【讀書筆記】Zygote 和 System 進程的啓動過程

· 這是《Android 系統源代碼情景分析》 一書中第 11 章， Zygote 和 System 進程的啓動過程，的讀書摘要；

一、Zygote 進程的過程

Zygote 進程是由 Android 系統的第一個進程腳本 init 啓動的；

Zygote 進程是通過複製自身的方式來創建 System 進程和應用程序的；

Android 系統中，所有的應用程序進程，以及用來運行系統關鍵服務的 System 進程都是 Zygote 進程負責創建的；

  

1、reigsterZyogteSocket()

創建一個 Server 段的本地 Socket,用來等待 ActivityManagerService 請求 Zygote 創建新的應用程序進程；

2、startSystemServer()

啓動 System 進程，以便它可以將系統關鍵服務啓動起來；

方法 Zygote.forkSystemServer()  創建子進程也就是 Android 系統的 System 進程；

System 進程的用戶 ID 和用戶組 ID 均爲 1000， 並且它還具有用戶組1001~1010， 1018以及3001~3003 的權限；

方法 handleSystemServiceProcess(...) 啓動 System 進程

3、runSelectLoop()

等待 ActivityManagerService 請求 Zygote 進程創建新的應用程序進程；

至此， Zygote 進程的啓動已經完成。

二、System 進程啓動過程

commonInit() 設置 System 進程的時區，鍵盤佈局等通用信息；

     zygoteInitNative()  在 native 層啓動一個 Bindler 線程池；

applicationInit(...)  調用 inovkeStaticMain(...) 方法， invokeStaticMain(...) 通過類加載器，加載 com.android.server.SystemServer, 進入 SystemServer.main() 方法， 啓動 SystemServer

SystemServer.main() 方法會調用 System.run() 方法，在此方法裏面啓動一些相關的 Service;

 private void run() {
        ...

        // Prepare the main looper thread (this thread).
        // 將線程的優先級設爲前臺線程，不能後臺取消， 啓動 主線程的 Looper
        android.os.Process.setThreadPriority(android.os.Process.THREAD_PRIORITY_FOREGROUND);
        android.os.Process.setCanSelfBackground(false);
        Looper.prepareMainLooper();

        // Initialize native services.
        System.loadLibrary("android_servers");
        nativeInit();

        // Check whether we failed to shut down last time we tried.
        // This call may not return.
        performPendingShutdown();

        // Initialize the system context.
        // 創建化系統的 context
        createSystemContext();

        // Create the system service manager.
        // 創建 系統服務管理
        mSystemServiceManager = new SystemServiceManager(mSystemContext);
        LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);

        // Start services.
        // 啓動服務
        try {
            startBootstrapServices();
            startCoreServices();
            startOtherServices();
        } catch (Throwable ex) {
            Slog.e("System", "******************************************");
            Slog.e("System", "************ Failure starting system services", ex);
            throw ex;
        }


        // Loop forever.
        // 進入 Looper
        Looper.loop();
        
        ...
    }

創建系統的 context

 // 創建系統的 context
    private void createSystemContext() {
        ActivityThread activityThread = ActivityThread.systemMain();
        mSystemContext = activityThread.getSystemContext();
        mSystemContext.setTheme(android.R.style.Theme_DeviceDefault_Light_DarkActionBar);
    }

   

  啓動根服務

/**
     * Starts the small tangle of critical services that are needed to get
     * the system off the ground.  These services have complex mutual dependencies
     * which is why we initialize them all in one place here.  Unless your service
     * is also entwined in these dependencies, it should be initialized in one of
     * the other functions.
     */
    private void startBootstrapServices() {
        // Wait for installd to finish starting up so that it has a chance to
        // create critical directories such as /data/user with the appropriate
        // permissions.  We need this to complete before we initialize other services.
        Installer installer = mSystemServiceManager.startService(Installer.class);

        // Activity manager runs the show.
        // Activity 管理
        mActivityManagerService = mSystemServiceManager.startService(ActivityManagerService.Lifecycle.class).getService();
        mActivityManagerService.setSystemServiceManager(mSystemServiceManager);
        mActivityManagerService.setInstaller(installer);

        // Power manager needs to be started early because other services need it.
        // Native daemons may be watching for it to be registered so it must be ready
        // to handle incoming binder calls immediately (including being able to verify
        // the permissions for those calls).
        // 電池服務管理
        mPowerManagerService = mSystemServiceManager.startService(PowerManagerService.class);

        // Now that the power manager has been started, let the activity manager
        // initialize power management features.
        mActivityManagerService.initPowerManagement();

        // Display manager is needed to provide display metrics before package manager
        // starts up.
        // 顯示服務
        mDisplayManagerService = mSystemServiceManager.startService(DisplayManagerService.class);

        // We need the default display before we can initialize the package manager.
        mSystemServiceManager.startBootPhase(SystemService.PHASE_WAIT_FOR_DEFAULT_DISPLAY);

      ...

        // Start the package manager.
        // 啓動 package 管理
        mPackageManagerService = PackageManagerService.main(mSystemContext, installer, mFactoryTestMode != FactoryTest.FACTORY_TEST_OFF, mOnlyCore);
        mFirstBoot = mPackageManagerService.isFirstBoot();
        mPackageManager = mSystemContext.getPackageManager();

        ServiceManager.addService(Context.USER_SERVICE, UserManagerService.getInstance());

        // Initialize attribute cache used to cache resources from packages.
        AttributeCache.init(mSystemContext);

        // Set up the Application instance for the system process and get started.
        // 啓動 ActivityMangerService
        mActivityManagerService.setSystemProcess();
    }

     

啓動一些核心服務

    /**
     * Starts some essential services that are not tangled up in the bootstrap process.
     */
    private void startCoreServices() {
        // Manages LEDs and display backlight.
        // 啓動屏幕燈光
        mSystemServiceManager.startService(LightsService.class);

        // Tracks the battery level.  Requires LightService.
        // 啓動電池服務
        mSystemServiceManager.startService(BatteryService.class);

        // Tracks application usage stats.
        mSystemServiceManager.startService(UsageStatsService.class);
        mActivityManagerService.setUsageStatsManager(LocalServices.getService(UsageStatsManagerInternal.class));
        // Update after UsageStatsService is available, needed before performBootDexOpt.
        mPackageManagerService.getUsageStatsIfNoPackageUsageInfo();

        // Tracks whether the updatable WebView is in a ready state and watches for update installs.
        // 更新 WebView
        mSystemServiceManager.startService(WebViewUpdateService.class);
    }

startOtherService()

啓動網絡，藍牙，電話等其他 Service;