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

 private void run() {
        try {
           ...
            //準備主線程lopper
            android.os.Process.setThreadPriority(
                    android.os.Process.THREAD_PRIORITY_FOREGROUND);
            android.os.Process.setCanSelfBackground(false);
            Looper.prepareMainLooper();
            Looper.getMainLooper().setSlowLogThresholdMs(
                    SLOW_DISPATCH_THRESHOLD_MS, SLOW_DELIVERY_THRESHOLD_MS);
            // 加載libandroid_servers.so庫
            System.loadLibrary("android_servers");
           //檢測上次關機過程是否失敗，這個調用可能不會返回
            performPendingShutdown();
             //初始化系統(tǒng)上下文
            createSystemContext();
           //創(chuàng)建系統(tǒng)服務管理者--SystemServiceManager
            mSystemServiceManager = new SystemServiceManager(mSystemContext);
            mSystemServiceManager.setStartInfo(mRuntimeRestart,
                    mRuntimeStartElapsedTime, mRuntimeStartUptime);
             //將mSystemServiceManager添加到本地服務中，至于什么是LocalServices，它有什么作用，
             //以后再單獨開一篇文章來講解
            LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);
            //為可以并行化的init任務準備線程池
            SystemServerInitThreadPool.get();
        } finally {
            traceEnd();  // InitBeforeStartServices
        }
        // 在這里開始啟動一系列服務了
        try {
            traceBeginAndSlog("StartServices");
            // 啟動引導服務
            startBootstrapServices();
            // 啟動核心服務
            startCoreServices();
            // 啟動其他服務
            startOtherServices();
            //停止init線程池
            SystemServerInitThreadPool.shutdown();
        } catch (Throwable ex) {
            Slog.e("System", "******************************************");
            Slog.e("System", "************ Failure starting system services", ex);
            throw ex;
        } finally {
            traceEnd();
        }
        ...
        // 死循環(huán)執(zhí)行
        Looper.loop();
        throw new RuntimeException("Main thread loop unexpectedly exited");
    }

private void createSystemContext() {
    //創(chuàng)建system_server進程的上下文信息
    ActivityThread activityThread = ActivityThread.systemMain();
    mSystemContext = activityThread.getSystemContext();
    //設置主題
    mSystemContext.setTheme(DEFAULT_SYSTEM_THEME);
    //獲取systemui上下文信息，并設置主題
    final Context systemUiContext = activityThread.getSystemUiContext();
    systemUiContext.setTheme(DEFAULT_SYSTEM_THEME);
}

 private void startBootstrapServices() {
        // 盡早啟動看門狗，這樣我們在早期啟動陷入死鎖時就可以使system server崩潰重啟。
        final Watchdog watchdog = Watchdog.getInstance();
        watchdog.start();
        //啟動Installer Service，這個Service 通過binder與installd進程通訊，負責apk安裝相關的工作
        Installer installer = mSystemServiceManager.startService(Installer.class);
        //設備標識符策略服務
        mSystemServiceManager.startService(DeviceIdentifiersPolicyService.class);
        // 管理uri授權
        mSystemServiceManager.startService(UriGrantsManagerService.Lifecycle.class);
        //啟動ActivityTaskManagerService和ActivityManagerService
        ActivityTaskManagerService atm = mSystemServiceManager.startService(
                ActivityTaskManagerService.Lifecycle.class).getService();
        mActivityManagerService = ActivityManagerService.Lifecycle.startService(
                mSystemServiceManager, atm);
        mActivityManagerService.setSystemServiceManager(mSystemServiceManager);
        mActivityManagerService.setInstaller(installer);
        mWindowManagerGlobalLock = atm.getGlobalLock();
        //電源管理器需要盡早啟動，因為其他服務需要它。
        mPowerManagerService = mSystemServiceManager.startService(PowerManagerService.class);
        //啟動熱緩解服務，目的是在手機開始過熱時進行有效的熱緩解
        mSystemServiceManager.startService(ThermalManagerService.class);
        // Now that the power manager has been started, let the activity manager
        // initialize power management features.
        mActivityManagerService.initPowerManagement();
        //啟動系統(tǒng)恢復服務，負責協(xié)調設備上與恢復有關的功能。
        mSystemServiceManager.startService(RecoverySystemService.class);
        //到這里為止，系統(tǒng)啟動的必須服務已經(jīng)加載完畢
        RescueParty.noteBoot(mSystemContext);
        //管理LED和屏幕背光，我們需要它來顯示
        mSystemServiceManager.startService(LightsService.class);
        //管理顯示設備
        //在package manager 啟動之前，需要啟動display manager 提供display metrics
        mDisplayManagerService = mSystemServiceManager.startService(DisplayManagerService.class);
     //只有DisplayManagerService會對PHASE_WAIT_FOR_DEFAULT_DISPLAY做處理
     //目的是在初始化包管理器之前，首先需要獲取一個默認的顯示設備
     mSystemServiceManager.startBootPhase(SystemService.PHASE_WAIT_FOR_DEFAULT_DISPLAY);
        // 啟動 package manager.
        if (!mRuntimeRestart) {
            MetricsLogger.histogram(null, "boot_package_manager_init_start",
                    (int) SystemClock.elapsedRealtime());
        }
        try {
            Watchdog.getInstance().pauseWatchingCurrentThread("packagemanagermain");
            mPackageManagerService = PackageManagerService.main(mSystemContext, installer,
                    mFactoryTestMode != FactoryTest.FACTORY_TEST_OFF, mOnlyCore);
        } finally {
            Watchdog.getInstance().resumeWatchingCurrentThread("packagemanagermain");
        }
        mFirstBoot = mPackageManagerService.isFirstBoot();
        mPackageManager = mSystemContext.getPackageManager();
        //啟動UserManager Service
        mSystemServiceManager.startService(UserManagerService.LifeCycle.class);
       //為系統(tǒng)進程設置應用程序實例并啟動
        mActivityManagerService.setSystemProcess();
        //使用ActivityManager實例完成看門狗設置并監(jiān)聽是否重啟
        watchdog.init(mSystemContext, mActivityManagerService);
        // DisplayManagerService needs to setup android.display scheduling related policies
        // since setSystemProcess() would have overridden policies due to setProcessGroup
        mDisplayManagerService.setupSchedulerPolicies();
        //負責動態(tài)資源overlay
        mSystemServiceManager.startService(new OverlayManagerService(mSystemContext, installer));
        mSystemServiceManager.startService(new SensorPrivacyService(mSystemContext));
        if (SystemProperties.getInt("persist.sys.displayinset.top", 0) > 0) {
            // DisplayManager needs the overlay immediately.
            mActivityManagerService.updateSystemUiContext();
            LocalServices.getService(DisplayManagerInternal.class).onOverlayChanged();
        }
          //傳感器服務需要訪問包管理器服務、app ops服務和權限服務，
         //因此我們在它們之后啟動它。
         //在單獨的線程中啟動傳感器服務。在使用它之前應該檢查完成情況。
        mSensorServiceStart = SystemServerInitThreadPool.get().submit(() -> {
            TimingsTraceLog traceLog = new TimingsTraceLog(
                    SYSTEM_SERVER_TIMING_ASYNC_TAG, Trace.TRACE_TAG_SYSTEM_SERVER);
            traceLog.traceBegin(START_SENSOR_SERVICE);
            startSensorService();
            traceLog.traceEnd();
        }, START_SENSOR_SERVICE);
    }

 private void startCoreServices() {
        // 追蹤電池充電狀態(tài)和電量。需要LightService
        mSystemServiceManager.startService(BatteryService.class);
         //跟蹤應用程序使用狀態(tài)
        mSystemServiceManager.startService(UsageStatsService.class);
        mActivityManagerService.setUsageStatsManager(
                LocalServices.getService(UsageStatsManagerInternal.class));
        // 跟蹤可更新的WebView是否處于就緒狀態(tài)，并監(jiān)視更新安裝。
        if (mPackageManager.hasSystemFeature(PackageManager.FEATURE_WEBVIEW)) {
            traceBeginAndSlog("StartWebViewUpdateService");
            mWebViewUpdateService = mSystemServiceManager.startService(WebViewUpdateService.class);
            traceEnd();
        }
       //跟蹤并緩存設備狀態(tài)。
        mSystemServiceManager.startService(CachedDeviceStateService.class);
        // 跟蹤在Binder調用中花費的cpu時間
        mSystemServiceManager.startService(BinderCallsStatsService.LifeCycle.class);
        // 跟蹤handlers中處理messages所花費的時間。
        mSystemServiceManager.startService(LooperStatsService.Lifecycle.class);
        //管理apk回滾
        mSystemServiceManager.startService(RollbackManagerService.class);
        // 用于捕獲bugreport,adb bugreport 命令調用的就是這個服務
        mSystemServiceManager.startService(BugreportManagerService.class);
        // 管理Gpu和Gpu驅動的服務
        mSystemServiceManager.startService(GpuService.class);
    }

 mAtmInternal.startHomeOnAllDisplays(currentUserId, "systemReady");

Intent getHomeIntent() {
        Intent intent = new Intent(mTopAction, mTopData != null ? Uri.parse(mTopData) : null);
        intent.setComponent(mTopComponent);
        intent.addFlags(Intent.FLAG_DEBUG_TRIAGED_MISSING);
        if (mFactoryTest != FactoryTest.FACTORY_TEST_LOW_LEVEL) {
            intent.addCategory(Intent.CATEGORY_HOME);
        }
        return intent;
    }