAG,
                            "startActivitySafely 2");
                }
                //关键代码，注释（2）；
                startActivity(intent, optsBundle);
                AppLaunchTracker.INSTANCE.get(this).onStartApp(intent.getComponent(),
                        Process.myUserHandle(), sourceContainer);
            } else {
                LauncherAppsCompat.getInstance(this).startActivityForProfile(
                        intent.getComponent(), user, intent.getSourceBounds(), optsBundle);
                AppLaunchTracker.INSTANCE.get(this).onStartApp(intent.getComponent(), user,
                        sourceContainer);
            }
            getUserEventDispatcher().logAppLaunch(v, intent);
            getStatsLogManager().logAppLaunch(v, intent);
            return true;
        } catch (ActivityNotFoundException|SecurityException e) {
            Toast.makeText(this, R.string.activity_not_found, Toast.LENGTH_SHORT).show();
            Log.e(TAG, "Unable to launch. tag=" + item + " intent=" + intent, e);
        }
        return false;
    }
    //...        
}

注释1:在一个新的Task中启动Activity；
注释2:根据startActivity(intent, optsBundle)方法往父类查看，发现在BaseActivity中没有该方法，继续往上追踪，最终调用的是Activity的startActivity(Intent intent, @Nullable Bundle options)方法；Launcher的继承关系如下：

|-- Activity(framework\base\core\java\android\app\Activity)
|-- -- BaseActivity
|-- -- -- BaseDraggingActivity
|-- -- -- -- Launcher

5. Activity

public class Activity extends ContextThemeWrapper
        implements LayoutInflater.Factory2,
        Window.Callback, KeyEvent.Callback,
        OnCreateContextMenuListener, ComponentCallbacks2,
        Window.OnWindowDismissedCallback, WindowControllerCallback,
        AutofillManager.AutofillClient, ContentCaptureManager.ContentCaptureClient {
    //...
    @Override
    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);
        }
    }
    
    public void startActivityForResult(@RequiresPermission Intent intent, int requestCode) {
        startActivityForResult(intent, requestCode, null);
    }
    
    public void startActivityForResult(@RequiresPermission Intent intent, int requestCode,
            @Nullable Bundle options) {
        if (mParent == null) {
            options = transferSpringboardActivityOptions(options);
            //关键代码；
            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;
            }

            cancelInputsAndStartExitTransition(options);
            // 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);
            }
        }
    }
    //...        
}

startActivityForResult(intent, -1)方法调用的是startActivityForResult(intent, requestCode, null)，所以最终还是执行的startActivityForResult(@RequiresPermission Intent intent, int requestCode,@Nullable Bundle options)方法。因为 Launcher 启动应用并不需要应用给予回复，所以这里 requestCode 为 -1。
最终调用了 Instrumentation 的 execStartActivity(this,mMainThread.getApplicationThread(), mToken, this,intent, requestCode, options) 方法。

6. Instrumentation

代码位于(framework\base\core\java\android\app)
Instrumentation 是 Android 系统中启动 Activity 的一个实际操作类，它用来监控应用程序和系统的交互。

public class Instrumentation {
    //...
    @UnsupportedAppUsage
    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);
                    ActivityResult result = null;
                    if (am.ignoreMatchingSpecificIntents()) {
                        result = am.onStartActivity(intent);
                    }
                    if (result != null) {
                        am.mHits++;
                        return result;
                    } else if (am.match(who, null, intent)) {
                        am.mHits++;
                        if (am.isBlocking()) {
                            return requestCode >= 0 ? am.getResult() : null;
                        }
                        break;
                    }
                }
            }
        }
        try {
            intent.migrateExtraStreamToClipData();
            intent.prepareToLeaveProcess(who);
            //关键代码；
            int result = ActivityTaskManager.getService()
                .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;
    }
    //...
}

调用ActivityTaskManager.getService().startActivity(whoThread, who.getBasePackageName(), intent,intent.resolveTypeIfNeeded(who.getContentResolver()),token, target != null ? target.mEmbeddedID : null,requestCode, 0, null, options)方法，那我们就先看看ActivityTaskManager类。

7. ActivityTaskManager

代码位于：(framework\base\core\java\android\app)

@TestApi
@SystemService(Context.ACTIVITY_TASK_SERVICE)
public class ActivityTaskManager {
    //...
    public static IActivityTaskManager getService() {
        //关键代码；
        return IActivityTaskManagerSingleton.get();
    }
    
    @UnsupportedAppUsage(trackingBug = 129726065)
    private static final Singleton<IActivityTaskManager> IActivityTaskManagerSingleton =
            new Singleton<IActivityTaskManager>() {
                @Override
                protected IActivityTaskManager create() {
                    final IBinder b = ServiceManager.getService(Context.ACTIVITY_TASK_SERVICE);
                    return IActivityTaskManager.Stub.asInterface(b);
                }
            };
    //...
}

在这里是使用的AIDL调用，此处是AIDL的client端。Server端是ActivityTaskManagerService。Instrumentation 中 execStartActivity 方法实际调用的是 ActivityTaskManagerService 中的 startActivity 方法。Instrumentation 中 execStartActivity 方法实际调用的是 ActivityTaskManagerService 中的 startActivity 方法。

8. ActivityTaskManagerService

代码位于：(framework\base\services\core\java\com\android\server\wm)
IActivityTaskManager代码：（frameworks\base\core\java\android\app\IActivityTaskManager.aidl）

public class ActivityTaskManagerService extends IActivityTaskManager.Stub {
    //...
    @Override
    public final int startActivity(IApplicationThread caller, String callingPackage,
            Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode,
            int startFlags, ProfilerInfo profilerInfo, Bundle bOptions) {
        return startActivityAsUser(caller, callingPackage, intent, resolvedType, resultTo,
                resultWho, requestCode, startFlags, profilerInfo, bOptions,
                UserHandle.getCallingUserId());
    }
    
    @Override
    public int startActivityAsUser(IApplicationThread caller, String callingPackage,
            Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode,
            int startFlags, ProfilerInfo profilerInfo, Bundle bOptions, int userId) {
        return startActivityAsUser(caller, callingPackage, intent, resolvedType, resultTo,
                resultWho, requestCode, startFlags, profilerInfo, bOptions, userId,
                true /*validateIncomingUser*/);
    }
    
    int startActivityAsUser(IApplicationThread caller, String callingPackage,
            Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode,
            int startFlags, ProfilerInfo profilerInfo, Bundle bOptions, int userId,
            boolean validateIncomingUser) {
        enforceNotIsolatedCaller("startActivityAsUser");

        userId = getActivityStartController().checkTargetUser(userId, validateIncomingUser,
                Binder.getCallingPid(), Binder.getCallingUid(), "startActivityAsUser");

        // TODO: Switch to user app stacks here.
        //关键代码；
        return getActivityStartController().obtainStarter(intent, "startActivityAsUser")
                .setCaller(caller)
                .setCallingPackage(callingPackage)
                .setResolvedType(resolvedType)
                .setResultTo(resultTo)
                .setResultWho(resultWho)
                .setRequestCode(requestCode)
                .setStartFlags(startFlags)
                .setProfilerInfo(profilerInfo)
                .setActivityOptions(bOptions)
                .setMayWait(userId)
                .execute();
    }
    
    ActivityStartController getActivityStartController() {
        return mActivityStartController;
    }
    //...
}

调用getActivityStartController()获取ActivityStartController的实例，然后调用obtainStarter()方法。

9. ActivityStartController

**代码位置：(framework\base\services\core\java\com\android\server\wm) **

public class ActivityStartController {
    //...
    ActivityStarter obtainStarter(Intent intent, String reason) {
        return mFactory.obtain().setIntent(intent).setReason(reason);
    }
    
    ActivityStartController(ActivityTaskManagerService service) {
        this(service, service.mStackSupervisor,
                new DefaultFactory(service, service.mStackSupervisor,
                    new ActivityStartInterceptor(service, service.mStackSupervisor)));
    }
    
    @VisibleForTesting
    ActivityStartController(ActivityTaskManagerService service, ActivityStackSupervisor supervisor,Factory factory) {
        mService = service;
        mSupervisor = supervisor;
        mHandler = new StartHandler(mService.mH.getLooper());
        mFactory = factory;
        mFactory.setController(this);
        mPendingRemoteAnimationRegistry = new PendingRemoteAnimationRegistry(service,
                service.mH);
    }
    //...
}

在obtainStarter()方法返回的是ActivityStarter对象，这里使用了工厂模式。mFactory是在ActivityStartController的构造方法中以参数的方式传递进来的，并且可以看到是DefaultFactory类实现的，而DefaultFactory是ActivityStarter的一个内部类。ActivityStartController的初始化是在ActivityTaskManagerService的initialize()方法中。

//ActivityTaskManagerService类
public void initialize(IntentFirewall intentFirewall, PendingIntentController intentController,
            Looper looper) {
        mH = new H(looper);
        mUiHandler = new UiHandler();
        mIntentFirewall = intentFirewall;
        final File systemDir = SystemServiceManager.ensureSystemDir();
        mAppWarnings = new AppWarnings(this, mUiContext, mH, mUiHandler, systemDir);
        mCompatModePackages = new CompatModePackages(this, systemDir, mH);
        mPendingIntentController = intentController;

        mTempConfig.setToDefaults();
        mTempConfig.setLocales(LocaleList.getDefault());
        mConfigurationSeq = mTempConfig.seq = 1;
        mStackSupervisor = createStackSupervisor();
        mRootActivityContainer = new RootActivityContainer(this);
        mRootActivityContainer.onConfigurationChanged(mTempConfig);

        mTaskChangeNotificationController =
                new TaskChangeNotificationController(mGlobalLock, mStackSupervisor, mH);
        mLockTaskController = new LockTaskController(mContext, mStackSupervisor, mH);
        //关键代码；
        mActivityStartController = new ActivityStartController(this);
        mRecentTasks = createRecentTasks();
        mStackSupervisor.setRecentTasks(mRecentTasks);
        mVrController = new VrController(mGlobalLock);
        mKeyguardController = mStackSupervisor.getKeyguardController();
    }

10. ActivityStarter

代码位置：(framework\base\services\core\java\com\android\server\wm)

class ActivityStarter {
    //...
    interface Factory {
        void setController(ActivityStartController controller);

        ActivityStarter obtain();

        void recycle(ActivityStarter starter);
    }
    
    static class DefaultFactory implements Factory {
        //...
        @Override
        public ActivityStarter obtain() {
            ActivityStarter starter = mStarterPool.acquire();

            if (starter == null) {
                starter = new ActivityStarter(mController, mService, mSupervisor, mInterceptor);
            }

            return starter;
        }
        //...
    }
    
    ActivityStarter setIntent(Intent intent) {
        mRequest.intent = intent;
        return this;
    }
    
    ActivityStarter setReason(String reason) {
        mRequest.reason = reason;
        return this;
    }
    
    int execute() {
        try {
            // TODO(b/64750076): Look into passing request directly to these methods to allow
            // for transactional diffs and preprocessing.
            if (mRequest.mayWait) {
                return startActivityMayWait(mRequest.caller, mRequest.callingUid,
                        mRequest.callingPackage, mRequest.realCallingPid, mRequest.realCallingUid,
                        mRequest.intent, mRequest.resolvedType,
                        mRequest.voiceSession, mRequest.voiceInteractor, mRequest.resultTo,
                        mRequest.resultWho, mRequest.requestCode, mRequest.startFlags,
                        mRequest.profilerInfo, mRequest.waitResult, mRequest.globalConfig,
                        mRequest.activityOptions, mRequest.ignoreTargetSecurity, mRequest.userId,
                        mRequest.inTask, mRequest.reason,
                        mRequest.allowPendingRemoteAnimationRegistryLookup,
                        mRequest.originatingPendingIntent, mRequest.allowBackgroundActivityStart);
            } else {
                return startActivity(mRequest.caller, mRequest.intent, mRequest.ephemeralIntent,
                        mRequest.resolvedType, mRequest.activityInfo, mRequest.resolveInfo,
                        mRequest.voiceSession, mRequest.voiceInteractor, mRequest.resultTo,
                        mRequest.resultWho, mRequest.requestCode, mRequest.callingPid,
                        mRequest.callingUid, mRequest.callingPackage, mRequest.realCallingPid,
                        mRequest.realCallingUid, mRequest.startFlags, mRequest.activityOptions,
                        mRequest.ignoreTargetSecurity, mRequest.componentSpecified,
                        mRequest.outActivity, mRequest.inTask, mRequest.reason,
                        mRequest.allowPendingRemoteAnimationRegistryLookup,
                        mRequest.originatingPendingIntent, mRequest.allowBackgroundActivityStart);
            }
        } finally {
            onExecutionComplete();
        }
    }
    
    private int startActivityMayWait(IApplicationThread caller, int callingUid,
            String callingPackage, int requestRealCallingPid, int requestRealCallingUid,
            Intent intent, String resolvedType, IVoiceInteractionSession voiceSession,
            IVoiceInteractor voiceInteractor, IBinder resultTo, String resultWho, int requestCode,
            int startFlags, ProfilerInfo profilerInfo, WaitResult outResult,
            Configuration globalConfig, SafeActivityOptions options, boolean ignoreTargetSecurity,
            int userId, TaskRecord inTask, String reason,
            boolean allowPendingRemoteAnimationRegistryLookup,
            PendingIntentRecord originatingPendingIntent, boolean allowBackgroundActivityStart) {
            //...
            final ActivityRecord[] outRecord = new ActivityRecord[1];
            //关键代码；
            int res = startActivity(caller, intent, ephemeralIntent, resolvedType, aInfo, rInfo,voiceSession, voiceInteractor, resultTo, resultWho, requestCode, callingPid,callingUid, callingPackage, realCallingPid, realCallingUid, startFlags, options,ignoreTargetSecurity, componentSpecified, outRecord, inTask, reason,allowPendingRemoteAnimationRegistryLookup, originatingPendingIntent,allowBackgroundActivityStart);

            Binder.restoreCallingIdentity(origId);
            //...
            return res;
        }
    }
    
    private int startActivity(IApplicationThread caller, Intent intent, Intent ephemeralIntent,
            String resolvedType, ActivityInfo aInfo, ResolveInfo rInfo,
            IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
            IBinder resultTo, String resultWho, int requestCode, int callingPid, int callingUid,
            String callingPackage, int realCallingPid, int realCallingUid, int startFlags,
            SafeActivityOptions options, boolean ignoreTargetSecurity, boolean componentSpecified,
            ActivityRecord[] outActivity, TaskRecord inTask, String reason,
            boolean allowPendingRemoteAnimationRegistryLookup,
            PendingIntentRecord originatingPendingIntent, boolean allowBackgroundActivityStart) {

        if (TextUtils.isEmpty(reason)) {
            throw new IllegalArgumentException("Need to specify a reason.");
        }
        mLastStartReason = reason;
        mLastStartActivityTimeMs = System.currentTimeMillis();
        mLastStartActivityRecord[0] = null;

        mLastStartActivityResult = startActivity(caller, intent, ephemeralIntent, resolvedType,
                aInfo, rInfo, voiceSession, voiceInteractor, resultTo, resultWho, requestCode,
                callingPid, callingUid, callingPackage, realCallingPid, realCallingUid, startFlags,
                options, ignoreTargetSecurity, componentSpecified, mLastStartActivityRecord,
                inTask, allowPendingRemoteAnimationRegistryLookup, originatingPendingIntent,
                allowBackgroundActivityStart);

        if (outActivity != null) {
            // mLastStartActivityRecord[0] is set in the call to startActivity above.
            outActivity[0] = mLastStartActivityRecord[0];
        }

        return getExternalResult(mLastStartActivityResult);
    }
    
    private int startActivity(IApplicationThread caller, Intent intent, Intent ephemeralIntent,
            String resolvedType, ActivityInfo aInfo, ResolveInfo rInfo,
            IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
            IBinder resultTo, String resultWho, int requestCode, int callingPid, int callingUid,String callingPackage, int realCallingPid, int realCallingUid, int startFlags,SafeActivityOptions options,boolean ignoreTargetSecurity, boolean componentSpecified, ActivityRecord[] outActivity,TaskRecord inTask, boolean allowPendingRemoteAnimationRegistryLookup,PendingIntentRecord originatingPendingIntent, boolean allowBackgroundActivityStart) {
            //...
            final int res = startActivity(r, sourceRecord, voiceSession, voiceInteractor, startFlags,true /* doResume */, checkedOptions, inTask, outActivity, restrictedBgActivity);
        mSupervisor.getActivityMetricsLogger().notifyActivityLaunched(res, outActivity[0]);
        return res;
    }
    
    private int startActivity(final ActivityRecord r, ActivityRecord sourceRecord,
                IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
                int startFlags, boolean doResume, ActivityOptions options, TaskRecord inTask,
                ActivityRecord[] outActivity, boolean restrictedBgActivity) {
        int result = START_CANCELED;
        final ActivityStack startedActivityStack;
        try {
            mService.mWindowManager.deferSurfaceLayout();
            //关键代码；
            result = startActivityUnchecked(r, sourceRecord, voiceSession, voiceInteractor,
                    startFlags, doResume, options, inTask, outActivity, restrictedBgActivity);
        } finally {
            final ActivityStack currentStack = r.getActivityStack();
            startedActivityStack = currentStack != null ? currentStack : mTargetStack;

            if (ActivityManager.isStartResultSuccessful(result)) {
                if (startedActivityStack != null) {
                    // If there is no state change (e.g. a resumed activity is reparented to
                    // top of another display) to trigger a visibility/configuration checking,
                    // we have to update the configuration for changing to different display.
                    final ActivityRecord currentTop =
                            startedActivityStack.topRunningActivityLocked();
                    if (currentTop != null && currentTop.shouldUpdateConfigForDisplayChanged()) {
                        mRootActivityContainer.ensureVisibilityAndConfig(
                                currentTop, currentTop.getDisplayId(),
                                true /* markFrozenIfConfigChanged */, false /* deferResume */);
                    }
                }
            } else {
                // If we are not able to proceed, disassociate the activity from the task.
                // Leaving an activity in an incomplete state can lead to issues, such as
                // performing operations without a window container.
                final ActivityStack stack = mStartActivity.getActivityStack();
                if (stack != null) {
                    stack.finishActivityLocked(mStartActivity, RESULT_CANCELED,
                            null /* intentResultData */, "startActivity", true /* oomAdj */);
                }

                // Stack should also be detached from display and be removed if it's empty.
                if (startedActivityStack != null && startedActivityStack.isAttached()
                        && startedActivityStack.numActivities() == 0
                        && !startedActivityStack.isActivityTypeHome()) {
                    startedActivityStack.remove();
                }
            }
            mService.mWindowManager.continueSurfaceLayout();
        }

        postStartActivityProcessing(r, result, startedActivityStack);

        return result;
    }
    
    private int startActivityUnchecked(final ActivityRecord r, ActivityRecord sourceRecord,
            IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
            int startFlags, boolean doResume, ActivityOptions options, TaskRecord inTask,
            ActivityRecord[] outActivity, boolean restrictedBgActivity) {
        //...
        TargetStack.startActivityLocked(mStartActivity, topFocused, newTask, mKeepCurTransition,
                mOptions);
        if (mDoResume) {
            final ActivityRecord topTaskActivity =
                    mStartActivity.getTaskRecord().topRunningActivityLocked();
            if (!mTargetStack.isFocusable()
                    || (topTaskActivity != null && topTaskActivity.mTaskOverlay
                    && mStartActivity != topTaskActivity)) {
                mTargetStack.ensureActivitiesVisibleLocked(mStartActivity, 0, !PRESERVE_WINDOWS);
                mTargetStack.getDisplay().mDisplayContent.executeAppTransition();
            } else {
                if (mTargetStack.isFocusable()
                        && !mRootActivityContainer.isTopDisplayFocusedStack(mTargetStack)) {
                    mTargetStack.moveToFront("startActivityUnchecked");
                }
                //关键代码；
                mRootActivityContainer.resumeFocusedStacksTopActivities(
                        mTargetStack, mStartActivity, mOptions);
            }
        } else if (mStartActivity != null) {
            mSupervisor.mRecentTasks.add(mStartActivity.getTaskRecord());
        }
        mRootActivityContainer.updateUserStack(mStartActivity.mUserId, mTargetStack);

        mSupervisor.handleNonResizableTaskIfNeeded(mStartActivity.getTaskRecord(),
                preferredWindowingMode, mPreferredDisplayId, mTargetStack);

        return START_SUCCESS;        
    }
    //...
}

在ActivityTaskManagerService中，最终调用的是execute()方法。因为之前有执行了setMayWait()方法，在execute()方法中，调用的是startActivityMayWait()方法。接下来调用一系列的startActivity()方法。最终在startActivityUnchecked()方法，调用RootActivityContainer的resumeFocusedStacksTopActivity()方法。

11. RootActivityContainer

代码位置：(framework\base\services\core\java\com\android\server\wm)
RootActivityContainer是10.0新引入的类，从类的注释可以看出这个类是暂时用来分担 ActivityStackSupervisor 的部分职责的。

class RootActivityContainer extends ConfigurationContainer
        implements DisplayManager.DisplayListener {
    //...
    boolean resumeFocusedStacksTopActivities(
            ActivityStack targetStack, ActivityRecord target, ActivityOptions targetOptions) {

        if (!mStackSupervisor.readyToResume()) {
            return false;
        }

        boolean result = false;
        if (targetStack != null && (targetStack.isTopStackOnDisplay()||getTopDisplayFocusedStack() == targetStack)) {
            //关键代码；
            result = targetStack.resumeTopActivityUncheckedLocked(target, targetOptions);
        }
        //...
        return result;
    }
    //...        
}

调用ActivityStack的resumeTopActivityUncheckedLocked()方法。

12. ActivityStack

**代码位置：(framework\base\services\core\java\com\android\server\wm) **

class ActivityStack extends ConfigurationContainer {
    //...
    @GuardedBy("mService")
    boolean resumeTopActivityUncheckedLocked(ActivityRecord prev, ActivityOptions options) {
        if (mInResumeTopActivity) {
            // Don't even start recursing.
            return false;
        }

        boolean result = false;
        try {
            // Protect against recursion.
            mInResumeTopActivity = true;
            //关键代码；
            result = resumeTopActivityInnerLocked(prev, options);
            // {@link ActivityStackSupervisor#checkReadyForSleepLocked} is skipped.
            final ActivityRecord next = topRunningActivityLocked(true /* focusableOnly */);
            if (next == null || !next.canTurnScreenOn()) {
                checkReadyForSleep();
            }
        } finally {
            mInResumeTopActivity = false;
        }

        return result;
    }
    
    @GuardedBy("mService")
    private boolean resumeTopActivityInnerLocked(ActivityRecord prev, ActivityOptions options) {
    boolean pausing = getDisplay().pauseBackStacks(userLeaving, next, false);
        //注释1；
        if (mResumedActivity != null) {
            if (DEBUG_STATES) Slog.d(TAG_STATES,"resumeTopActivityLocked: Pausing " + mResumedActivity);
            //关键代码；
            pausing |= startPausingLocked(userLeaving, false, next, false);
        }
 
        //...
        if (next.attachedToProcess()) {
            //...
            //注释2；
            //关键代码；
            mStackSupervisor.startSpecificActivityLocked(next, true, false);
            return true;
        }
            // From this point on, if something goes wrong there is no way
            // to recover the activity.
            try {
                next.completeResumeLocked();
            } catch (Exception e) {
                // If any exception gets thrown, toss away this
                // activity and try the next one.
                Slog.w(TAG, "Exception thrown during resume of " + next, e);
                requestFinishActivityLocked(next.appToken, Activity.RESULT_CANCELED, null,
                        "resume-exception", true);
                return true;
            }
        } else {
            // Whoops, need to restart this activity!
            if (!next.hasBeenLaunched) {
                next.hasBeenLaunched = true;
            } else {
                if (SHOW_APP_STARTING_PREVIEW) {
                    next.showStartingWindow(null /* prev */, false /* newTask */,
                            false /* taskSwich */);
                }
                if (DEBUG_SWITCH) Slog.v(TAG_SWITCH, "Restarting: " + next);
            }
            if (DEBUG_STATES) Slog.d(TAG_STATES, "resumeTopActivityLocked: Restarting " + next);
            //注释3；
            //关键代码；
            mStackSupervisor.startSpecificActivityLocked(next, true, true); 
        }
    }
    
    final boolean startPausingLocked(boolean userLeaving, boolean uiSleeping,
            ActivityRecord resuming, boolean pauseImmediately) {
          //...
          if (prev.attachedToProcess()) {
            if (DEBUG_PAUSE) Slog.v(TAG_PAUSE, "Enqueueing pending pause: " + prev);
            try {
                EventLogTags.writeAmPauseActivity(prev.mUserId, System.identityHashCode(prev),
                        prev.shortComponentName, "userLeaving=" + userLeaving);
                //关键代码；
                mService.getLifecycleManager().scheduleTransaction(prev.app.getThread(),
prev.appToken, PauseActivityItem.obtain(prev.finishing, userLeaving,prev.configChangeFlags, pauseImmediately));
            } catch (Exception e) {
                // Ignore exception, if process died other code will cleanup.
                Slog.w(TAG, "Exception thrown during pause", e);
                mPausingActivity = null;
                mLastPausedActivity = null;
                mLastNoHistoryActivity = null;
            }
        } else {
            mPausingActivity = null;
            mLastPausedActivity = null;
            mLastNoHistoryActivity = null;
        }
        //...  
    }
    
    //...
}

注释1:判断是否有Activity处于Resume状态，有的话会调用startPausingLocked()方法让Activity执行Pausing过程。接着才会调用注释2和注释3中的ActivityStackSupervisor 中的 startSpecificActivityLocked()方法。
在startPausingLocked()中，执行了mService.getLifecycleManager().scheduleTransaction()，这里的mService是ActivityTaskManagerService。getLifecycleManager方法获取到的是ClientLifecycleManager。

13. ClientLifecycleManager

**代码位置： (framework\base\services\core\java\com\android\server\wm) **

class ClientLifecycleManager {
    void scheduleTransaction(ClientTransaction transaction) throws RemoteException {
        final IApplicationThread client = transaction.getClient();
        //关键代码；
        transaction.schedule();
        if (!(client instanceof Binder)) {
            // If client is not an instance of Binder - it's a remote call and at this point it is
            // safe to recycle the object. All objects used for local calls will be recycled after
            // the transaction is executed on client in ActivityThread.
            transaction.recycle();
        }
    }
    
    void scheduleTransaction(@NonNull IApplicationThread client,
            @NonNull ClientTransactionItem callback) throws RemoteException {
        final ClientTransaction clientTransaction = transactionWithCallback(client,
                null /* activityToken */, callback);
        scheduleTransaction(clientTransaction);
    }
    
    void scheduleTransaction(@NonNull IApplicationThread client, @NonNull IBinder activityToken,
            @NonNull ActivityLifecycleItem stateRequest) throws RemoteException {
        final ClientTransaction clientTransaction = transactionWithState(client, activityToken,
                stateRequest);
        scheduleTransaction(clientTransaction);
    }
    
    void scheduleTransaction(@NonNull IApplicationThread client, @NonNull IBinder activityToken,
            @NonNull ClientTransactionItem callback) throws RemoteException {
        final ClientTransaction clientTransaction = transactionWithCallback(client, activityToken,
                callback);
        scheduleTransaction(clientTransaction);
    }
    
}

最后调用的是scheduleTransaction(ClientTransaction transaction)方法，通过 ClientTransaction 的 getClient 方法获取到 client 对象，client 是 IApplicationThread 类型，在 ClientTransaction 的静态方法 obtain 中进行初始化。执行了transaction.schedule()，我们重点关注的就是这个方法。

14. ClientTransaction

代码位置：(framework\base\core\java\android\app\servertransaction)

public class ClientTransaction implements Parcelable, ObjectPoolItem {
    //...
    public IApplicationThread getClient() {
        return mClient;
    }
    
    public static ClientTransaction obtain(IApplicationThread client, IBinder activityToken) {
        ClientTransaction instance = ObjectPool.obtain(ClientTransaction.class);
        if (instance == null) {
            instance = new ClientTransaction();
        }
        instance.mClient = client;
        instance.mActivityToken = activityToken;

        return instance;
    }
    
    public void schedule() throws RemoteException {
        mClient.scheduleTransaction(this);
    }
    //...
}

mClient是IApplicationThread，实际实现是在ActivityThread中的内部类ApplicationThread。

15. ActivityThread

代码位置：(framework\base\core\java\android\app)

public final class ActivityThread extends ClientTransactionHandler {
    //...
    private class ApplicationThread extends IApplicationThread.Stub {
        //...
        @Override
        public void scheduleTransaction(ClientTransaction transaction) throws RemoteException {
            //关键代码；
            ActivityThread.this.scheduleTransaction(transaction);
        }
        //...
    }
    //...
}

scheduleTransaction()方法在ActivityThread类中不存在，在父类ClientTransactionHandler中。

16. ClientTransactionHandler

代码位置：(framework\base\core\java\android\app)

public abstract class ClientTransactionHandler {
    //...
    void scheduleTransaction(ClientTransaction transaction) {
        transaction.preExecute(this);
        sendMessage(ActivityThread.H.EXECUTE_TRANSACTION, transaction);
    }
    
    abstract void sendMessage(int what, Object obj);
    //...
}

sendMessage()方法在子类ActivityThread中实现。

17. ActivityThread/H

public final class ActivityThread extends ClientTransactionHandler {
    //...
    void sendMessage(int what, Object obj) {
        sendMessage(what, obj, 0, 0, false);
    }

    private void sendMessage(int what, Object obj, int arg1) {
        sendMessage(what, obj, arg1, 0, false);
    }

    private void sendMessage(int what, Object obj, int arg1, int arg2) {
        sendMessage(what, obj, arg1, arg2, 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);
    }
    
    class H extends Handler {
        //...
        public void handleMessage(Message msg) {
            switch (msg.what) {
                case EXECUTE_TRANSACTION:
                    final ClientTransaction transaction = (ClientTransaction) msg.obj;
                    mTransactionExecutor.execute(transaction);
                    if (isSystem()) {
                        transaction.recycle();
                    }
                    break;
            }
        }
        //...
    }
    //...
}

调用了 TransactionExecutor 的方法 execute(transaction)。

18. TransactionExecutor

代码位置：(framework\base\core\java\android\app\servertransaction)

public class TransactionExecutor {
    //...
    public void execute(ClientTransaction transaction) {
        if (DEBUG_RESOLVER) Slog.d(TAG, tId(transaction) + "Start resolving transaction");

        final IBinder token = transaction.getActivityToken();
        if (token != null) {
            final Map<IBinder, ClientTransactionItem> activitiesToBeDestroyed =
                    mTransactionHandler.getActivitiesToBeDestroyed();
            final ClientTransactionItem destroyItem = activitiesToBeDestroyed.get(token);
            if (destroyItem != null) {
                if (transaction.getLifecycleStateRequest() == destroyItem) {
                    // It is going to execute the transaction that will destroy activity with the
                    // token, so the corresponding to-be-destroyed record can be removed.
                    activitiesToBeDestroyed.remove(token);
                }
                if (mTransactionHandler.getActivityClient(token) == null) {
                    // The activity has not been created but has been requested to destroy, so all
                    // transactions for the token are just like being cancelled.
                    Slog.w(TAG, tId(transaction) + "Skip pre-destroyed transaction:\n"
                            + transactionToString(transaction, mTransactionHandler));
                    return;
                }
            }
        }

        if (DEBUG_RESOLVER) Slog.d(TAG, transactionToString(transaction, mTransactionHandler));

        executeCallbacks(transaction);
        //关键代码；
        executeLifecycleState(transaction);
        mPendingActions.clear();
        if (DEBUG_RESOLVER) Slog.d(TAG, tId(transaction) + "End resolving transaction");
    }
    
    private void executeLifecycleState(ClientTransaction transaction) {
        //关键代码；
        final ActivityLifecycleItem lifecycleItem = transaction.getLifecycleStateRequest();
        if (lifecycleItem == null) {
            // No lifecycle request, return early.
            return;
        }

        final IBinder token = transaction.getActivityToken();
        final ActivityClientRecord r = mTransactionHandler.getActivityClient(token);
        if (DEBUG_RESOLVER) {
            Slog.d(TAG, tId(transaction) + "Resolving lifecycle state: "
                    + lifecycleItem + " for activity: "
                    + getShortActivityName(token, mTransactionHandler));
        }

        if (r == null) {
            // Ignore requests for non-existent client records for now.
            return;
        }

        // Cycle to the state right before the final requested state.
        cycleToPath(r, lifecycleItem.getTargetState(), true /* excludeLastState */, transaction);

        // Execute the final transition with proper parameters.
        //关键代码；
        lifecycleItem.execute(mTransactionHandler, token, mPendingActions);
        lifecycleItem.postExecute(mTransactionHandler, token, mPendingActions);
    }
    //...
}

返回一个 ActivityLifecycleItem 类型的对象，而 ActivityLifecycleItem 是一个抽象类，必然要有实现类，通过全局搜索存在三个实现类：PauseActivityItem，ResumeActivityItem，StopActivityItem，由于这里是要让 Activity 进入 Pause 状态，所以这里是 PauseActivityItem。

19. PauseActivityItem

代码位置：(framework\base\core\java\android\app\servertransaction)

public class PauseActivityItem extends ActivityLifecycleItem {
    //...
    @Override
    public void execute(ClientTransactionHandler client, IBinder token,
            PendingTransactionActions pendingActions) {
        Trace.traceBegin(TRACE_TAG_ACTIVITY_MANAGER, "activityPause");
        client.handlePauseActivity(token, mFinished, mUserLeaving, mConfigChanges, pendingActions,
                "PAUSE_ACTIVITY_ITEM");
        Trace.traceEnd(TRACE_TAG_ACTIVITY_MANAGER);
    }
    //...
}

调用了handlePauseActivity()方法，但这个方法在ClientTransactionHandler中只是一个抽象方法，具体实现是在ActivityThread中。

20. ActivityThread

public final class ActivityThread extends ClientTransactionHandler {
    //...
    @Override
    public void handlePauseActivity(IBinder token, boolean finished, boolean userLeaving,
            int configChanges, PendingTransactionActions pendingActions, String reason) {
        ActivityClientRecord r = mActivities.get(token);
        if (r != null) {
            if (userLeaving) {
                performUserLeavingActivity(r);
            }

            r.activity.mConfigChangeFlags |= configChanges;
            //关键代码；
            performPauseActivity(r, finished, reason, pendingActions);

            // Make sure any pending writes are now committed.
            if (r.isPreHoneycomb()) {
                QueuedWork.waitToFinish();
            }
            mSomeActivitiesChanged = true;
        }
    }
    
    private Bundle performPauseActivity(ActivityClientRecord r, boolean finished, String reason,
            PendingTransactionActions pendingActions) {
        if (r.paused) {
            if (r.activity.mFinished) {
                // If we are finishing, we won't call onResume() in certain cases.
                // So here we likewise don't want to call onPause() if the activity
                // isn't resumed.
                return null;
            }
            RuntimeException e = new RuntimeException(
                    "Performing pause of activity that is not resumed: "
                    + r.intent.getComponent().toShortString());
            Slog.e(TAG, e.getMessage(), e);
        }
        if (finished) {
            r.activity.mFinished = true;
        }

        // Pre-Honeycomb apps always save their state before pausing
        final boolean shouldSaveState = !r.activity.mFinished && r.isPreHoneycomb();
        if (shouldSaveState) {
            callActivityOnSaveInstanceState(r);
        }

        performPauseActivityIfNeeded(r, reason);

        // Notify any outstanding on paused listeners
        ArrayList<OnActivityPausedListener> listeners;
        synchronized (mOnPauseListeners) {
            listeners = mOnPauseListeners.remove(r.activity);
        }
        int size = (listeners != null ? listeners.size() : 0);
        for (int i = 0; i < size; i++) {
            listeners.get(i).onPaused(r.activity);
        }

        final Bundle oldState = pendingActions != null ? pendingActions.getOldState() : null;
        if (oldState != null) {
            if (r.isPreHoneycomb()) {
                r.state = oldState;
            }
        }

        return shouldSaveState ? r.state : null;
    }
    
    private void performPauseActivityIfNeeded(ActivityClientRecord r, String reason) {
        if (r.paused) {
            // You are already paused silly...
            return;
        }

        // Always reporting top resumed position loss when pausing an activity. If necessary, it
        // will be restored in performResumeActivity().
        reportTopResumedActivityChanged(r, false /* onTop */, "pausing");

        try {
            r.activity.mCalled = false;
            //关键代码；
            mInstrumentation.callActivityOnPause(r.activity);
            if (!r.activity.mCalled) {
                throw new SuperNotCalledException("Activity " + safeToComponentShortString(r.intent)
                        + " 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 "
                        + safeToComponentShortString(r.intent) + ": " + e.toString(), e);
            }
        }
        r.setState(ON_PAUSE);
    }
    //...
}

最终调用的是Instrumentation的callActivityOnPause()方法。

21. Instrumentation

public class Instrumentation{
    //...
    public void callActivityOnPause(Activity activity) {
        activity.performPause();
    }
    //...
}

回到了Activity的performPause()方法。

22. Activity

public class Activity extends ContextThemeWrapper
        implements LayoutInflater.Factory2,
        Window.Callback, KeyEvent.Callback,
        OnCreateContextMenuListener, ComponentCallbacks2,
        Window.OnWindowDismissedCallback, WindowControllerCallback,
        AutofillManager.AutofillClient, ContentCaptureManager.ContentCaptureClient {
    //...
    final void performPause() {
        dispatchActivityPrePaused();
        mDoReportFullyDrawn = false;
        mFragments.dispatchPause();
        mCalled = false;
        //关键代码；
        onPause();
        writeEventLog(LOG_AM_ON_PAUSE_CALLED, "performPause");
        mResumed = false;
        if (!mCalled && getApplicationInfo().targetSdkVersion
                >= android.os.Build.VERSION_CODES.GINGERBREAD) {
            throw new SuperNotCalledException(
                    "Activity " + mComponent.toShortString() +
                    " did not call through to super.onPause()");
        }
        dispatchActivityPostPaused();
    }
    //...   
}

这里，我们可以看到onPause()方法，也就是我们平时会用到的Activity的一个生命周期方法onPause()。

23. ActivityStackSupervisor

代码位置：
在ActivityStack中，我们分析代码出现了分支，会先将Activity切换为onPause()状态，然后再去启动Activity。
也就是我们接下来要分析的startSpecificActivityLocked()方法。

public class ActivityStackSupervisor implements RecentTasks.Callbacks {
    //...
    void startSpecificActivityLocked(ActivityRecord r, boolean andResume, boolean checkConfig) {
        // Is this activity's application already running?
        final WindowProcessController wpc =
                mService.getProcessController(r.processName, r.info.applicationInfo.uid);

        boolean knownToBeDead = false;
        //注释1；
        if (wpc != null && wpc.hasThread()) {
            try {
                //注释2；
                realStartActivityLocked(r, wpc, andResume, checkConfig);
                return;
            } catch (RemoteException e) {
                Slog.w(TAG, "Exception when starting activity "
                        + r.intent.getComponent().flattenToShortString(), e);
            }

            // If a dead object exception was thrown -- fall through to
            // restart the application.
            knownToBeDead = true;
        }
        if (getKeyguardController().isKeyguardLocked()) {
            r.notifyUnknownVisibilityLaunched();
        }

        try {
            if (Trace.isTagEnabled(TRACE_TAG_ACTIVITY_MANAGER)) {
                Trace.traceBegin(TRACE_TAG_ACTIVITY_MANAGER, "dispatchingStartProcess:"
                        + r.processName);
            }
            // Post message to start process to avoid possible deadlock of calling into AMS with the
            // ATMS lock held.
            //注释3；
            final Message msg = PooledLambda.obtainMessage(
                    ActivityManagerInternal::startProcess, mService.mAmInternal, r.processName,
                    r.info.applicationInfo, knownToBeDead, "activity", r.intent.getComponent());
            mService.mH.sendMessage(msg);
        } finally {
            Trace.traceEnd(TRACE_TAG_ACTIVITY_MANAGER);
        }
    }
    //...
}

注释1:判断 App 进程是否存在，并且已经启动，如果存在而且已经启动，则调用方法 realStartActivityLocked()。
注释3:如果 App 进程不存在，从注释可以看出通过 sendMessage 启动新的进程。因为这里的是从 Launcher 启动 App，所以进程必然不存在，这里追如果创建进程。
这里的 mService 是 ActivityTaskManagerService，它的内部和 ActivityThread 一样也存在一个 H 类，只是它比 ActivityThread 简单很多。

24. ActivityTaskManagerService

public class ActivityTaskManagerService extends IActivityTaskManager.Stub {
    //...
    final class H extends Handler {
        @Override
        public void handleMessage(Message msg) {
            switch (msg.what) {
                case REPORT_TIME_TRACKER_MSG: {
                    AppTimeTracker tracker = (AppTimeTracker) msg.obj;
                    tracker.deliverResult(mContext);
                } break;
            }
        }
    }
    //...
}

但是在ActivityTaskManagerService不是启动进程的地方。而是在ActivityManagerInternal::startProcess中。

25. ActivityManagerInternal

代码位置：(frameworks\base\core\java\android\app)

public abstract class ActivityManagerInternal {
    //...
    public abstract void startProcess(String processName, ApplicationInfo info,boolean knownToBeDead, String hostingType, ComponentName hostingName);
    //...
}

关于ActivityManagerInternal 类，在它的内部也有方法 startProcess()，只不过此类为抽象类，方法也为抽象方法。在庞大的 ActivityManagerService 类中存在一个内部类 LocalService，它的父类就是ActivityManagerInternal。

26. ActivityManagerService

代码位置：(framework\base\services\core\java\com\android\server\am)

public class ActivityManagerService extends IActivityManager.Stub
        implements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback {
    //...
    @VisibleForTesting
    public final class LocalService extends ActivityManagerInternal {
        //...
        @Override
        public void startProcess(String processName, ApplicationInfo info,
                boolean knownToBeDead, String hostingType, ComponentName hostingName) {
            try {
                if (Trace.isTagEnabled(Trace.TRACE_TAG_ACTIVITY_MANAGER)) {
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "startProcess:"
                            + processName);
                }
                synchronized (ActivityManagerService.this) {
                    //关键代码；
                    startProcessLocked(processName, info, knownToBeDead, 0 /* intentFlags */,
                            new HostingRecord(hostingType, hostingName),
                            false /* allowWhileBooting */, false /* isolated */,
                            true /* keepIfLarge */);
                }
            } finally {
                Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
            }
        }
        //...
    }
    
    @GuardedBy("this")
    final ProcessRecord startProcessLocked(String processName,
            ApplicationInfo info, boolean knownToBeDead, int intentFlags,
            HostingRecord hostingRecord, boolean allowWhileBooting,
            boolean isolated, boolean keepIfLarge) {
        //关键代码；
        return mProcessList.startProcessLocked(processName, info, knownToBeDead, intentFlags,
                hostingRecord, allowWhileBooting, isolated, 0 /* isolatedUid */, keepIfLarge,
                null /* ABI override */, null /* entryPoint */, null /* entryPointArgs */,
                null /* crashHandler */);
    }
    //...        
}

27. ProcessList

代码位置：(framework\base\services\core\java\com\android\server\am)

public final class ProcessList {
    //...
    @GuardedBy("mService")
    boolean startProcessLocked(ProcessRecord app, HostingRecord hostingRecord,
            boolean disableHiddenApiChecks, boolean mountExtStorageFull,
            String abiOverride) {
        //...
        return startProcessLocked(hostingRecord, entryPoint, app, uid, gids,runtimeFlags, mountExternal, seInfo, requiredAbi, instructionSet, invokeWith,startTime);
        //...        
    }
    
    @GuardedBy("mService")
    boolean startProcessLocked(HostingRecord hostingRecord,String entryPoint,ProcessRecord app, int uid, int[] gids, int runtimeFlags, int mountExternal,String seInfo, String requiredAbi, String instructionSet, String invokeWith,long startTime) {
        //...
        if (mService.mConstants.FLAG_PROCESS_START_ASYNC) {
            if (DEBUG_PROCESSES) Slog.i(TAG_PROCESSES,
                    "Posting procStart msg for " + app.toShortString());
            mService.mProcStartHandler.post(() -> {
                try {
                    //关键代码；
                    final Process.ProcessStartResult startResult = startProcess(app.hostingRecord,
                            entryPoint, app, app.startUid, gids, runtimeFlags, mountExternal,
                            app.seInfo, requiredAbi, instructionSet, invokeWith, app.startTime);
                    synchronized (mService) {
                        handleProcessStartedLocked(app, startResult, startSeq);
                    }
                } catch (RuntimeException e) {
                    synchronized (mService) {
                        Slog.e(ActivityManagerService.TAG, "Failure starting process "
                                + app.processName, e);
                        mPendingStarts.remove(startSeq);
                        app.pendingStart = false;
                        mService.forceStopPackageLocked(app.info.packageName,
                                UserHandle.getAppId(app.uid),
                                false, false, true, false, false, app.userId, "start failure");
                    }
                }
            });
            return true;
        } else {
            try {
                //关键代码；
                final Process.ProcessStartResult startResult = startProcess(hostingRecord,
                        entryPoint, app,
                        uid, gids, runtimeFlags, mountExternal, seInfo, requiredAbi, instructionSet,
                        invokeWith, startTime);
                handleProcessStartedLocked(app, startResult.pid, startResult.usingWrapper,
                        startSeq, false);
            } catch (RuntimeException e) {
                Slog.e(ActivityManagerService.TAG, "Failure starting process "
                        + app.processName, e);
                app.pendingStart = false;
                mService.forceStopPackageLocked(app.info.packageName, UserHandle.getAppId(app.uid),
                        false, false, true, false, false, app.userId, "start failure");
            }
            return app.pid > 0;
        }
        //...    
    }
    
    private Process.ProcessStartResult startProcess(HostingRecord hostingRecord, String entryPoint,
            ProcessRecord app, int uid, int[] gids, int runtimeFlags, int mountExternal,
            String seInfo, String requiredAbi, String instructionSet, String invokeWith,
            long startTime) {
        try {
            Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "Start proc: " +
                    app.processName);
            checkSlow(startTime, "startProcess: asking zygote to start proc");
            final Process.ProcessStartResult startResult;
            if (hostingRecord.usesWebviewZygote()) {
                startResult = startWebView(entryPoint,
                        app.processName, uid, uid, gids, runtimeFlags, mountExternal,
                        app.info.targetSdkVersion, seInfo, requiredAbi, instructionSet,
                        app.info.dataDir, null, app.info.packageName,
                        new String[] {PROC_START_SEQ_IDENT + app.startSeq});
            } else if (hostingRecord.usesAppZygote()) {
                final AppZygote appZygote = createAppZygoteForProcessIfNeeded(app);

                startResult = appZygote.getProcess().start(entryPoint,
                        app.processName, uid, uid, gids, runtimeFlags, mountExternal,
                        app.info.targetSdkVersion, seInfo, requiredAbi, instructionSet,
                        app.info.dataDir, null, app.info.packageName,
                        /*useUsapPool=*/ false,
                        new String[] {PROC_START_SEQ_IDENT + app.startSeq});
            } else {
                //关键代码；
                startResult = Process.start(entryPoint,
                        app.processName, uid, uid, gids, runtimeFlags, mountExternal,
                        app.info.targetSdkVersion, seInfo, requiredAbi, instructionSet,
                        app.info.dataDir, invokeWith, app.info.packageName,
                        new String[] {PROC_START_SEQ_IDENT + app.startSeq});
            }
            checkSlow(startTime, "startProcess: returned from zygote!");
            return startResult;
        } finally {
            Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
        }
    }
    //...
}

28. Process

代码位置：(framework\base\core\java\android\os)

public class Process {
    //...
    public static ProcessStartResult start(@NonNull final String processClass,
                                           @Nullable final String niceName,
                                           int uid, int gid, @Nullable int[] gids,
                                           int runtimeFlags,
                                           int mountExternal,
                                           int targetSdkVersion,
                                           @Nullable String seInfo,
                                           @NonNull String abi,
                                           @Nullable String instructionSet,
                                           @Nullable String appDataDir,
                                           @Nullable String invokeWith,
                                           @Nullable String packageName,
                                           @Nullable String[] zygoteArgs) {
        return ZYGOTE_PROCESS.start(processClass, niceName, uid, gid, gids,
                    runtimeFlags, mountExternal, targetSdkVersion, seInfo,
                    abi, instructionSet, appDataDir, invokeWith, packageName,
                    /*useUsapPool=*/ true, zygoteArgs);
    }
    //...
}

调用了ZygoteProcess的start()方法。

29. ZygoteProcess

代码位置：(framework\base\core\java\android\os)

class ZygoteStartFailedEx extends Exception {
    //...
    public final Process.ProcessStartResult start(@NonNull final String processClass,
                                                  final String niceName,
                                                  int uid, int gid, @Nullable int[] gids,
                                                  int runtimeFlags, int mountExternal,
                                                  int targetSdkVersion,
                                                  @Nullable String seInfo,
                                                  @NonNull String abi,
                                                  @Nullable String instructionSet,
                                                  @Nullable String appDataDir,
                                                  @Nullable String invokeWith,
                                                  @Nullable String packageName,
                                                  boolean useUsapPool,
                                                  @Nullable String[] zygoteArgs) {
        // TODO (chriswailes): Is there a better place to check this value?
        if (fetchUsapPoolEnabledPropWithMinInterval()) {
            informZygotesOfUsapPoolStatus();
        }

        try {
            return startViaZygote(processClass, niceName, uid, gid, gids,
                    runtimeFlags, mountExternal, targetSdkVersion, seInfo,
                    abi, instructionSet, appDataDir, invokeWith, /*startChildZygote=*/ false,
                    packageName, useUsapPool, zygoteArgs);
        } catch (ZygoteStartFailedEx ex) {
            Log.e(LOG_TAG,
                    "Starting VM process through Zygote failed");
            throw new RuntimeException(
                    "Starting VM process through Zygote failed", ex);
        }
    }
    
    private Process.ProcessStartResult startViaZygote(@NonNull final String processClass,
                                                      @Nullable final String niceName,
                                                      final int uid, final int gid,
                                                      @Nullable final int[] gids,
                                                      int runtimeFlags, int mountExternal,
                                                      int targetSdkVersion,
                                                      @Nullable String seInfo,
                                                      @NonNull String abi,
                                                      @Nullable String instructionSet,
                                                      @Nullable String appDataDir,
                                                      @Nullable String invokeWith,
                                                      boolean startChildZygote,
                                                      @Nullable String packageName,
                                                      boolean useUsapPool,
                                                      @Nullable String[] extraArgs)
                                                      throws ZygoteStartFailedEx {
        ArrayList<String> argsForZygote = new ArrayList<>();

        // --runtime-args, --setuid=, --setgid=,
        // and --setgroups= must go first
        argsForZygote.add("--runtime-args");
        argsForZygote.add("--setuid=" + uid);
        argsForZygote.add("--setgid=" + gid);
        argsForZygote.add("--runtime-flags=" + runtimeFlags);
        if (mountExternal == Zygote.MOUNT_EXTERNAL_DEFAULT) {
            argsForZygote.add("--mount-external-default");
        } else if (mountExternal == Zygote.MOUNT_EXTERNAL_READ) {
            argsForZygote.add("--mount-external-read");
        } else if (mountExternal == Zygote.MOUNT_EXTERNAL_WRITE) {
            argsForZygote.add("--mount-external-write");
        } else if (mountExternal == Zygote.MOUNT_EXTERNAL_FULL) {
            argsForZygote.add("--mount-external-full");
        } else if (mountExternal == Zygote.MOUNT_EXTERNAL_INSTALLER) {
            argsForZygote.add("--mount-external-installer");
        } else if (mountExternal == Zygote.MOUNT_EXTERNAL_LEGACY) {
            argsForZygote.add("--mount-external-legacy");
        }

        argsForZygote.add("--target-sdk-version=" + targetSdkVersion);

        // --setgroups is a comma-separated list
        if (gids != null && gids.length > 0) {
            StringBuilder sb = new StringBuilder();
            sb.append("--setgroups=");

            int sz = gids.length;
            for (int i = 0; i < sz; i++) {
                if (i != 0) {
                    sb.append(',');
                }
                sb.append(gids[i]);
            }

            argsForZygote.add(sb.toString());
        }

        if (niceName != null) {
            argsForZygote.add("--nice-name=" + niceName);
        }

        if (seInfo != null) {
            argsForZygote.add("--seinfo=" + seInfo);
        }

        if (instructionSet != null) {
            argsForZygote.add("--instruction-set=" + instructionSet);
        }

        if (appDataDir != null) {
            argsForZygote.add("--app-data-dir=" + appDataDir);
        }

        if (invokeWith != null) {
            argsForZygote.add("--invoke-with");
            argsForZygote.add(invokeWith);
        }

        if (startChildZygote) {
            argsForZygote.add("--start-child-zygote");
        }

        if (packageName != null) {
            argsForZygote.add("--package-name=" + packageName);
        }

        argsForZygote.add(processClass);

        if (extraArgs != null) {
            Collections.addAll(argsForZygote, extraArgs);
        }

        synchronized(mLock) {
            return zygoteSendArgsAndGetResult(openZygoteSocketIfNeeded(abi),
                                              useUsapPool,
                                              argsForZygote);
        }
    }
    
    private Process.ProcessStartResult zygoteSendArgsAndGetResult(
            ZygoteState zygoteState, boolean useUsapPool, @NonNull ArrayList<String> args)
            throws ZygoteStartFailedEx {
        for (String arg : args) {
            if (arg.indexOf('\n') >= 0) {
                throw new ZygoteStartFailedEx("Embedded newlines not allowed");
            } else if (arg.indexOf('\r') >= 0) {
                throw new ZygoteStartFailedEx("Embedded carriage returns not allowed");
            }
        }

        String msgStr = args.size() + "\n" + String.join("\n", args) + "\n";

        if (useUsapPool && mUsapPoolEnabled && canAttemptUsap(args)) {
            try {
                return attemptUsapSendArgsAndGetResult(zygoteState, msgStr);
            } catch (IOException ex) {
                Log.e(LOG_TAG, "IO Exception while communicating with USAP pool - "
                        + ex.getMessage());
            }
        }

        return attemptZygoteSendArgsAndGetResult(zygoteState, msgStr);
    }
    
    private Process.ProcessStartResult attemptZygoteSendArgsAndGetResult(
            ZygoteState zygoteState, String msgStr) throws ZygoteStartFailedEx {
        try {
            final BufferedWriter zygoteWriter = zygoteState.mZygoteOutputWriter;
            final DataInputStream zygoteInputStream = zygoteState.mZygoteInputStream;

            zygoteWriter.write(msgStr);
            zygoteWriter.flush();

            Process.ProcessStartResult result = new Process.ProcessStartResult();
            result.pid = zygoteInputStream.readInt();
            result.usingWrapper = zygoteInputStream.readBoolean();

            if (result.pid < 0) {
                throw new ZygoteStartFailedEx("fork() failed");
            }

            return result;
        } catch (IOException ex) {
            zygoteState.close();
            Log.e(LOG_TAG, "IO Exception while communicating with Zygote - "
                    + ex.toString());
            throw new ZygoteStartFailedEx(ex);
        }
    }
    //...
}

在attemptConnectionToPrimaryZygote()方法向主 Zygote 发起连接请求。通过 socket 向 Zygote 进程发送参数列表，然后进入“阻塞等待”状态，直到 socket 服务端发送回来新创建的进程 pid。Zygote 进程源码的最后调用到了 ActivityThread.main() 方法。

30. ActivityThread

public final class ActivityThread extends ClientTransactionHandler {
    //...
    public static void main(String[] args) {
        //...
        //注释1；
        Looper.prepareMainLooper();
        //注释2；
        ActivityThread thread = new ActivityThread();
        thread.attach(false, startSeq);

        if (sMainThreadHandler == null) {
            sMainThreadHandler = thread.getHandler();
        }

        if (false) {
            Looper.myLooper().setMessageLogging(new LogPrinter(Log.DEBUG, "ActivityThread"));
        }

        // End of event ActivityThreadMain.
        Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
        Looper.loop();
        //...
    }
    
    @UnsupportedAppUsage
    private void attach(boolean system, long startSeq) {
        sCurrentActivityThread = this;
        mSystemThread = system;
        if (!system) {
            android.ddm.DdmHandleAppName.setAppName("<pre-initialized>",
                                                    UserHandle.myUserId());
            RuntimeInit.setApplicationObject(mAppThread.asBinder());
            //注释3；
            final IActivityManager mgr = ActivityManager.getService();
            try {
                //注释4；
                mgr.attachApplication(mAppThread, startSeq);
            } catch (RemoteException ex) {
                throw ex.rethrowFromSystemServer();
            }
            // 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 {
                            ActivityTaskManager.getService().releaseSomeActivities(mAppThread);
                        } catch (RemoteException e) {
                            throw e.rethrowFromSystemServer();
                        }
                    }
                }
            });
        } 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();
                mInstrumentation.basicInit(this);
                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);
            }
        }

        ViewRootImpl.ConfigChangedCallback configChangedCallback
                = (Configuration globalConfig) -> {
            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(globalConfig,
                        null /* compat */)) {
                    updateLocaleListFromAppContext(mInitialApplication.getApplicationContext(),
                            mResourcesManager.getConfiguration().getLocales());

                    // This actually changed the resources! Tell everyone about it.
                    if (mPendingConfiguration == null
                            || mPendingConfiguration.isOtherSeqNewer(globalConfig)) {
                        mPendingConfiguration = globalConfig;
                        sendMessage(H.CONFIGURATION_CHANGED, globalConfig);
                    }
                }
            }
        };
        ViewRootImpl.addConfigCallback(configChangedCallback);
    }
    
    //...
}

注释1:创建主线程的 Looper，主线程的 Handler 不需要创建 Looper。
注释2:初始化 ActivityThread，并且 attach 到系统进程。
注释3:这里 IActivityManager 类型对象是 ActivityManagerService。
注释4:通过 ActivityManagerService 的 attachApplication 方法绑定一个 Application，这里的 mAppThread 是 ApplicationThread 对象。

31. ActivityManagerService

public class ActivityManagerService extends IActivityManager.Stub
        implements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback {
    //...
    @Override
    public final void attachApplication(IApplicationThread thread, long startSeq) {
        synchronized (this) {
            int callingPid = Binder.getCallingPid();
            final int callingUid = Binder.getCallingUid();
            final long origId = Binder.clearCallingIdentity();
            attachApplicationLocked(thread, callingPid, callingUid, startSeq);
            Binder.restoreCallingIdentity(origId);
        }
    }
    
    @GuardedBy("this")
    private final boolean attachApplicationLocked(IApplicationThread thread,int pid, int callingUid, long startSeq) {
        //...
        boolean badApp = false;
        boolean didSomething = false;

        // See if the top visible activity is waiting to run in this process...
        //注释1；
        if (normalMode) {
            try {
                didSomething = mAtmInternal.attachApplication(app.getWindowProcessController());
            } catch (Exception e) {
                Slog.wtf(TAG, "Exception thrown launching activities in " + app, e);
                badApp = true;
            }
        }

        // Find any services that should be running in this process...
        //注释2；
        if (!badApp) {
            try {
                didSomething |= mServices.attachApplicationLocked(app, processName);
                checkTime(startTime, "attachApplicationLocked: after mServices.attachApplicationLocked");
            } catch (Exception e) {
                Slog.wtf(TAG, "Exception thrown starting services in " + app, e);
                badApp = true;
            }
        }

        // Check if a next-broadcast receiver is in this process...
        //注释3；
        if (!badApp && isPendingBroadcastProcessLocked(pid)) {
            try {
                didSomething |= sendPendingBroadcastsLocked(app);
                checkTime(startTime, "attachApplicationLocked: after sendPendingBroadcastsLocked");
            } catch (Exception e) {
                // If the app died trying to launch the receiver we declare it 'bad'
                Slog.wtf(TAG, "Exception thrown dispatching broadcasts in " + app, e);
                badApp = true;
            }
        }
        //...
    }
    //...        
}

注释1:对于Activity的处理逻辑。调用了 ActivityTaskManagerInternal 的 attachApplication() 方法。ActivityTaskManagerInternal 在 ActivityManagerService 的构造方法中进行初始化的。
注释2:对于Service的处理逻辑。
注释3:对于BroadcastReceiver的处理逻辑。

32. ActivityTaskManagerInternal

代码位置：(framework\base\services\core\java\com\android\server\wm)
ActivityTaskManagerInternal 是一个抽象类，attachApplication() 也是一个抽象方法。
ActivityTaskManagerInternal 的实现类是 ActivityTaskManagerService 中的内部类 LocalService 继承了此抽象方法。

public abstract class ActivityTaskManagerInternal {
    //...
    public abstract boolean attachApplication(WindowProcessController wpc) throws RemoteException;
    //...
}

33. ActivityTaskManagerService/LocalService

public class ActivityTaskManagerService extends IActivityTaskManager.Stub {
    //...
    final class LocalService extends ActivityTaskManagerInternal {
        //...
        @HotPath(caller = HotPath.PROCESS_CHANGE)
        @Override
        public boolean attachApplication(WindowProcessController wpc) throws RemoteException {
            synchronized (mGlobalLockWithoutBoost) {
                return mRootActivityContainer.attachApplication(wpc);
            }
        }
        //...
    }
    //...        
}

调用的是RootActivityContainer的attachApplication()方法。

34. RootActivityContainer

class RootActivityContainer extends ConfigurationContainer implements DisplayManager.DisplayListener {
    //...
    boolean attachApplication(WindowProcessController app) throws RemoteException {
        final String processName = app.mName;
        boolean didSomething = false;
        for (int displayNdx = mActivityDisplays.size() - 1; displayNdx >= 0; --displayNdx) {
            final ActivityDisplay display = mActivityDisplays.get(displayNdx);
            final ActivityStack stack = display.getFocusedStack();
            if (stack != null) {
                stack.getAllRunningVisibleActivitiesLocked(mTmpActivityList);
                final ActivityRecord top = stack.topRunningActivityLocked();
                final int size = mTmpActivityList.size();
                for (int i = 0; i < size; i++) {
                    final ActivityRecord activity = mTmpActivityList.get(i);
                    if (activity.app == null && app.mUid == activity.info.applicationInfo.uid
                            && processName.equals(activity.processName)) {
                        try {
                            //关键代码；
                            if (mStackSupervisor.realStartActivityLocked(activity, app,top == activity /* andResume */, true /* checkConfig */)) {
                                didSomething = true;
                            }
                        } catch (RemoteException e) {
                            Slog.w(TAG, "Exception in new application when starting activity "
                                    + top.intent.getComponent().flattenToShortString(), e);
                            throw e;
                        }
                    }
                }
            }
        }
        if (!didSomething) {
            ensureActivitiesVisible(null, 0, false /* preserve_windows */);
        }
        return didSomething;
    }
    //...
}

调用 ActivityStackSupervisor 的 realStartActivityLocked() 方法。

35. ActivityStackSupervisor

public class ActivityStackSupervisor implements RecentTasks.Callbacks {
    //...
    boolean realStartActivityLocked(ActivityRecord r, WindowProcessController proc,
            boolean andResume, boolean checkConfig) throws RemoteException {
        //...
        // Schedule transaction.
        mService.getLifecycleManager().scheduleTransaction(clientTransaction);
        //...        
    }
    //...
}

mService.getLifecycleManager().scheduleTransaction，这里的 mService 是 ActivityTaskManagerService。而

getLifecycleManager 方法获取到的是 ClientLifecycleManager。又再次回到了 ClientLifecycleManager 的 scheduleTransaction 方法了。接下来的流程在前面已经分析过了，最终都会走到 Activity 的生命周期。

BubbleTextView是Launcher的应用图标控件，继承自TextView。

public class BubbleTextView extends TextView implements ItemInfoUpdateReceiver, OnResumeCallback {
    //...
    
    @Override
    public void reapplyItemInfo(ItemInfoWithIcon info) {
        if (getTag() == info) {
            mIconLoadRequest = null;
            mDisableRelayout = true;

            // Optimization: Starting in N, pre-uploads the bitmap to RenderThread.
            info.iconBitmap.prepareToDraw();

            if (info instanceof AppInfo) {
                applyFromApplicationInfo((AppInfo) info);
            } else if (info instanceof WorkspaceItemInfo) {
                applyFromWorkspaceItem((WorkspaceItemInfo) info);
                mActivity.invalidateParent(info);
            } else if (info instanceof PackageItemInfo) {
                applyFromPackageItemInfo((PackageItemInfo) info);
            }

            mDisableRelayout = false;
        }
    }
    
    public void applyFromWorkspaceItem(WorkspaceItemInfo info) {
        applyFromWorkspaceItem(info, false);
    }
    
    public void applyFromWorkspaceItem(WorkspaceItemInfo info, boolean promiseStateChanged) {
        applyIconAndLabel(info);
        setTag(info);
        if (promiseStateChanged || (info.hasPromiseIconUi())) {
            applyPromiseState(promiseStateChanged);
        }

        applyDotState(info, false /* animate */);
    }
    
    public void applyFromApplicationInfo(AppInfo info) {
        applyIconAndLabel(info);

        // We don't need to check the info since it's not a WorkspaceItemInfo
        super.setTag(info);

        // Verify high res immediately
        verifyHighRes();

        if (info instanceof PromiseAppInfo) {
            PromiseAppInfo promiseAppInfo = (PromiseAppInfo) info;
            applyProgressLevel(promiseAppInfo.level);
        }
        applyDotState(info, false /* animate */);
    }
    
    public void applyFromPackageItemInfo(PackageItemInfo info) {
        applyIconAndLabel(info);
        // We don't need to check the info since it's not a WorkspaceItemInfo
        super.setTag(info);

        // Verify high res immediately
        verifyHighRes();
    }
    
    private void applyIconAndLabel(ItemInfoWithIcon info) {
        FastBitmapDrawable iconDrawable = DrawableFactory.INSTANCE.get(getContext())
                .newIcon(getContext(), info);
        mDotParams.color = IconPalette.getMutedColor(info.iconColor, 0.54f);

        setIcon(iconDrawable);
        setText(info.title);
        if (info.contentDescription != null) {
            setContentDescription(info.isDisabled()
                    ? getContext().getString(R.string.disabled_app_label, info.contentDescription)
                    : info.contentDescription);
        }
    }
    
    //...
}

可以发现，桌面的应用图标都是在reapplyItemInfo(ItemInfoWithIcon info)方法中设置的，那么我们追踪这个方法，是在ItemInfoUpdateReceiver接口中声明的，在IconCache.java文件中调用。
其中关于桌面应用图标的数据信息，类关系如下：

总结

启动流程：
1 . 点击桌面app图标，Launcher进程采用Binder IPC向system_server进程发起startActivity请求；
2 . system_server进程接收到请求后，向zygote进程发送创建进程的请求；
3 . zygote进程fork出新的子进程，即app进程；
4 . app进程通过Binder IPC向system_server进程发起attachApplication请求；
5 . system_server进程在收到请求后，进行一系列的准备工作后，再通过Binder IPC向app进程发送LAUNCH_ACTIVITY消息；
6 . 主线程在收到Message后，通过发射机制创建目标Activity，并回调Activity.onCreate()等方法；
7 . 进行到这里，app便正式启动，开始进入Activity生命周期，执行完onCreate()、onStart()、onResume()方法后，UI渲染结束后便可以看到app的界面了；

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