（M）getSystemService解析

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经常看到有类似于如下的语句，之前不知道这个究竟是如何返回一个TelephonyManager对象的，今天简单地看了下，发现其实这个也没有太过复杂

(TelephonyManager) context.getSystemService(Context.TELEPHONY_SERVICE);

主要是调用了Context的getSystemService方法

public abstract Object getSystemService(@ServiceName @NonNull String name);

这个地方是一个抽象方法，那么一定是在其子类中调用的，因此ContextImpl.java文件中

@Override    public Object getSystemService(String name) {        return SystemServiceRegistry.getSystemService(this, name);    }

好，继续看SystemServiceRegistry的getSystemService方法

/**     * Gets a system service from a given context.     */    public static Object getSystemService(ContextImpl ctx, String name) {        ServiceFetcher<?> fetcher = SYSTEM_SERVICE_FETCHERS.get(name);        return fetcher != null ? fetcher.getService(ctx) : null;    }

那么，SYSTEM_SERVICE_FETCHERS是什么？

/**     * Statically registers a system service with the context.     * This method must be called during static initialization only.     */    private static <T> void registerService(String serviceName, Class<T> serviceClass,            ServiceFetcher<T> serviceFetcher) {        SYSTEM_SERVICE_NAMES.put(serviceClass, serviceName);        SYSTEM_SERVICE_FETCHERS.put(serviceName, serviceFetcher);    }

是一个private方法，在哪儿调用的呢？

static {    ......    registerService(Context.TELEPHONY_SERVICE, TelephonyManager.class,                new CachedServiceFetcher<TelephonyManager>() {            @Override            public TelephonyManager createService(ContextImpl ctx) {                return new TelephonyManager(ctx.getOuterContext());            }});    ......}

回到registerService方法，SYSTEM_SERVICE_FETCHERS中添加的是这个static的CachedServiceFetcher对象，getSystemService返回的应该是fetcher.getService(ctx)才对，那么看一下CachedServiceFetcher类的getService方法

/**     * Override this class when the system service constructor needs a     * ContextImpl and should be cached and retained by that context.     */    static abstract class CachedServiceFetcher<T> implements ServiceFetcher<T> {        private final int mCacheIndex;        public CachedServiceFetcher() {            mCacheIndex = sServiceCacheSize++;        }        @Override        @SuppressWarnings("unchecked")        public final T getService(ContextImpl ctx) {            final Object[] cache = ctx.mServiceCache;            synchronized (cache) {                // Fetch or create the service.                Object service = cache[mCacheIndex];                if (service == null) {                    service = createService(ctx);                    cache[mCacheIndex] = service;                }                return (T)service;            }        }        public abstract T createService(ContextImpl ctx);    }

getService方法中调用了createService方法，而createService方法是在SystemServiceRegistry类的静态区域中定义的，回到上述registerService调用的地方

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

createService返回的是新建的一个TelephonyManager对象，而CachedServiceFetcher的getService方法中return的就是这个

所以

(TelephonyManager) context.getSystemService(Context.TELEPHONY_SERVICE);

这个返回的就是new TelephonyManager（context）对象了

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