Zygote家的大兒子 —— SystemServer

本文基於 Android 9.0 , 代碼倉庫地址： "android_9.0.0_r45" 文中源碼鏈接： "SystemServer.java" "SystemServiceManager.java" "SystemService.java" 首先來回顧一下上篇文章 "Java 世界的盤古和女 ...

本文基於 Android 9.0 , 代碼倉庫地址： android_9.0.0_r45

文中源碼鏈接：

SystemServer.java

SystemServiceManager.java

SystemService.java

首先來回顧一下上篇文章 Java 世界的盤古和女媧 —— Zygote ，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：

registerServerSocketFromEnv()，註冊服務端 socket，用於和客戶端進程通信
preload()，預載入一系列資源，提高應用啟動速度
forkSystemServer()，創建 system_server 進程
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 Gityuan 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。下麵就以此為起點，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);
        }

        // 未設置時區的話預設設為 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!
        // 清除虛擬機記憶體增長限制，允許應用申請更多記憶體
        VMRuntime.getRuntime().clearGrowthLimit();

        // 設置堆記憶體的有效利用率為 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順序
        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，載入 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。我在註釋里標記了比較重要的 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深入理解 Handler 消息機制。最後會調用 Looper.loop() 開啟消息迴圈，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() {
    // 判斷是否是大記憶體設備,在低記憶體設備上不啟用硬體加速
    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.PHASE_WAIT_FOR_DEFAULT_DISPLAY (100)

PackageManagerService UserManagerService OverlayManagerService SensorService

一共啟動了十二個核心服務。註意中間的 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。下麵儘量精簡一下代碼：

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。另外註意 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？下麵以 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 extends SystemService> T startService(Class<T> 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<T> 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<SystemService> 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，下麵簡單總結一下：

語言、時區、地區等設置
虛擬機記憶體設置
指紋信息，Binder 調用設置
Looper.prepareMainLooper() ，創建主線程 Looper
初始化 native 服務，載入 libandroid_servers.so
createSystemContext()，初始化系統上下文
創建系統服務管理 SystemServiceManager
startBootstrapServices，啟動系統引導服務
startCoreServices，啟動系統核心服務
startOtherServices，啟動其他服務
Looper.loop()，開啟消息迴圈

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！

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