Android系统服务是如何获取的

val wm = getSystemService(Context.WINDOW_SERVICE) as WindowManager

//ContextImpl.java
  public Object getSystemService(String name) {
    return SystemServiceReGIStry.getSystemService(this, name);
  }

//SystemServiceRegistry.java  
  public static Object getSystemService(ContextImpl ctx, String name) {
    final ServiceFetcher<?> fetcher = SYSTEM_SERVICE_FETCHERS.get(name);
    final Object ret = fetcher.getService(ctx);
    return ret;
  }

//SystemServiceRegistry.java  
  static abstract interface ServiceFetcher<T> {
    T getService(ContextImpl ctx);
  }

//SystemServiceRegistry.java  
  static abstract class CachedServiceFetcher<T> implements ServiceFetcher<T> {
    private final int mCacheIndex;

    CachedServiceFetcher() {
      mCacheIndex = sServiceCacheSize++;
    }

    @Override
    public final T getService(ContextImpl ctx) {
      final Object[] cache = ctx.mServiceCache;
      T ret = null;

      T service = (T) cache[mCacheIndex];
      if (service != null) {
        ret = service;
      } else {
        service = createService(ctx);
        cache[mCacheIndex] = service;
        ret = service;
      }
      return ret;
    }

    public abstract T createService(ContextImpl ctx) throws ServiceNotFoundException;
  }

假象的映射（从String到系统服务） <=> SYSTEM_SERVICE_FETCHERS映射（从String到ServiceFetcher）
                    + ctx.mServiceCache数组（ServiceFetcher中的mCacheIndex下标到系统服务）

//SystemServiceRegistry.java
  static {
    registerService(Context.WINDOW_SERVICE, WindowManager.class,
        new CachedServiceFetcher<WindowManager>() {
      @Override
      public WindowManager createService(ContextImpl ctx) {
        return new WindowManagerImpl(ctx);
      }}); 
  }
  private static <T> void registerService(@NonNull String serviceName,
      @NonNull Class<T> serviceClass, @NonNull ServiceFetcher<T> serviceFetcher) {
    SYSTEM_SERVICE_FETCHERS.put(serviceName, serviceFetcher);
  }

//SystemServiceRegistry.java    
	registerService(Context.ALARM_SERVICE, AlarmManager.class,
        new CachedServiceFetcher<AlarmManager>() {
      @Override
      public AlarmManager createService(ContextImpl ctx) throws ServiceNotFoundException {
        IBinder b = ServiceManager.getServiceOrThrow(Context.ALARM_SERVICE);
        IAlarmManager service = IAlarmManager.Stub.asInterface(b);
        return new AlarmManager(service, ctx);
      }});

//SystemServiceRegistry.java    
    registerService(Context.ACTIVITY_SERVICE, ActivityManager.class,
        new CachedServiceFetcher<ActivityManager>() {
      @Override
      public ActivityManager createService(ContextImpl ctx) {
        return new ActivityManager(ctx.getOuterContext(), ctx.mMainThread.getHandler());
      }});

//ActivityManager.java
  public void killUid(int uid, String reason) {
    try {
      getService().killUid(UserHandle.getAppId(uid),
          UserHandle.getUserId(uid), reason);
    } catch (RemoteException e) {
      throw e.rethrowFromSystemServer();
    }
  }
  
  public List<RunningTaskInfo> getRunningTasks(int maxNum)
      throws SecurityException {
    try {
      return getTaskService().getTasks(maxNum);
    } catch (RemoteException e) {
      throw e.rethrowFromSystemServer();
    }
  }
  
  public static IActivityManager getService() {
    return IActivityManagerSingleton.get();
  }

  private static IActivityTaskManager getTaskService() {
    return ActivityTaskManager.getService();
  }

    registerService(Context.WINDOW_SERVICE, WindowManager.class,
        new CachedServiceFetcher<WindowManager>() {
      @Override
      public WindowManager createService(ContextImpl ctx) {
        return new WindowManagerImpl(ctx);
      }});