RequestMappingHandlerMapping--Spring MVC優先級最高的HandlerMapping

DispatcherServlet在啟動時會根據配置文件創建HandlerMapping，這些HandlerMapping都實現了Ordered接口，DispatcherServlet實例化所有的HandlerMapping后放到一個集合中，並根據order對HandlerMapping進行排序。

DispatcherServlet在接受請求時會循環遍歷有序的HandlerMapping集合，RequestMappingHandlerMapping的order是0，開始接收請求。

RequestMappingHandlerMappin繼承AbstractHandlerMethodMapping，AbstractHandlerMethodMapping實現了InitializingBean，在實例化后對調用afterPropertiesSet方法，該方法完成method handler的注冊：

public void afterPropertiesSet() {
        initHandlerMethods();
    }

    /**
     * Scan beans in the ApplicationContext, detect and register handler methods.
     * @see #isHandler(Class)
     * @see #getMappingForMethod(Method, Class)
     * @see #handlerMethodsInitialized(Map)
     */
    protected void initHandlerMethods() {
        if (logger.isDebugEnabled()) {
            logger.debug("Looking for request mappings in application context: " + getApplicationContext());
        }

        String[] beanNames = (this.detectHandlerMethodsInAncestorContexts ?
                BeanFactoryUtils.beanNamesForTypeIncludingAncestors(getApplicationContext(), Object.class) :
                getApplicationContext().getBeanNamesForType(Object.class));

        for (String beanName : beanNames) {
            if (isHandler(getApplicationContext().getType(beanName))){
                detectHandlerMethods(beanName);
            }
        }
        handlerMethodsInitialized(getHandlerMethods());
    }