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

 private void run() {
        try {
           ...
            //準(zhǔn)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(guān)機(jī)過程是否失敗，這個(gè)調(diào)用可能不會(huì)返回
            performPendingShutdown();
             //初始化系統(tǒng)上下文
            createSystemContext();
           //創(chuàng)建系統(tǒng)服務(wù)管理者--SystemServiceManager
            mSystemServiceManager = new SystemServiceManager(mSystemContext);
            mSystemServiceManager.setStartInfo(mRuntimeRestart,
                    mRuntimeStartElapsedTime, mRuntimeStartUptime);
             //將mSystemServiceManager添加到本地服務(wù)中，至于什么是LocalServices，它有什么作用，
             //以后再單獨(dú)開一篇文章來講解
            LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);
            //為可以并行化的init任務(wù)準(zhǔn)備線程池
            SystemServerInitThreadPool.get();
        } finally {
            traceEnd();  // InitBeforeStartServices
        }
        // 在這里開始啟動(dòng)一系列服務(wù)了
        try {
            traceBeginAndSlog("StartServices");
            // 啟動(dòng)引導(dǎo)服務(wù)
            startBootstrapServices();
            // 啟動(dòng)核心服務(wù)
            startCoreServices();
            // 啟動(dòng)其他服務(wù)
            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(jìn)程的上下文信息
    ActivityThread activityThread = ActivityThread.systemMain();
    mSystemContext = activityThread.getSystemContext();
    //設(shè)置主題
    mSystemContext.setTheme(DEFAULT_SYSTEM_THEME);
    //獲取systemui上下文信息，并設(shè)置主題
    final Context systemUiContext = activityThread.getSystemUiContext();
    systemUiContext.setTheme(DEFAULT_SYSTEM_THEME);
}

 private void startBootstrapServices() {
        // 盡早啟動(dòng)看門狗，這樣我們在早期啟動(dòng)陷入死鎖時(shí)就可以使system server崩潰重啟。
        final Watchdog watchdog = Watchdog.getInstance();
        watchdog.start();
        //啟動(dòng)Installer Service，這個(gè)Service 通過binder與installd進(jìn)程通訊，負(fù)責(zé)apk安裝相關(guān)的工作
        Installer installer = mSystemServiceManager.startService(Installer.class);
        //設(shè)備標(biāo)識(shí)符策略服務(wù)
        mSystemServiceManager.startService(DeviceIdentifiersPolicyService.class);
        // 管理uri授權(quán)
        mSystemServiceManager.startService(UriGrantsManagerService.Lifecycle.class);
        //啟動(dòng)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(dòng)，因?yàn)槠渌?wù)需要它。
        mPowerManagerService = mSystemServiceManager.startService(PowerManagerService.class);
        //啟動(dòng)熱緩解服務(wù)，目的是在手機(jī)開始過熱時(shí)進(jìn)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(dòng)系統(tǒng)恢復(fù)服務(wù)，負(fù)責(zé)協(xié)調(diào)設(shè)備上與恢復(fù)有關(guān)的功能。
        mSystemServiceManager.startService(RecoverySystemService.class);
        //到這里為止，系統(tǒng)啟動(dòng)的必須服務(wù)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(shè)備
        //在package manager 啟動(dòng)之前，需要啟動(dòng)display manager 提供display metrics
        mDisplayManagerService = mSystemServiceManager.startService(DisplayManagerService.class);
     //只有DisplayManagerService會(huì)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(gè)默認(rèn)的顯示設(shè)備
     mSystemServiceManager.startBootPhase(SystemService.PHASE_WAIT_FOR_DEFAULT_DISPLAY);
        // 啟動(dòng) 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(dòng)UserManager Service
        mSystemServiceManager.startService(UserManagerService.LifeCycle.class);
       //為系統(tǒng)進(jìn)程設(shè)置應(yīng)用程序?qū)嵗?dòng)
        mActivityManagerService.setSystemProcess();
        //使用ActivityManager實(shí)例完成看門狗設(shè)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(fù)責(zé)動(dòng)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(wù)需要訪問包管理器服務(wù)、app ops服務(wù)和權(quán)限服務(wù)，
         //因此我們在它們之后啟動(dòng)它。
         //在單獨(dú)的線程中啟動(dòng)傳感器服務(wù)。在使用它之前應(yīng)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(yīng)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(shè)備狀態(tài)。
        mSystemServiceManager.startService(CachedDeviceStateService.class);
        // 跟蹤在Binder調(diào)用中花費(fèi)的cpu時(shí)間
        mSystemServiceManager.startService(BinderCallsStatsService.LifeCycle.class);
        // 跟蹤handlers中處理messages所花費(fèi)的時(shí)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(diào)用的就是這個(gè)服務(wù)
        mSystemServiceManager.startService(BugreportManagerService.class);
        // 管理Gpu和Gpu驅(qū)動(dòng)的服務(wù)
        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;
    }