文中相关源码链接：
之前介绍 SystemServer 启动流程 的时候说到，SystemServer 进程启动了一系列的系统服务，ActivityManagerService 是其中最核心的服务之一。它和四大组件的启动、切换、调度及应用进程的管理和调度息息相关，其重要性不言而喻。本文主要介绍其启动流程，它是在 SystemServer 的 main() 中启动的，整个启动流程经历了 startBootstrapServices 、startCoreService() 、startOtherService()。下面就顺着源码来捋一捋 ActivityManagerService 的启动流程，下文中简称 AMS。
private void startBootstrapServices() { ... // 1. AMS 初始化 mActivityManagerService = mSystemServiceManager.startService( ActivityManagerService.Lifecycle.class).getService(); // 设置 AMS 的系统服务管理器 mActivityManagerService.setSystemServiceManager(mSystemServiceManager); // 设置 AMS 的应用安装器 mActivityManagerService.setInstaller(installer); ... mActivityManagerService.initPowerManagement(); ... // 2. AMS.setSystemProcess() mActivityManagerService.setSystemProcess(); ... } private void startCoreServices{ ... mActivityManagerService.setUsageStatsManager( LocalServices.getService(UsageStatsManagerInternal.class)); ... } private void startherService{ ... // 3. 安装系统 Provider mActivityManagerService.installSystemProviders(); ... final Watchdog watchdog = Watchdog.getInstance(); watchdog.init(context, mActivityManagerService); ... mActivityManagerService.setWindowManager(wm); ... networkPolicy = new NetworkPolicyManagerService(context, mActivityManagerService, networkManagement); ... if (safeMode) { traceBeginAndSlog("EnterSafeModeAndDisableJitCompilation"); mActivityManagerService.enterSafeMode(); // Disable the JIT for the system_server process VMRuntime.getRuntime().disableJitCompilation(); traceEnd(); } ... mPowerManagerService.systemReady(mActivityManagerService.getAppOpsService()); ... // 4. AMS.systemReady() mActivityManagerService.systemReady(() -> { ... mActivityManagerService.startObservingNativeCrashes(); } } 复制代码
AMS 初始化
mActivityManagerService = mSystemServiceManager.startService( ActivityManagerService.Lifecycle.class).getService(); 复制代码
AMS 通过 SystemServiceManager.startService() 方法初始化，startService() 在之前的文章中已经分析过，其作用是根据参数传入的类通过反射进行实例化，并回调其 onStart() 方法。注意这里传入的是 ActivityManagerService.Lifecycle.class，LifeCycle 是 AMS 的一个静态内部类。
public static final class Lifecycle extends SystemService { private final ActivityManagerService mService; // 构造函数中新建 ActivityManagerService 对象 public Lifecycle(Context context) { super(context); mService = new ActivityManagerService(context); } @Override public void onStart() { mService.start(); } @Override public void onBootPhase(int phase) { mService.mBootPhase = phase; if (phase == PHASE_SYSTEM_SERVICES_READY) { mService.mBatteryStatsService.systemServicesReady(); mService.mServices.systemServicesReady(); } } @Override public void onCleanupUser(int userId) { mService.mBatteryStatsService.onCleanupUser(userId); } public ActivityManagerService getService() { return mService; } } 复制代码
Lifecycle 的构造函数中初始化了 AMS。再来看看 AMS 的构造函数。
public ActivityManagerService(Context systemContext) { mInjector = new Injector(); // AMS 上下文 mContext = systemContext; mFactoryTest = FactoryTest.getMode(); // ActivityThread 对象 mSystemThread = ActivityThread.currentActivityThread(); // ContextImpl 对象 mUiContext = mSystemThread.getSystemUiContext(); mPermissionReviewRequired = mContext.getResources().getBoolean( com.android.internal.R.bool.config_permissionReviewRequired); // 线程名为 ActivityManager 的前台线程，ServiceThread 继承于 HandlerThread mHandlerThread = new ServiceThread(TAG, THREAD_PRIORITY_FOREGROUND, false /*allowIo*/); mHandlerThread.start(); // 获取 mHandlerThread 的 Handler 对象 mHandler = new MainHandler(mHandlerThread.getLooper()); // 创建名为 android.ui 的线程 mUiHandler = mInjector.getUiHandler(this); // 不知道什么作用 mProcStartHandlerThread = new ServiceThread(TAG + ":procStart", THREAD_PRIORITY_FOREGROUND, false /* allowIo */); mProcStartHandlerThread.start(); mProcStartHandler = new Handler(mProcStartHandlerThread.getLooper()); mConstants = new ActivityManagerConstants(this, mHandler); /* static; one-time init here */ // 根据优先级 kill 后台应用进程 if (sKillHandler == null) { sKillThread = new ServiceThread(TAG + ":kill", THREAD_PRIORITY_BACKGROUND, true /* allowIo */); sKillThread.start(); sKillHandler = new KillHandler(sKillThread.getLooper()); } // 前台广播队列，超时时间为 10 秒 mFgBroadcastQueue = new BroadcastQueue(this, mHandler, "foreground", BROADCAST_FG_TIMEOUT, false); // 后台广播队列，超时时间为 60 秒 mBgBroadcastQueue = new BroadcastQueue(this, mHandler, "background", BROADCAST_BG_TIMEOUT, true); mBroadcastQueues[0] = mFgBroadcastQueue; mBroadcastQueues[1] = mBgBroadcastQueue; // 创建 ActiveServices mServices = new ActiveServices(this); mProviderMap = new ProviderMap(this); // 创建 AppErrors，用于处理应用中的错误 mAppErrors = new AppErrors(mUiContext, this); // 创建 /data/system 目录 File dataDir = Environment.getDataDirectory(); File systemDir = new File(dataDir, "system"); systemDir.mkdirs(); mAppWarnings = new AppWarnings(this, mUiContext, mHandler, mUiHandler, systemDir); // TODO: Move creation of battery stats service outside of activity manager service. // 创建 BatteryStatsService，其信息保存在 /data/system/procstats 中 // 这里有个 TODO，打算把 BatteryStatsService 的创建移除 AMS mBatteryStatsService = new BatteryStatsService(systemContext, systemDir, mHandler); mBatteryStatsService.getActiveStatistics().readLocked(); mBatteryStatsService.scheduleWriteToDisk(); mOnBattery = DEBUG_POWER ? true : mBatteryStatsService.getActiveStatistics().getIsOnBattery(); mBatteryStatsService.getActiveStatistics().setCallback(this); // 创建 ProcessStatsService，并将其信息保存在 /data/system/procstats 中 mProcessStats = new ProcessStatsService(this, new File(systemDir, "procstats")); mAppOpsService = mInjector.getAppOpsService(new File(systemDir, "appops.xml"), mHandler); // 定义 ContentProvider 访问指定 Uri 数据的权限 mGrantFile = new AtomicFile(new File(systemDir, "urigrants.xml"), "uri-grants"); // 多用户管理 mUserController = new UserController(this); mVrController = new VrController(this); // 获取 OpenGL 版本 GL_ES_VERSION = SystemProperties.getInt("ro.opengles.version", ConfigurationInfo.GL_ES_VERSION_UNDEFINED); if (SystemProperties.getInt("sys.use_fifo_ui", 0) != 0) { mUseFifoUiScheduling = true; } mTrackingAssociations = "1".equals(SystemProperties.get("debug.track-associations")); mTempConfig.setToDefaults(); mTempConfig.setLocales(LocaleList.getDefault()); mConfigurationSeq = mTempConfig.seq = 1; // 创建 ActivityStackSupervisor ，用于管理 Activity 任务栈 mStackSupervisor = createStackSupervisor(); mStackSupervisor.onConfigurationChanged(mTempConfig); mKeyguardController = mStackSupervisor.getKeyguardController(); mCompatModePackages = new CompatModePackages(this, systemDir, mHandler); mIntentFirewall = new IntentFirewall(new IntentFirewallInterface(), mHandler); mTaskChangeNotificationController = new TaskChangeNotificationController(this, mStackSupervisor, mHandler); // 创建 ActivityStartController 对象，用于管理 Activity 的启动 mActivityStartController = new ActivityStartController(this); // 创建最近任务栈 RecentTask 对象 mRecentTasks = createRecentTasks(); mStackSupervisor.setRecentTasks(mRecentTasks); mLockTaskController = new LockTaskController(mContext, mStackSupervisor, mHandler); mLifecycleManager = new ClientLifecycleManager(); // 创建 CpuTracker 线程，追踪 CPU 状态 mProcessCpuThread = new Thread("CpuTracker") { @Override public void run() { synchronized (mProcessCpuTracker) { mProcessCpuInitLatch.countDown(); mProcessCpuTracker.init(); // 初始化 ProcessCpuTracker。注意同步问题 } while (true) { try { try { synchronized(this) { final long now = SystemClock.uptimeMillis(); long nextCpuDelay = (mLastCpuTime.get()+MONITOR_CPU_MAX_TIME)-now; long nextWriteDelay = (mLastWriteTime+BATTERY_STATS_TIME)-now; //Slog.i(TAG, "Cpu delay=" + nextCpuDelay // + ", write delay=" + nextWriteDelay); if (nextWriteDelay < nextCpuDelay) { nextCpuDelay = nextWriteDelay; } if (nextCpuDelay > 0) { mProcessCpuMutexFree.set(true); this.wait(nextCpuDelay); } } } catch (InterruptedException e) { } // 更新 Cpu 统计信息 updateCpuStatsNow(); } catch (Exception e) { Slog.e(TAG, "Unexpected exception collecting process stats", e); } } } }; // hidden api 设置 mHiddenApiBlacklist = new HiddenApiSettings(mHandler, mContext); // 设置 Watchdog 监控 Watchdog.getInstance().addMonitor(this); Watchdog.getInstance().addThread(mHandler); // bind background thread to little cores // this is expected to fail inside of framework tests because apps can't touch cpusets directly // make sure we've already adjusted system_server's internal view of itself first // 更新进程的 oom_adj 值 updateOomAdjLocked(); try { Process.setThreadGroupAndCpuset(BackgroundThread.get().getThreadId(), Process.THREAD_GROUP_BG_NONINTERACTIVE); } catch (Exception e) { Slog.w(TAG, "Setting background thread cpuset failed"); } } 复制代码
AMS 的构造函数中做了很多事情，代码中作了很多注释，这里就不再展开细说了。
LifeCycle 的构造函数中初始化了 AMS，然后会调用 LifeCycle.onStart()，最终调用的是 AMS.start() 方法。
private void start() { // 移除所有进程组 removeAllProcessGroups(); // 启动构造函数中创建的 CpuTracker 线程，监控 cpu 使用情况 mProcessCpuThread.start(); // 启动 BatteryStatsService，统计电池信息 mBatteryStatsService.publish(); mAppOpsService.publish(mContext); // 启动 LocalService ，将 ActivityManagerInternal 加入服务列表 LocalServices.addService(ActivityManagerInternal.class, new LocalService()); // 等待 mProcessCpuThread 线程中的同步代码块执行完毕。 // 在执行 mProcessCpuTracker.init() 方法时访问 mProcessCpuTracker 将阻塞 try { mProcessCpuInitLatch.await(); } catch (InterruptedException e) { Thread.currentThread().interrupt(); throw new IllegalStateException("Interrupted wait during start"); } } 复制代码
AMS 的初始化工作到这里就基本结束了，我们再回到 startBootstrapServices() 中，看看 AMS 的下一步动作 setSystemProcess() 。
AMS.setSystemProcess()
public void setSystemProcess() { try { // 注册各种服务 // 注册 AMS ServiceManager.addService(Context.ACTIVITY_SERVICE, this, /* allowIsolated= */ true, DUMP_FLAG_PRIORITY_CRITICAL | DUMP_FLAG_PRIORITY_NORMAL | DUMP_FLAG_PROTO); // 注册进程统计服务 ServiceManager.addService(ProcessStats.SERVICE_NAME, mProcessStats); // 注册内存信息服务 ServiceManager.addService("meminfo", new MemBinder(this), /* allowIsolated= */ false, DUMP_FLAG_PRIORITY_HIGH); // 注册 GraphicsBinder ServiceManager.addService("gfxinfo", new GraphicsBinder(this)); // 注册 DbBinder ServiceManager.addService("dbinfo", new DbBinder(this)); if (MONITOR_CPU_USAGE) { // 注册 DbBinder ServiceManager.addService("cpuinfo", new CpuBinder(this), /* allowIsolated= */ false, DUMP_FLAG_PRIORITY_CRITICAL); } // 注册权限管理者 PermissionController ServiceManager.addService("permission", new PermissionController(this // 注册进程信息服务 ProcessInfoService ServiceManager.addService("processinfo", new ProcessInfoService(this)); // 获取包名为 android 的应用信息，framework-res.apk ApplicationInfo info = mContext.getPackageManager().getApplicationInfo( "android", STOCK_PM_FLAGS | MATCH_SYSTEM_ONLY); mSystemThread.installSystemApplicationInfo(info, getClass().getClassLoader()); synchronized (this) { // 创建 ProcessRecord ProcessRecord app = newProcessRecordLocked(info, info.processName, false, 0); app.persistent = true; app.pid = MY_PID; app.maxAdj = ProcessList.SYSTEM_ADJ; app.makeActive(mSystemThread.getApplicationThread(), mProcessStats); synchronized (mPidsSelfLocked) { mPidsSelfLocked.put(app.pid, app); } // 更新 mLruProcesses updateLruProcessLocked(app, false, null); // 更新进程对应的 oom_adj 值 updateOomAdjLocked(); } } catch (PackageManager.NameNotFoundException e) { throw new RuntimeException( "Unable to find android system package", e); } // Start watching app ops after we and the package manager are up and running. // 当 packager manager 启动并运行时开始监听 app ops mAppOpsService.startWatchingMode(AppOpsManager.OP_RUN_IN_BACKGROUND, null, new IAppOpsCallback.Stub() { @Override public void opChanged(int op, int uid, String packageName) { if (op == AppOpsManager.OP_RUN_IN_BACKGROUND && packageName != null) { if (mAppOpsService.checkOperation(op, uid, packageName) != AppOpsManager.MODE_ALLOWED) { runInBackgroundDisabled(uid); } } } }); } 复制代码
setSystemProcess() 的主要工作就是向 ServiceManager 注册关联的系统服务。
AMS.installSystemProviders()
public final void installSystemProviders() { List<ProviderInfo> providers; synchronized (this) { ProcessRecord app = mProcessNames.get("system", SYSTEM_UID); providers = generateApplicationProvidersLocked(app); if (providers != null) { for (int i=providers.size()-1; i>=0; i--) { ProviderInfo pi = (ProviderInfo)providers.get(i); if ((pi.applicationInfo.flags&ApplicationInfo.FLAG_SYSTEM) == 0) { Slog.w(TAG, "Not installing system proc provider " + pi.name + ": not system .apk"); // 移除非系统 Provier providers.remove(i); } } } } // 安装系统 Provider if (providers != null) { mSystemThread.installSystemProviders(providers); } synchronized (this) { mSystemProvidersInstalled = true; } mConstants.start(mContext.getContentResolver()); // 创建 CoreSettingsObserver ，监控核心设置的变化 mCoreSettingsObserver = new CoreSettingsObserver(this); // 创建 FontScaleSettingObserver，监控字体的变化 mFontScaleSettingObserver = new FontScaleSettingObserver(); // 创建 DevelopmentSettingsObserver mDevelopmentSettingsObserver = new DevelopmentSettingsObserver(); GlobalSettingsToPropertiesMapper.start(mContext.getContentResolver()); // Now that the settings provider is published we can consider sending // in a rescue party. RescueParty.onSettingsProviderPublished(mContext); //mUsageStatsService.monitorPackages(); } 复制代码
installSystemProviders() 的主要工作是安装系统 Proviers。
AMS.systemReady()
AMS.systemReady() 是 AMS 启动流程的最后一步了。
public void systemReady(final Runnable goingCallback, TimingsTraceLog traceLog) { ... synchronized(this) { if (mSystemReady) { // 首次调用 mSystemReady 为 false // If we're done calling all the receivers, run the next "boot phase" passed in // by the SystemServer if (goingCallback != null) { goingCallback.run(); } return; } // 一系列 systemReady() mHasHeavyWeightFeature = mContext.getPackageManager().hasSystemFeature( PackageManager.FEATURE_CANT_SAVE_STATE); mLocalDeviceIdleController = LocalServices.getService(DeviceIdleController.LocalService.class); mAssistUtils = new AssistUtils(mContext); mVrController.onSystemReady(); // Make sure we have the current profile info, since it is needed for security checks. mUserController.onSystemReady(); mRecentTasks.onSystemReadyLocked(); mAppOpsService.systemReady(); mSystemReady = true; } ... ArrayList<ProcessRecord> procsToKill = null; synchronized(mPidsSelfLocked) { for (int i=mPidsSelfLocked.size()-1; i>=0; i--) { ProcessRecord proc = mPidsSelfLocked.valueAt(i); if (!isAllowedWhileBooting(proc.info)){ if (procsToKill == null) { procsToKill = new ArrayList<ProcessRecord>(); } procsToKill.add(proc); } } } synchronized(this) { if (procsToKill != null) { for (int i=procsToKill.size()-1; i>=0; i--) { ProcessRecord proc = procsToKill.get(i); removeProcessLocked(proc, true, false, "system update done"); } } // Now that we have cleaned up any update processes, we // are ready to start launching real processes and know that // we won't trample on them any more. mProcessesReady = true; } ... if (goingCallback != null) goingCallback.run(); mBatteryStatsService.noteEvent(BatteryStats.HistoryItem.EVENT_USER_RUNNING_START, Integer.toString(currentUserId), currentUserId); mBatteryStatsService.noteEvent(BatteryStats.HistoryItem.EVENT_USER_FOREGROUND_START, Integer.toString(currentUserId), currentUserId); // 回调所有 SystemService 的 onStartUser() 方法 mSystemServiceManager.startUser(currentUserId); synchronized (this) { // Only start up encryption-aware persistent apps; once user is // unlocked we'll come back around and start unaware apps startPersistentApps(PackageManager.MATCH_DIRECT_BOOT_AWARE); // Start up initial activity. mBooting = true; // Enable home activity for system user, so that the system can always boot. We don't // do this when the system user is not setup since the setup wizard should be the one // to handle home activity in this case. if (UserManager.isSplitSystemUser() && Settings.Secure.getInt(mContext.getContentResolver(), Settings.Secure.USER_SETUP_COMPLETE, 0) != 0) { ComponentName cName = new ComponentName(mContext, SystemUserHomeActivity.class); try { AppGlobals.getPackageManager().setComponentEnabledSetting(cName, PackageManager.COMPONENT_ENABLED_STATE_ENABLED, 0, UserHandle.USER_SYSTEM); } catch (RemoteException e) { throw e.rethrowAsRuntimeException(); } } // 启动桌面 Home 应用 startHomeActivityLocked(currentUserId, "systemReady"); ... long ident = Binder.clearCallingIdentity(); try { // 发送广播 USER_STARTED Intent intent = new Intent(Intent.ACTION_USER_STARTED); intent.addFlags(Intent.FLAG_RECEIVER_REGISTERED_ONLY | Intent.FLAG_RECEIVER_FOREGROUND); intent.putExtra(Intent.EXTRA_USER_HANDLE, currentUserId); broadcastIntentLocked(null, null, intent, null, null, 0, null, null, null, OP_NONE, null, false, false, MY_PID, SYSTEM_UID, currentUserId); // 发送广播 USER_STARTING intent = new Intent(Intent.ACTION_USER_STARTING); intent.addFlags(Intent.FLAG_RECEIVER_REGISTERED_ONLY); intent.putExtra(Intent.EXTRA_USER_HANDLE, currentUserId); broadcastIntentLocked(null, null, intent, null, new IIntentReceiver.Stub() { @Override public void performReceive(Intent intent, int resultCode, String data, Bundle extras, boolean ordered, boolean sticky, int sendingUser) throws RemoteException { } }, 0, null, null, new String[] {INTERACT_ACROSS_USERS}, OP_NONE, null, true, false, MY_PID, SYSTEM_UID, UserHandle.USER_ALL); } catch (Throwable t) { Slog.wtf(TAG, "Failed sending first user broadcasts", t); } finally { Binder.restoreCallingIdentity(ident); } mStackSupervisor.resumeFocusedStackTopActivityLocked(); mUserController.sendUserSwitchBroadcasts(-1, currentUserId); BinderInternal.nSetBinderProxyCountWatermarks(6000,5500); BinderInternal.nSetBinderProxyCountEnabled(true); BinderInternal.setBinderProxyCountCallback( new BinderInternal.BinderProxyLimitListener() { @Override public void onLimitReached(int uid) { if (uid == Process.SYSTEM_UID) { Slog.i(TAG, "Skipping kill (uid is SYSTEM)"); } else { killUid(UserHandle.getAppId(uid), UserHandle.getUserId(uid), "Too many Binders sent to SYSTEM"); } } }, mHandler); } } 复制代码
systemReady() 方法源码很长，上面做了很多删减。注意其中的 startHomeActivityLocked() 方法会启动桌面 Activity 。
boolean startHomeActivityLocked(int userId, String reason) { ... Intent intent = getHomeIntent(); ActivityInfo aInfo = resolveActivityInfo(intent, STOCK_PM_FLAGS, userId); if (aInfo != null) { intent.setComponent(new ComponentName(aInfo.applicationInfo.packageName, aInfo.name)); // Don't do this if the home app is currently being // instrumented. aInfo = new ActivityInfo(aInfo); aInfo.applicationInfo = getAppInfoForUser(aInfo.applicationInfo, userId); ProcessRecord app = getProcessRecordLocked(aInfo.processName, aInfo.applicationInfo.uid, true); if (app == null || app.instr == null) { intent.setFlags(intent.getFlags() | FLAG_ACTIVITY_NEW_TASK); final int resolvedUserId = UserHandle.getUserId(aInfo.applicationInfo.uid); // For ANR debugging to verify if the user activity is the one that actually // launched. final String myReason = reason + ":" + userId + ":" + resolvedUserId; // 启动桌面 Activity mActivityStartController.startHomeActivity(intent, aInfo, myReason); } } else { Slog.wtf(TAG, "No home screen found for " + intent, new Throwable()); } return true; } 复制代码
ActivityStartController 负责启动 Activity，至此，桌面应用就启动了。
最后
整篇写下来感觉就像小学生流水日记一样，但是读源码，就像有钱人的生活一样，往往是那么朴实无华，且枯燥。我不经意间看了看我的劳力士，太晚了，时序图后面再补上吧！
最后，下集预告，接着这篇，Activity 的启动流程分析，敬请期待！
