Android中获取各种服务是哪里来的

      当app第一次加载时就开始注册了那些服务，具体来说就是在应用创建ContextImpl实例的时候调用了SystemServiceRegistry类来进行注册，其注册的服务（就是具体的相关的服务的实例）保存在HashMap的对象SYSTEM_SERVICE_FETCHERS中：

      private static final HashMap<String, ServiceFetcher<?>> SYSTEM_SERVICE_FETCHERS = new HashMap<String, ServiceFetcher<?>>();


       以下就是常用的服务注册：
    registerService(Context.ACTIVITY_SERVICE, ActivityManager.class,
new CachedServiceFetcher<ActivityManager>() {
@Override
public ActivityManager createService(ContextImpl ctx) {
return new ActivityManager(ctx.getOuterContext(),
ctx.mMainThread.getHandler());
}
});

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

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

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

registerService(Context.BLUETOOTH_SERVICE, BluetoothManager.class,
new CachedServiceFetcher<BluetoothManager>() {
@Override
public BluetoothManager createService(ContextImpl ctx) {
return new BluetoothManager(ctx);
}
});
。。。。。。。。。。。。。。。。。。。。。。。
 
        注册时调用了registerService（）方法：
     private static <T> void registerService(String serviceName,
Class<T> serviceClass, ServiceFetcher<T> serviceFetcher) {
//保存服务名称
SYSTEM_SERVICE_NAMES.put(serviceClass, serviceName);
//保存服务实例
SYSTEM_SERVICE_FETCHERS.put(serviceName, serviceFetcher);
}