具体实现类 base/core/java/android/app/ContextImpl.java

里面含有静态代码块，registerService(SENSOR_SERVICE, new ServiceFetcher() {

                  public Object createService(ContextImpl ctx) {

                      return new SystemSensorManager(ctx.mMainThread.getHandler().getLooper());

                  }});

各种实例都在其中，主要是通过Map来集中管理，

        private static final HashMap<String, ServiceFetcher> SYSTEM_SERVICE_MAP =

 268             new HashMap<String, ServiceFetcher>();

 269

 270     private static int sNextPerContextServiceCacheIndex = 0;

 271     private static void registerService(String serviceName, ServiceFetcher fetcher) {

 272         if (!(fetcher instanceof StaticServiceFetcher)) {

 273             fetcher.mContextCacheIndex = sNextPerContextServiceCacheIndex++;

 274         }

 275         SYSTEM_SERVICE_MAP.put(serviceName, fetcher);

 276     }

   @Override

1478     public Object getSystemService(String name) {

1479         ServiceFetcher fetcher = SYSTEM_SERVICE_MAP.get(name);

1480         return fetcher == null ? null : fetcher.getService(this);

1481     }

              fetch.getService会调用createService，然后直接返回 代码如下

 public Object getService(ContextImpl ctx) {

 215             ArrayList<Object> cache = ctx.mServiceCache;

 216             Object service;

 217             synchronized (cache) {

 218                 if (cache.size() == 0) {

 219                     // Initialize the cache vector on first access.

 220                     // At this point sNextPerContextServiceCacheIndex

 221                     // is the number of potential services that are

 222                     // cached per-Context.

 223                     for (int i = 0; i < sNextPerContextServiceCacheIndex; i++) {

 224                         cache.add(null);

 225                     }

 226                 } else {

 227                     service = cache.get(mContextCacheIndex);

 228                     if (service != null) {

 229                         return service;

 230                     }

 231                 }

 232                 service = createService(ctx);

 233                 cache.set(mContextCacheIndex, service);

 234                 return service;

 235             }

 236         }

 

          获取的时候直接在SYSTEM_SERVICE_MAP取出来，

    @Override

1478     public Object getSystemService(String name) {

1479         ServiceFetcher fetcher = SYSTEM_SERVICE_MAP.get(name);

1480         return fetcher == null ? null : fetcher.getService(this);

1481     }