AMS 啓動過程

SystemServer.java

  /**
     * The main entry point from zygote.
     */
    public static void main(String[] args) {
        new SystemServer().run();
    }

創建SystemServer對象運行run()方法，點入run() 方法

//啓動服務的關鍵代碼
Start services.
        try {
            traceBeginAndSlog("StartServices");
            startBootstrapServices();
            startCoreServices();
            startOtherServices();
            SystemServerInitThreadPool.shutdown();
        } catch (Throwable ex) {
            Slog.e("System", "******************************************");
            Slog.e("System", "************ Failure starting system services", ex);
            throw ex;
        } finally {
            traceEnd();
        }

進入startBootstrapServices() 可以看到啓動ActivityManagerService的代碼

//mActivityManagerService 通過反射啓動AMS 服務
  // Activity manager runs the show.
        traceBeginAndSlog("StartActivityManager");
        mActivityManagerService = mSystemServiceManager.startService(
                ActivityManagerService.Lifecycle.class).getService();
        mActivityManagerService.setSystemServiceManager(mSystemServiceManager);
        mActivityManagerService.setInstaller(installer);
        traceEnd();

SystemServiceManager 啓動服務的代碼，可以看到到就是就是通過Class.forName()

    public SystemService startService(String className) {
        final Class<SystemService> serviceClass;
        try {
            serviceClass = (Class<SystemService>)Class.forName(className);
        } catch (ClassNotFoundException ex) {
            Slog.i(TAG, "Starting " + className);
            throw new RuntimeException("Failed to create service " + className
                    + ": service class not found, usually indicates that the caller should "
                    + "have called PackageManager.hasSystemFeature() to check whether the "
                    + "feature is available on this device before trying to start the "
                    + "services that implement it", ex);
        }
        return startService(serviceClass);
    }

AMS 通過靜態內部類啓動

   public static final class Lifecycle extends SystemService {
        private final ActivityManagerService mService;

        public Lifecycle(Context context) {
            super(context);
            //創建AMS實例
            mService = new ActivityManagerService(context);
        }

        @Override
        public void onStart() {
        //啓動AMS 服務對象
            mService.start();
        }

        public ActivityManagerService getService() {
        //獲取實例對象
            return mService;
        }
    }

我們的AMS已經啓動了，接着我們點擊桌面上的圖標以後就要啓動我們的app了。
先上一張時序圖

用戶在Launcher程序裏點擊應用圖標時，會通知ActivityManagerService啓動應用的主Activity，ActivityManagerService發現這個應用還未啓動，則會通知Zygote進程孵化出應用進程，然後在這個新孵化的應用進程裏執行ActivityThread的main方法。

讓我們先來看看ActivityThread 這個線程幹了什麼

   public static void main(String[] args) {

        ····
        Looper.prepareMainLooper();
        ActivityThread thread = new ActivityThread();
        thread.attach(false);
        if (sMainThreadHandler == null) {
            sMainThreadHandler = thread.getHandler();
        }

       ···
        Looper.loop();
        ···
    }

… 未完待續