Spring MVC中的HandlerMapping

如何根据http请求选择合适的controller是MVC中一项十分关键的功能，在Spring MVC中，HandlerMapping接口是这一活动的抽象。同时，Spring MVC也提供了一系列HandlerMapping的实现，根据一定的规则选择controller。如果当前的HandlerMappign实现中没有能够满足你所需要的规则是，可以通过实现HandlerMapping接口进行扩展。下面，就来看一下HandlerMapping相关的类图：

在Spring MVC中，关于HandlerMapping的使用，主要包括两个部分：注册和查找。在HandlerMapping的实现中，持有一个handlerMap这样一个HashMap<String, Object>，其中key是http请求的path信息，value可以是一个字符串，或者是一个处理请求的HandlerExecutionChain，如果是String类型，则会将其视为Spring的bean名称。在HandlerMapping对象的创建中，IoC容器执行了一个容器回调方法setApplicationContext，在这个方法中调用initApplicationContext方法进行初始化，各个子类可以根据需求的不同覆写这个方法。关于handlerMap信息的注册就是在initApplicationContext方法中被执行的。下面就来看一下注册url到controller映射信息的实现：

SimpleUrlHandlerMapping中的注册实现代码：

protected void registerHandlers(Map<String, Object> urlMap) throws BeansException {
		if (urlMap.isEmpty()) {
			logger.warn("Neither 'urlMap' nor 'mappings' set on SimpleUrlHandlerMapping");
		}
		else {
//urlMap信息是根据配置文件注入进来的
			for (Map.Entry<String, Object> entry : urlMap.entrySet()) {
				String url = entry.getKey();
				Object handler = entry.getValue();
				// Prepend with slash if not already present.
				if (!url.startsWith("/")) {
					url = "/" + url;
				}
				// Remove whitespace from handler bean name.
				if (handler instanceof String) {
					handler = ((String) handler).trim();
				}
				registerHandler(url, handler);
			}
		}
	}

AbstractDetectingUrlHandlerMapping中的注册实现代码：

protected void detectHandlers() throws BeansException {
		if (logger.isDebugEnabled()) {
			logger.debug("Looking for URL mappings in application context: " + getApplicationContext());
		}
		String[] beanNames = (this.detectHandlersInAncestorContexts ?
				BeanFactoryUtils.beanNamesForTypeIncludingAncestors(getApplicationContext(), Object.class) :
				getApplicationContext().getBeanNamesForType(Object.class));

		// Take any bean name that we can determine URLs for.
		for (String beanName : beanNames) {
			String[] urls = determineUrlsForHandler(beanName);
			if (!ObjectUtils.isEmpty(urls)) {
				// URL paths found: Let's consider it a handler.
				registerHandler(urls, beanName);
			}
			else {
				if (logger.isDebugEnabled()) {
					logger.debug("Rejected bean name '" + beanName + "': no URL paths identified");
				}
			}
		}
	}

单个的映射关系注册是在registerHandler方法中执行的：

protected void registerHandler(String urlPath, Object handler) throws BeansException, IllegalStateException {
		Assert.notNull(urlPath, "URL path must not be null");
		Assert.notNull(handler, "Handler object must not be null");
		Object resolvedHandler = handler;

		// Eagerly resolve handler if referencing singleton via name.
//如果给定的handeler是字符串，则认为是bean name，直接到IoC容器中取得bean instance
		if (!this.lazyInitHandlers && handler instanceof String) {
			String handlerName = (String) handler;
			if (getApplicationContext().isSingleton(handlerName)) {
				resolvedHandler = getApplicationContext().getBean(handlerName);
			}
		}

		Object mappedHandler = this.handlerMap.get(urlPath);
//同一path不能对应多个处理对象
		if (mappedHandler != null) {
			if (mappedHandler != resolvedHandler) {
				throw new IllegalStateException(
						"Cannot map handler [" + handler + "] to URL path [" + urlPath +
						"]: There is already handler [" + resolvedHandler + "] mapped.");
			}
		}
		else {
			if (urlPath.equals("/")) {
				if (logger.isInfoEnabled()) {
					logger.info("Root mapping to handler [" + resolvedHandler + "]");
				}
				setRootHandler(resolvedHandler);
			}
			else if (urlPath.equals("/*")) {
				if (logger.isInfoEnabled()) {
					logger.info("Default mapping to handler [" + resolvedHandler + "]");
				}
				setDefaultHandler(resolvedHandler);
			}
			else {
//将path和handler放在handlerMap中，保存了它们之间的映射关系
				this.handlerMap.put(urlPath, resolvedHandler);
				if (logger.isInfoEnabled()) {
					logger.info("Mapped URL path [" + urlPath + "] onto handler [" + resolvedHandler + "]");
				}
			}
		}
	}

通过以上代码，在HandlerMapping对象被创建的时候，即完成了url到handler之间的映射关系的注册。handlerMap已经被赋值，可以被使用了。

在完成url到handler映射关系的注册后，就可以使用getHandler方法，根据http请求获得handler对象了。下面，首先看一下getHandler方法的调用时序：

下面就主要关注一下上图中涉及到的方法的代码实现。

	public final HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
		Object handler = getHandlerInternal(request);
                //如果没有找到匹配的handler，则使用默认handler
		if (handler == null) {
			handler = getDefaultHandler();
		}
		if (handler == null) {
			return null;
		}
		// Bean name or resolved handler?
                //如果给定的handler是字符串类型，则认为是bean name
		if (handler instanceof String) {
			String handlerName = (String) handler;
			handler = getApplicationContext().getBean(handlerName);
		}
                //如果Handler是HandlerExecutionChain类型，则只是添加拦截器，否则会创建一个新的HandlerExecutionChain
		return getHandlerExecutionChain(handler, request);
	}

将url信息与handler进行匹配查找的操作是在lookupHandler方法中执行的，下面就来看一下lookupHandler方法的实现：

//这个方法可能的返回值是HandlerExecutionChain对象或者是null
//在HandlerExecutionChain对象中的handler，是根据handlerMap中取出来的bean name获得到的bean instance
protected Object lookupHandler(String urlPath, HttpServletRequest request) throws Exception {
		// Direct match?
                //直接匹配
		Object handler = this.handlerMap.get(urlPath);
		if (handler != null) {
			// Bean name or resolved handler?
			if (handler instanceof String) {
				String handlerName = (String) handler;
                                //从IoC容器中取出handler
				handler = getApplicationContext().getBean(handlerName);
			}
			validateHandler(handler, request);
                        //创建一个HandlerExecutionChain对象并返回
			return buildPathExposingHandler(handler, urlPath, urlPath, null);
		}
		// Pattern match?
                //根据一定的模式匹配规则
		List<String> matchingPatterns = new ArrayList<String>();
		for (String registeredPattern : this.handlerMap.keySet()) {
			if (getPathMatcher().match(registeredPattern, urlPath)) {
				matchingPatterns.add(registeredPattern);
			}
		}
		String bestPatternMatch = null;
		if (!matchingPatterns.isEmpty()) {
			Collections.sort(matchingPatterns, getPathMatcher().getPatternComparator(urlPath));
			if (logger.isDebugEnabled()) {
				logger.debug("Matching patterns for request [" + urlPath + "] are " + matchingPatterns);
			}
			bestPatternMatch = matchingPatterns.get(0);
		}
		if (bestPatternMatch != null) {
                        //处理最佳匹配
			handler = this.handlerMap.get(bestPatternMatch);
			// Bean name or resolved handler?
			if (handler instanceof String) {
				String handlerName = (String) handler;
				handler = getApplicationContext().getBean(handlerName);
			}
			validateHandler(handler, request);
			String pathWithinMapping = getPathMatcher().extractPathWithinPattern(bestPatternMatch, urlPath);
			Map<String, String> uriTemplateVariables =
					getPathMatcher().extractUriTemplateVariables(bestPatternMatch, urlPath);
                        //返回一个HandlerExecutionChain对象
			return buildPathExposingHandler(handler, bestPatternMatch, pathWithinMapping, uriTemplateVariables);
		}
		// No handler found...
		return null;
	}

最后在关注一下getHandlerInternal的代码实现

protected Object getHandlerInternal(HttpServletRequest request) throws Exception {
		String lookupPath = this.urlPathHelper.getLookupPathForRequest(request);
                //查找符合匹配规则的handler。可能的结果是HandlerExecutionChain对象或者是null
		Object handler = lookupHandler(lookupPath, request);
                //如果没有找到匹配的handler，则需要处理下default handler
		if (handler == null) {
			// We need to care for the default handler directly, since we need to
			// expose the PATH_WITHIN_HANDLER_MAPPING_ATTRIBUTE for it as well.
			Object rawHandler = null;
			if ("/".equals(lookupPath)) {
				rawHandler = getRootHandler();
			}
			if (rawHandler == null) {
				rawHandler = getDefaultHandler();
			}
                        //在getRootHandler和getDefaultHandler方法中，可能持有的是bean name。
			if (rawHandler != null) {
				// Bean name or resolved handler?
				if (rawHandler instanceof String) {
					String handlerName = (String) rawHandler;
					rawHandler = getApplicationContext().getBean(handlerName);
				}
				validateHandler(rawHandler, request);
				handler = buildPathExposingHandler(rawHandler, lookupPath, lookupPath, null);
			}
		}
                //如果handler还是为空，则抛出错误。
		if (handler != null && this.mappedInterceptors != null) {
			Set<HandlerInterceptor> mappedInterceptors =
					this.mappedInterceptors.getInterceptors(lookupPath, this.pathMatcher);
			if (!mappedInterceptors.isEmpty()) {
				HandlerExecutionChain chain;
				if (handler instanceof HandlerExecutionChain) {
					chain = (HandlerExecutionChain) handler;
				} else {
					chain = new HandlerExecutionChain(handler);
				}
				chain.addInterceptors(mappedInterceptors.toArray(new HandlerInterceptor[mappedInterceptors.size()]));
			}
		}
		if (handler != null && logger.isDebugEnabled()) {
			logger.debug("Mapping [" + lookupPath + "] to handler '" + handler + "'");
		}
		else if (handler == null && logger.isTraceEnabled()) {
			logger.trace("No handler mapping found for [" + lookupPath + "]");
		}
		return handler;
	}

经过上述代码的处理，就可以使用getHandler方法获得到HandlerExecutionChain对象了。DispatcherServlet获得到HandlerExecutionChain对象后，便可以获得真正的handler，调用相应的Controller了。

这篇文章主要包括两个方面的内容：HandlerMapping中url到handler映射关系的注册和根据url查找handler。第一部分是在HandlerMapping对象创建后，使用IoC容器的回调方法触发的，第二部分是在DispatchServlet中，调用getHandler方法触发的。