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

 private void run() {
       <--開始!-->
            // Prepare the main looper thread (this thread).
            <--準備主線程(該線程)-->
            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);
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            <--初始化native服務-->
            System.loadLibrary("android_servers");
            <--初始化系統上下文-->
            createSystemContext();
            <--創建system service manager!!!-->    
            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();
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       <--打開系統服務-->
            <--啟動引導服務-->
            <--用SystemServiceManager啟動了ActivityManagerService、PowerManagerService、 PackageManagerService等服務-->
            startBootstrapServices();
            <--核心服務-->
            <--啟動BatteryService、UsageStatsService和WebViewUpdateService-->
            startCoreServices();
            <--啟動其他服務-->
            <--啟動了WMS,CameraService、AlarmManagerService、VrManagerService等服務-->
            startOtherServices();
      <--Loop forever-->
        Looper.loop();
        throw new RuntimeException("Main thread loop unexpectedly exited");
    }

SystemServer.java
//啟動AMS
 private void startBootstrapServices() {
        ...
        <--調用SystemServiceManager的startSErvice()方法, 傳入Lifecycle.class字節碼文件的參數, 
        通過反射實例化Lifecycle對象,并啟動AMS(通過這個參數"ActivityManagerService.Lifecycle.class"可以看出
        Lifecycle是AMS的一個內部類)-->
        mActivityManagerService = mSystemServiceManager.startService(
                ActivityManagerService.Lifecycle.class).getService();
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        mActivityManagerService.setSystemServiceManager(mSystemServiceManager);
        mActivityManagerService.setInstaller(installer);
 }
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SystemServiceManager.java
 public SystemService startService(String className) {
        <--使用反射獲取.class文件-->
        Class serviceClass = Class.forName(className);
        return this.startService(serviceClass);
    }
SystemServiceManager.java
 public <T extends SystemService> T startService(Class<T> serviceClass) {
        String name = serviceClass.getName();
        SystemService service;
        Constructor<T> constructor = serviceClass.getConstructor(Context.class);
        <--通過反射,調用constructor的newInstance方法來創建Lifecycle類型的service對象-->
        service = (SystemService)constructor.newInstance(this.mContext);
        <--把該服務加入到系統服務集合當中, 該系統服務集合就是SystemServiceManager類的list類型的成員變量-->
        this.mServices.add(service);
        <--調用反射類的onStart()方法開啟AMS服務(實際上Lifecycle內部類雖然是靜態的, 
        但是顯示的擁有一個AMS的對象, 該方法就是利用這個AMS對象調用AMS的start()方法)-->
        service.onStart();
        <--返回該服務-->
        return service; 
  }
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startBootstrapServices() 啟動引導服務，在這里實際上已經new了AMS，在new方法里，已經初始化了AMS的大部分重要的屬性。AMS的Looper和各種handler也是在這里準備好的。
private void startBootstrapServices() {
    ...
       ActivityTaskManagerService atm = mSystemServiceManager.startService(
               ActivityTaskManagerService.Lifecycle.class).getService();
            mActivityManagerService = ActivityManagerService.Lifecycle.startService(
               mSystemServiceManager, atm);
            mActivityManagerService.setSystemServiceManager(mSystemServiceManager);
            mActivityManagerService.setInstaller(installer);
       ...
}
   public static final class Lifecycle extends SystemService {
       private final ActivityManagerService mService;
       private static ActivityTaskManagerService sAtm;
       public Lifecycle(Context var1) {
           super(var1);
           this.mService = new ActivityManagerService(var1, sAtm);
       }
       public static ActivityManagerService startService(SystemServiceManager var0, ActivityTaskManagerService var1) {
           sAtm = var1;
           return ((ActivityManagerService.Lifecycle)var0.startService(ActivityManagerService.Lifecycle.class)).getService();
       }

public void systemReady(final Runnable goingCallback, TimingsTraceLog traceLog) {
            if (bootingSystemUser) {
                mAtmInternal.startHomeOnAllDisplays(currentUserId, "systemReady");
            }
 }