首先来回顾一下上篇文章 Java 世界的盘古和女娲 —— Zygote ，主要介绍了 Android 世界中的第一个 Java 进程 Zygote，它的主要工作流程如下：
registerServerSocketFromEnv()， 注册服务端 socket，用于和客户端进程通信preload()，预加载一系列资源，提高应用启动速度forkSystemServer()，创建system_server进程- 功成身退，调用
runSelectLoop()等待响应客户端请求，创建应用进程
本篇文章的主角 system_server 进程是 Zygote 进程 fork 出的第一个进程，它负责管理和启动整个 Framework 层。
再来看看 Gityuan 的这张图片，找一下 System Server 的位置，它承载了各类系统服务的创建和启动。关于 system_server 进程的创建流程，上篇文章中已经做了详细介绍，这里再简单看一下流程图：
最终会调用到 SystemServer.main() 方法。下面就以此为起点，来具体分析 SystemServer 都做了些什么。
SystemServer 启动流程
public static void main(String[] args) { new SystemServer().run(); } 复制代码
接着看 run() 方法。
private void run() { try { ...... // 如果设备时间早于 1970 年，很多 API 处理负数时会 crash。所以直接设置为 1970 年 1 月 1 日 if (System.currentTimeMillis() < EARLIEST_SUPPORTED_TIME) { Slog.w(TAG, "System clock is before 1970; setting to 1970."); SystemClock.setCurrentTimeMillis(EARLIEST_SUPPORTED_TIME); } // 未设置时区的话默认设为 GMT String timezoneProperty = SystemProperties.get("persist.sys.timezone"); if (timezoneProperty == null || timezoneProperty.isEmpty()) { Slog.w(TAG, "Timezone not set; setting to GMT."); SystemProperties.set("persist.sys.timezone", "GMT"); } // 语言地区设置 if (!SystemProperties.get("persist.sys.language").isEmpty()) { final String languageTag = Locale.getDefault().toLanguageTag(); SystemProperties.set("persist.sys.locale", languageTag); SystemProperties.set("persist.sys.language", ""); SystemProperties.set("persist.sys.country", ""); SystemProperties.set("persist.sys.localevar", ""); } // The system server should never make non-oneway calls Binder.setWarnOnBlocking(true); // The system server should always load safe labels PackageItemInfo.setForceSafeLabels(true); // Default to FULL within the system server. SQLiteGlobal.sDefaultSyncMode = SQLiteGlobal.SYNC_MODE_FULL; // Deactivate SQLiteCompatibilityWalFlags until settings provider is initialized SQLiteCompatibilityWalFlags.init(null); // Here we go! Slog.i(TAG, "Entered the Android system server!"); int uptimeMillis = (int) SystemClock.elapsedRealtime(); EventLog.writeEvent(EventLogTags.BOOT_PROGRESS_SYSTEM_RUN, uptimeMillis); if (!mRuntimeRestart) { MetricsLogger.histogram(null, "boot_system_server_init", uptimeMillis); } // 设置虚拟机运行库路径 SystemProperties.set("persist.sys.dalvik.vm.lib.2", VMRuntime.getRuntime().vmLibrary()); // Mmmmmm... more memory! // 清除虚拟机内存增长限制，允许应用申请更多内存 VMRuntime.getRuntime().clearGrowthLimit(); // 设置堆内存的有效利用率为 0.8，(可能被忽略) VMRuntime.getRuntime().setTargetHeapUtilization(0.8f); // 确保指纹信息已经定义 Build.ensureFingerprintProperty(); // Within the system server, it is an error to access Environment paths without // explicitly specifying a user. Environment.setUserRequired(true); // Within the system server, any incoming Bundles should be defused // to avoid throwing BadParcelableException. BaseBundle.setShouldDefuse(true); // Within the system server, when parceling exceptions, include the stack trace Parcel.setStackTraceParceling(true); // 确保系统的 Binder 调用总是运行在前台优先级 BinderInternal.disableBackgroundScheduling(true); // Increase the number of binder threads in system_server BinderInternal.setMaxThreads(sMaxBinderThreads); // Prepare the main looper thread (this thread). android.os.Process.setThreadPriority( android.os.Process.THREAD_PRIORITY_FOREGROUND); android.os.Process.setCanSelfBackground(false); // 1. 创建主线程 Looper Looper.prepareMainLooper(); Looper.getMainLooper().setSlowLogThresholdMs( SLOW_DISPATCH_THRESHOLD_MS, SLOW_DELIVERY_THRESHOLD_MS); // 初始化 native 服务，加载 libandroid_servers.so System.loadLibrary("android_servers"); // 检查上次关机是否失败，可能不会有返回值 performPendingShutdown(); // 2. 初始化系统上下文 createSystemContext(); // 3. 创建系统服务管理 SystemServiceManager // 并将 mSystemServiceManager 注册到 sLocalServiceObjects 中 mSystemServiceManager = new SystemServiceManager(mSystemContext); mSystemServiceManager.setStartInfo(mRuntimeRestart, mRuntimeStartElapsedTime, mRuntimeStartUptime); LocalServices.addService(SystemServiceManager.class, mSystemServiceManager); // Prepare the thread pool for init tasks that can be parallelized SystemServerInitThreadPool.get(); } finally { traceEnd(); // InitBeforeStartServices } // Start services. try { traceBeginAndSlog("StartServices"); startBootstrapServices(); // 4. 启动系统引导服务 startCoreServices(); // 5. 启动系统核心服务 startOtherServices(); // 6. 启动其他服务 SystemServerInitThreadPool.shutdown(); } catch (Throwable ex) { Slog.e("System", "******************************************"); Slog.e("System", "************ Failure starting system services", ex); throw ex; } finally { traceEnd(); } StrictMode.initVmDefaults(null); if (!mRuntimeRestart && !isFirstBootOrUpgrade()) { int uptimeMillis = (int) SystemClock.elapsedRealtime(); MetricsLogger.histogram(null, "boot_system_server_ready", uptimeMillis); final int MAX_UPTIME_MILLIS = 60 * 1000; if (uptimeMillis > MAX_UPTIME_MILLIS) { Slog.wtf(SYSTEM_SERVER_TIMING_TAG, "SystemServer init took too long. uptimeMillis=" + uptimeMillis); } } // 7. Loop forever. Looper.loop(); throw new RuntimeException("Main thread loop unexpectedly exited"); } 复制代码
代码虽然比较长，但是逻辑很清晰。我在注释里标记了比较重要的 7 个步骤，逐一分析。
Looper.prepareMainLooper()
初始化 Looper。关于 Handler 消息机制，可以阅读我的另一篇文章 深入理解 Handler 消息机制 。最后会调用 Looper.loop() 开启消息循环，开始处理消息。
createSystemContext()
private void createSystemContext() { // 创建 system_server 上下文信息 ActivityThread activityThread = ActivityThread.systemMain(); mSystemContext = activityThread.getSystemContext(); mSystemContext.setTheme(DEFAULT_SYSTEM_THEME); final Context systemUiContext = activityThread.getSystemUiContext(); // 设置主题，用于系统 dialog 等 systemUiContext.setTheme(DEFAULT_SYSTEM_THEME); } 复制代码
创建系统上下文。首先调用 ActivityThread.systemMain() 方法获取 ActivityThread 对象，然后再获取上下文。
public static ActivityThread systemMain() { // 判断是否是大内存设备,在低内存设备上不启用硬件加速 if (!ActivityManager.isHighEndGfx()) { ThreadedRenderer.disable(true); } else { ThreadedRenderer.enableForegroundTrimming(); } ActivityThread thread = new ActivityThread(); thread.attach(true, 0); return thread; } 复制代码
关于 ActivityThread.attach() 方法这里不做具体分析了，后面文章说到应用启动时再来详细解析。
创建完系统上下文，接下来就是启动各种系统服务了。源码中把服务大致分为了三类，再来回顾一下：
startBootstrapServices(); // 4. 启动系统引导服务 startCoreServices(); // 5. 启动系统核心服务 startOtherServices(); // 6. 启动其他服务 复制代码
逐一进行分析。
startBootstrapServices()
private void startBootstrapServices() { final String TAG_SYSTEM_CONFIG = "ReadingSystemConfig"; SystemServerInitThreadPool.get().submit(SystemConfig::getInstance, TAG_SYSTEM_CONFIG); // 阻塞等待与 installd 建立 socket 通道 Installer installer = mSystemServiceManager.startService(Installer.class); // 启动 DeviceIdentifiersPolicyService，在 ActivityManagerService 之前 mSystemServiceManager.startService(DeviceIdentifiersPolicyService.class); // 启动服务 ActivityManagerService mActivityManagerService = mSystemServiceManager.startService( ActivityManagerService.Lifecycle.class).getService(); mActivityManagerService.setSystemServiceManager(mSystemServiceManager); mActivityManagerService.setInstaller(installer); // 启动服务 PowerManagerService mPowerManagerService = mSystemServiceManager.startService(PowerManagerService.class); // Now that the power manager has been started, let the activity manager // initialize power management features. mActivityManagerService.initPowerManagement(); // 启动服务 RecoverySystemService mSystemServiceManager.startService(RecoverySystemService.class); // Now that we have the bare essentials of the OS up and running, take // note that we just booted, which might send out a rescue party if // we're stuck in a runtime restart loop. RescueParty.noteBoot(mSystemContext); // 启动服务 LightsService mSystemServiceManager.startService(LightsService.class); // Package manager isn't started yet; need to use SysProp not hardware feature if (SystemProperties.getBoolean("config.enable_sidekick_graphics", false)) { mSystemServiceManager.startService(WEAR_SIDEKICK_SERVICE_CLASS); } // 启动 DisplayManagerService，在 PackageManagerService 之前 mDisplayManagerService = mSystemServiceManager.startService(DisplayManagerService.class); // We need the default display before we can initialize the package manager. mSystemServiceManager.startBootPhase(SystemService.PHASE_WAIT_FOR_DEFAULT_DISPLAY); // 正在加密设备时只运行核心 app String cryptState = SystemProperties.get("vold.decrypt"); if (ENCRYPTING_STATE.equals(cryptState)) { Slog.w(TAG, "Detected encryption in progress - only parsing core apps"); mOnlyCore = true; } else if (ENCRYPTED_STATE.equals(cryptState)) { Slog.w(TAG, "Device encrypted - only parsing core apps"); mOnlyCore = true; } // 启动服务 PackageManagerService if (!mRuntimeRestart) { MetricsLogger.histogram(null, "boot_package_manager_init_start", (int) SystemClock.elapsedRealtime()); } mPackageManagerService = PackageManagerService.main(mSystemContext, installer, mFactoryTestMode != FactoryTest.FACTORY_TEST_OFF, mOnlyCore); mFirstBoot = mPackageManagerService.isFirstBoot(); mPackageManager = mSystemContext.getPackageManager(); if (!mRuntimeRestart && !isFirstBootOrUpgrade()) { MetricsLogger.histogram(null, "boot_package_manager_init_ready", (int) SystemClock.elapsedRealtime()); } if (!mOnlyCore) { boolean disableOtaDexopt = SystemProperties.getBoolean("config.disable_otadexopt", false); if (!disableOtaDexopt) { traceBeginAndSlog("StartOtaDexOptService"); try { OtaDexoptService.main(mSystemContext, mPackageManagerService); } catch (Throwable e) { reportWtf("starting OtaDexOptService", e); } finally { traceEnd(); } } } // 启动服务 UserManagerService mSystemServiceManager.startService(UserManagerService.LifeCycle.class); // 初始化属性 cache 以缓存包资源 AttributeCache.init(mSystemContext); // 设置 AMS mActivityManagerService.setSystemProcess(); // DisplayManagerService needs to setup android.display scheduling related policies // since setSystemProcess() would have overridden policies due to setProcessGroup mDisplayManagerService.setupSchedulerPolicies(); // 启动服务 OverlayManagerService OverlayManagerService overlayManagerService = new OverlayManagerService( mSystemContext, installer); mSystemServiceManager.startService(overlayManagerService); if (SystemProperties.getInt("persist.sys.displayinset.top", 0) > 0) { // DisplayManager needs the overlay immediately. overlayManagerService.updateSystemUiContext(); LocalServices.getService(DisplayManagerInternal.class).onOverlayChanged(); } // 在单独的线程中启动 SensorService mSensorServiceStart = SystemServerInitThreadPool.get().submit(() -> { TimingsTraceLog traceLog = new TimingsTraceLog( SYSTEM_SERVER_TIMING_ASYNC_TAG, Trace.TRACE_TAG_SYSTEM_SERVER); startSensorService(); }, START_SENSOR_SERVICE); } 复制代码
startBootstrapServices() 方法中的都是系统启动过程中的关键服务，且相互依赖，主要下列服务 ：
InstallerDeviceIdentifiersPolicyServiceActivityManagerServicePowerManagerServiceRecoverySystemServiceLightsServiceStartSidekickServiceDisplayManagerServiceSystemService.PHASE_WAIT_FOR_DEFAULT_DISPLAY (100)
PackageManagerServiceUserManagerServiceOverlayManagerServiceSensorService
一共启动了十二个核心服务。注意中间的 SystemService.PHASE_WAIT_FOR_DEFAULT_DISPLAY，它并不是代表什么系统服务，而是一个 int 值 100,类似的 int 值还有一些，定义在 SystemService 类中，它的作用是给服务启动过程划分阶段，每个阶段都有特定的含义，可以做不同的事情。这里先混个脸熟，等介绍完所有的服务，再回过头来总结一下有哪些阶段。
startCoreServices()
private void startCoreServices() { // 启动服务 BatteryService，需要 LightService mSystemServiceManager.startService(BatteryService.class); // 启动服务 UsageStatsService，统计应用使用情况 mSystemServiceManager.startService(UsageStatsService.class); mActivityManagerService.setUsageStatsManager( LocalServices.getService(UsageStatsManagerInternal.class)); // 检查是否存在可更新的 WebView。存在就启动服务 WebViewUpdateService if (mPackageManager.hasSystemFeature(PackageManager.FEATURE_WEBVIEW)) { mWebViewUpdateService = mSystemServiceManager.startService(WebViewUpdateService.class); } // 启动服务 BinderCallsStatsService，跟踪 Binder 调用的 cpu 时间消耗 BinderCallsStatsService.start(); } 复制代码
启动了四个服务，BatteryServiceUsageStatsServiceWebViewUpdateService 和 BinderCallsStatsService 。
startOtherServices()
startOtherServices() 源码有一千多行，就像一个杂货铺，启动了一系列的服务。下面尽量精简一下代码：
KeyAttestationApplicationIdProviderService/KeyChainSystemService SchedulingPolicyService/TelecomLoaderService/TelephonyRegistry mContentResolver = context.getContentResolver(); AccountManagerService/ContentService mActivityManagerService.installSystemProviders(); DropBoxManagerService/VibratorService/ConsumerIrService/AlarmManagerService final Watchdog watchdog = Watchdog.getInstance(); watchdog.init(context, mActivityManagerService); InputManagerService/WindowManagerService/VrManagerService/BluetoothService IpConnectivityMetrics/NetworkWatchlistService/PinnerService InputMethodManagerService/AccessibilityManagerService/StorageManagerService StorageStatsService/UiModeManagerService/LockSettingsService PersistentDataBlockService/OemLockService/DeviceIdleController DevicePolicyManagerService/StatusBarManagerService/ClipboardService NetworkManagementService/IpSecService/TextServicesManagerService TextClassificationManagerService/NetworkScoreService/NetworkStatsService NetworkPolicyManagerService/WifiScanningService/RttService WifiAware/WifiP2P/Lowpan/Ethernet/ConnectivityService/NsdService SystemUpdateManagerService/UpdateLockService/NotificationManagerService DeviceStorageMonitorService/LocationManagerService/CountryDetectorService SearchManagerService/WallpaperManagerService/AudioService/BroadcastRadioService DockObserver/ThermalObserver/WiredAccessoryManager/MidiManager/UsbService SerialService/HardwarePropertiesManagerService/TwilightService ColorDisplayService/JobSchedulerService/SoundTriggerService/TrustManagerService BackupManager/AppWidgerService/VoiceRecognitionManager/GestureLauncherService SensorNotificationService/ContextHubSystemService/DiskStatsService TimeZoneRulesManagerService/NetworkTimeUpdateService/CommonTimeManagementService CertBlacklister/EmergencyAffordanceService/DreamManagerService/GraphicsStatsService CoverageService/PrintManager/CompanionDeviceManager/RestrictionsManagerService MediaSessionService/MediaUpdateService/HdmiControlService/TvInputManagerService MediaResourceMonitorService/TvRemoteService/MediaRouterService/FingerprintService BackgroundDexOptService/PruneInstantAppsJobService/ShortcutService LauncherAppsService/CrossProfileAppsService/MediaProjectionManagerService WearConfigService/WearConnectivityService/WearTimeService/WearLeftyService WearGlobalActionsService/SliceManagerService/CameraServiceProxy/IoTSystemService MmsServiceBroker/AutoFillService // It is now time to start up the app processes... vibrator.systemReady(); lockSettings.systemReady(); // 480 mSystemServiceManager.startBootPhase(SystemService.PHASE_LOCK_SETTINGS_READY); // 500 mSystemServiceManager.startBootPhase(SystemService.PHASE_SYSTEM_SERVICES_READY); wm.systemReady(); mPowerManagerService.systemReady(mActivityManagerService.getAppOpsService()); mPackageManagerService.systemReady(); mDisplayManagerService.systemReady(safeMode, mOnlyCore); // Start device specific services final String[] classes = mSystemContext.getResources().getStringArray( R.array.config_deviceSpecificSystemServices); for (final String className : classes) { try { mSystemServiceManager.startService(className); } catch (Throwable e) { reportWtf("starting " + className, e); } } // 520 mSystemServiceManager.startBootPhase(SystemService.PHASE_DEVICE_SPECIFIC_SERVICES_READY); mActivityManagerService.systemReady(() -> { // 550 mSystemServiceManager.startBootPhase(SystemService.PHASE_ACTIVITY_MANAGER_READY); startSystemUi(context, windowManagerF); networkManagementF.systemReady(); ipSecServiceF.systemReady(); networkStatsF.systemReady(); connectivityF.systemReady(); Watchdog.getInstance().start mPackageManagerService.waitForAppDataPrepared(); // 600 mSystemServiceManager.startBootPhase(SystemService.PHASE_THIRD_PARTY_APPS_CAN_START); locationF.systemRunning(); countryDetectorF.systemRunning(); networkTimeUpdaterF.systemRunning(); commonTimeMgmtServiceF.systemRunning(); inputManagerF.systemRunning(); telephonyRegistryF.systemRunning(); mediaRouterF.systemRunning(); mmsServiceF.systemRunning(); incident.systemRunning(); } 复制代码
通过上面的代码可以看到启动了相当多的系统服务。startOtherServices() 方法共经历了五个启动阶段，如下所示：
SystemService.PHASE_LOCK_SETTINGS_READY // 480 SystemService.PHASE_SYSTEM_SERVICES_READY // 500 SystemService.PHASE_DEVICE_SPECIFIC_SERVICES_READY // 520 SystemService.PHASE_ACTIVITY_MANAGER_READY // 550 SystemService.PHASE_THIRD_PARTY_APPS_CAN_START // 600 复制代码
最后调用的 mActivityManagerService.systemReady() 方法。该方法中会调用 startHomeActivityLocked 来启动桌面 Activity，这样桌面应用就启动了。
Looper.loop()
至此，system_server 进程的主要工作就算完成了，进入 Looper.loop() 状态,等待其他线程通过 Handler 发送消息到主线程并处理。
SystemServer 启动阶段分类
回过头再来看看前面提到的启动阶段分类，定义在 com.android.server.SystemService 类中：
/* * Boot Phases * * 启动阶段 */ public static final int PHASE_WAIT_FOR_DEFAULT_DISPLAY = 100; // maybe should be a dependency? /** * After receiving this boot phase, services can obtain lock settings data. */ public static final int PHASE_LOCK_SETTINGS_READY = 480; /** * After receiving this boot phase, services can safely call into core system services * such as the PowerManager or PackageManager. * * 在这个阶段之后，可以安全的调用系统核心服务，如 PowerManager 和 PackageManager */ public static final int PHASE_SYSTEM_SERVICES_READY = 500; /** * After receiving this boot phase, services can safely call into device specific services. * * 在这个阶段之后，可以安全调用设备特定的服务 */ public static final int PHASE_DEVICE_SPECIFIC_SERVICES_READY = 520; /** * After receiving this boot phase, services can broadcast Intents. * * 在这个阶段之后，服务可以广播 */ public static final int PHASE_ACTIVITY_MANAGER_READY = 550; /** * After receiving this boot phase, services can start/bind to third party apps. * Apps will be able to make Binder calls into services at this point. * * 在这个阶段之后，服务可以启动/绑定第三方应用 * 应用此时可以进行 Binder 调用 */ public static final int PHASE_THIRD_PARTY_APPS_CAN_START = 600; /** * After receiving this boot phase, services can allow user interaction with the device. * This phase occurs when boot has completed and the home application has started. * System services may prefer to listen to this phase rather than registering a * broadcast receiver for ACTION_BOOT_COMPLETED to reduce overall latency. * * 在这个阶段之后，允许用户和设备交互。 * 这个阶段发生在启动完成，home 应用已经开始。 * 系统服务更倾向于监听这个阶段，而不是监听启动广播 ACTION_BOOT_COMPLETED，以降低延迟 */ public static final int PHASE_BOOT_COMPLETED = 1000; 复制代码
在 system_server 启动过程中各个阶段的位置大致如下：
private void startBootstrapServices() { ... // 100 mSystemServiceManager.startBootPhase(SystemService.PHASE_WAIT_FOR_DEFAULT_DISPLAY); ... } private void startOtherServices() { ... // 480 mSystemServiceManager.startBootPhase(SystemService.PHASE_LOCK_SETTINGS_READY); // 500 mSystemServiceManager.startBootPhase(SystemService.PHASE_SYSTEM_SERVICES_READY); ... // 520 mSystemServiceManager.startBootPhase(SystemService.PHASE_DEVICE_SPECIFIC_SERVICES_READY); mActivityManagerService.systemReady(() -> { mSystemServiceManager.startBootPhase( SystemService.PHASE_ACTIVITY_MANAGER_READY); // 550 ... mSystemServiceManager.startBootPhase( SystemService.PHASE_THIRD_PARTY_APPS_CAN_START); // 600 } } 复制代码
最后的 SystemService.PHASE_BOOT_COMPLETED（1000） 在 AMS 的 finishBooting() 方法中调用。另外注意 480 和 500 两个阶段是连在一起的，中间没有发生任何事情。
那么，划分阶段的具体作用是什么呢？答案就在 startBootPhase() 方法中：
public void startBootPhase(final int phase) { if (phase <= mCurrentPhase) { throw new IllegalArgumentException("Next phase must be larger than previous"); } mCurrentPhase = phase; try { final int serviceLen = mServices.size(); for (int i = 0; i < serviceLen; i++) { final SystemService service = mServices.get(i); long time = SystemClock.elapsedRealtime(); try { // 回调系统服务的 onBootPhase() 方法 service.onBootPhase(mCurrentPhase); } catch (Exception ex) { throw new RuntimeException("Failed to boot service " + service.getClass().getName() + ": onBootPhase threw an exception during phase " + mCurrentPhase, ex); } warnIfTooLong(SystemClock.elapsedRealtime() - time, service, "onBootPhase"); } } finally { Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER); } } 复制代码
核心就在于 service.onBootPhase(mCurrentPhase);。所有系统服务都是继承于 SystemService 的，startBootPhase() 方法会回调当前阶段已经加入 mServices 的所有系统服务的 onBootPhase() 方法，在合适的阶段做一些合适的事情。以 AMS 为例：
@Override public void onBootPhase(int phase) { mService.mBootPhase = phase; if (phase == PHASE_SYSTEM_SERVICES_READY) { mService.mBatteryStatsService.systemServicesReady(); mService.mServices.systemServicesReady(); } } 复制代码
SystemServer 是如何启动服务的 ？
看完 SystemServer 的源码，它最重要的工作就是创建和启动各种系统服务。那么服务一般是如何创建的呢？下面以 startBootstrapServices() 中创建的第一个服务 Installer 为例来看一下：
Installer installer = mSystemServiceManager.startService(Installer.class); 复制代码
进入 SystemServiceManager 的 startService() 方法：
public <T extends SystemService> T startService(Class<T> serviceClass) { try { // 获取服务名称 final String name = serviceClass.getName(); // Create the service. if (!SystemService.class.isAssignableFrom(serviceClass)) { throw new RuntimeException("Failed to create " + name + ": service must extend " + SystemService.class.getName()); } final T service; try { // 获取服务类的构造器 Constructor<T> constructor = serviceClass.getConstructor(Context.class); // 反射创建 service service = constructor.newInstance(mContext); } catch (InstantiationException ex) { throw new RuntimeException("Failed to create service " + name + ": service could not be instantiated", ex); } catch (IllegalAccessException ex) { throw new RuntimeException("Failed to create service " + name + ": service must have a public constructor with a Context argument", ex); } catch (NoSuchMethodException ex) { throw new RuntimeException("Failed to create service " + name + ": service must have a public constructor with a Context argument", ex); } catch (InvocationTargetException ex) { throw new RuntimeException("Failed to create service " + name + ": service constructor threw an exception", ex); } startService(service); return service; } finally { Trace.traceEnd(Trace.TRACE_TAG_SYSTEM_SERVER); } } 复制代码
创建并启动一个系统服务。这个系统服务必须是 com.android.server.SystemService 的子类。根据参数传入的 Class 对象反射创建其实例，再调用重载方法 startService() ：
public void startService(@NonNull final SystemService service) { // Register it. mServices.add(service); // Start it. long time = SystemClock.elapsedRealtime(); try { // 回调系统服务的 onStart() 方法 service.onStart(); } catch (RuntimeException ex) { throw new RuntimeException("Failed to start service " + service.getClass().getName() + ": onStart threw an exception", ex); } warnIfTooLong(SystemClock.elapsedRealtime() - time, service, "onStart"); } 复制代码
就两步。第一步，注册服务，mServices 是一个 ArrayList<SystemService> 对象，用来保存已经创建的系统服务。第二步，回调服务的 onStart() 方法，还是以 Installer 为例：
@Override public void onStart() { if (mIsolated) { mInstalld = null; } else { connect(); } } 复制代码
这样一个服务就启动完成了。这是一种比较普遍的启动方式，当然还有一些系统服务具有不一样的启动方式，这里就不一一分析了，后面有机会解析具体服务的时候再来分析。
总结
SystemServer 的启动流程比较耿直，没有那么多弯弯绕，下面简单总结一下：
- 语言、时区、地区等设置
- 虚拟机内存设置
- 指纹信息，Binder 调用设置
Looper.prepareMainLooper()，创建主线程 Looper- 初始化 native 服务，加载
libandroid_servers.so createSystemContext()，初始化系统上下文- 创建系统服务管理
SystemServiceManager startBootstrapServices，启动系统引导服务startCoreServices，启动系统核心服务startOtherServices，启动其他服务Looper.loop()，开启消息循环
另外，在 startOtherServices 的最后会调用 AMS 的 onSystemReady() 方法启动桌面 Activity。
预告
还记得 Zygote 进程的 runSelectLoop() 方法吗？Zygote 在创建完 system_server 进程之后，就开始默默的等待客户端请求创建应用进程。下一篇，我们将从源码角度来捋一遍客户端是如何发送请求，Zygote 是如何处理请求，应用进程是如何创建的，敬请期待！
