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。