Android getSystemService

getSystemService的實際調用者-SystemServiceRegistry

getSystemService是Context中的方法，經過SystemServiceRegistry類獲取SystemService實例，具體的實如今ContextImpl中。有關Context的簡介，能夠參考http://www.javashuo.com/article/p-yegqpqxb-hz.html

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

getSystemService在SystemServiceRegistry中的實現方式

一、SystemServiceRegistry中聲明瞭兩個靜態HashMap

// Service registry information.
// This information is never changed once static initialization has completed.
private static final HashMap<Class<?>, String> SYSTEM_SERVICE_NAMES =
        new HashMap<Class<?>, String>();
private static final HashMap<String, ServiceFetcher<?>> SYSTEM_SERVICE_FETCHERS =
        new HashMap<String, ServiceFetcher<?>>();

SYSTEM_SERVICE_NAMES存放着SystemService的class和名稱；

SYSTEM_SERVICE_FETCHERS存放着SystemService的名稱和接口ServiceFetcher的實例。

 

二、在SystemServiceRegistry類加載時會建立服務實例並經過registerService方法將ServiceFetcher存放在HashMap中。

ServiceFetcher是一個用來獲取Service實例的接口：

/**
 * Base interface for classes that fetch services.
 * These objects must only be created during static initialization.
 */
static abstract interface ServiceFetcher<T> {
    T getService(ContextImpl ctx);
}

static {
    registerService(Context.ACCESSIBILITY_SERVICE, AccessibilityManager.class,
            new CachedServiceFetcher<AccessibilityManager>() {
        @Override
        public AccessibilityManager createService(ContextImpl ctx) {
            return AccessibilityManager.getInstance(ctx);
        }});
    // 省略代碼
}

/**
 * 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);
}

 

三、調用getSystemService時根據名稱從HashMap獲取實例

/**
 * 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;
}

getSystemService中的設計模式

SystemService的兩個靜態HashMap其實就是「使用容器實現單例」

有關單例模式能夠參考：http://www.javashuo.com/article/p-riuvmrsx-ht.html