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());
	}