Android上下文服務管理查詢過程

在開發Android應用程序時，如果需要使用系統提供的服務，可以通過服務名稱調用山下文的getSystemService(String name)來獲取服務管理者，那麼該函數是如何實現服務查詢的呢？

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

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

SYSTEM_SERVICE_MAP被定義為HashMap類型變量，該變量以服務名稱—ServiceFetcher對象這種鍵值對的方式保存了一系列系統服務管理對象，該變量定義為靜態類型，因此在ContextImpl類加載時，該變量就已經創建，同時在ContextImpl類中通過Static塊語句來初始化SYSTEM_SERVICE_MAP變量：

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

	registerService(ACCOUNT_SERVICE, new ServiceFetcher() {
			public Object createService(ContextImpl ctx) {
				IBinder b = ServiceManager.getService(ACCOUNT_SERVICE);
				IAccountManager service = IAccountManager.Stub.asInterface(b);
				return new AccountManager(ctx, service);
			}});

	if (SystemProperties.getBoolean("universe_ui_support",false)) {
		registerService(THEME_SERVICE, new ServiceFetcher() {
			public Object createService(ContextImpl ctx) {
			IBinder b = ServiceManager.getService(THEME_SERVICE);
			IThemeManager service = IThemeManager.Stub.asInterface(b);
			return new ThemeManager(ctx, service);
			}});
	}

	registerService(ACTIVITY_SERVICE, new ServiceFetcher() {
			public Object createService(ContextImpl ctx) {
				return new ActivityManager(ctx.getOuterContext(), ctx.mMainThread.getHandler());
			}});

	registerService(ALARM_SERVICE, new StaticServiceFetcher() {
			public Object createStaticService() {
				IBinder b = ServiceManager.getService(ALARM_SERVICE);
				IAlarmManager service = IAlarmManager.Stub.asInterface(b);
				return new AlarmManager(service);
			}});

	registerService(AUDIO_SERVICE, new ServiceFetcher() {
			public Object createService(ContextImpl ctx) {
				return new AudioManager(ctx);
			}});

	registerService(MEDIA_ROUTER_SERVICE, new ServiceFetcher() {
			public Object createService(ContextImpl ctx) {
				return new MediaRouter(ctx);
			}});

	registerService(CLIPBOARD_SERVICE, new ServiceFetcher() {
			public Object createService(ContextImpl ctx) {
				return new ClipboardManager(ctx.getOuterContext(),
						ctx.mMainThread.getHandler());
			}});

	registerService(CONNECTIVITY_SERVICE, new StaticServiceFetcher() {
			public Object createStaticService() {
				IBinder b = ServiceManager.getService(CONNECTIVITY_SERVICE);
				return new ConnectivityManager(IConnectivityManager.Stub.asInterface(b));
			}});

	registerService(COUNTRY_DETECTOR, new StaticServiceFetcher() {
			public Object createStaticService() {
				IBinder b = ServiceManager.getService(COUNTRY_DETECTOR);
				return new CountryDetector(ICountryDetector.Stub.asInterface(b));
			}});

	registerService(DEVICE_POLICY_SERVICE, new ServiceFetcher() {
			public Object createService(ContextImpl ctx) {
				return DevicePolicyManager.create(ctx, ctx.mMainThread.getHandler());
			}});

	registerService(DOWNLOAD_SERVICE, new ServiceFetcher() {
			public Object createService(ContextImpl ctx) {
				return new DownloadManager(ctx.getContentResolver(), ctx.getPackageName());
			}});

	registerService(NFC_SERVICE, new ServiceFetcher() {
			public Object createService(ContextImpl ctx) {
				return new NfcManager(ctx);
			}});

	registerService(DROPBOX_SERVICE, new StaticServiceFetcher() {
			public Object createStaticService() {
				return createDropBoxManager();
			}});

	registerService(INPUT_SERVICE, new StaticServiceFetcher() {
			public Object createStaticService() {
				return InputManager.getInstance();
			}});

	registerService(INPUT_METHOD_SERVICE, new ServiceFetcher() {
			public Object createService(ContextImpl ctx) {
				return InputMethodManager.getInstance(ctx);
			}});

	registerService(TEXT_SERVICES_MANAGER_SERVICE, new ServiceFetcher() {
			public Object createService(ContextImpl ctx) {
				return TextServicesManager.getInstance();
			}});

	registerService(KEYGUARD_SERVICE, new ServiceFetcher() {
			public Object getService(ContextImpl ctx) {
				// TODO: why isn't this caching it?  It wasn't
				// before, so I'm preserving the old behavior and
				// using getService(), instead of createService()
				// which would do the caching.
				return new KeyguardManager();
			}});

	registerService(LAYOUT_INFLATER_SERVICE, new ServiceFetcher() {
			public Object createService(ContextImpl ctx) {
				return PolicyManager.makeNewLayoutInflater(ctx.getOuterContext());
			}});

	registerService(LOCATION_SERVICE, new ServiceFetcher() {
			public Object createService(ContextImpl ctx) {
				IBinder b = ServiceManager.getService(LOCATION_SERVICE);
				return new LocationManager(ctx, ILocationManager.Stub.asInterface(b));
			}});

	registerService(NETWORK_POLICY_SERVICE, new ServiceFetcher() {
		@Override
		public Object createService(ContextImpl ctx) {
			return new NetworkPolicyManager(INetworkPolicyManager.Stub.asInterface(
					ServiceManager.getService(NETWORK_POLICY_SERVICE)));
		}
	});

	registerService(NOTIFICATION_SERVICE, new ServiceFetcher() {
			public Object createService(ContextImpl ctx) {
				final Context outerContext = ctx.getOuterContext();
				return new NotificationManager(
					new ContextThemeWrapper(outerContext,
							Resources.selectSystemTheme(0,
									outerContext.getApplicationInfo().targetSdkVersion,
									com.android.internal.R.style.Theme_Dialog,
									com.android.internal.R.style.Theme_Holo_Dialog,
									com.android.internal.R.style.Theme_DeviceDefault_Dialog)),
					ctx.mMainThread.getHandler());
			}});

	registerService(NSD_SERVICE, new ServiceFetcher() {
			@Override
			public Object createService(ContextImpl ctx) {
				IBinder b = ServiceManager.getService(NSD_SERVICE);
				INsdManager service = INsdManager.Stub.asInterface(b);
				return new NsdManager(ctx.getOuterContext(), service);
			}});

	// Note: this was previously cached in a static variable, but
	// constructed using mMainThread.getHandler(), so converting
	// it to be a regular Context-cached service...
	registerService(POWER_SERVICE, new ServiceFetcher() {
			public Object createService(ContextImpl ctx) {
				IBinder b = ServiceManager.getService(POWER_SERVICE);
				IPowerManager service = IPowerManager.Stub.asInterface(b);
				return new PowerManager(service, ctx.mMainThread.getHandler());
			}});

	registerService(SEARCH_SERVICE, new ServiceFetcher() {
			public Object createService(ContextImpl ctx) {
				return new SearchManager(ctx.getOuterContext(),
						ctx.mMainThread.getHandler());
			}});

	registerService(SENSOR_SERVICE, new ServiceFetcher() {
			public Object createService(ContextImpl ctx) {
				return new SystemSensorManager(ctx.mMainThread.getHandler().getLooper());
			}});

	registerService(STATUS_BAR_SERVICE, new ServiceFetcher() {
			public Object createService(ContextImpl ctx) {
				return new StatusBarManager(ctx.getOuterContext());
			}});

	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;
				}
			}});
	registerService(TELEPHONY_SERVICE, new ServiceFetcher() {
			public Object createService(ContextImpl ctx) {
				return new TelephonyManager(ctx.getOuterContext());
			}});

	registerService(THROTTLE_SERVICE, new StaticServiceFetcher() {
			public Object createStaticService() {
				IBinder b = ServiceManager.getService(THROTTLE_SERVICE);
				return new ThrottleManager(IThrottleManager.Stub.asInterface(b));
			}});

	registerService(UI_MODE_SERVICE, new ServiceFetcher() {
			public Object createService(ContextImpl ctx) {
				return new UiModeManager();
			}});

	registerService(USB_SERVICE, new ServiceFetcher() {
			public Object createService(ContextImpl ctx) {
				IBinder b = ServiceManager.getService(USB_SERVICE);
				return new UsbManager(ctx, IUsbManager.Stub.asInterface(b));
			}});

	registerService(SERIAL_SERVICE, new ServiceFetcher() {
			public Object createService(ContextImpl ctx) {
				IBinder b = ServiceManager.getService(SERIAL_SERVICE);
				return new SerialManager(ctx, ISerialManager.Stub.asInterface(b));
			}});

	registerService(VIBRATOR_SERVICE, new ServiceFetcher() {
			public Object createService(ContextImpl ctx) {
				return new SystemVibrator();
			}});

	registerService(WALLPAPER_SERVICE, WALLPAPER_FETCHER);

	registerService(WIFI_SERVICE, new ServiceFetcher() {
			public Object createService(ContextImpl ctx) {
				IBinder b = ServiceManager.getService(WIFI_SERVICE);
				IWifiManager service = IWifiManager.Stub.asInterface(b);
				return new WifiManager(service, ctx.mMainThread.getHandler());
			}});

	registerService(WIFI_P2P_SERVICE, new ServiceFetcher() {
			public Object createService(ContextImpl ctx) {
				IBinder b = ServiceManager.getService(WIFI_P2P_SERVICE);
				IWifiP2pManager service = IWifiP2pManager.Stub.asInterface(b);
				return new WifiP2pManager(service);
			}});

	registerService(WINDOW_SERVICE, new ServiceFetcher() {
			public Object getService(ContextImpl ctx) {
				return WindowManagerImpl.getDefault(ctx.mPackageInfo.mCompatibilityInfo);
			}});
}

在該Static塊語句裡，通過調用registerService函數向SYSTEM_SERVICE_MAP哈希變量中添加服務管理對象：

private static void registerService(String serviceName, ServiceFetcher fetcher) {
	if (!(fetcher instanceof StaticServiceFetcher)) {
		fetcher.mContextCacheIndex = sNextPerContextServiceCacheIndex++;
	}
	SYSTEM_SERVICE_MAP.put(serviceName, fetcher);
}

首先通過服務名稱從SYSTEM_SERVICE_MAP哈希表中查找出服務對應的ServiceFetcher對象，然後調用ServiceFetcher對象的getService函數來獲取服務管理Manager對象

public final Object getService(ContextImpl unused) {
	synchronized (StaticServiceFetcher.this) {
		Object service = mCachedInstance;
		if (service != null) {
			return service;
		}
		return mCachedInstance = createStaticService();
	}
}

這裡以單例模式獲取服務管理對象，如果變量mCachedInstance為空，則表示系統還未使用過該服務，則調用createStaticService()函數來構造服務管理對象，該函數是一個抽象函數，在注冊每一個服務管理對象時實現該函數，在該函數裡構造對應的服務管理對象。