【读书笔记】Zygote 和 System 进程的启动过程

· 这是《Android 系统源代码情景分析》 一书中第 11 章， Zygote 和 System 进程的启动过程，的读书摘要；

一、Zygote 进程的过程

Zygote 进程是由 Android 系统的第一个进程脚本 init 启动的；

Zygote 进程是通过复制自身的方式来创建 System 进程和应用程序的；

Android 系统中，所有的应用程序进程，以及用来运行系统关键服务的 System 进程都是 Zygote 进程负责创建的；

  

1、reigsterZyogteSocket()

创建一个 Server 段的本地 Socket,用来等待 ActivityManagerService 请求 Zygote 创建新的应用程序进程；

2、startSystemServer()

启动 System 进程，以便它可以将系统关键服务启动起来；

方法 Zygote.forkSystemServer()  创建子进程也就是 Android 系统的 System 进程；

System 进程的用户 ID 和用户组 ID 均为 1000， 并且它还具有用户组1001~1010， 1018以及3001~3003 的权限；

方法 handleSystemServiceProcess(...) 启动 System 进程

3、runSelectLoop()

等待 ActivityManagerService 请求 Zygote 进程创建新的应用程序进程；

至此， Zygote 进程的启动已经完成。

二、System 进程启动过程

commonInit() 设置 System 进程的时区，键盘布局等通用信息；

      zygoteInitNative()  在 native 层启动一个 Bindler 线程池；

applicationInit(...)  调用 inovkeStaticMain(...) 方法， invokeStaticMain(...) 通过类加载器，加载 com.android.server.SystemServer, 进入 SystemServer.main() 方法， 启动 SystemServer

SystemServer.main() 方法会调用 System.run() 方法，在此方法里面启动一些相关的 Service;

 private void run() {
        ...

        // Prepare the main looper thread (this thread).
        // 将线程的优先级设为前台线程，不能后台取消， 启动 主线程的 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.
        // 创建化系统的 context
        createSystemContext();

        // Create the system service manager.
        // 创建 系统服务管理
        mSystemServiceManager = new SystemServiceManager(mSystemContext);
        LocalServices.addService(SystemServiceManager.class, mSystemServiceManager);

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


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

创建系统的 context

 // 创建系统的 context
    private void createSystemContext() {
        ActivityThread activityThread = ActivityThread.systemMain();
        mSystemContext = activityThread.getSystemContext();
        mSystemContext.setTheme(android.R.style.Theme_DeviceDefault_Light_DarkActionBar);
    }

   

  启动根服务

/**
     * 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).
        // 电池服务管理
        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.
        // 显示服务
        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.
        // 启动 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.
        // 启动 ActivityMangerService
        mActivityManagerService.setSystemProcess();
    }

     

启动一些核心服务

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

        // Tracks the battery level.  Requires LightService.
        // 启动电池服务
        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()

启动网络，蓝牙，电话等其他 Service;