Android源码分析-Android9.0下的Activity启动流程

前言

最近在阅读Android源码时，发现最新的Android9.0源码中startActivity启动Activity的流程相比于低版本的Android源码来说改动较大，且本人在网上也没有找到基于Android9.0的相关源码分析文章。故写下此文，记录下源码追踪流程，方便以后自己复查，同时也分享给有需要的读者。

Activity->startActivity

    @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);
        }
    }

直接调用startActivityForResult

Activity->startActivityForResult

    public void startActivityForResult(@RequiresPermission Intent intent, int requestCode,
                                       @Nullable Bundle options) {
        // mParent指的是Activity 赋值是在内部API调用setParent方法
        if (mParent == null) {
            options = transferSpringboardActivityOptions(options);
            //Instrumentation为工具类ActivityResult为其静态内部类  工具类调用执行开始Activity方法execStartActity
            //mMainThread为ActivityThread ，getApplicationThread()方法获取的是ApplicationThread实例
            //ApplicationThread是ActivityThread的内部类  该类继承ApplicationThreadNative抽象类，
            //而ApplicationThreadNative继承Binder类并实现IApplicationThread接口
            //IApplictionThread继承了IInterface接口
            //Binder类继承IBinder接口，这就是为什么execStartActivity方法的第二个参数定义为IBinder
            Instrumentation.ActivityResult ar =
                    mInstrumentation.execStartActivity(
                            this, mMainThread.getApplicationThread(), mToken, this,
                            intent, requestCode, options);
            if (ar != null) {
                // ar不为空说明Activity启动成功
                mMainThread.sendActivityResult(
                        mToken, mEmbeddedID, requestCode, ar.getResultCode(),
                        ar.getResultData());
            }
            if (requestCode >= 0) {
                mStartedActivity = true;
            }

            cancelInputsAndStartExitTransition(options);
        } 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);
            }
        }
    }

调用了mInstrumentation.execStartActivity方法

Instrumentation->execStartActivity

    public ActivityResult execStartActivity(
        Context who, IBinder contextThread, IBinder token, String target,
        Intent intent, int requestCode, Bundle options) {
        IApplicationThread whoThread = (IApplicationThread) contextThread;
        if (mActivityMonitors != null) {
            synchronized (mSync) {
                final int N = mActivityMonitors.size();
                for (int i=0; i<N; i++) {
                    //ActivityMonitor为Activity的监测器是Instrumentation的静态内部类，
                    //当一个Activity启动时将会被检测
                    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);
            // result 为启动Activity返回的状态码
            // 这里通过IPC机制，调用了ActivityManagerService中的startActivity方法
            int result = ActivityManager.getService()
                .startActivity(whoThread, who.getBasePackageName(), intent,
                        intent.resolveTypeIfNeeded(who.getContentResolver()),
                        token, target, requestCode, 0, null, options);
            //检查Activity是否启动成功若未启动成功则抛出对应异常
            checkStartActivityResult(result, intent);
        } catch (RemoteException e) {
            throw new RuntimeException("Failure from system", e);
        }
        return null;
    }

可以看到，这个方法最后是调用了ActivityManager.getService().startActivity()方法。
查看ActivityManager源码

ActivityManager->getService

   public static IActivityManager getService() {
        return IActivityManagerSingleton.get();
    }

    private static final Singleton<IActivityManager> IActivityManagerSingleton =
            new Singleton<IActivityManager>() {
                @Override
                protected IActivityManager create() {
                    // 通过Binder机制获取ActivityManagerService
                    final IBinder b = ServiceManager.getService(Context.ACTIVITY_SERVICE);
                    final IActivityManager am = IActivityManager.Stub.asInterface(b);
                    return am;
                }
            };

这里是通过Binder的跨进程通信获取到了系统服务ActivityManagerService，所以ActivityManager.getService().startActivity()是调用了ActivityManagerService中的startActivity方法。

ActivityManagerService->startActivity

    @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 final int startActivityAsUser(IApplicationThread caller, String callingPackage,
            Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode,
            int startFlags, ProfilerInfo profilerInfo, Bundle bOptions, int userId) {
        return startActivityAsUser(caller, callingPackage, intent, resolvedType, resultTo,
                resultWho, requestCode, startFlags, profilerInfo, bOptions, userId,
                true /*validateIncomingUser*/);
    }

    public final int startActivityAsUser(IApplicationThread caller, String callingPackage,
            Intent intent, String resolvedType, IBinder resultTo, String resultWho, int requestCode,
            int startFlags, ProfilerInfo profilerInfo, Bundle bOptions, int userId,
            boolean validateIncomingUser) {
        enforceNotIsolatedCaller("startActivity");

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

        // TODO: Switch to user app stacks here.
        // 通过mActivityStartController获取ActivityStarter，进行后续页面跳转
        return mActivityStartController.obtainStarter(intent, "startActivityAsUser")
                .setCaller(caller)
                .setCallingPackage(callingPackage)
                .setResolvedType(resolvedType)
                .setResultTo(resultTo)
                .setResultWho(resultWho)
                .setRequestCode(requestCode)
                .setStartFlags(startFlags)
                .setProfilerInfo(profilerInfo)
                .setActivityOptions(bOptions)
                .setMayWait(userId)
                .execute();
    }

这里最后通过mActivityStartController.obtainStarter获取到了一个ActivityStarter对象，然后设置了一些属性，最后调用了execute()方法。所以流程走到ActivityStarter中的execute()方法。

ActivityStarter->execute

    int execute() {
        try {
            // TODO(b/64750076): Look into passing request directly to these methods to allow
            // for transactional diffs and preprocessing.
            // ActivityManagerService中的startActivityAsUser方法中调用了setMayWait方法
            // setMayWait方法会将mRequest.mayWait置为true，所以这里会走startActivityMayWait方法
            //    ActivityStarter setMayWait(int userId) {
            //        mRequest.mayWait = true;
            //        mRequest.userId = userId;
            //
            //        return this;
            //    }
            if (mRequest.mayWait) {
                return startActivityMayWait(mRequest.caller, mRequest.callingUid,
                        mRequest.callingPackage, 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);
            } 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);
            }
        } finally {
            onExecutionComplete();
        }
    }

查看setMayWait方法

    ActivityStarter setMayWait(int userId) {
        mRequest.mayWait = true;
        mRequest.userId = userId;

        return this;
    }

ActivityManagerService中的startActivityAsUser方法中调用了setMayWait方法。setMayWait方法会将mRequest.mayWait置为true，所以这里会走startActivityMayWait方法

ActivityStarter->startActivityMayWait

    private int startActivityMayWait(IApplicationThread caller, int callingUid,
                                     String callingPackage, 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) {
        
                 ...略

        // Collect information about the target of the Intent.
        // 对Intent参数进行解析获取Activity的相关信息，并把数据保存在aInfo变量中
        ActivityInfo aInfo = mSupervisor.resolveActivity(intent, rInfo, startFlags, profilerInfo);
        synchronized (mService) {

                  ...略

            final ActivityRecord[] outRecord = new ActivityRecord[1];
            // 调用ActivityStarter的startActivity方法
            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);

                  ....略

            //通知跟踪器Activity正在启动
            mSupervisor.getActivityMetricsLogger().notifyActivityLaunched(res, outRecord[0]);
            return res;
        }
    }

调用ActivityStarter的startActivity方法

ActivityStarter->startActivity

    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) {
       
        ...一些必要的判断...

        // 检查是否有开启新Activity的权限
        boolean abort = !mSupervisor.checkStartAnyActivityPermission(intent, aInfo, resultWho,
                requestCode, callingPid, callingUid, callingPackage, ignoreTargetSecurity,
                inTask != null, callerApp, resultRecord, resultStack);
        abort |= !mService.mIntentFirewall.checkStartActivity(intent, callingUid,
                callingPid, resolvedType, aInfo.applicationInfo);

         ...

        // 如果我们开始的活动与当前恢复的活动的UID不同，检查是否允许应用程序切换。
        if (voiceSession == null && (stack.getResumedActivity() == null
                || stack.getResumedActivity().info.applicationInfo.uid != realCallingUid)) {
            if (!mService.checkAppSwitchAllowedLocked(callingPid, callingUid,
                    realCallingPid, realCallingUid, "Activity start")) {
                mController.addPendingActivityLaunch(new PendingActivityLaunch(r,
                        sourceRecord, startFlags, stack, callerApp));
                ActivityOptions.abort(checkedOptions);
                return ActivityManager.START_SWITCHES_CANCELED;
            }
        }

        // 加载PendingActivity
        mController.doPendingActivityLaunches(false);

        // 继续调用方法
        return startActivity(r, sourceRecord, voiceSession, voiceInteractor, startFlags,
                true /* doResume */, checkedOptions, inTask, outActivity);
    }

继续调用同名方法startActivity

ActivityStarter->startActivity

    private int startActivity(final ActivityRecord r, ActivityRecord sourceRecord,
                              IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
                              int startFlags, boolean doResume, ActivityOptions options, TaskRecord inTask,
                              ActivityRecord[] outActivity) {
        int result = START_CANCELED;
        try {
            mService.mWindowManager.deferSurfaceLayout();
            // 调用startActivityUnchecked方法，返回结果
            result = startActivityUnchecked(r, sourceRecord, voiceSession, voiceInteractor,
                    startFlags, doResume, options, inTask, outActivity);
        } finally {
            // 如果我们无法继续，请解除活动与任务的关联。使活动处于不完整状态可能会导致问题，例如在没有窗口容器的情况下执行操作。
            final ActivityStack stack = mStartActivity.getStack();
            if (!ActivityManager.isStartResultSuccessful(result) && stack != null) {
                stack.finishActivityLocked(mStartActivity, RESULT_CANCELED,
                        null /* intentResultData */, "startActivity", true /* oomAdj */);
            }
            mService.mWindowManager.continueSurfaceLayout();
        }

        postStartActivityProcessing(r, result, mTargetStack);

        return result;
    }

调用startActivityUnchecked方法

ActivityStarter->startActivityUnchecked

    private int startActivityUnchecked(final ActivityRecord r, ActivityRecord sourceRecord,
                                       IVoiceInteractionSession voiceSession, IVoiceInteractor voiceInteractor,
                                       int startFlags, boolean doResume, ActivityOptions options, TaskRecord inTask,
                                       ActivityRecord[] outActivity) {

        // 初始化ActivityStarter全局变量
        setInitialState(r, options, inTask, doResume, startFlags, sourceRecord, voiceSession,
                voiceInteractor);

        computeLaunchingTaskFlags();

        computeSourceStack();

        mIntent.setFlags(mLaunchFlags);

        // 判断是否应将新Activity插入到现有任务栈中
        // 如果不需要，则返回null，如果需要，则返回该任务栈信息
        ActivityRecord reusedActivity = getReusableIntentActivity();

        ......

        // 使新Activity可见
        if (mDoResume) {
            final ActivityRecord topTaskActivity =
                    mStartActivity.getTask().topRunningActivityLocked();
            if (!mTargetStack.isFocusable()
                    || (topTaskActivity != null && topTaskActivity.mTaskOverlay
                    && mStartActivity != topTaskActivity)) {
                mTargetStack.ensureActivitiesVisibleLocked(null, 0, !PRESERVE_WINDOWS);
                mService.mWindowManager.executeAppTransition();
            } else {
                if (mTargetStack.isFocusable() && !mSupervisor.isFocusedStack(mTargetStack)) {
                    mTargetStack.moveToFront("startActivityUnchecked");
                }
                // 关注这里
                mSupervisor.resumeFocusedStackTopActivityLocked(mTargetStack, mStartActivity,
                        mOptions);
            }
        } else if (mStartActivity != null) {
            mSupervisor.mRecentTasks.add(mStartActivity.getTask());
        }
        mSupervisor.updateUserStackLocked(mStartActivity.userId, mTargetStack);

        mSupervisor.handleNonResizableTaskIfNeeded(mStartActivity.getTask(), preferredWindowingMode,
                preferredLaunchDisplayId, mTargetStack);

        return START_SUCCESS;
    }

这里需要关注的是mSupervisor.resumeFocusedStackTopActivityLocked方法

ActivityStackSupervisor->resumeFocusedStackTopActivityLocked

    boolean resumeFocusedStackTopActivityLocked(
            ActivityStack targetStack, ActivityRecord target, ActivityOptions targetOptions) {

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

        if (targetStack != null && isFocusedStack(targetStack)) {
            return targetStack.resumeTopActivityUncheckedLocked(target, targetOptions);
        }

        final ActivityRecord r = mFocusedStack.topRunningActivityLocked();
        if (r == null || !r.isState(RESUMED)) {
            mFocusedStack.resumeTopActivityUncheckedLocked(null, null);
        } else if (r.isState(RESUMED)) {
            // Kick off any lingering app transitions form the MoveTaskToFront operation.
            mFocusedStack.executeAppTransition(targetOptions);
        }

        return false;
    }

可以看到，这里调用了resumeTopActivityUncheckedLocked方法

ActivityStackSupervisor->resumeTopActivityUncheckedLocked

    boolean resumeTopActivityUncheckedLocked(ActivityRecord prev, ActivityOptions options) {
        if (mStackSupervisor.inResumeTopActivity) {
            // Don't even start recursing.
            return false;
        }

        boolean result = false;
        try {
            // Protect against recursion.
            mStackSupervisor.inResumeTopActivity = 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 {
            mStackSupervisor.inResumeTopActivity = false;
        }

        return result;
    }

这里调用了resumeTopActivityInnerLocked方法，resumeTopActivityInnerLocked方法中会调用startSpecificActivityLocked方法，然后再调用realStartActivityLocked方法。Activity的启动事务就是在realStartActivityLocked方法中启动的。

ActivityStackSupervisor->realStartActivityLocked

    final boolean realStartActivityLocked(ActivityRecord r, ProcessRecord app,
            boolean andResume, boolean checkConfig) throws RemoteException {

            ......           

            try {
                
                ......

                // Create activity launch transaction.
                // 创建Activity启动事务
                // ClientTransaction.obtain方法如下：
                // 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;
                // }
                // 这里传入的app.thread会赋值给ClientTransaction的成员变量mClient，
                // 而ClientTransaction会调用mClient.scheduleTransaction(this)来执行事务
                // 所以事务最终是调用app.thread的scheduleTransaction执行。
                // 而这个app.thread是ActivityThread的内部类ApplicationThread。
                final ClientTransaction clientTransaction = ClientTransaction.obtain(app.thread,
                        r.appToken);
                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, app.repProcState, r.icicle,
                        r.persistentState, results, newIntents, mService.isNextTransitionForward(),
                        profilerInfo));

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

                // Schedule transaction.
                // 执行Activity启动事务
                mService.getLifecycleManager().scheduleTransaction(clientTransaction);
              
                ......

            } catch (RemoteException e) {
                if (r.launchFailed) {
                    // 第二次启动失败的异常处理
                    return false;
                }

                // 第一次启动失败，重试
                r.launchFailed = true;
                app.activities.remove(r);
                throw e;
            }
        } finally {
            endDeferResume();
        }

        ......

        return true;
    }

方法中需要关注的部分已经用注释说明了，在realStartActivityLocked中最主要的工作就是创建了Activity的启动事务ClientTransaction，并调用ClientLifecycleManager的scheduleTransaction方法启动它。接下来，看ClientTransaction事务中是怎么启动Activity的。

ClientLifecycleManager->scheduleTransaction

    void scheduleTransaction(ClientTransaction transaction) throws RemoteException {
        final IApplicationThread client = transaction.getClient();
        // 执行事务
        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();
        }
    }

调用ClientTransaction的schedule方法

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

这里调用了mClient的scheduleTransaction方法，那么这里的mClient是从哪来的呢。很容易想到，是在创建ClientTransaction事务对象的时候赋值的，也就是调用obtain方法时。

ClientTransaction->obtain

    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;
    }

我们回到ActivityStackSupervisor的realStartActivityLocked方法

ActivityStackSupervisor->realStartActivityLocked

    final boolean realStartActivityLocked(ActivityRecord r, ProcessRecord app,
            boolean andResume, boolean checkConfig) throws RemoteException {

            ......           

            try {
                
                ......

                // Create activity launch transaction.
                // 创建Activity启动事务
                // ClientTransaction.obtain方法如下：
                // 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;
                // }
                // 这里传入的app.thread会赋值给ClientTransaction的成员变量mClient，
                // 而ClientTransaction会调用mClient.scheduleTransaction(this)来执行事务
                // 所以事务最终是调用app.thread的scheduleTransaction执行。
                // 而这个app.thread是ActivityThread的内部类ApplicationThread。
                final ClientTransaction clientTransaction = ClientTransaction.obtain(app.thread,
                        r.appToken);
                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, app.repProcState, r.icicle,
                        r.persistentState, results, newIntents, mService.isNextTransitionForward(),
                        profilerInfo));

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

                // Schedule transaction.
                // 执行Activity启动事务
                mService.getLifecycleManager().scheduleTransaction(clientTransaction);
              
                ......

            } catch (RemoteException e) {
                if (r.launchFailed) {
                    // 第二次启动失败的异常处理
                    return false;
                }

                // 第一次启动失败，重试
                r.launchFailed = true;
                app.activities.remove(r);
                throw e;
            }
        } finally {
            endDeferResume();
        }

        ......

        return true;
    }

可以看到，赋值给mClient的是app.thread，所以事务最终是调用的app.thread的scheduleTransaction方法来执行，而这个app.thread是ActivityThread的内部类ApplicationThread。所以流程转到了ActivityThread的内部类ApplicationThread中。

ActivityThread->ApplicationThread->scheduleTransaction

    @Override
    public void scheduleTransaction(ClientTransaction transaction) throws RemoteException {
        ActivityThread.this.scheduleTransaction(transaction);
    }

这里还是调用了ActivityThread的scheduleTransaction方法。但是在查看ActivityThread类就会发现ActivityThread类中并没有scheduleTransaction这个方法。因此自然会想到很可能是继承的父类中的方法。ActivityThread继承的是ClientTransactionHandler类。到ClientTransactionHandler类中一查，果然发现了scheduleTransaction方法。所以这里最终调用的就是ClientTransactionHandler中的scheduleTransaction方法。

ClientTransactionHandler->scheduleTransaction

    void scheduleTransaction(ClientTransaction transaction) {
        transaction.preExecute(this);
        sendMessage(ActivityThread.H.EXECUTE_TRANSACTION, transaction);
    }

这里调用了sendMessage方法，而sendMessage是一个抽象方法，所以这里调用的是ActivityThread类中的sendMessage实现。

ActivityThread->sendMessage

    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);
    }

这里就是生成了一个Message对象，并向mH这个Handler发送该消息。所以这里mH将会接受到一个what为ActivityThread.H.EXECUTE_TRANSACTION的消息。去查看Handler中对EXECUTE_TRANSACTION消息的处理。

ActivityThread->H->handleMessage

    case EXECUTE_TRANSACTION:
        final ClientTransaction transaction = (ClientTransaction) msg.obj;
        // 调用TransactionExecutor的execute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;

这里调用了TransactionExecutor的execute方法

TransactionExecutor->execute

    public void execute(ClientTransaction transaction) {
        final IBinder token = transaction.getActivityToken();
        log("Start resolving transaction for client: " + mTransactionHandler + ", token: " + token);

        executeCallbacks(transaction);

        executeLifecycleState(transaction);
        mPendingActions.clear();
        log("End resolving transaction");
    }

这里调用了executeCallbacks和executeLifecycleState两个方法，查看两个方法就会发现，这两个方法最后都会调用cycleToPath这个方法。

TransactionExecutor->cycleToPath

    private void cycleToPath(ActivityClientRecord r, int finish,
            boolean excludeLastState) {
        final int start = r.getLifecycleState();
        log("Cycle from: " + start + " to: " + finish + " excludeLastState:" + excludeLastState);
        final IntArray path = mHelper.getLifecyclePath(start, finish, excludeLastState);
        performLifecycleSequence(r, path);
    }

TransactionExecutor->performLifecycleSequence

    private void performLifecycleSequence(ActivityClientRecord r, IntArray path) {
        final int size = path.size();
        for (int i = 0, state; i < size; i++) {
            state = path.get(i);
            log("Transitioning to state: " + state);
            switch (state) {
                case ON_CREATE:
                    mTransactionHandler.handleLaunchActivity(r, mPendingActions,
                            null /* customIntent */);
                    break;
                case ON_START:
                    mTransactionHandler.handleStartActivity(r, mPendingActions);
                    break;
                case ON_RESUME:
                    mTransactionHandler.handleResumeActivity(r.token, false /* finalStateRequest */,
                            r.isForward, "LIFECYCLER_RESUME_ACTIVITY");
                    break;
                case ON_PAUSE:
                    mTransactionHandler.handlePauseActivity(r.token, false /* finished */,
                            false /* userLeaving */, 0 /* configChanges */, mPendingActions,
                            "LIFECYCLER_PAUSE_ACTIVITY");
                    break;
                case ON_STOP:
                    mTransactionHandler.handleStopActivity(r.token, false /* show */,
                            0 /* configChanges */, mPendingActions, false /* finalStateRequest */,
                            "LIFECYCLER_STOP_ACTIVITY");
                    break;
                case ON_DESTROY:
                    mTransactionHandler.handleDestroyActivity(r.token, false /* finishing */,
                            0 /* configChanges */, false /* getNonConfigInstance */,
                            "performLifecycleSequence. cycling to:" + path.get(size - 1));
                    break;
                case ON_RESTART:
                    mTransactionHandler.performRestartActivity(r.token, false /* start */);
                    break;
                default:
                    throw new IllegalArgumentException("Unexpected lifecycle state: " + state);
            }
        }
    }

看到这里就豁然开朗了，Activity的生命周期就是在这里进行一个相关方法的调用。
这里的成员变量mTransactionHandler是一个ClientTransactionHandler对象，在ClientTransactionHandler中这些方法都是抽象方法，这里执行的是ClientTransactionHandler的实现类ActivityThread中的handleLaunchActivity方法。

ActivityThread->handleLaunchActivity

    public Activity handleLaunchActivity(ActivityClientRecord r,
            PendingTransactionActions pendingActions, Intent customIntent) {
        // If we are getting ready to gc after going to the background, well
        // we are back active so skip it.
        unscheduleGcIdler();
        mSomeActivitiesChanged = true;

        if (r.profilerInfo != null) {
            mProfiler.setProfiler(r.profilerInfo);
            mProfiler.startProfiling();
        }

        // Make sure we are running with the most recent config.
        handleConfigurationChanged(null, null);

        if (localLOGV) Slog.v(
            TAG, "Handling launch of " + r);

        // Initialize before creating the activity
        if (!ThreadedRenderer.sRendererDisabled) {
            GraphicsEnvironment.earlyInitEGL();
        }
        //创建WindowManagerServer
        WindowManagerGlobal.initialize();

        // 通过反射创建指定的Activity，并回调Activity的performCreate方法执行onCreate
        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 {
                ActivityManager.getService()
                        .finishActivity(r.token, Activity.RESULT_CANCELED, null,
                                Activity.DONT_FINISH_TASK_WITH_ACTIVITY);
            } catch (RemoteException ex) {
                throw ex.rethrowFromSystemServer();
            }
        }

        return a;
    }

到这，Activity的启动流程也就走完了。

可以看到，一个startActivity方法在源码中的处理是多么的复杂。如果我们不去阅读源码的话，根本不会对Activity的启动有一个更深层的理解。所以作为开发，保持一个阅读源码的习惯是非常重要的。

希望本文能对各位读者有一点帮助，也欢迎指出文章中的错误，谢谢大家。