//这是我们获取服务的路口
 @Override
 public Object getSystemService(String name) {
  //可以看到我们是从一个HashMap中拿的服务.
  ServiceFetcher fetcher = SYSTEM_SERVICE_MAP.get(name);
  return fetcher == null ? null : fetcher.getService(this);
 }

 private static final HashMap<String, ServiceFetcher> SYSTEM_SERVICE_MAP =
    new HashMap<String, ServiceFetcher>();
 //这是注册服务的方法,请注意是静态方法
 private static void registerService(String serviceName, ServiceFetcher fetcher) {
  if (!(fetcher instanceof StaticServiceFetcher)) {
   fetcher.mContextCacheIndex = sNextPerContextServiceCacheIndex++;
  }
  SYSTEM_SERVICE_MAP.put(serviceName, fetcher);
 } 

 static {
   registerService(ACCESSIBILITY_SERVICE, new ServiceFetcher() {
     public Object getService(ContextImpl ctx) {
      return AccessibilityManager.getInstance(ctx);
     }});

   registerService(CAPTIONING_SERVICE, new ServiceFetcher() {
     public Object getService(ContextImpl ctx) {
      return new CaptioningManager(ctx);
     }});
  ....
 }

 @Override
 public Object getSystemService(String name) {
  return SystemServiceRegistry.getSystemService(this, name);
 }

final class SystemServiceRegistry {
 private final static String TAG = "SystemServiceRegistry";

 // Service registry information.
 // This information is never changed once static initialization has completed.
 private static final HashMap<Class<?>, String> SYSTEM_SERVICE_NAMES =
   new HashMap<Class<?>, String>();
 private static final HashMap<String, ServiceFetcher<?>> SYSTEM_SERVICE_FETCHERS =
   new HashMap<String, ServiceFetcher<?>>();
 private static int sServiceCacheSize;

 // Not instantiable.
 private SystemServiceRegistry() { }

 static {
  registerService(Context.ACCESSIBILITY_SERVICE, AccessibilityManager.class,
    new CachedServiceFetcher<AccessibilityManager>() {
   @Override
   public AccessibilityManager createService(ContextImpl ctx) {
    return AccessibilityManager.getInstance(ctx);
   }});

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