AMS 由 SystemServer.java 启动，后者负责诸多 Service 的启动，比如 WMS，PWS 等。

private void run(){
  ...
  startBootstrapServices();
  startCoreServices();
  startOtherServices();
  ...
}

启动我们的 AMS 在startBootstrapServices()方法中初始化:

...
mActivityManagerService = mySystemServiceManager.startService(ActivityManagerService.Lifecycle.class).getService();
mActivityManagerService.setSystemServiceManager(mSystemServiceManager);
...
//重点有将 AMS 的服务注册到 SM
mActivityManagerService.setSystemProcess();

首先这个mSystemServiceManager 是 SystemServiceManager.java,可以看到，这里是利用反射实例化一个 SystemService对象并且加入内部的 ArrayList中

@SuppressWarnings("unchecked")
    public <T extends SystemService> T startService(Class<T> serviceClass) {
        final String name = serviceClass.getName();
        ...
        final T service;
         ...
        Constructor<T> constructor = serviceClass.getConstructor(Context.class);
            service = constructor.newInstance(mContext);
        ...
        // Register it.
        mServices.add(service);

        // Start it.
       ...
            service.onStart();
       ...
        return service;
    }

//以下是 ActivityManagerService.Lifecycle

 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();
        }

        public ActivityManagerService getService() {
            return mService;
        }
    }

由上面可以知道，实例化了一个 Lifecycle 对象，后者的构造函数实例化了 AMS，并且通过Lifecycle 的onStart()调用了 AMS.start()

private void start() {
        Process.removeAllProcessGroups();
        //启动 CpuTracker 线程
        mProcessCpuThread.start();
        //启动电池统计服务
        mBatteryStatsService.publish(mContext);
        mAppOpsService.publish(mContext);
        Slog.d("AppOps", "AppOpsService published");
        //创建 LocalService 并且添加
        LocalServices.addService(ActivityManagerInternal.class, new LocalService());
    }

随后的一个关键函数是在 SystemServer.java 中startOtherServices()方法中:

...
private void startOtherServices(){
  mActivityManagerService.systemReady(new Runnable(){
    ...
  }
);
}
...

在ActivityManagerService.java 中，通过 startHomeActivityLocked()启动桌面程序，即 Launcher

public void systemReady(final Runnable goingCallback){
  ...
  startHomeActivityLocked(mCurrentUserid,"systemReady");
  ...
}

private void start() {
        Process.removeAllProcessGroups();
        //启动 CpuTracker 线程
        mProcessCpuThread.start();
        //启动电池统计服务
        mBatteryStatsService.publish(mContext);
        mAppOpsService.publish(mContext);
        Slog.d("AppOps", "AppOpsService published");
        //创建 LocalService 并且天机
        LocalServices.addService(ActivityManagerInternal.class, new LocalService());
    }

public void setSystemProcess(){
  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));
  ...//注册各种子服务
  //创建了 ProcessRecord 对象
  ProcessRecord app = newProcessRecordLocked(info, info.processName, false, o);
  ...

}

AMS 在system 进程，主要负责 和其他进程进行通信处理 Activity(主页)，其通信机制是利用 Binder 驱动进行转发。因此在研究 AMS 时，要主要所研究的对象所在的进程。