Activity的启动流程（基于Android10）

一个Activity的启动过程可以说是面试必问的问题，这其中的过程十分复杂，面试过程中能不能把核心的东西说全说对非常重要。今天我就详细的跟一遍这个流程，总结出来。目标用5个小时的时间。

关键类

ActivityThread		应用进程的主线程代码
ApplicationThread   应用进程端，IPC的对象，用来与服务端通信
Instrument 	负责调用Activity和Application的生命周期
ActivityTaskManagerService  负责管理AMS
ActivityStarter		负责启动模式，启动Flag相关
ActivityStack		
ActivityStackSuprevisor
ClientTransactionItem：执行具体activity生命周期的抽象类，子类：LaunchActivityItem等
ClientTransaction:处理相关的信息和生命周期状态
TransactionExecutor 主要作用是执行ClientTransaction
ClientLifecycleManager 生命周期的管理调用

点击桌面图标触发Laucher流程

Launcher应用在创建桌面图标的时候会为每个图标注册一个点击回调

	// packages/apps/Launcher3/src/com/android/launcher3/Launcher.java

    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);	// 回调方法
        favorite.setOnFocusChangeListener(mFocusHandler);
        return favorite;
    }

当点击图标时，触发ItemClickHandler的onClick方法，然后会执行startAppShortcutOrInfoActivity方法

public class ItemClickHandler {
    private static void onClick(View v, String sourceContainer) {
        // 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) return;

        Launcher launcher = Launcher.getLauncher(v.getContext());
        if (!launcher.getWorkspace().isFinishedSwitchingState()) return;

        Object tag = v.getTag();
        if (tag instanceof WorkspaceItemInfo) {
            // 点击桌面组件
            onClickAppShortcut(v, (WorkspaceItemInfo) tag, launcher, sourceContainer);
        } else if (tag instanceof FolderInfo) {
            // 点击文件夹图标
            if (v instanceof FolderIcon) {
                onClickFolderIcon(v);
            }
        } else if (tag instanceof AppInfo) {
            // 点击应用图标
            startAppShortcutOrInfoActivity(v, (AppInfo) tag, launcher,
                    sourceContainer == null ? CONTAINER_ALL_APPS: sourceContainer);
        } else if (tag instanceof LauncherAppWidgetInfo) {
            // 点击启动小组件
            if (v instanceof PendingAppWidgetHostView) {
                onClickPendingWidget((PendingAppWidgetHostView) v, launcher);
            }
        }
    }
    
    private static void startAppShortcutOrInfoActivity(View v, ItemInfo item, Launcher launcher,
            @Nullable String sourceContainer) {
        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 */);
        }
        
        // 调用Launcher的startActivitySafely方法
        launcher.startActivitySafely(v, intent, item, sourceContainer);
    }
}

public class Launcher extends BaseDraggingActivity implements LauncherExterns,
        Callbacks, LauncherProviderChangeListener, UserEventDelegate,
        InvariantDeviceProfile.OnIDPChangeListener {
            
    public boolean startActivitySafely(View v, Intent intent, ItemInfo item,
            @Nullable String sourceContainer) {
        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;
        }

        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;
    }
}

然后调用父类BaseDraggingActivity的startActivitySafely方法，在其中调用Activity的startActity方法，并且添加了FLAG_ACTIVITY_NEW_TASK标记 ，表示在新的任务栈中拉起应用。

public abstract class BaseDraggingActivity extends BaseActivity
        implements WallpaperColorInfo.OnChangeListener {
    public boolean startActivitySafely(View v, Intent intent, @Nullable ItemInfo item,
            @Nullable String sourceContainer) {
        if (mIsSafeModeEnabled && !PackageManagerHelper.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;

        // 添加FLAG_ACTIVITY_NEW_TASK标志
        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
                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 (NullPointerException|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;
    }

Activity流程

值得注意的是mMainThread.getApplicationThread()获取的是Launcher应用进程的ApplicationThread的对象，该对象是一个IPC通信的服务端对象，后面用于AMS端通过该对象调度应用。

public class Activity {
	public void startActivityForResult(@RequiresPermission Intent intent, int requestCode,
            @Nullable Bundle options) {
        if (mParent == null) {
            options = transferSpringboardActivityOptions(options);
            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);
            }
        }
    }
}

Instrumentation是Android系统中启动Activity的一个实际操作类，用来监控应用程序和系统的交互。

public class Instrumentation {
	public ActivityResult execStartActivity(
            Context who, IBinder contextThread, IBinder token, Activity target,
            Intent intent, int requestCode, Bundle options) {
        // IPC通信的对象
        IApplicationThread whoThread = (IApplicationThread) contextThread;
        Uri referrer = target != null ? target.onProvideReferrer() : null;
        if (referrer != null) {
            intent.putExtra(Intent.EXTRA_REFERRER, referrer);
        }
        ... ...
        try {
            intent.migrateExtraStreamToClipData();
            intent.prepareToLeaveProcess(who);
            // 调用ActivityTaskManager的startActivity方法
            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;
    }
}

此处通过IPC通信，调用到系统服务端ActivityTaskManagerService中。

ActivityTaskManagerService流程

这其中通过建造者模式构造一个ActivityStarter对象，ActivityStarter的内部类Request对象维护了启动一个Activity所需要的所有信息。

public class ActivityTaskManagerService extends IActivityTaskManager.Stub {
	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());
    }
    
    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");

        // 获取ActivityStarter对象
        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();

    }
}

class ActivityStarter {
	int execute() {
        try {
            if (mRequest.mayWait) {
                // 走这里
                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);
            } 
            ··· ···
        } finally {
            onExecutionComplete();
        }
    }
    
    // 经过一系列的startActivity的调用，最终走到startActivityUnchecked
    private int startActivityUnchecked(final ActivityRecord r, ActivityRecord sourceRecord,
            IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
            int startFlags, boolean doResume, ActivityOptions options, TaskRecord inTask,
            ActivityRecord[] outActivity, boolean restrictedBgActivity) {
        
        // 初始化启动Activity的各种配置
        setInitialState(r, options, inTask, doResume, startFlags, sourceRecord, voiceSession,
                voiceInteractor, restrictedBgActivity);

        final int preferredWindowingMode = mLaunchParams.mWindowingMode;
	
        // 计算出启动的Flag，并赋值给mLaunchFlags
        computeLaunchingTaskFlags();

        computeSourceStack();

        mIntent.setFlags(mLaunchFlags);
		
        // 判断是否有可以复用的Activity
        ActivityRecord reusedActivity = getReusableIntentActivity();

        mSupervisor.getLaunchParamsController().calculate(
                reusedActivity != null ? reusedActivity.getTaskRecord() : mInTask,
                r.info.windowLayout, r, sourceRecord, options, PHASE_BOUNDS, mLaunchParams);
        mPreferredDisplayId =
                mLaunchParams.hasPreferredDisplay() ? mLaunchParams.mPreferredDisplayId
                        : DEFAULT_DISPLAY;

        // Do not start home activity if it cannot be launched on preferred display. We are not
        // doing this in ActivityStackSupervisor#canPlaceEntityOnDisplay because it might
        // fallback to launch on other displays.
        if (r.isActivityTypeHome() && !mRootActivityContainer.canStartHomeOnDisplay(r.info,
                mPreferredDisplayId, true /* allowInstrumenting */)) {
            Slog.w(TAG, "Cannot launch home on display " + mPreferredDisplayId);
            return START_CANCELED;
        }

        if (reusedActivity != null) {
        	// reusedActivity为null
            ··· ···
        }

        ··· ···
            
        mTargetStack.startActivityLocked(mStartActivity, topFocused, newTask, mKeepCurTransition,
                mOptions);
        if (mDoResume) {
            final ActivityRecord topTaskActivity =
                    mStartActivity.getTaskRecord().topRunningActivityLocked();
            if (!mTargetStack.isFocusable()
                    || (topTaskActivity != null && topTaskActivity.mTaskOverlay
                    && mStartActivity != topTaskActivity)) {
                // If the activity is not focusable, we can't resume it, but still would like to
                // make sure it becomes visible as it starts (this will also trigger entry
                // animation). An example of this are PIP activities.
                // Also, we don't want to resume activities in a task that currently has an overlay
                // as the starting activity just needs to be in the visible paused state until the
                // over is removed.
                mTargetStack.ensureActivitiesVisibleLocked(mStartActivity, 0, !PRESERVE_WINDOWS);
                // Go ahead and tell window manager to execute app transition for this activity
                // since the app transition will not be triggered through the resume channel.
                mTargetStack.getDisplay().mDisplayContent.executeAppTransition();
            } else {
                // If the target stack was not previously focusable (previous top running activity
                // on that stack was not visible) then any prior calls to move the stack to the
                // will not update the focused stack.  If starting the new activity now allows the
                // task stack to be focusable, then ensure that we now update the focused stack
                // accordingly.
                if (mTargetStack.isFocusable()
                        && !mRootActivityContainer.isTopDisplayFocusedStack(mTargetStack)) {
                    mTargetStack.moveToFront("startActivityUnchecked");
                }
                // 最关键的一步
                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;
    }
}

其中关键的对象是mRootActivityContainer，他是一个用来临时分担一些类ActivityStackSupervisor的对象。

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)) {
            result = targetStack.resumeTopActivityUncheckedLocked(target, targetOptions);
        }
		
       ··· ···
        return result;
    }
}

再到ActivityStack的resumeTopActivityUncheckedLocked方法，其中有一个重要步骤：判断当前是否有处于Resume状态的Activity，先将其转换为onPause状态。当前场景就是将Launcher切换到onPause状态

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

        boolean result = false;
        try {
            // Protect against recursion.
            mInResumeTopActivity = true;
            result = resumeTopActivityInnerLocked(prev, options);

            // When resuming the top activity, it may be necessary to pause the top activity (for
            // example, returning to the lock screen. We suppress the normal pause logic in
            // {@link #resumeTopActivityUncheckedLocked}, since the top activity is resumed at the
            // end. We call the {@link ActivityStackSupervisor#checkReadyForSleepLocked} again here
            // to ensure any necessary pause logic occurs. In the case where the Activity will be
            // shown regardless of the lock screen, the call to
            // {@link ActivityStackSupervisor#checkReadyForSleepLocked} is skipped.
            final ActivityRecord next = topRunningActivityLocked(true /* focusableOnly */);
            if (next == null || !next.canTurnScreenOn()) {
                checkReadyForSleep();
            }
        } finally {
            mInResumeTopActivity = false;
        }

        return result;
    }
    
    private boolean resumeTopActivityInnerLocked(ActivityRecord prev, ActivityOptions options) {
        if (!mService.isBooting() && !mService.isBooted()) {
            // Not ready yet!
            return false;
        }

        boolean pausing = getDisplay().pauseBackStacks(userLeaving, next, false);
        // 如果当前有处于Resumed状态的Activity，那么先将其转换到Pause状态
        if (mResumedActivity != null) {
            if (DEBUG_STATES) Slog.d(TAG_STATES,
                    "resumeTopActivityLocked: Pausing " + mResumedActivity);
            pausing |= startPausingLocked(userLeaving, false, next, false);
        }
        
        // 再启动指定的Activity
        mStackSupervisor.startSpecificActivityLocked(next, true, true);

        return true;
    }
    
}

先分析一下startPausingLocked流程，然后再回来startSpecificActivityLocked方法拉起指定应用。

class ActivityStack extends ConfigurationContainer {
	final boolean startPausingLocked(boolean userLeaving, boolean uiSleeping,
            ActivityRecord resuming, boolean pauseImmediately) {
        ··· ···
        ActivityRecord prev = mResumedActivity;

        if (prev == null) {
            if (resuming == null) {
                Slog.wtf(TAG, "Trying to pause when nothing is resumed");
                mRootActivityContainer.resumeFocusedStacksTopActivities();
            }
            return false;
        }

        if (prev == resuming) {
            Slog.wtf(TAG, "Trying to pause activity that is in process of being resumed");
            return false;
        }

        if (DEBUG_STATES) Slog.v(TAG_STATES, "Moving to PAUSING: " + prev);
        else if (DEBUG_PAUSE) Slog.v(TAG_PAUSE, "Start pausing: " + prev);
        mPausingActivity = prev;
        mLastPausedActivity = prev;
        mLastNoHistoryActivity = (prev.intent.getFlags() & Intent.FLAG_ACTIVITY_NO_HISTORY) != 0
                || (prev.info.flags & ActivityInfo.FLAG_NO_HISTORY) != 0 ? prev : null;
        prev.setState(PAUSING, "startPausingLocked");
        prev.getTaskRecord().touchActiveTime();
        clearLaunchTime(prev);

        mService.updateCpuStats();

        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);
				
                // 获取ClientLifecycleManager对象来调度应用的生命周期
                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;
        }
    }
}

ClientLifecycleManager对象是用来辅助管理应用生命周期的类，内部保存了一个ApplicationThread的对象，用于IPC通信。

class ClientLifecycleManager {
	void scheduleTransaction(ClientTransaction transaction) throws RemoteException {
        final IApplicationThread client = transaction.getClient();
        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();
        }
    }
}

public class ClientTransaction implements Parcelable, ObjectPoolItem {
	public void schedule() throws RemoteException {
        mClient.scheduleTransaction(this);
    }
}

通过IPC通信，调用到了ActivityThread的scheduleTransaction方法，最终调用到ActivityThread的内部类H的handleMessage方法中

public final class ActivityThread extends ClientTransactionHandler {
	public void scheduleTransaction(ClientTransaction transaction) throws RemoteException {
            ActivityThread.this.scheduleTransaction(transaction);
    }
    
    // what为EXECUTE_TRANSACTION
    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) {
    	case EXECUTE_TRANSACTION:
            final ClientTransaction transaction = (ClientTransaction) msg.obj;
        	// 重要方法
            mTransactionExecutor.execute(transaction);
            if (isSystem()) {
                // Client transactions inside system process are recycled on the client side
                // instead of ClientLifecycleManager to avoid being cleared before this
                // message is handled.
                transaction.recycle();
            }
            // TODO(lifecycler): Recycle locally scheduled transactions.
            break;
    }
}

此时又来到了TransactionExecutor的execute方法，其中executeLifecycleState会执行PauseActivityItem.execute方法

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));
		
		// 重要方法1
        executeCallbacks(transaction);
		
    	// 重要方法2
        executeLifecycleState(transaction);
        mPendingActions.clear();
        if (DEBUG_RESOLVER) Slog.d(TAG, tId(transaction) + "End resolving transaction");
    }

private void executeLifecycleState(ClientTransaction transaction) {
    	// 这个lifecycleItem是ATMS带过来的，实际上是PauseActivityItem对象
        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.
        lifecycleItem.execute(mTransactionHandler, token, mPendingActions);
        lifecycleItem.postExecute(mTransactionHandler, token, mPendingActions);
    }

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

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;
            performPauseActivity(r, finished, reason, pendingActions);

            // Make sure any pending writes are now committed.
            if (r.isPreHoneycomb()) {
                QueuedWork.waitToFinish();
            }
            mSomeActivitiesChanged = true;
        }
    }
    
    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;
            // 继续来到Instrument的callActivityOnPause
            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);
    }
}

继续来到Instrument的callActivityOnPause方法

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

这个Activity就是用户的Activity，在这个场景里就是Launcher.java

public class Activity {
	final void performPause() {
        dispatchActivityPrePaused();
        mDoReportFullyDrawn = false;
        mFragments.dispatchPause();
        mCalled = false;
        // 模版方法，用户自定义的onPause方法
        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();
    }
}

好了，至此，Launcher Activity完成了onPause的回调，下面要回到ActivityStackManagerService中的startSpecificActivityLocked方法，拉起指定的应用。

启动应用进程

启动Activity分为两种：1. Activity所在的Application已经启动，不需要创建进程；2. 需要先创建Activity所需的进程；

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;
        if (wpc != null && wpc.hasThread()) {
            try {
                // 如果Activity所属的Application已经启动
                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;
        }

        // Suppress transition until the new activity becomes ready, otherwise the keyguard can
        // appear for a short amount of time before the new process with the new activity had the
        // ability to set its showWhenLocked flags.
        if (getKeyguardController().isKeyguardLocked()) {
            r.notifyUnknownVisibilityLaunched();
        }

        try {
            if (Trace.isTagEnabled(TRACE_TAG_ACTIVITY_MANAGER)) {
                Trace.traceBegin(TRACE_TAG_ACTIVITY_MANAGER, "dispatchingStartProcess:"
                        + r.processName);
            }
            
            // 通过一个Handler消息，触发startProcess创建新的进程
            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);
        }
    }
}

mService.mH.sendMessage(msg);这个消息会触发ActivityManagerService到内部类LocalService的startProcess方法，经过层层调用，最终调用Process.start方法。

public final class LocalService extends ActivityManagerInternal {
	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) {
                startProcessLocked(processName, info, knownToBeDead, 0 /* intentFlags */,
                                   new HostingRecord(hostingType, hostingName),
                                   false /* allowWhileBooting */, false /* isolated */,
                                   true /* keepIfLarge */);
            }
        } finally {
            Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
        }
    }
    
    final ProcessRecord startProcessLocked(String processName,
            ApplicationInfo info, boolean knownToBeDead, int intentFlags,
            HostingRecord hostingRecord, boolean allowWhileBooting,
            boolean isolated, boolean keepIfLarge) {
        return mProcessList.startProcessLocked(processName, info, knownToBeDead, intentFlags,
                hostingRecord, allowWhileBooting, isolated, 0 /* isolatedUid */, keepIfLarge,
                null /* ABI override */, null /* entryPoint */, null /* entryPointArgs */,
                null /* crashHandler */);
    }
    
    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 {
                // 重要方法，调用Porcess.start方法
                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);
        }
    }
}

Process.start通过Zygote创建应用进程

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);
    }
}

Zygote创建进程留在另外一篇文章去分析，Zygote通过Socket消息发送到Zygote的服务端，从而Fork出一个新进程，然后新进程会执行ActivityThread主线程的main方法。

ActivityThread流程

当应用进程被Zygote孵化后，会首先执行ActivityThread.main方法，在Main方法中会执行一些初始化的工作，包括创建主线程的Looper。最重要的是attach方法，该方法通过IPC消息通知AMS，应用进程已经拉起来了。

public final class ActivityThread extends ClientTransactionHandler {
    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());
            // ActivityManagerService的ipc对象
            final IActivityManager mgr = ActivityManager.getService();
            try {
                mgr.attachApplication(mAppThread, startSeq);
            } catch (RemoteException ex) {
                throw ex.rethrowFromSystemServer();
            }
            // Watch for getting close to heap limit.
            ··· ···
    } 
}

public class ActivityManagerService extends IActivityManager.Stub
        implements Watchdog.Monitor, BatteryStatsImpl.BatteryCallback {
	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);
        }
    }
}

然后会走到LocalService的attachApplication方法

public final class LocalService extends ActivityManagerInternal {
	public boolean attachApplication(WindowProcessController wpc) throws RemoteException {
            synchronized (mGlobalLockWithoutBoost) {
                return mRootActivityContainer.attachApplication(wpc);
            }
        }
}

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 {
                            // 调用ActivityStackSuperVisor的realStartActivityLocked方法
                            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;
    }
}

public class ActivityStackSupervisor implements RecentTasks.Callbacks {
    boolean realStartActivityLocked(ActivityRecord r, WindowProcessController proc,
            boolean andResume, boolean checkConfig) throws RemoteException {

        try {
            // 创建启动应用的Transaction
            final ClientTransaction clientTransaction = ClientTransaction.obtain(
                proc.getThread(), r.appToken);

            final DisplayContent dc = r.getDisplay().mDisplayContent;
            // LaunchActivityITtem对象
            clientTransaction.addCallback(LaunchActivityItem.obtain(new Intent(r.intent),
                   System.identityHashCode(r), r.info,
                   // TODO: Have this take the merged configuration instead of separate global
                   // and override configs.
                   mergedConfiguration.getGlobalConfiguration(),
                   mergedConfiguration.getOverrideConfiguration(), r.compat,
                   r.launchedFromPackage, task.voiceInteractor, proc.getReportedProcState(),
                   r.icicle, r.persistentState, results, newIntents,
                   dc.isNextTransitionForward(), proc.createProfilerInfoIfNeeded(),
                   	r.assistToken));

            // Set desired final state.
            final ActivityLifecycleItem lifecycleItem;
            if (andResume) {
                lifecycleItem = ResumeActivityItem.obtain(dc.isNextTransitionForward());
            } else {
                lifecycleItem = PauseActivityItem.obtain();
            }
            clientTransaction.setLifecycleStateRequest(lifecycleItem);

            // Schedule transaction.
            mService.getLifecycleManager().scheduleTransaction(clientTransaction);
    }
}

之前已经分析过clientTransaction的执行，此方法通过IPC通信发送到ApplicationThread中，然后执行LaunchActivtyItem的execute方法

public class LaunchActivityItem extends ClientTransactionItem {
    public void execute(ClientTransactionHandler client, IBinder token,
            PendingTransactionActions pendingActions) {
        Trace.traceBegin(TRACE_TAG_ACTIVITY_MANAGER, "activityStart");
        ActivityClientRecord r = new ActivityClientRecord(token, mIntent, mIdent, mInfo,
                mOverrideConfig, mCompatInfo, mReferrer, mVoiceInteractor, mState, mPersistentState,
                mPendingResults, mPendingNewIntents, mIsForward,
                mProfilerInfo, client, mAssistToken);
        // client对象是ActivityThread对象
        client.handleLaunchActivity(r, pendingActions, null /* customIntent */);
        Trace.traceEnd(TRACE_TAG_ACTIVITY_MANAGER);
    }
}

public final class ActivityThread extends ClientTransactionHandler {
    public Activity handleLaunchActivity(ActivityClientRecord r,
            PendingTransactionActions pendingActions, Intent customIntent) {
       	
        // 创建Activity 
        final Activity a = performLaunchActivity(r, customIntent);

        if (a != null) {
            r.createdConfig = new Configuration(mConfiguration);
            reportSizeConfigurations(r);
            if (!r.activity.mFinished && pendingActions != null) {
                pendingActions.setOldState(r.state);
                pendingActions.setRestoreInstanceState(true);
                pendingActions.setCallOnPostCreate(true);
            }
        } else {
            // If there was an error, for any reason, tell the activity manager to stop us.
            try {
                ActivityTaskManager.getService()
                        .finishActivity(r.token, Activity.RESULT_CANCELED, null,
                                Activity.DONT_FINISH_TASK_WITH_ACTIVITY);
            } catch (RemoteException ex) {
                throw ex.rethrowFromSystemServer();
            }
        }

        return a;
    }
    
    private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
        ActivityInfo aInfo = r.activityInfo;
		
        // 创建Context
        ContextImpl appContext = createBaseContextForActivity(r);
        Activity activity = null;
        try {
            java.lang.ClassLoader cl = appContext.getClassLoader();
            // Instrument通过反射创建Activity对象
            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) {
        }

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

            if (activity != null) {
                CharSequence title = r.activityInfo.loadLabel(appContext.getPackageManager());
                Configuration config = new Configuration(mCompatConfiguration);
                if (r.overrideConfig != null) {
                    config.updateFrom(r.overrideConfig);
                }
                Window window = null;
                if (r.mPendingRemoveWindow != null && r.mPreserveWindow) {
                    window = r.mPendingRemoveWindow;
                    r.mPendingRemoveWindow = null;
                    r.mPendingRemoveWindowManager = null;
                }
                appContext.setOuterContext(activity);
                // Activity绑定context
                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, r.configCallback,
                        r.assistToken);

                activity.mCalled = false;
                if (r.isPersistable()) {
                    mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
                } else {
                    // 执行onCreate方法
                    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;
            }

        return activity;
    }
}

此时Activity已经被创建了，并且也执行了onCreate方法，后续执行onStart，onResume回调。在新的Activity执行完onResume后，原来的栈顶Activity开始执行onStop生命周期。