Zygote家的大兒子——SystemServer

本文基於 Android 9.0 , 代碼倉庫地址 ： https://github.com/lulululbj/android_9.0.0_r45

文中源碼鏈接：

https://github.com/lulululbj/android_9.0.0_r45/blob/master/frameworks/base/services/java/com/android/server/SystemServer.java

https://github.com/lulululbj/android_9.0.0_r45/blob/master/frameworks/base/services/core/java/com/android/server/SystemServiceManager.java

https://github.com/lulululbj/android_9.0.0_r45/blob/master/frameworks/base/services/core/java/com/android/server/SystemService.java

首先來回顧一下上篇文章 https://juejin.im/post/5d8f73bf51882555b149dc64 ，主要介紹了 Android 世界中的第一個 Java 進程 Zygote，它的主要工作流程如下：

1. registerServerSocketFromEnv()， 註冊服務端 socket，用於和客戶端進程通信
2. preload()，預加載一系列資源，提高應用啟動速度
3. forkSystemServer()，創建 system_server 進程
4. 功成身退，調用 runSelectLoop() 等待響應客戶端請求，創建應用進程

本篇文章的主角 system_server 進程是 Zygote 進程 fork 出的第一個進程，它負責管理和啟動整個 Framework 層。

再來看看 http://gityuan.com 的這張圖片，找一下 System Server 的位置，它承載了各類系統服務的創建和啟動。關於 system_server 進程的創建流程，上篇文章中已經做了詳細介紹，這裡再簡單看一下流程圖：

最終會調用到 SystemServer.main() 方法。下面就以此為起點，來具體分析 SystemServer 都做了些什麼。

SystemServer 啟動流程

public static void main(String[] args) {
 new SystemServer().run();
}

接著看 run() 方法。

private void run() {
 try {
 ......
 // 如果設備時間早於 1970 年，很多 API 處理負數時會 crash。所以直接設置為 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);
 }
 // 未設置時區的話默認設為 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");
 }
 // 語言地區設置
 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);
 }
 // 設置虛擬機運行庫路徑
 SystemProperties.set("persist.sys.dalvik.vm.lib.2", VMRuntime.getRuntime().vmLibrary());
 // Mmmmmm... more memory!
 // 清除虛擬機內存增長限制，允許應用申請更多內存
 VMRuntime.getRuntime().clearGrowthLimit();
 // 設置堆內存的有效利用率為 0.8，(可能被忽略)
 VMRuntime.getRuntime().setTargetHeapUtilization(0.8f);
 // 確保指紋信息已經定義
 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);
 // 確保系統的 Binder 調用總是運行在前臺優先級
 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. 創建主線程 Looper
 Looper.prepareMainLooper();
 Looper.getMainLooper().setSlowLogThresholdMs(
 SLOW_DISPATCH_THRESHOLD_MS, SLOW_DELIVERY_THRESHOLD_MS);
 // 初始化 native 服務，加載 libandroid_servers.so
 System.loadLibrary("android_servers");
 // 檢查上次關機是否失敗，可能不會有返回值
 performPendingShutdown();
 // 2. 初始化系統上下文
 createSystemContext();
 // 3. 創建系統服務管理 SystemServiceManager
 // 並將 mSystemServiceManager 註冊到 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. 啟動系統引導服務
 startCoreServices(); // 5. 啟動系統核心服務
 startOtherServices(); // 6. 啟動其他服務
 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");
}

代碼雖然比較長，但是邏輯很清晰。我在註釋裡標記了比較重要的 7 個步驟，逐一分析。

Looper.prepareMainLooper()

初始化 Looper。關於 Handler 消息機制，可以閱讀我的另一篇文章 https://juejin.im/post/5d712cedf265da03ea5a9ecf 。最後會調用 Looper.loop() 開啟消息循環，開始處理消息。

createSystemContext()

private void createSystemContext() {
 // 創建 system_server 上下文信息
 ActivityThread activityThread = ActivityThread.systemMain();
 mSystemContext = activityThread.getSystemContext();
 mSystemContext.setTheme(DEFAULT_SYSTEM_THEME);
 final Context systemUiContext = activityThread.getSystemUiContext();
 // 設置主題，用於系統 dialog 等
 systemUiContext.setTheme(DEFAULT_SYSTEM_THEME);
}

創建系統上下文。首先調用 ActivityThread.systemMain() 方法獲取 ActivityThread 對象，然後再獲取上下文。

public static ActivityThread systemMain() {
 // 判斷是否是大內存設備,在低內存設備上不啟用硬件加速
 if (!ActivityManager.isHighEndGfx()) {
 ThreadedRenderer.disable(true);
 } else {
 ThreadedRenderer.enableForegroundTrimming();
 }
 ActivityThread thread = new ActivityThread();
 thread.attach(true, 0);
 return thread;
}

關於 ActivityThread.attach() 方法這裡不做具體分析了，後面文章說到應用啟動時再來詳細解析。

創建完系統上下文，接下來就是啟動各種系統服務了。源碼中把服務大致分為了三類，再來回顧一下：

startBootstrapServices(); // 4. 啟動系統引導服務
startCoreServices(); // 5. 啟動系統核心服務
startOtherServices(); // 6. 啟動其他服務

逐一進行分析。

startBootstrapServices()

private void startBootstrapServices() {
 final String TAG_SYSTEM_CONFIG = "ReadingSystemConfig";
 SystemServerInitThreadPool.get().submit(SystemConfig::getInstance, TAG_SYSTEM_CONFIG);
 // 阻塞等待與 installd 建立 socket 通道
 Installer installer = mSystemServiceManager.startService(Installer.class);
 // 啟動 DeviceIdentifiersPolicyService，在 ActivityManagerService 之前
 mSystemServiceManager.startService(DeviceIdentifiersPolicyService.class);
 // 啟動服務 ActivityManagerService
 mActivityManagerService = mSystemServiceManager.startService(
 ActivityManagerService.Lifecycle.class).getService();
 mActivityManagerService.setSystemServiceManager(mSystemServiceManager);
 mActivityManagerService.setInstaller(installer);
 // 啟動服務 PowerManagerService
 mPowerManagerService = mSystemServiceManager.startService(PowerManagerService.class);
 // Now that the power manager has been started, let the activity manager
 // initialize power management features.
 mActivityManagerService.initPowerManagement();
 // 啟動服務 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);
 // 啟動服務 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);
 }
 // 啟動 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);
 // 正在加密設備時只運行核心 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;
 }
 // 啟動服務 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();
 }
 }
 }
 // 啟動服務 UserManagerService
 mSystemServiceManager.startService(UserManagerService.LifeCycle.class);
 // 初始化屬性 cache 以緩存包資源
 AttributeCache.init(mSystemContext);
 // 設置 AMS
 mActivityManagerService.setSystemProcess();
 // DisplayManagerService needs to setup android.display scheduling related policies
 // since setSystemProcess() would have overridden policies due to setProcessGroup
 mDisplayManagerService.setupSchedulerPolicies();
 // 啟動服務 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();
 }
 // 在單獨的線程中啟動 SensorService
 mSensorServiceStart = SystemServerInitThreadPool.get().submit(() -> {
 TimingsTraceLog traceLog = new TimingsTraceLog(
 SYSTEM_SERVER_TIMING_ASYNC_TAG, Trace.TRACE_TAG_SYSTEM_SERVER);
 startSensorService();
 }, START_SENSOR_SERVICE);
}

startBootstrapServices() 方法中的都是系統啟動過程中的關鍵服務，且相互依賴，主要下列服務 ：

Installer DeviceIdentifiersPolicyService ActivityManagerService PowerManagerService RecoverySystemService LightsService StartSidekickService DisplayManagerService

SystemService.PHASEWAITFORDEFAULTDISPLAY (100)

PackageManagerService UserManagerService OverlayManagerService SensorService

一共啟動了十二個核心服務。注意中間的 SystemService.PHASE_WAIT_FOR_DEFAULT_DISPLAY，它並不是代表什麼系統服務，而是一個 int 值 100,類似的 int 值還有一些，定義在 SystemService 類中，它的作用是給服務啟動過程劃分階段，每個階段都有特定的含義，可以做不同的事情。這裡先混個臉熟，等介紹完所有的服務，再回過頭來總結一下有哪些階段。

startCoreServices()

private void startCoreServices() {
 // 啟動服務 BatteryService，需要 LightService
 mSystemServiceManager.startService(BatteryService.class);
 // 啟動服務 UsageStatsService，統計應用使用情況
 mSystemServiceManager.startService(UsageStatsService.class);
 mActivityManagerService.setUsageStatsManager(
 LocalServices.getService(UsageStatsManagerInternal.class));
 // 檢查是否存在可更新的 WebView。存在就啟動服務 WebViewUpdateService
 if (mPackageManager.hasSystemFeature(PackageManager.FEATURE_WEBVIEW)) {
 mWebViewUpdateService = mSystemServiceManager.startService(WebViewUpdateService.class);
 }
 // 啟動服務 BinderCallsStatsService，跟蹤 Binder 調用的 cpu 時間消耗
 BinderCallsStatsService.start();
}

啟動了四個服務，BatteryService UsageStatsService WebViewUpdateService 和 BinderCallsStatsService 。

startOtherServices()

startOtherServices() 源碼有一千多行，就像一個雜貨鋪，啟動了一系列的服務。下面儘量精簡一下代碼：

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();
}

通過上面的代碼可以看到啟動了相當多的系統服務。startOtherServices() 方法共經歷了五個啟動階段，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

最後調用的 mActivityManagerService.systemReady() 方法。該方法中會調用 startHomeActivityLocked 來啟動桌面 Activity，這樣桌面應用就啟動了。

Looper.loop()

至此，system_server 進程的主要工作就算完成了，進入 Looper.loop() 狀態,等待其他線程通過 Handler 發送消息到主線程並處理。

SystemServer 啟動階段分類

回過頭再來看看前面提到的啟動階段分類，定義在 com.android.server.SystemService 類中：

/*
 * Boot Phases
 * 
 * 啟動階段
 */
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.
 * 
 * 在這個階段之後，可以安全的調用系統核心服務，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.
 * 
 * 在這個階段之後，可以安全調用設備特定的服務
 */
public static final int PHASE_DEVICE_SPECIFIC_SERVICES_READY = 520;
/**
 * After receiving this boot phase, services can broadcast Intents.
 * 
 * 在這個階段之後，服務可以廣播
 */
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.
 * 
 * 在這個階段之後，服務可以啟動/綁定第三方應用
 * 應用此時可以進行 Binder 調用
 */
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.
 * 
 * 在這個階段之後，允許用戶和設備交互。
 * 這個階段發生在啟動完成，home 應用已經開始。
 * 系統服務更傾向於監聽這個階段，而不是監聽啟動廣播 ACTION_BOOT_COMPLETED，以降低延遲
 */
public static final int PHASE_BOOT_COMPLETED = 1000;

在 system_server 啟動過程中各個階段的位置大致如下：

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
 }
}

最後的 SystemService.PHASE_BOOT_COMPLETED（1000） 在 AMS 的 finishBooting() 方法中調用。另外注意 480 和 500 兩個階段是連在一起的，中間沒有發生任何事情。

那麼，劃分階段的具體作用是什麼呢？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 {
 // 回調系統服務的 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);
 }
}

核心就在於 service.onBootPhase(mCurrentPhase);。所有系統服務都是繼承於 SystemService 的，startBootPhase() 方法會回調當前階段已經加入 mServices 的所有系統服務的 onBootPhase() 方法，在合適的階段做一些合適的事情。以 AMS 為例：

@Override
public void onBootPhase(int phase) {
 mService.mBootPhase = phase;
 if (phase == PHASE_SYSTEM_SERVICES_READY) {
 mService.mBatteryStatsService.systemServicesReady();
 mService.mServices.systemServicesReady();
 }
}

SystemServer 是如何啟動服務的 ？

看完 SystemServer 的源碼，它最重要的工作就是創建和啟動各種系統服務。那麼服務一般是如何創建的呢？下面以 startBootstrapServices() 中創建的第一個服務 Installer 為例來看一下：

Installer installer = mSystemServiceManager.startService(Installer.class);

進入 SystemServiceManager 的 startService() 方法：

public  T startService(Class serviceClass) {
 try {
 // 獲取服務名稱
 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 {
 // 獲取服務類的構造器
 Constructor constructor = serviceClass.getConstructor(Context.class);
 // 反射創建 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);
 }
}

創建並啟動一個系統服務。這個系統服務必須是 com.android.server.SystemService 的子類。根據參數傳入的 Class 對象反射創建其實例，再調用重載方法 startService() ：

public void startService(@NonNull final SystemService service) {
 // Register it.
 mServices.add(service);
 // Start it.
 long time = SystemClock.elapsedRealtime();
 try {
 // 回調系統服務的 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");
}

就兩步。第一步，註冊服務，mServices 是一個 ArrayList 對象，用來保存已經創建的系統服務。第二步，回調服務的 onStart() 方法，還是以 Installer 為例：

@Override
public void onStart() {
 if (mIsolated) {
 mInstalld = null;
 } else {
 connect();
 }
}

這樣一個服務就啟動完成了。這是一種比較普遍的啟動方式，當然還有一些系統服務具有不一樣的啟動方式，這裡就不一一分析了，後面有機會解析具體服務的時候再來分析。

總結

SystemServer 的啟動流程比較耿直，沒有那麼多彎彎繞，下面簡單總結一下：

1. 語言、時區、地區等設置
2. 虛擬機內存設置
3. 指紋信息，Binder 調用設置
4. Looper.prepareMainLooper() ，創建主線程 Looper
5. 初始化 native 服務，加載 libandroid_servers.so
6. createSystemContext()，初始化系統上下文
7. 創建系統服務管理 SystemServiceManager
8. startBootstrapServices，啟動系統引導服務
9. startCoreServices，啟動系統核心服務
10. startOtherServices，啟動其他服務
11. Looper.loop()，開啟消息循環

另外，在 startOtherServices 的最後會調用 AMS 的 onSystemReady() 方法啟動桌面 Activity。

預告

還記得 Zygote 進程的 runSelectLoop() 方法嗎？Zygote 在創建完 system_server 進程之後，就開始默默的等待客戶端請求創建應用進程。下一篇，我們將從源碼角度來捋一遍客戶端是如何發送請求，Zygote 是如何處理請求，應用進程是如何創建的，敬請期待！