本文是探索Activity启动源码的第二篇, 其余参考第一篇.

Activity

第一篇的流程图:

流程图

第一篇已经探索至关键位置, 即ActivityStackSupervisor的realStartActivityLocked方法, 方法如其名, 从此开始, 才是真正地(Real)启动(Start). Let's start!

ActivityManagerService

ActivityStackSupervisor#realStartActivityLocked:

final boolean realStartActivityLocked(ActivityRecord r, ProcessRecord app,boolean andResume, boolean checkConfig) throws RemoteException{// 当屏幕方向修改时, 推迟恢复, 防止冗余启动Activity.if (checkConfig) {Configuration config = mWindowManager.updateOrientationFromAppTokens(mService.mConfiguration,r.mayFreezeScreenLocked(app) ? r.appToken : null);mService.updateConfigurationLocked(config, r, false, true /* deferResume */);}// 将进程描述符(ProcessRecord)设置入Activity描述符(ActivityRecord)r.app = app;app.waitingToKill = null; // 避免在后台被杀死r.launchCount++; // 增加启动次数r.lastLaunchTime = SystemClock.uptimeMillis(); // 最新启动时间// 当Activity描述符不在进程的Activity列表中, 将Activity添加入进程的Activity列表int idx = app.activities.indexOf(r);if (idx < 0) {app.activities.add(r);}// AMS更新进程描述符为最少最新使用(LRU).mService.updateLruProcessLocked(app, true, null); // ...try {// ...// 远程调用ApplicationThread的scheduleLaunchActivityapp.thread.scheduleLaunchActivity(new Intent(r.intent), r.appToken,System.identityHashCode(r), r.info, new Configuration(mService.mConfiguration),new Configuration(task.mOverrideConfig), r.compat, r.launchedFromPackage,task.voiceInteractor, app.repProcState, r.icicle, r.persistentState, results,newIntents, !andResume, mService.isNextTransitionForward(), profilerInfo);// ...} catch (RemoteException e) {// ...}// ...return true;
}

ActivityRecord即Activity描述符(变量r), 含有被启动的Activity信息; ProcessRecord即进程描述符(变量app), 含有当前应用的进程信息.
app.thread类型是IApplicationThread, 通过IApplicationThread的代理ApplicationThreadProxy, 远程调用ApplicationThread的scheduleLaunchActivity方法, ApplicationThread是IApplicationThread的最终实现.

启动Activity再次由AMS(ActivityManagerService)通过远程调用(Binder)交给应用进程(ActivityThread)处理. ApplicationThread是ActivityThread的私有类.

ApplicationThread#scheduleLaunchActivity:

@Override
public final void scheduleLaunchActivity(Intent intent, IBinder token, 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 notResumed, boolean isForward, ProfilerInfo profilerInfo){// ...// 即来自客户端(AMS)的Activity描述符ActivityClientRecord r = new ActivityClientRecord();// 将AMS的Activity描述符, 转换为当前进程的Activity描述符r.token = token;// ...// 封装, 发送启动消息给H类处理, 并传递Activity描述符sendMessage(H.LAUNCH_ACTIVITY, r);
}

sendMessage是重载方法, 最终调用H类处理数据.

private void sendMessage(int what, Object obj, int arg1, int arg2, boolean async){// ...// 最终由H类(Handler)处理数据mH.sendMessage(msg);
}

将启动Activity交给ActivityThread的Handler H类处理.

private class H extends Handler{public void handleMessage(Message msg){switch (msg.what) {case LAUNCH_ACTIVITY: {Trace.traceBegin(Trace.TRACE_TAG_ACTIVITY_MANAGER, "activityStart");ActivityClientRecord r = (ActivityClientRecord)msg.obj;// 设置Activity描述符的包信息r.packageInfo = getPackageInfoNoCheck(r.activityInfo.applicationInfo, r.compatInfo);// 至此, 完成启动Activity已经由AMS交给当前应用, 在当前应用中启动Activity.handleLaunchActivity(r, null);Trace.traceEnd(Trace.TRACE_TAG_ACTIVITY_MANAGER);} break;}}
}

启动Activity, 先由当前进程, 通过ActivityManagerNative#getDefault#startActivity方法, 交给AMS处理, AMS管理Activity的Stack和Task, 并设置Activity描述符(Record); 再通过app.thread#scheduleLaunchActivity方法, 继续交给当前进程处理.

AMS切换当前线程的流程图:

流程图

ActivityThread

ActivityThread#handleLaunchActivity:

handleLaunchActivity调用performLaunchActivity方法, 继续执行启动, 在成功后, 调用handleResumeActivity方法, 执行显示Activity.

    private void handleLaunchActivity(ActivityClientRecord r, Intent customIntent, String reason){// 因为当前进程正在活跃, 所以跳过在后台中执行GC.unscheduleGcIdler();// ...// 确保使用最近的环境配置handleConfigurationChanged(null, null);if (localLOGV) Slog.v(TAG, "Handling launch of " + r);// 在创建Activity前, 初始化WindowManagerGlobal, 即WindowManagerServiceWindowManagerGlobal.initialize();// 执行启动ActivityActivity a = performLaunchActivity(r, customIntent);// 在启动成功后, 处理恢复Activityif (a != null) {r.createdConfig = new Configuration(mConfiguration);reportSizeConfigurations(r);Bundle oldState = r.state;// 恢复ActivityhandleResumeActivity(r.token, false, r.isForward,!r.activity.mFinished && !r.startsNotResumed, r.lastProcessedSeq, reason);// ...}} else {// 如果发生错误, 则AMS停止启动Activitytry {ActivityManagerNative.getDefault().finishActivity(r.token, Activity.RESULT_CANCELED, null,Activity.DONT_FINISH_TASK_WITH_ACTIVITY);} catch (RemoteException ex) {throw ex.rethrowFromSystemServer();}}}

ActivityThread#performLaunchActivity:

performLaunchActivity方法是Activity启动的核心:

1. 获取Activity的组件信息(ComponentName);
2. 根据组件使用反射创建Activity(newActivity);
3. 将Activity绑定(attach)Application和BaseContext;
4. 相继调用onCreate, onStart, onRestoreInstanceState, onPostCreate等方法.
5. 放入Activity列表(Map)中统一管理, token是key.

private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {ActivityInfo aInfo = r.activityInfo;if (r.packageInfo == null) {// 通过应用信息, 兼容信息, 从PMS, 获取Activity的包信息r.packageInfo = getPackageInfo(aInfo.applicationInfo, r.compatInfo,Context.CONTEXT_INCLUDE_CODE);}// 获取组件信息ComponentName component = r.intent.getComponent();if (component == null) {component = r.intent.resolveActivity(mInitialApplication.getPackageManager());r.intent.setComponent(component);}// 当指定Activity时, 使用包名和类名创建组件if (r.activityInfo.targetActivity != null) {component = new ComponentName(r.activityInfo.packageName,r.activityInfo.targetActivity);}Activity activity = null;try {java.lang.ClassLoader cl = r.packageInfo.getClassLoader();// 通过反射, 使用ClassLoader创建Activityactivity = mInstrumentation.newActivity(cl, component.getClassName(), r.intent);// ...} catch (Exception e) {// ...}try {// 使用单例模式, 创建ApplicationApplication app = r.packageInfo.makeApplication(false, mInstrumentation);// ...if (activity != null) {// 创建Activity的上下文BaseContextContext appContext = createBaseContextForActivity(r, activity);// ...// 将Activity绑定上下文appContext, 和应用appactivity.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);// ...// 设置主题int theme = r.activityInfo.getThemeResource();if (theme != 0) {activity.setTheme(theme);}activity.mCalled = false; // 判断执行是否成功, 成功会置true// 调用Activity的onCreate方法.if (r.isPersistable()) {mInstrumentation.callActivityOnCreate(activity, r.state, r.persistentState);} else {mInstrumentation.callActivityOnCreate(activity, r.state);}// ...// 调用Activity的onStart方法.if (!r.activity.mFinished) {activity.performStart();r.stopped = false;}// 调用Activity的onRestoreInstanceState方法.if (!r.activity.mFinished) {if (r.isPersistable()) {if (r.state != null || r.persistentState != null) {mInstrumentation.callActivityOnRestoreInstanceState(activity, r.state,r.persistentState);}} else if (r.state != null) {mInstrumentation.callActivityOnRestoreInstanceState(activity, r.state);}}// 调用Activity的onPostCreate方法.if (!r.activity.mFinished) {activity.mCalled = false; // 判断执行是否成功, 成功会置trueif (r.isPersistable()) {mInstrumentation.callActivityOnPostCreate(activity, r.state,r.persistentState);} else {mInstrumentation.callActivityOnPostCreate(activity, r.state);}// ...}}r.paused = true;// 将Activity放入ActivityThread的Activity数组中统一管理.mActivities.put(r.token, r);} // ...return activity;
}

onPostCreate: Called when activity start-up is complete (after onStart() and onRestoreInstanceState(Bundle) have been called). Applications will generally not implement this method; it is intended for system classes to do final initialization after application code has run.
在调用时, 表明Activity已经完全启动, 只剩下显示(onResume).

通过分析performLaunchActivity, 我们也更加清晰Activity的生命周期, 顺序如下, onCreate, onStart, onRestoreInstanceState, onPostCreate. 注意, onStart是Activity处理; 其余三个是Instrumentation处理, 支持继承重写相应方法, 自行处理.

至此, Activity已经完全启动, 并调用相应的生命周期方法.

OK, that's all! Enjoy it!