getSystemService，

getSystemService，

frameworks/base/core/java/android/app/Activity.java

frameworks/base/core/java/android/app/ContextImpl.java

frameworks/base/core/java/android/content/Context.java

frameworks/base/core/java/android/content/ContextWrapper.java

frameworks/base/core/java/android/view/ContextThemeWrapper.java

其中ContextWrapper，ContextImpl，都继承自Context。而Context没有实现getSystemService。

public abstract Object getSystemService(String name); 

Activity继承自ContextThemeWrapper，ContextThemeWrapper继承自ContextWrapper。ContexWrapper也是调用的父类的getSystemService。

所以getSystemService基本上都是调用的ContexImpl中的实现。

/** 
 * Common implementation of Context API, which provides the base 
 * context object for Activity and other application components. 
 */  
class ContextImpl extends Context {  
    private final static String TAG = "ApplicationContext";  

    @Override
    public Object getSystemService(String name) {
        ServiceFetcher fetcher = SYSTEM_SERVICE_MAP.get(name);
        return fetcher == null ? null : fetcher.getService(this);
    }

    static {  
  
        registerService(STORAGE_SERVICE, new ServiceFetcher() {  
                public Object createService(ContextImpl ctx) {  
                    try {  
                        return new StorageManager(ctx.mMainThread.getHandler().getLooper());  
                    } catch (RemoteException rex) {  
                        Log.e(TAG, "Failed to create StorageManager", rex);  
                        return null;                     
                    }  
                }});  

扩展阅读：

http://www.360doc.com/content/12/0229/23/6541311_190694454.shtml

Override Context.getSystemService()

getSystemService() in Android