AMS流程

创建AMS对象
调用ams.setSystemProcess
调用ams.installSystemProviders
调用ams.systemReady

ActivityThread创建

在startBootstrapServices之前有一个流程是 createSystemContext,该过程会创建对象有ActivityThread，Instrumentation, ContextImpl，LoadedApk，Application,这些都是后面AMS要用到的对象,请注意它们是在这里创建的。

private void createSystemContext() {
        ActivityThread activityThread = ActivityThread.systemMain();
        mSystemContext = activityThread.getSystemContext();
        mSystemContext.setTheme(DEFAULT_SYSTEM_THEME);

        final Context systemUiContext = activityThread.getSystemUiContext();
        systemUiContext.setTheme(DEFAULT_SYSTEM_THEME);
    }

创建AMS对象

startBootstrapServices

private void startBootstrapServices() {
    ...
    //启动AMS服务
    mActivityManagerService = mSystemServiceManager.startService(
            ActivityManagerService.Lifecycle.class).getService();

    //设置AMS的系统服务管理器
    mActivityManagerService.setSystemServiceManager(mSystemServiceManager);
    //设置AMS的APP安装器
    mActivityManagerService.setInstaller(installer);
    //初始化AMS相关的PMS
    mActivityManagerService.initPowerManagement();
    ...

    //设置SystemServer
    mActivityManagerService.setSystemProcess();
}

startService

public SystemService startService(String className) {
        final Class<SystemService> serviceClass;
        try {
            serviceClass = (Class<SystemService>)Class.forName(className);
        } catch (ClassNotFoundException ex) {
            ......
        }
        return startService(serviceClass);
    }

    public void startService(@NonNull final SystemService service) {
        // Register it.
        mServices.add(service);
        // Start it.
        long time = SystemClock.elapsedRealtime();
        try {
            service.onStart();
        } catch (RuntimeException ex) {
            .....
        }
        warnIfTooLong(SystemClock.elapsedRealtime() - time, service, "onStart");
    }

这部分比较简单,主要是添加服务,并调用服务的 onStart 函数

ActivityManagerService.Lifecycle.class

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

        public Lifecycle(Context context) {
            super(context);
            mService = new ActivityManagerService(context);
        }

        @Override
        public void onStart() {
            mService.start();
        }

        @Override
        public void onCleanupUser(int userId) {
            mService.mBatteryStatsService.onCleanupUser(userId);
        }

        public ActivityManagerService getService() {
            return mService;
        }
    }

继承SystemService,代表是一个系统服务,里面主要是创建 ActivityManagerService

ActivityManagerService对象创建

public ActivityManagerService(Context systemContext) {
    mContext = systemContext;
    mFactoryTest = FactoryTest.getMode();
    mSystemThread = ActivityThread.currentActivityThread();

    //创建名为"ActivityManager"的前台线程，并获取mHandler
    mHandlerThread = new ServiceThread(TAG, android.os.Process.THREAD_PRIORITY_FOREGROUND, false);
    mHandlerThread.start();

    mHandler = new MainHandler(mHandlerThread.getLooper());

    //通过UiThread类，创建名为"android.ui"的线程
    mUiHandler = new UiHandler();

    ......

    //创建目录/data/system
    File dataDir = Environment.getDataDirectory();
    File systemDir = new File(dataDir, "system");
    systemDir.mkdirs();

    //创建服务BatteryStatsService
    mBatteryStatsService = new BatteryStatsService(systemDir, mHandler);
    mBatteryStatsService.getActiveStatistics().readLocked();
    ......

    //创建进程统计服务，信息保存在目录/data/system/procstats，
    mProcessStats = new ProcessStatsService(this, new File(systemDir, "procstats"));
    ......
    //CPU使用情况的追踪器执行初始化
    mProcessCpuTracker.init();
    ......
    mRecentTasks = new RecentTasks(this);
    // 创建ActivityStackSupervisor对象
    mStackSupervisor = new ActivityStackSupervisor(this, mRecentTasks);
    mTaskPersister = new TaskPersister(systemDir, mStackSupervisor, mRecentTasks);

    //创建名为"CpuTracker"的线程
    mProcessCpuThread = new Thread("CpuTracker") {
        public void run() {
         ......
        }
    };
    ......
}

该过程共创建了3个线程
ActivityManager
android.ui
CpuTracker

start

mSystemServiceManager.startService 会调用服务的onStart,在AMS里onStart会调用他的start

private void start() {
    Process.removeAllProcessGroups(); //移除所有的进程组
    mProcessCpuThread.start(); //开启CpuTracker线程

    mBatteryStatsService.publish(mContext); //启动电池统计服务
    mAppOpsService.publish(mContext);
    //创建LocalService，并添加到LocalServices
    LocalServices.addService(ActivityManagerInternal.class, new LocalService());
}

调用ams.setSystemProcess

这个步骤还是在 startBootstrapServices 中执行的

public void setSystemProcess() {
        try {
            ServiceManager.addService(Context.ACTIVITY_SERVICE, this, true);
            ServiceManager.addService(ProcessStats.SERVICE_NAME, mProcessStats);
            ServiceManager.addService("meminfo", new MemBinder(this));
            ServiceManager.addService("gfxinfo", new GraphicsBinder(this));
            ServiceManager.addService("dbinfo", new DbBinder(this));
            if (MONITOR_CPU_USAGE) {
                ServiceManager.addService("cpuinfo", new CpuBinder(this));
            }
            ServiceManager.addService("permission", new PermissionController(this));
            ServiceManager.addService("processinfo", new ProcessInfoService(this));

            ApplicationInfo info = mContext.getPackageManager().getApplicationInfo(
                    "android", STOCK_PM_FLAGS | MATCH_SYSTEM_ONLY);
            mSystemThread.installSystemApplicationInfo(info, getClass().getClassLoader());

            ......
        } catch (PackageManager.NameNotFoundException e) {
            throw new RuntimeException(
                    "Unable to find android system package", e);
        }
    }

如果之前看过笔者写的binder系列,应该知道 ServiceManager.addService 主要就是添加服务
该函数的主要作用是注册服务,以及为系统进程设置Application单例并启动,是由 installSystemApplicationInfo 函数完成的
该函数最终会执行到 LoadedApk 的 installSystemApplicationInfo 函数,来设置包名为 'android' 的 package 信息

/**
     * Sets application info about the system package.
     */
    void installSystemApplicationInfo(ApplicationInfo info, ClassLoader classLoader) {
        assert info.packageName.equals("android");
        mApplicationInfo = info;
        mClassLoader = classLoader;
    }

调用ams.installSystemProviders

在执行完startBootstrapServices后,最后会运行 startOtherServices 函数

private void startOtherServices() {
  ......
  //安装系统Provider 
  mActivityManagerService.installSystemProviders();
  ......

  mActivityManagerService.systemReady(new Runnable() {
     public void run() {
         ......
      }
  }
}

安装服务,就是在某某链表中添加
installSystemProviders 中会调用 mSystemThread.installSystemProviders(providers)

try {
        ActivityManager.getService().publishContentProviders(
                getApplicationThread(), results);
        } catch (RemoteException ex) {
            throw ex.rethrowFromSystemServer();
        }

那最终调用到 AMS的 publishContentProviders 函数,这个函数就是把providers放到map里等等操作

调用ams.systemReady

AMS是最后一个启动的服务,会调用 mActivityManagerService.systemReady

mActivityManagerService.systemReady(new Runnable() {
    public void run() {
        //启动WebView
      WebViewFactory.prepareWebViewInSystemServer();
      //启动系统UI
      startSystemUi(context);

      // 执行一系列服务的systemReady方法
      networkScoreF.systemReady();
      networkManagementF.systemReady();
      networkStatsF.systemReady();

      ......

      //执行一系列服务的systemRunning方法
      wallpaper.systemRunning();
      inputMethodManager.systemRunning(statusBarF);
      location.systemRunning();

    }
});

systemReady大致流程

public final class ActivityManagerService{

    public void systemReady(final Runnable goingCallback) {
        ...//更新操作
        mSystemReady = true; //系统处于ready状态
        removeProcessLocked(proc, true, false, "system update done");//杀掉所有非persistent进程
        mProcessesReady = true;  //进程处于ready状态

        goingCallback.run(); //这里有可能启动进程

        addAppLocked(info, false, null); //启动所有的persistent进程
        mBooting = true;  //正在启动中
        startHomeActivityLocked(mCurrentUserId, "systemReady"); //启动桌面
        mStackSupervisor.resumeTopActivitiesLocked(); //恢复栈顶的Activity
    }
}

来自：https://www.yuque.com/mikaelzero/blog/qu8hg3