RequestMappingHandlerMapping--Spring MVC优先级最高的HandlerMapping

DispatcherServlet在启动时会根据配置文件创建HandlerMapping，这些HandlerMapping都实现了Ordered接口，DispatcherServlet实例化所有的HandlerMapping后放到一个集合中，并根据order对HandlerMapping进行排序。

DispatcherServlet在接受请求时会循环遍历有序的HandlerMapping集合，RequestMappingHandlerMapping的order是0，开始接收请求。

RequestMappingHandlerMappin继承AbstractHandlerMethodMapping，AbstractHandlerMethodMapping实现了InitializingBean，在实例化后对调用afterPropertiesSet方法，该方法完成method handler的注册：

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());
    }