public final class SystemServer {

    //zygote的主入口
    public static void main(String[] args) {
        new SystemServer().run();
    }

    public SystemServer() {
        // Check for factory test mode.
        mFactoryTestMode = FactoryTest.getMode();
    }

    private void run() {

        ...ignore some code...

         //加载本地系统服务库，并进行初始化 
        System.loadLibrary("android_servers");
        nativeInit();

        // 创建系统上下文
        createSystemContext();

        //初始化SystemServiceManager对象，下面的系统服务开启都需要调用SystemServiceManager.startService(Class<T>)，这个方法通过反射来启动对应的服务
        mSystemServiceManager = new SystemServiceManager(mSystemContext);

        //开启服务
        try {
            startBootstrapServices();
            startCoreServices();
            startOtherServices();
        } catch (Throwable ex) {
            Slog.e("System", "******************************************");
            Slog.e("System", "************ Failure starting system services", ex);
            throw ex;
        }

        ...ignore some code...

    }

    //初始化系统上下文对象mSystemContext，并设置默认的主题,mSystemContext实际上是一个ContextImpl对象。调用ActivityThread.systemMain()的时候，会调用ActivityThread.attach(true)，而在attach()里面，则创建了Application对象，并调用了Application.onCreate()。
    private void createSystemContext() {
        ActivityThread activityThread = ActivityThread.systemMain();
        mSystemContext = activityThread.getSystemContext();
        mSystemContext.setTheme(android.R.style.Theme_DeviceDefault_Light_DarkActionBar);
    }

    //在这里开启了几个核心的服务，因为这些服务之间相互依赖，所以都放在了这个方法里面。
    private void startBootstrapServices() {

        ...ignore some code...

        //初始化ActivityManagerService
        mActivityManagerService = mSystemServiceManager.startService(
                ActivityManagerService.Lifecycle.class).getService();
        mActivityManagerService.setSystemServiceManager(mSystemServiceManager);

        //初始化PowerManagerService，因为其他服务需要依赖这个Service，因此需要尽快的初始化
        mPowerManagerService = mSystemServiceManager.startService(PowerManagerService.class);

        // 现在电源管理已经开启，ActivityManagerService负责电源管理功能
        mActivityManagerService.initPowerManagement();

        // 初始化DisplayManagerService
        mDisplayManagerService = mSystemServiceManager.startService(DisplayManagerService.class);

    //初始化PackageManagerService
    mPackageManagerService = PackageManagerService.main(mSystemContext, mInstaller,
       mFactoryTestMode != FactoryTest.FACTORY_TEST_OFF, mOnlyCore);

    ...ignore some code...

    }

}