Activity 创建流程详解
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创建流程
ActivityThread作为主应用程序的主线程管理类，我们都从main方法开始分析。main方法主要功能是创建ActivityThread且关联，创建Looper死循环不让程序退出。
public static void main(String[] args) { ... //为主线程创建loop对象，我们在主线程使用Handler时候没有初始化都可以使用，因为这里做了初始化。 Looper.prepareMainLooper(); ... ActivityThread thread = new ActivityThread(); thread.attach(false, startSeq); if (sMainThreadHandler == null) { sMainThreadHandler = thread.getHandler(); } //开始死循环获得消息 Looper.loop(); }接下来我们分析一下ActivityThread的attach方法，在调用attach时候第一个参数传递的是false，我们可以看出它通过AMS跨进程调用了attachApplication方法。
private void attach(boolean system, long startSeq) { if (!system) { ... final IActivityManager mgr = ActivityManager.getService(); try { mgr.attachApplication(mAppThread, startSeq); } catch (RemoteException ex) { throw ex.rethrowFromSystemServer(); } } ... }接下来我们需要去framework看AMS源码才能只知道里面到底做了什么事情，
最后调用了attachApplicationLocked方法，然后回调到主进程ApplicationThread的bindApplication方法里面去。
//frameworks/base/services/core/java/com/android/server/am/ActivityManagerService.java private final boolean attachApplicationLocked(IApplicationThread thread, int pid, int callingUid, long startSeq) { ... thread.bindApplication(processName, appInfo, providers, 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); ... didSomething = mAtmInternal.attachApplication(app.getWindowProcessController()); ... }我们继续分析主进程ActivityThread，AMS通知它创建Application，且两个进程之间数据同步。
public final void bindApplication(String processName, ApplicationInfo appInfo, List<ProviderInfo> providers, 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) { ... AppBindData data = new AppBindData(); data.processName = processName; data.appInfo = appInfo; data.providers = providers; data.instrumentationName = instrumentationName; data.instrumentationArgs = instrumentationArgs; data.instrumentationWatcher = instrumentationWatcher; data.instrumentationUiAutomationConnection = instrumentationUiConnection; data.debugMode = debugMode; data.enableBinderTracking = enableBinderTracking; data.trackAllocation = trackAllocation; data.restrictedBackupMode = isRestrictedBackupMode; data.persistent = persistent; data.config = config; data.compatInfo = compatInfo; data.initProfilerInfo = profilerInfo; data.buildSerial = buildSerial; data.autofillOptions = autofillOptions; data.contentCaptureOptions = contentCaptureOptions; sendMessage(H.BIND_APPLICATION, data);//1 }我们从上面代码看出第一点，在ApplicationThread通过H类是继承Handler，通过发送消息到主线程管理类ActivityThread执行绑定Application，调用handleBindApplication方法。创建LoadedApk对象并且用于创建Application。
private void handleBindApplication(AppBindData data) { //设置进程名字 Process.setArgV0(data.processName); //创建LoadedApk实例 data.info = getPackageInfoNoCheck(data.appInfo, data.compatInfo); app = data.info.makeApplication(data.restrictedBackupMode, null); } public Application makeApplication(boolean forceDefaultAppClass, Instrumentation instrumentation) { //1 if (mApplication != null) { return mApplication; } Application app = null; try { ClassLoader cl = getClassLoader(); ContextImpl appContext = ContextImpl.createAppContext(mActivityThread, this); //2 app = mActivityThread.mInstrumentation.newApplication( cl, appClass, appContext); appContext.setOuterContext(app); } catch (Exception e) { } mApplication = app; if (instrumentation != null) { //3 instrumentation.callApplicationOnCreate(app); } return app; }从上述代码我们可以看出，
1：如果有已经创建好了Applicaiton对象及返回。
2：创建Application对象，并且与appContext建立关联关系，且调用attach方法
3：通过instrumentation调用执行onCreate方法
然后我们在回到AMS代码里面及方法attachApplicationLocked，看看是如何执行跳转Activity的。
//frameworks/base/services/core/java/com/android/server/wm/ActivityTaskManagerService.java public boolean attachApplication(WindowProcessController wpc) throws RemoteException { synchronized (mGlobalLockWithoutBoost) { return mRootActivityContainer.attachApplication(wpc); }我们可以看出下面代码realStartActivityLocked，这个都是我们真正发起创建Activity的方法，然后我们继续分析代码。
//frameworks/base/services/core/java/com/android/server/wm/RootActivityContainer.java 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 { //执行跳转 if (mStackSupervisor.realStartActivityLocked(activity, app, top == activity /* andResume */, true /* checkConfig */)) { didSomething = true; } } catch (RemoteException e) { } } } } } return didSomething; }在下面的代码主要作用是，发送指令到应用进程创建Activity，LaunchActivityItem表示一个创建任务。
//frameworks/base/services/core/java/com/android/server/wm/ActivityStackSupervisor.java boolean realStartActivityLocked(ActivityRecord r, WindowProcessController proc, boolean andResume, boolean checkConfig) throws RemoteException { //启动指令 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. 跨进程调用到应用进程 mService.getLifecycleManager().scheduleTransaction(clientTransaction); return true; }于是开始执行命令到,
//ClientLifecycleManager.java void scheduleTransaction(ClientTransaction transaction) throws RemoteException { final IApplicationThread client = transaction.getClient(); transaction.schedule(); if (!(client instanceof Binder)) { transaction.recycle(); } }//ClientTransaction.javapublic void schedule() throws RemoteException { mClient.scheduleTransaction(this); }上述代码mClient实际都是ApplicationThread，那我们由此知道，马上都会创建真正的Activity了，接下来我们继续分析，我们来到ApplicationThread这个类里面的这个scheduleTransaction方法。
//ClientTransactionHandler.java void scheduleTransaction(ClientTransaction transaction) { transaction.preExecute(this); sendMessage(ActivityThread.H.EXECUTE_TRANSACTION, transaction); }上述代码看出，最终代买执行到了ActivityThread，然后通过发送消息到主线程管理执行创建Activity，ActivityThread继承了ClientTransactionHandler。
//TransactionExecutor.java public void execute(ClientTransaction transaction) { executeCallbacks(transaction); //之前执行暂停调用这个方法 executeLifecycleState(transaction); } public void executeCallbacks(ClientTransaction transaction) { for (int i = 0; i < size; ++i) { final ClientTransactionItem item = callbacks.get(i); item.execute(mTransactionHandler, token, mPendingActions); item.postExecute(mTransactionHandler, token, mPendingActions); } }经过上述代码的表演，我们来到了LaunchActivityItem这个类。
@Override public void execute(ClientTransactionHandler client, IBinder token, PendingTransactionActions pendingActions) { Trace.traceBegin(TRACE_TAG_ACTIVITY_MANAGER, "activityStart"); //与 ActivityRecord 信息同步 token信息通讯 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); }走到这里我们开始创建ActivityClientRecord来记录Activity的信息，并且通过token与AMS的ActivityRecord相应关联。继续看handleLaunchActivity的源码，如下。
public Activity handleLaunchActivity(ActivityClientRecord r, PendingTransactionActions pendingActions, Intent customIntent) { final Activity a = performLaunchActivity(r, customIntent); return a; } private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) { ContextImpl appContext = createBaseContextForActivity(r); Activity activity = null; try { ClassLoader cl = appContext.getClassLoader(); activity = mInstrumentation.newActivity( cl, component.getClassName(), r.intent); } catch (Exception e) { } try { Application app = r.packageInfo.makeApplication(false, mInstrumentation); if (activity != null) { appContext.setOuterContext(activity); 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 (r.isPersistable()) { mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState); } else { mInstrumentation.callActivityOnCreate(activity, r.state); } r.activity = activity; } } catch (SuperNotCalledException e) { throw e; } catch (Exception e) { } return activity; }原创声明：本文系作者授权腾讯云开发者社区发表，未经许可，不得转载。
如有侵权，请联系 cloudcommunity@tencent.com 删除。
原创声明：本文系作者授权腾讯云开发者社区发表，未经许可，不得转载。
如有侵权，请联系 cloudcommunity@tencent.com 删除。
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