辣麼他的大致是什麼樣的流程呢？下面跟蹤一下source code。 代碼基於Android 5.1.1
一、相關的類文件：
framework/base/core/java/android/app/ContextImpl.java
二、相關流程
this.getSystemService(Context.LAYOUT_INFLATER_SERVICE) ;
@Override public Object getSystemService(String name) { ServiceFetcher fetcher = SYSTEM_SERVICE_MAP.get(name); return fetcher == null ? null : fetcher.getService(this); }從上面getSystemService方法中看到，首先是從SYSTEM_SERVICE_MAP中獲取ServiceFetcher對象，然後調用ServiceFetcher對象的getService方法獲取管理的service.
這裏的SYSTEM_SERVICE_MAP 是一個HashMap，鍵值對的方式存儲service相關信息。
private static final HashMap<String, ServiceFetcher> SYSTEM_SERVICE_MAP = new HashMap<String, ServiceFetcher>();
SYSTEM_SERVICE_MAP 是static 因此在加載ContextImpl.java這個類的時候，就初始化了。
下面用的是一個靜態初始化塊將service添加到對應的SYSTEM_SERVICE_MAP 中：
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); }}); 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); }}); registerService(ACTIVITY_SERVICE, new ServiceFetcher() { public Object createService(ContextImpl ctx) { return new ActivityManager(ctx.getOuterContext(), ctx.mMainThread.getHandler()); }}); registerService(ALARM_SERVICE, new ServiceFetcher() { public Object createService(ContextImpl ctx) { IBinder b = ServiceManager.getService(ALARM_SERVICE); IAlarmManager service = IAlarmManager.Stub.asInterface(b); return new AlarmManager(service, ctx); }}); 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(BLUETOOTH_SERVICE, new ServiceFetcher() { public Object createService(ContextImpl ctx) { return new BluetoothManager(ctx); }}); registerService(HDMI_CONTROL_SERVICE, new StaticServiceFetcher() { public Object createStaticService() { IBinder b = ServiceManager.getService(HDMI_CONTROL_SERVICE); return new HdmiControlManager(IHdmiControlService.Stub.asInterface(b)); }}); registerService(CLIPBOARD_SERVICE, new ServiceFetcher() { public Object createService(ContextImpl ctx) { return new ClipboardManager(ctx.getOuterContext(), ctx.mMainThread.getHandler()); }}); registerService(CONNECTIVITY_SERVICE, new ServiceFetcher() { public Object createService(ContextImpl ctx) { 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(BATTERY_SERVICE, new ServiceFetcher() { public Object createService(ContextImpl ctx) { return new BatteryManager(); }}); 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(DISPLAY_SERVICE, new ServiceFetcher() { @Override public Object createService(ContextImpl ctx) { return new DisplayManager(ctx.getOuterContext()); }}); registerService(INPUT_METHOD_SERVICE, new StaticServiceFetcher() { public Object createStaticService() { return InputMethodManager.getInstance(); }}); 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))); } });三、關係結構
這裏先來研究一下ServiceFetcher這個內部類
/** * Override this class when the system service constructor needs a * ContextImpl. Else, use StaticServiceFetcher below. */ /*package*/ static class ServiceFetcher { int mContextCacheIndex = -1; /** * Main entrypoint; only override if you don't need caching. */ public Object getService(ContextImpl ctx) { //先從緩存中獲取，如果沒有則創建，並緩存起來。 ArrayList<Object> cache = ctx.mServiceCache; Object service; synchronized (cache) { if (cache.size() == 0) { //這段代碼是第一次調用的時候會執行，sNextPerContextServiceCacheIndex =0 貌似沒什麼意義啊？！ // Initialize the cache vector on first access. // At this point sNextPerContextServiceCacheIndex // is the number of potential services that are // cached per-Context. for (int i = 0; i < sNextPerContextServiceCacheIndex; i++) { cache.add(null); } } else { service = cache.get(mContextCacheIndex);//以當前ServiceFetcher對象的mContextCacheIndex獲取service if (service != null) { return service; } } //如果緩存沒有則創建service實例，緩存起來並返回。 service = createService(ctx); //crateService方法會被重寫，從registerService方法中可以看到 cache.set(mContextCacheIndex, service); return service; } } /** * Override this to create a new per-Context instance of the * service. getService() will handle locking and caching. */ public Object createService(ContextImpl ctx) { throw new RuntimeException("Not implemented");//從這裏可以看出該方法必須被重寫 } }
ServiceFetcher類中，mContextCacheIndex是存儲在list中的index，index的值是在registerService的時候賦值的。
mServiceCache 屬於ContextImpl的全局變量，應該是用於緩存service的，當我們在程序中使用到了某個service，它就會緩存創建出來的這個service，以供其他的使用、。
從getService的方法邏輯我們就可以看出來。
另外，大家可以看到在static靜態初始化塊中，、，ServiceFetcher並不是全部都重寫了createService方法，而是直接重寫了getService返回了service
實例，也就是說不緩存該service，爲啥呢？ 具體的原因在代碼後面有註釋。google大佬說以前都沒有緩存，現在也不緩存.....
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(); }});