Activity的启动流程分析

Posted on | In | Visitors
Words count in article 5744 | Reading time 28

1 概述

基于Android10源码分析更新

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Intent intent = new Intent(this, MainActivity.class);
this.startActivity(intent);

相信每位Android开发者对上面的代码都写过无数次了，那么问题来了：

  • 根activity（第一个activity）又是如何启动的？
  • Application是如何创建的？
  • 这个代码是如何启动一个Activity的？
  • 内部做了哪些事情？
  • onCreate这些生命周期是何时执行的？
  • Activity对象何时创建的？

我们不了解内部实现的原理，在平时开发中好像也没有什么问题。其实不然，解决这些问题后，你会对Android系统有更深层次的理解，也会学习到系统源码优秀的设计。并且对解决一些高级问题和深入的性能优化问题有很大帮助，阅读优秀的源码是技术进阶的必要阶段。但另一方面，系统源码是很庞大繁杂的，我们需要带着问题抓住主流程，不能陷入代码细节——这是阅读系统源码以及其他第三方库源码的正确姿势。

2 根Activity启动流程

我们app应用进程是怎么创建的呢？当点击app图标后，Launcher会在桌面activity（此activity就叫Launcher）内调用startActivitySafely方法，startActivitySafely方法会调用startActivity方法,如下图: 此时应用的进程就已经创建出来了。

2.1 入口main()

学过Java的都知道，应用程序入口是public static void main(String[] args)方法，基于Java的Android也一样，Android应用程序的入口在ActivityThread.main()方法中，一切的起源都从main()方法开始：

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    ...
    final ApplicationThread mAppThread = new ApplicationThread();
    final H mH = new H();
    ...

    public static void main(String[] args){
         ...
        //初始化Looper
        Looper.prepareMainLooper();

        ...

        //创建ActivityThread对象 实例化了上面的ApplicationThread和mH，就是handler 这个mH很重要后续会详解
        ActivityThread thread = new ActivityThread();
        //建立Binder通道 (创建新线程) 发送出创建Application的消息
        thread.attach(false, startSeq);

        ...
        
        //开启循环 主线程进入无限循环状态，等待接收消息
        Looper.loop();
    }

ActivityThread thread = new ActivityThread()创建ActivityThread实例，同时会创建ApplicationThread实例，ApplicationThread是ActivityThread中的一个内部类。然后调用了attach方法：

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    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());
            //获得IActivityManager实例
            final IActivityManager mgr = ActivityManager.getService();
            try {
            	//把ApplicationThread实例关联到AMS中
                mgr.attachApplication(mAppThread, startSeq);
            } catch (RemoteException ex) {
                throw ex.rethrowFromSystemServer();
            }
            ...
        } 
        ...
    }

2.2 应用进程和系统服务IPC通信

我们看下ActivityManager.getService()内部执行流程

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    /**
     * @hide
     */
    public static IActivityManager getService() {
        return IActivityManagerSingleton.get();
    }

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

看到IBinder这个标志，这里你应该明白了：这里是获取一个跨进程的服务。 IActivityManager.Stub.asInterface用于将服务端的Binder对象转换成客户端所需的AIDL接口类型的对象，这种转换过程是区分进程的，如果客户端和服务端位于同一进程，那么此方法返回的 就是服务端的Stub对象本身，否则返回的是系统封装后的Stub.proxy对象。这里因为是跨进程调用的，返回的是Binder代理对象，这个代理对象实际上代理的是ActivityManagerService,它实现了IActivityManager.Stub接口是Binder本地对象，见android Binder机制。

接着看下mgr.attachApplication(mAppThread, startSeq)方法，前面说过会走到AMS的attachApplication：

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    @Override
    public final void attachApplication(IApplicationThread thread, long startSeq) {
        if (thread == null) {
              throw new SecurityException("Invalid application interface");
        }
        synchronized (this) {
            int callingPid = Binder.getCallingPid();
            final int callingUid = Binder.getCallingUid();
            final long origId = Binder.clearCallingIdentity();
            attachApplicationLocked(thread, callingPid, callingUid, startSeq);
            Binder.restoreCallingIdentity(origId);
          }
      }

接着走到attachApplicationLocked()方法很长，这里保留重要的几点：

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    private final boolean attachApplicationLocked(IApplicationThread thread,
            int pid, int callingUid, long startSeq) {
            ...
            //1、IPC操作，创建绑定Application
			if (app.isolatedEntryPoint != null) {
                // This is an isolated process which should just call an entry point instead of
                // being bound to an application.
                thread.runIsolatedEntryPoint(app.isolatedEntryPoint, app.isolatedEntryPointArgs);
            } else if (instr2 != null) {
                thread.bindApplication(processName, appInfo, providerList,
                        instr2.mClass,
                        profilerInfo, instr2.mArguments,
                        instr2.mWatcher,
                        instr2.mUiAutomationConnection, testMode,
                        mBinderTransactionTrackingEnabled, enableTrackAllocation,
                        isRestrictedBackupMode || !normalMode, app.isPersistent(),
                        new Configuration(app.getWindowProcessController().getConfiguration()),
                        app.compat, getCommonServicesLocked(app.isolated),
                        mCoreSettingsObserver.getCoreSettingsLocked(),
                        buildSerial, autofillOptions, contentCaptureOptions,
                        app.mDisabledCompatChanges);
            } else {
                thread.bindApplication(processName, appInfo, providerList, null, profilerInfo,
                        null, null, null, testMode,
                        mBinderTransactionTrackingEnabled, enableTrackAllocation,
                        isRestrictedBackupMode || !normalMode, app.isPersistent(),
                        new Configuration(app.getWindowProcessController().getConfiguration()),
                        app.compat, getCommonServicesLocked(app.isolated),
                        mCoreSettingsObserver.getCoreSettingsLocked(),
                        buildSerial, autofillOptions, contentCaptureOptions,
                        app.mDisabledCompatChanges);
            }
            ...
            // 2、绑定之后激活Application 赋值IApplicationThread
            app.makeActive(thread, mProcessStats);
			...
			
        // See if the top visible activity is waiting to run in this process...
        if (normalMode) {
            try {
            	//3、通过ATMS启动 根activity
                didSomething = mAtmInternal.attachApplication(app.getWindowProcessController());
            } catch (Exception e) {
                Slog.wtf(TAG, "Exception thrown launching activities in " + app, e);
                badApp = true;
            }
        }
		...
}

AMS的attachApplicationLocked方法主要三件事：

  1. 调用IApplicationThread的bindApplication方法，IPC操作，创建绑定Application；
  2. 通过makeActive方法赋值IApplicationThread
  3. 通过ATMS启动根activity

先看makeActive方法：

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    //WindowProcessController
    public void makeActive(IApplicationThread _thread, ProcessStatsService tracker){
        ...
        thread = _thread;
        mWindowProcessController.setThread(thread);
    }

mWindowProcessController.setThread(thread)完成了IApplicationThread的赋值。这样就可以依据IApplicationThread是否为空判断进程是否存在了。

再看创建绑定Application的过程：

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//ApplicationThread
public final void bindApplication(String processName, ApplicationInfo appInfo,
                ProviderInfoList providerList, ComponentName instrumentationName,
                ProfilerInfo profilerInfo, Bundle instrumentationArgs,
                IInstrumentationWatcher instrumentationWatcher,
                IUiAutomationConnection instrumentationUiConnection, int debugMode,
                boolean enableBinderTracking, boolean trackAllocation,
                boolean isRestrictedBackupMode, boolean persistent, Configuration config,
                CompatibilityInfo compatInfo, Map services, Bundle coreSettings,
                String buildSerial, AutofillOptions autofillOptions,
                ContentCaptureOptions contentCaptureOptions, long[] disabledCompatChanges) {
            ...
            sendMessage(H.BIND_APPLICATION, data);
        }

IApplicationThread的bindApplication方法实现是客户端的ApplicationThread的bindApplication方法，它又使用H（之前说过H就是一个Handler）,转移到了ActivityThread的handleBindApplication方法（从Binder线程池转移到主线程），看下handleBindApplication方法：

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private void handleBindApplication(AppBindData data) {
	...
	final LoadedApk pi = getPackageInfo(instrApp, data.compatInfo,
        appContext.getClassLoader(), false, true, false);

          
    final ContextImpl instrContext = ContextImpl.createAppContext(this, pi,
        appContext.getOpPackageName());

    try {
        //创建Instrumentation
        final ClassLoader cl = instrContext.getClassLoader();
        mInstrumentation = (Instrumentation)
                cl.loadClass(data.instrumentationName.getClassName()).newInstance();
            } 
        ...
        final ComponentName component = new ComponentName(ii.packageName, ii.name);
        mInstrumentation.init(this, instrContext, appContext, component,
                data.instrumentationWatcher, data.instrumentationUiAutomationConnection);
	...
	//创建Application
    app = data.info.makeApplication(data.restrictedBackupMode, null);

	...
    mInitialApplication = app;
    try {
        mInstrumentation.onCreate(data.instrumentationArgs);
    }
    ...
    try {
        //内部调用Application的onCreate方法
        mInstrumentation.callApplicationOnCreate(app);
    }
	...
}

主要就是创建Application，并且调用生命周期onCreate方法。创建Application后，内部会attach方法，attach内部会调用attachBaseContext方法，attachBaseContext方法是我们能接触到的一个方法，接着才是onCreate方法。

再来看 根activity 的启动，回到上面AMS的attachApplicationLocked方法，调用了mAtmInternal.attachApplication方法，mAtmInternal是ActivityTaskManagerInternal实例，具体实现是在ActivityTaskManagerService的内部类LocalService，去看看：

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    //ActivityTaskManagerService#LocalService
    public boolean attachApplication(WindowProcessController wpc) throws RemoteException {
        synchronized (mGlobalLockWithoutBoost) {
            return mRootActivityContainer.attachApplication(wpc);
        }
    }

mRootActivityContainer是RootActivityContainer实例，看下它的attachApplication方法：

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    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 {
                            if (mStackSupervisor.realStartActivityLocked(activity, app,
                                    top == activity /* andResume */, true /* checkConfig */)) {
                                didSomething = true;
                            }
                        } 
                        ...
                    }
                }
            }
        }
        if (!didSomething) {
            ensureActivitiesVisible(null, 0, false /* preserve_windows */);
        }
        return didSomething;
    }

遍历activity栈，因为我们是第一次启动理论上应该只有一个根activity，然后调用mStackSupervisor.realStartActivityLocked方法

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    boolean realStartActivityLocked(ActivityRecord r, WindowProcessController proc,
            boolean andResume, boolean checkConfig) throws RemoteException {
	
			...

                // Create activity launch transaction.
                final ClientTransaction clientTransaction = ClientTransaction.obtain(
                        proc.getThread(), r.appToken);

                final DisplayContent dc = r.getDisplay().mDisplayContent;
                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);

                ...

        return true;
    }

中间有段代码如上，通过 ClientTransaction.obtain( proc.getThread(), r.appToken)获取了clientTransaction，其中参数proc.getThread()是IApplicationThread，就是前面提到的ApplicationThread在系统进程的代理。

ClientTransaction是包含一系列的待客户端处理的事务的容器，客户端接收后取出事务并执行。 接着看，使用clientTransaction.addCallback添加了LaunchActivityItem实例：

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    //ClientTransaction
    //都是用来发送到客户端的
	private List<ClientTransactionItem> mActivityCallbacks;
	
    public void addCallback(ClientTransactionItem activityCallback) {
        if (mActivityCallbacks == null) {
            mActivityCallbacks = new ArrayList<>();
        }
        mActivityCallbacks.add(activityCallback);
    }

看下LaunchActivityItem实例的获取：

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    /** Obtain an instance initialized with provided params. */
    public static LaunchActivityItem obtain(Intent intent, int ident, ActivityInfo info,
            Configuration curConfig, Configuration overrideConfig, CompatibilityInfo compatInfo,
            String referrer, IVoiceInteractor voiceInteractor, int procState, Bundle state,
            PersistableBundle persistentState, List<ResultInfo> pendingResults,
            List<ReferrerIntent> pendingNewIntents, boolean isForward, ProfilerInfo profilerInfo,
            IBinder assistToken) {
        LaunchActivityItem instance = ObjectPool.obtain(LaunchActivityItem.class);
        if (instance == null) {
            instance = new LaunchActivityItem();
        }
        setValues(instance, intent, ident, info, curConfig, overrideConfig, compatInfo, referrer,
                voiceInteractor, procState, state, persistentState, pendingResults,
                pendingNewIntents, isForward, profilerInfo, assistToken);

        return instance;
    }

new了一个LaunchActivityItem然后设置各种值。我们从名字就能看出，它就是用来启动activity的。它是怎么发挥作用的呢？接着看： 回到realStartActivityLocked方法，接着调用了mService.getLifecycleManager().scheduleTransaction(clientTransaction)，mService是ActivityTaskManagerService，getLifecycleManager()方法获取的是ClientLifecycleManager实例，它的scheduleTransaction方法如下：

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    void scheduleTransaction(ClientTransaction transaction) throws RemoteException {
        final IApplicationThread client = transaction.getClient();
        transaction.schedule();
        if (!(client instanceof Binder)) {
            transaction.recycle();
        }
    }

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

mClient.scheduleTransaction调用IApplicationThread的scheduleTransaction方法。由于IApplicationThread是ApplicationThread在系统进程的代理，所以真正执行的地方就是 客户端的ApplicationThread中了。也就是说，Activity启动的操作又跨进程的还给了客户端。

接着上面的分析，我们找到ApplicationThread的scheduleTransaction方法：

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

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

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

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

2.3 线程切换及消息处理——mH

实际上最后就是调用了mH.sendMessage(msg)方法，之前我们有说过mH实际上是一个Handler,我们看看内部实现：

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//
class H extends Handler {
        public static final int BIND_APPLICATION        = 110;
        @UnsupportedAppUsage
        public static final int EXIT_APPLICATION        = 111;
        @UnsupportedAppUsage
        public static final int RECEIVER                = 113;
        @UnsupportedAppUsage
        public static final int CREATE_SERVICE          = 114;
        @UnsupportedAppUsage
        public static final int SERVICE_ARGS            = 115;
        @UnsupportedAppUsage
        public static final int STOP_SERVICE            = 116;

        public static final int CONFIGURATION_CHANGED   = 118;
        public static final int CLEAN_UP_CONTEXT        = 119;
        @UnsupportedAppUsage
        public static final int GC_WHEN_IDLE            = 120;
        @UnsupportedAppUsage
        public static final int BIND_SERVICE            = 121;
        @UnsupportedAppUsage
        public static final int UNBIND_SERVICE          = 122;

        ...

        public static final int EXECUTE_TRANSACTION = 159;
        public static final int RELAUNCH_ACTIVITY = 160;
        public static final int PURGE_RESOURCES = 161;
        public static final int ATTACH_STARTUP_AGENTS = 162;

            
        public void handleMessage(Message msg) {
            if (DEBUG_MESSAGES) Slog.v(TAG, ">>> handling: " + codeToString(msg.what));

            ...

            switch (msg.what) {
                case BIND_APPLICATION:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "bindApplication");
                    AppBindData data = (AppBindData)msg.obj;
                    handleBindApplication(data);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case EXIT_APPLICATION:
                    if (mInitialApplication != null) {
                        mInitialApplication.onTerminate();
                    }
                    Looper.myLooper().quit();
                    break;
                case RECEIVER:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "broadcastReceiveComp");
                    handleReceiver((ReceiverData)msg.obj);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case CREATE_SERVICE:
                    if (Trace.isTagEnabled(Trace.TRACE_TAG_ACTIVITY_MANAGER)) {
                        Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER,
                                ("serviceCreate: " + String.valueOf(msg.obj)));
                    }
                    handleCreateService((CreateServiceData)msg.obj);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case BIND_SERVICE:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "serviceBind");
                    handleBindService((BindServiceData)msg.obj);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case UNBIND_SERVICE:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "serviceUnbind");
                    handleUnbindService((BindServiceData)msg.obj);
                    schedulePurgeIdler();
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case SERVICE_ARGS:
                    if (Trace.isTagEnabled(Trace.TRACE_TAG_ACTIVITY_MANAGER)) {
                        Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER,
                                ("serviceStart: " + String.valueOf(msg.obj)));
                    }
                    handleServiceArgs((ServiceArgsData)msg.obj);
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;
                case STOP_SERVICE:
                    Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "serviceStop");
                    handleStopService((IBinder)msg.obj);
                    schedulePurgeIdler();
                    Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);
                    break;

                ...

                case EXECUTE_TRANSACTION:
                    final ClientTransaction transaction = (ClientTransaction) msg.obj;
                    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;
                case RELAUNCH_ACTIVITY:
                    handleRelaunchActivityLocally((IBinder) msg.obj);
                    break;
                case PURGE_RESOURCES:
                    schedulePurgeIdler();
                    break;
                case ATTACH_STARTUP_AGENTS:
                    handleAttachStartupAgents((String) msg.obj);
                    break;
            }
            Object obj = msg.obj;
            if (obj instanceof SomeArgs) {
                ((SomeArgs) obj).recycle();
            }
            if (DEBUG_MESSAGES) Slog.v(TAG, "<<< done: " + codeToString(msg.what));
        }
    }

mH是在创建ActivityThread实例时赋值的，是自定义Handler子类H的实例，也就是在ActivityThread的main方法中，并且初始化是已经主线程已经有了mainLooper，所以，使用这个mH来sendMessage就把消息发送到了主线程。

那么是从哪个线程发送的呢？那就要看看ApplicationThread的scheduleTransaction方法是执行在哪个线程了。根据IPC知识，我们知道，服务器的Binder方法运行在Binder的线程池中，也就是说系统进行跨进程调用ApplicationThread的scheduleTransaction就是执行在Binder的线程池中的。但是通过mH一转换，消息最终到了应用的主线程处理了。

那么是怎么处理Activity的启动的呢？接着看。我们找到ActivityThread.H.EXECUTE_TRANSACTION这个消息的处理：取出ClientTransaction实例，调用TransactionExecutor的execute方法，那就看看：

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

        final IBinder token = transaction.getActivityToken();
        ...
        executeCallbacks(transaction);

        executeLifecycleState(transaction);
        ...
    }

跟进executeCallbacks方法：

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    public void executeCallbacks(ClientTransaction transaction) {
        final List<ClientTransactionItem> callbacks = transaction.getCallbacks();
        if (callbacks == null || callbacks.isEmpty()) {
            // No callbacks to execute, return early.
            return;
        }
        if (DEBUG_RESOLVER) Slog.d(TAG, tId(transaction) + "Resolving callbacks in transaction");

        final IBinder token = transaction.getActivityToken();
        ActivityClientRecord r = mTransactionHandler.getActivityClient(token);

        // In case when post-execution state of the last callback matches the final state requested
        // for the activity in this transaction, we won't do the last transition here and do it when
        // moving to final state instead (because it may contain additional parameters from server).
        final ActivityLifecycleItem finalStateRequest = transaction.getLifecycleStateRequest();
        final int finalState = finalStateRequest != null ? finalStateRequest.getTargetState()
                : UNDEFINED;
        // Index of the last callback that requests some post-execution state.
        final int lastCallbackRequestingState = lastCallbackRequestingState(transaction);

        final int size = callbacks.size();
        for (int i = 0; i < size; ++i) {
            final ClientTransactionItem item = callbacks.get(i);
            ...
            item.execute(mTransactionHandler, token, mPendingActions);
            item.postExecute(mTransactionHandler, token, mPendingActions);
            ...
        }

遍历callbacks，调用ClientTransactionItem的execute方法，而我们这里要关注的是ClientTransactionItem的子类LaunchActivityItem，看下它的execute方法：

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    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.handleLaunchActivity(r, pendingActions, null /* customIntent */);
        Trace.traceEnd(TRACE_TAG_ACTIVITY_MANAGER);
    }

里面调用了client.handleLaunchActivity方法，client是ClientTransactionHandler的实例，是在TransactionExecutor构造方法传入的，TransactionExecutor创建是在ActivityThread中：

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    //ActivityThread
    private final TransactionExecutor mTransactionExecutor = new TransactionExecutor(this);

所以，client.handleLaunchActivity方法就是ActivityThread的handleLaunchActivity方法。

到这里 ApplicationThread把启动Activity的操作，通过mH切到了主线程，走到了ActivityThread的handleLaunchActivity方法。

2.4 Activity初始化及生命周期

接着来看handleLaunchActivity方法：

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    public Activity handleLaunchActivity(ActivityClientRecord r,
            PendingTransactionActions pendingActions, Intent customIntent) {
        ...
        final Activity a = performLaunchActivity(r, customIntent);
        ...
        return a;
    }

    // activity 启动的核心实现
    private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
    	//1、从ActivityClientRecord获取待启动的Activity的组件信息
        ActivityInfo aInfo = r.activityInfo;
        if (r.packageInfo == null) {
            r.packageInfo = getPackageInfo(aInfo.applicationInfo, r.compatInfo,
                    Context.CONTEXT_INCLUDE_CODE);
        }

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

        if (r.activityInfo.targetActivity != null) {
            component = new ComponentName(r.activityInfo.packageName,
                    r.activityInfo.targetActivity);
        }
		//创建ContextImpl对象
        ContextImpl appContext = createBaseContextForActivity(r);
        Activity activity = null;
        try {
        	//2、创建activity实例
            java.lang.ClassLoader cl = appContext.getClassLoader();
            activity = mInstrumentation.newActivity(
                    cl, component.getClassName(), r.intent);
            StrictMode.incrementExpectedActivityCount(activity.getClass());
            r.intent.setExtrasClassLoader(cl);
            r.intent.prepareToEnterProcess();
            if (r.state != null) {
                r.state.setClassLoader(cl);
            }
        } catch (Exception e) {
            ..
        }
        try {
        	//3、创建Application对象（如果没有的话）
            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);
                
                //4、attach方法为activity关联上下文环境
                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);

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

                activity.mCalled = false;
                
                //5、调用生命周期onCreate
                if (r.isPersistable()) {
                    mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);
                } else {
                    mInstrumentation.callActivityOnCreate(activity, r.state);
                }
                if (!activity.mCalled) {
                    throw new SuperNotCalledException(
                        "Activity " + r.intent.getComponent().toShortString() +
                        " did not call through to super.onCreate()");
                }
                r.activity = activity;
            }
            r.setState(ON_CREATE);
            
            synchronized (mResourcesManager) {
                mActivities.put(r.token, r);
            }

        } 
        ...

        return activity;
    }

performLaunchActivity主要完成以下事情：

  1. 从ActivityClientRecord获取待启动的Activity的组件信息
  2. 通过mInstrumentation.newActivity方法使用类加载器创建activity实例
  3. 通过LoadedApk的makeApplication方法创建Application对象，内部也是通过mInstrumentation使用类加载器，创建后就调用了instrumentation.callApplicationOnCreate方法，也就是Application的onCreate方法。
  4. 创建ContextImpl对象并通过activity.attach方法对重要数据初始化，关联了Context的具体实现ContextImpl，attach方法内部还完成了window创建，这样Window接收到外部事件后就能传递给Activity了。
  5. 通过 mInstrumentation.callActivityOnCreate方法调用Activity的onCreate。

到这里Activity的onCreate方法执行完，那么onStart、onResume呢？ 上面看到LaunchActivityItem，是用来启动Activity的，也就是走到Activity的onCreate，那么是不是有 “XXXActivityItem”呢? 有的：

  • LaunchActivityItem 远程App端的onCreate生命周期事务
  • ResumeActivityItem 远程App端的onResume生命周期事务
  • PauseActivityItem 远程App端的onPause生命周期事务
  • StopActivityItem 远程App端的onStop生命周期事务
  • DestroyActivityItem 远程App端onDestroy生命周期事务

这些对应的都会走到handlexxxActivity方法，这里我们就不再一一分析了。至此根Activity的启动已经完成了，那么普通activity的启动流程呢？

3 普通activity的启动流程

3.1 Activity启动的发起

从startActivity方法开始，会走到startActivityForResult方法：

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    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) {
                mStartedActivity = true;
            }

            cancelInputsAndStartExitTransition(options);
        } else {
            ...
        }
    }

看到里面调用了mInstrumentation.execStartActivity方法，其中一个参数mMainThread.getApplicationThread()，它的类型是ApplicationThread，之前我们说ApplicationThread是ActivityThread的内部类，继承IApplicationThread.Stub，也是个Binder对象，是用于IPC通讯使用的。而Instrumentation具有跟踪application及activity生命周期的功能，用于android 应用测试框架中代码检测。接着看下mInstrumentation.execStartActivity方法：

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    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);
        }
        ...

        try {
            intent.migrateExtraStreamToClipData();
            intent.prepareToLeaveProcess(who);
            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;
    }

这里看到Activity的启动又交给了ActivityTaskManager.getService()：

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    //ActivityTaskManager
    public static IActivityTaskManager getService() {
        return IActivityTaskManagerSingleton.get();
    }
    
    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);
                }
            };

这儿的代码有没有很熟悉？不错，跟我们之前分析的获取ActivityManagerService服务是一样的，不过这儿获取的是ActivityTaskManagerService（ATMS）。ATMS是用于管理Activity及其容器（任务、堆栈、显示等）的系统服务，运行在系统服务进程（system_server）之中。

值得说明的是，ATMS是在Android10中新增的，分担了之前ActivityManagerService（AMS）的一部分功能（activity task相关）。 在Android10 之前 ，这个地方获取的是服务是AMS。查看Android10的AMS，你会发现startActivity方法内也是调用了ATMS的startActivity方法。所以在理解上，ATMS就隶属于AMS。

接着看，ActivityTaskManager.getService().startActivity有个返回值result，且调用了checkStartActivityResult(result, intent)：

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    public static void checkStartActivityResult(int res, Object intent) {
        if (!ActivityManager.isStartResultFatalError(res)) {
            return;
        }

        switch (res) {
            case ActivityManager.START_INTENT_NOT_RESOLVED:
            case ActivityManager.START_CLASS_NOT_FOUND:
                if (intent instanceof Intent && ((Intent)intent).getComponent() != null)
                    throw new ActivityNotFoundException(
                            "Unable to find explicit activity class "
                            + ((Intent)intent).getComponent().toShortString()
                            + "; have you declared this activity in your AndroidManifest.xml?");
                throw new ActivityNotFoundException(
                        "No Activity found to handle " + intent);
            case ActivityManager.START_PERMISSION_DENIED:
                throw new SecurityException("Not allowed to start activity "
                        + intent);
            ...
            
            case ActivityManager.START_CANCELED:
                throw new AndroidRuntimeException("Activity could not be started for "
                        + intent);
            default:
                throw new AndroidRuntimeException("Unknown error code "
                        + res + " when starting " + intent);
        }
    }

这是用来检查Activity启动的结果，如果发生致命错误，就会抛出对应的异常。看到第一个case中就抛出了 have you declared this activity in your AndroidManifest.xml?——如果Activity没在Manifest中注册就会有这个错误。

3.2 ATMS

到这里，Activity的启动就跨进程（IPC）的转移到系统进程提供的服务ATMS中了，接着看ATMS的startActivity：

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    //ActivityTaskManagerService
    @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.
        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();

    }

跟到startActivityAsUser中，通过getActivityStartController().obtainStarter方法获取ActivityStarter实例 然后调用一系列方法，最后的execute()方法是开始启动activity：

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    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();
        }
    }

这儿分了两种情况，不过 不论startActivityMayWait还是startActivity最终都是走到下面这个startActivity方法：

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    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();
            result = startActivityUnchecked(r, sourceRecord, voiceSession, voiceInteractor,
                    startFlags, doResume, options, inTask, outActivity, restrictedBgActivity);
        } finally {
            final ActivityStack currentStack = r.getActivityStack();
            startedActivityStack = currentStack != null ? currentStack : mTargetStack;

           ...
        }

        postStartActivityProcessing(r, result, startedActivityStack);
        return result;
    }

里面有调用了startActivityUnchecked方法，之后调用RootActivityContainer的resumeFocusedStacksTopActivities方法。RootActivityContainer是Android10新增的类，分担了之前ActivityStackSupervisor的部分功能。接着跳转到ActivityStack的resumeTopActivityUncheckedLocked方法：

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    //ActivityStack
    boolean resumeTopActivityUncheckedLocked(ActivityRecord prev, ActivityOptions options) {
        if (mInResumeTopActivity) {
            // Don't even start recursing.
            return false;
        }
        boolean result = false;
        try {
            mInResumeTopActivity = true;
            result = resumeTopActivityInnerLocked(prev, options);

            final ActivityRecord next = topRunningActivityLocked(true /* focusableOnly */);
            if (next == null || !next.canTurnScreenOn()) {
                checkReadyForSleep();
            }
        } finally {
            mInResumeTopActivity = false;
        }

        return result;
    }

跟进resumeTopActivityInnerLocked方法：

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    private boolean resumeTopActivityInnerLocked(ActivityRecord prev, ActivityOptions options) {
        ...
        boolean pausing = getDisplay().pauseBackStacks(userLeaving, next, false);
        if (mResumedActivity != null) {
            if (DEBUG_STATES) Slog.d(TAG_STATES,
                    "resumeTopActivityLocked: Pausing " + mResumedActivity);
             // 暂停上一个Activity
            pausing |= startPausingLocked(userLeaving, false, next, false);
        }
        ...
        //这里next.attachedToProcess()，只有启动了的Activity才会返回true
        if (next.attachedToProcess()) {
            ...
            
            try {
                final ClientTransaction transaction =
                        ClientTransaction.obtain(next.app.getThread(), next.appToken);
                ...
                //启动了的Activity就发送ResumeActivityItem事务给客户端了，后面会讲到
                transaction.setLifecycleStateRequest(
                        ResumeActivityItem.obtain(next.app.getReportedProcState(),
                                getDisplay().mDisplayContent.isNextTransitionForward()));
                mService.getLifecycleManager().scheduleTransaction(transaction);
               ....
            } catch (Exception e) {
                ....
                mStackSupervisor.startSpecificActivityLocked(next, true, false);
                return true;
            }
            ....
        } else {
            ....
            if (SHOW_APP_STARTING_PREVIEW) {
            	    //这里就是 冷启动时 出现白屏 的原因了：取根activity的主题背景 展示StartingWindow
                    next.showStartingWindow(null , false ,false);
                }
            // 继续当前Activity，普通activity的正常启动 关注这里即可
            mStackSupervisor.startSpecificActivityLocked(next, true, true);
        }
        return true;
    }

先对上一个Activity执行pause操作，再执行当前创建操作，代码最终进入到了ActivityStackSupervisor.startSpecificActivityLocked方法中。这里有个点注意下，启动activity前调用了next.showStartingWindow方法来展示一个window，这就是 冷启动时 出现白屏 的原因了。我们继续看ActivityStackSupervisor.startSpecificActivityLocked方法：

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    //ActivityStackSupervisor
    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 {
                realStartActivityLocked(r, wpc, andResume, checkConfig);
                return;
            } catch (RemoteException e) {
                Slog.w(TAG, "Exception when starting activity "
                        + r.intent.getComponent().flattenToShortString(), e);
            }
            knownToBeDead = true;
        }

        ...
        
        try {
            if (Trace.isTagEnabled(TRACE_TAG_ACTIVITY_MANAGER)) {
                Trace.traceBegin(TRACE_TAG_ACTIVITY_MANAGER, "dispatchingStartProcess:"
                        + r.processName);
            }
            // 上面的wpc != null && wpc.hasThread()不满足的话，说明没有进程，就会取创建进程
            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);
        }
    }

有个判断条件if (wpc != null && wpc.hasThread())，意思是是否启动了应用进程，内部是通过IApplicationThread是否为空来判断,之前我们讲根activity启动时有说到在WindowProcessController.makeActive方法内部给IApplicationThread赋值，就是用于这儿的判断。这里我们看已启动应用进程的情况，即调用了realStartActivityLocked方法，看到这个realStartActivityLocked方法是不是很熟悉？没错，就是我们前面讲根activity启动流程时调用的mStackSupervisor.realStartActivityLocked方法，之后的流程就跟前面分析的一样了点击查看。

4 总结

完整关系如下：