MVC 处理 HTTP 分发请求

HandlerMapping 配置与设计

在初始化完成时，在上下文环境中已定义的所有 HandlerMapping 都已经被加载了，这些加载的 handlerMapping 被放在一个 List 中并被排序，存储着 HTTP 请求对应的映射数据。这个 List 中的每一个元素都对应着一个具体 handlerMapping 的配置，一般每一个 handlerMapping 可以持有一系列从 URL 请求到 Controller 的映射，而 Spring MVC 提供了一系列的 HandlerMapping 实现。如下：

通过这些在 HandlerMapping 中定义的映射关系，即这些 URL 请求和控制器的对应关系，使 Spring MVC 应用可以根据 HTTP 请求确定一个对应的 Controller。具体来说，这些映射关系是通过接口类 HandlerMapping 来封装的，在 HandlerMapping 接口中定义了一个 getHandler 方法，通过这个方法，可以获得与 HTTP 请求对应的 HandlerExecutionChain，而这个 HandlerExecutionChain，封装了具体的 Controller 对象：

public interface HandlerMapping {String PATH_WITHIN_HANDLER_MAPPING_ATTRIBUTE = HandlerMapping.class.getName() + ".pathWithinHandlerMapping";String BEST_MATCHING_PATTERN_ATTRIBUTE = HandlerMapping.class.getName() + ".bestMatchingPattern";String INTROSPECT_TYPE_LEVEL_MAPPING = HandlerMapping.class.getName() + ".introspectTypeLevelMapping";String URI_TEMPLATE_VARIABLES_ATTRIBUTE = HandlerMapping.class.getName() + ".uriTemplateVariables";String MATRIX_VARIABLES_ATTRIBUTE = HandlerMapping.class.getName() + ".matrixVariables";String PRODUCIBLE_MEDIA_TYPES_ATTRIBUTE = HandlerMapping.class.getName() + ".producibleMediaTypes";// 调用 getHandler 实际上返回的是一个 HandlerExecutionChain ，这是典型的 Command 的模式的使用，这个 HandlerExecutionChain // 不但持有 handler 本身，还包括了处理这个 HTTP 请求相关的拦截器HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception;
}

HandlerExecutionChain 看起来比较简洁，它持有一个 Interceptor 链和一个 handler 对象，这个 handler 对象实际上就是 HTTP 请求对应的 Controller，在持有这个 handler 对象的同时，还在 HandlerExecutionChain 中设置了一个拦截器链，通过这个拦截器链中的拦截器，可以为 handler 对象提供功能的增强。要完成这些工作，需要对拦截器链和 handler 都进行配置，这些配置都是在 HandlerExecutionChain 的初始化函数中完成的。为了维护这个拦截器链和 handler，handlerExecutionChain 还提供了一系列与拦截器链维护相关的一些操作，比如可以为拦截器链增加拦截器的 addInterceptor 方法等。

public class HandlerExecutionChain {private static final Log logger = LogFactory.getLog(HandlerExecutionChain.class);private final Object handler;private HandlerInterceptor[] interceptors;private List<HandlerInterceptor> interceptorList;private int interceptorIndex = -1;public HandlerExecutionChain(Object handler) {this(handler, (HandlerInterceptor[]) null);}public HandlerExecutionChain(Object handler, HandlerInterceptor... interceptors) {if (handler instanceof HandlerExecutionChain) {HandlerExecutionChain originalChain = (HandlerExecutionChain) handler;this.handler = originalChain.getHandler();this.interceptorList = new ArrayList<HandlerInterceptor>();CollectionUtils.mergeArrayIntoCollection(originalChain.getInterceptors(), this.interceptorList);CollectionUtils.mergeArrayIntoCollection(interceptors, this.interceptorList);}else {this.handler = handler;this.interceptors = interceptors;}}public Object getHandler() {return this.handler;}public void addInterceptor(HandlerInterceptor interceptor) {initInterceptorList().add(interceptor);}public void addInterceptors(HandlerInterceptor... interceptors) {if (!ObjectUtils.isEmpty(interceptors)) {initInterceptorList().addAll(Arrays.asList(interceptors));}}private List<HandlerInterceptor> initInterceptorList() {if (this.interceptorList == null) {this.interceptorList = new ArrayList<HandlerInterceptor>();if (this.interceptors != null) {// An interceptor array specified through the constructorthis.interceptorList.addAll(Arrays.asList(this.interceptors));}}this.interceptors = null;return this.interceptorList;}public HandlerInterceptor[] getInterceptors() {if (this.interceptors == null && this.interceptorList != null) {this.interceptors = this.interceptorList.toArray(new HandlerInterceptor[this.interceptorList.size()]);}return this.interceptors;}boolean applyPreHandle(HttpServletRequest request, HttpServletResponse response) throws Exception {HandlerInterceptor[] interceptors = getInterceptors();if (!ObjectUtils.isEmpty(interceptors)) {for (int i = 0; i < interceptors.length; i++) {HandlerInterceptor interceptor = interceptors[i];if (!interceptor.preHandle(request, response, this.handler)) {triggerAfterCompletion(request, response, null);return false;}this.interceptorIndex = i;}}return true;}void applyPostHandle(HttpServletRequest request, HttpServletResponse response, ModelAndView mv) throws Exception {HandlerInterceptor[] interceptors = getInterceptors();if (!ObjectUtils.isEmpty(interceptors)) {for (int i = interceptors.length - 1; i >= 0; i--) {HandlerInterceptor interceptor = interceptors[i];interceptor.postHandle(request, response, this.handler, mv);}}}void triggerAfterCompletion(HttpServletRequest request, HttpServletResponse response, Exception ex)throws Exception {HandlerInterceptor[] interceptors = getInterceptors();if (!ObjectUtils.isEmpty(interceptors)) {for (int i = this.interceptorIndex; i >= 0; i--) {HandlerInterceptor interceptor = interceptors[i];try {interceptor.afterCompletion(request, response, this.handler, ex);}catch (Throwable ex2) {logger.error("HandlerInterceptor.afterCompletion threw exception", ex2);}}}}void applyAfterConcurrentHandlingStarted(HttpServletRequest request, HttpServletResponse response) {HandlerInterceptor[] interceptors = getInterceptors();if (!ObjectUtils.isEmpty(interceptors)) {for (int i = interceptors.length - 1; i >= 0; i--) {if (interceptors[i] instanceof AsyncHandlerInterceptor) {try {AsyncHandlerInterceptor asyncInterceptor = (AsyncHandlerInterceptor) interceptors[i];asyncInterceptor.afterConcurrentHandlingStarted(request, response, this.handler);}catch (Throwable ex) {logger.error("Interceptor [" + interceptors[i] + "] failed in afterConcurrentHandlingStarted", ex);}}}}}@Overridepublic String toString() {if (this.handler == null) {return "HandlerExecutionChain with no handler";}StringBuilder sb = new StringBuilder();sb.append("HandlerExecutionChain with handler [").append(this.handler).append("]");if (!CollectionUtils.isEmpty(this.interceptorList)) {sb.append(" and ").append(this.interceptorList.size()).append(" interceptor");if (this.interceptorList.size() > 1) {sb.append("s");}}return sb.toString();}
}

下面我们以 SimpleUrlHandlerMapping 说明在实际中是如何应用的，SimpleUrlHandlerMapping 定义了一个 Map 变量（自己定义一个 Map 主要有两个作用，第一是方便配置，第二是可以在注册前做一些预处理，如确保所以 url 都以”/“开头），将所有的 url 和 Handler 的对应关系放在里面，最后注册到父类的 Map 中。

SimpleUrlHandlerMapping.java

public void initApplicationContext() throws BeansException {super.initApplicationContext();registerHandlers(this.urlMap);
}protected void registerHandlers(Map<String, Object> urlMap) throws BeansException {if (urlMap.isEmpty()) {logger.warn("Neither 'urlMap' nor 'mappings' set on SimpleUrlHandlerMapping");}else {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);}}
}

AbstractUrlHandlerMapping 对 handler 的注册。

AbstractUrlHandlerMapping.java

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.if (!this.lazyInitHandlers && handler instanceof String) {String handlerName = (String) handler;if (getApplicationContext().isSingleton(handlerName)) {resolvedHandler = getApplicationContext().getBean(handlerName);}}Object mappedHandler = this.handlerMap.get(urlPath);if (mappedHandler != null) {if (mappedHandler != resolvedHandler) {throw new IllegalStateException("Cannot map " + getHandlerDescription(handler) + " to URL path [" + urlPath +"]: There is already " + getHandlerDescription(mappedHandler) + " mapped.");}}else {if (urlPath.equals("/")) {if (logger.isInfoEnabled()) {logger.info("Root mapping to " + getHandlerDescription(handler));}setRootHandler(resolvedHandler);}else if (urlPath.equals("/*")) {if (logger.isInfoEnabled()) {logger.info("Default mapping to " + getHandlerDescription(handler));}setDefaultHandler(resolvedHandler);}else {this.handlerMap.put(urlPath, resolvedHandler);if (logger.isInfoEnabled()) {logger.info("Mapped URL path [" + urlPath + "] onto " + getHandlerDescription(handler));}}}
}

HandlerMapping 完成请求和映射处理

在各种准备工作完成后，就是使用 HandlerMapping 来完成请求的映射处理了，而具体执行过程是在 AbstactHandlerMapping 中的 getHandler 来完成的。

AbstactHandlerMapping.java

@Override
public final HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {Object handler = getHandlerInternal(request);// 使用默认的 Handler，也就是"/"对应的 handlerif (handler == null) {handler = getDefaultHandler();}// 这里通过名称取出对应的 Handler Beanif (handler == null) {return null;}// Bean name or resolved handler?if (handler instanceof String) {String handlerName = (String) handler;handler = getApplicationContext().getBean(handlerName);}// 这里把 Handler 封装到 HandlerExecutionChain 中并加上拦截器HandlerExecutionChain executionChain = getHandlerExecutionChain(handler, request);if (CorsUtils.isCorsRequest(request)) {CorsConfiguration globalConfig = this.corsConfigSource.getCorsConfiguration(request);CorsConfiguration handlerConfig = getCorsConfiguration(handler, request);CorsConfiguration config = (globalConfig != null ? globalConfig.combine(handlerConfig) : handlerConfig);executionChain = getCorsHandlerExecutionChain(request, executionChain, config);}return executionChain;
}protected HandlerExecutionChain getHandlerExecutionChain(Object handler, HttpServletRequest request) {HandlerExecutionChain chain = (handler instanceof HandlerExecutionChain ?(HandlerExecutionChain) handler : new HandlerExecutionChain(handler));String lookupPath = this.urlPathHelper.getLookupPathForRequest(request);for (HandlerInterceptor interceptor : this.adaptedInterceptors) {if (interceptor instanceof MappedInterceptor) {MappedInterceptor mappedInterceptor = (MappedInterceptor) interceptor;if (mappedInterceptor.matches(lookupPath, this.pathMatcher)) {chain.addInterceptor(mappedInterceptor.getInterceptor());}}else {chain.addInterceptor(interceptor);}}return chain;
}

在取得 handler 的具体过程在 getHandlerInternal 方法中实现。这个方法接受 HTTP 请求作为参数，它的实现在 AbstractUrlHandlerMapping 中，这个实现过程包括从 HTTP 请求中得到 URL ，并根据 URL 到 urlMapping 中获取 handler。

AbstractUrlHandlerMapping .java

protected Object getHandlerInternal(HttpServletRequest request) throws Exception {// 从request 中得到请求的 URL 路径String lookupPath = getUrlPathHelper().getLookupPathForRequest(request);// 将得到的 URL 路径与 Handler 进行匹配Object handler = lookupHandler(lookupPath, request);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();}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);}}if (handler != null && logger.isDebugEnabled()) {logger.debug("Mapping [" + lookupPath + "] to " + handler);}else if (handler == null && logger.isTraceEnabled()) {logger.trace("No handler mapping found for [" + lookupPath + "]");}return handler;
}// lookupHandler 根据 URL 路径启动在 HandlerMap 中对 handler 的检索，并最终返回 handler 对象
protected Object lookupHandler(String urlPath, HttpServletRequest request) throws Exception {// Direct match?Object handler = this.handlerMap.get(urlPath);if (handler != null) {// Bean name or resolved handler?if (handler instanceof String) {String handlerName = (String) handler;handler = getApplicationContext().getBean(handlerName);}validateHandler(handler, request);return buildPathExposingHandler(handler, urlPath, urlPath, null);}// Pattern match?List<String> matchingPatterns = new ArrayList<String>();for (String registeredPattern : this.handlerMap.keySet()) {if (getPathMatcher().match(registeredPattern, urlPath)) {matchingPatterns.add(registeredPattern);}else if (useTrailingSlashMatch()) {if (!registeredPattern.endsWith("/") && getPathMatcher().match(registeredPattern + "/", urlPath)) {matchingPatterns.add(registeredPattern +"/");}}}String bestPatternMatch = null;Comparator<String> patternComparator = getPathMatcher().getPatternComparator(urlPath);if (!matchingPatterns.isEmpty()) {Collections.sort(matchingPatterns, patternComparator);if (logger.isDebugEnabled()) {logger.debug("Matching patterns for request [" + urlPath + "] are " + matchingPatterns);}bestPatternMatch = matchingPatterns.get(0);}if (bestPatternMatch != null) {handler = this.handlerMap.get(bestPatternMatch);if (handler == null) {Assert.isTrue(bestPatternMatch.endsWith("/"));handler = this.handlerMap.get(bestPatternMatch.substring(0, bestPatternMatch.length() - 1));}// 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);// There might be multiple 'best patterns', let's make sure we have the correct URI template variables// for all of themMap<String, String> uriTemplateVariables = new LinkedHashMap<String, String>();for (String matchingPattern : matchingPatterns) {if (patternComparator.compare(bestPatternMatch, matchingPattern) == 0) {Map<String, String> vars = getPathMatcher().extractUriTemplateVariables(matchingPattern, urlPath);Map<String, String> decodedVars = getUrlPathHelper().decodePathVariables(request, vars);uriTemplateVariables.putAll(decodedVars);}}if (logger.isDebugEnabled()) {logger.debug("URI Template variables for request [" + urlPath + "] are " + uriTemplateVariables);}return buildPathExposingHandler(handler, bestPatternMatch, pathWithinMapping, uriTemplateVariables);}// No handler found...return null;
}

至此，HTTP 请求的解析和匹配 Handler 过程就全部完成了，剩下的就是 HTTP 请求的分发处理了。

Spring MVC 对 HTTP 请求的分发处理

让我们重回到 DispatcherServlet，毫无疑问，它是 Spring MVC 中非常重要的一个类，他不仅仅建立了自己的持有 Ioc 容器，还肩负请求分布处理的任务。而 HTTP 的请求发布处理是在 doService 方法中完成的。

DispatcherServlet.java

protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {if (logger.isDebugEnabled()) {String resumed = WebAsyncUtils.getAsyncManager(request).hasConcurrentResult() ? " resumed" : "";logger.debug("DispatcherServlet with name '" + getServletName() + "'" + resumed +" processing " + request.getMethod() + " request for [" + getRequestUri(request) + "]");}// Keep a snapshot of the request attributes in case of an include,// to be able to restore the original attributes after the include.Map<String, Object> attributesSnapshot = null;if (WebUtils.isIncludeRequest(request)) {attributesSnapshot = new HashMap<String, Object>();Enumeration<?> attrNames = request.getAttributeNames();while (attrNames.hasMoreElements()) {String attrName = (String) attrNames.nextElement();if (this.cleanupAfterInclude || attrName.startsWith("org.springframework.web.servlet")) {attributesSnapshot.put(attrName, request.getAttribute(attrName));}}}// 对 HTTP 请求参数进行处理request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());FlashMap inputFlashMap = this.flashMapManager.retrieveAndUpdate(request, response);if (inputFlashMap != null) {request.setAttribute(INPUT_FLASH_MAP_ATTRIBUTE, Collections.unmodifiableMap(inputFlashMap));}request.setAttribute(OUTPUT_FLASH_MAP_ATTRIBUTE, new FlashMap());request.setAttribute(FLASH_MAP_MANAGER_ATTRIBUTE, this.flashMapManager);try {// 这里是 doDispatch 的分发请求的人口doDispatch(request, response);}finally {if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {// Restore the original attribute snapshot, in case of an include.if (attributesSnapshot != null) {restoreAttributesAfterInclude(request, attributesSnapshot);}}}
}

对于请求的实际处理是由 doDispatch() 来完成的，这个方法很长，但过程简单明了。这个 doDispatch 方法是 DispatcherServlet 完成 Dispatcher 的主要方法，包括准备 ModelAndView，调用 getHandler 来响应 HTTP 请求，然后通过执行 Handler 的处理来得到返回的 ModelAndView 结果，最后把这个 ModelAndView 对象交给相应的视图对象去呈现。这里是 MVC 模式的核心地区：

DispatcherServlet.java

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {HttpServletRequest processedRequest = request;HandlerExecutionChain mappedHandler = null;boolean multipartRequestParsed = false;WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);// 这里为视图准备好一个 ModelAndView，这个 ModelAndView 持有 handler 处理请求的结果try {ModelAndView mv = null;Exception dispatchException = null;try {processedRequest = checkMultipart(request);multipartRequestParsed = (processedRequest != request);// 根据请求得到对应的 handler，handler 的注册以及 getHandler 的实现mappedHandler = getHandler(processedRequest);if (mappedHandler == null || mappedHandler.getHandler() == null) {noHandlerFound(processedRequest, response);return;}// 调用 Handler 的地方HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());// Process last-modified header, if supported by the handler.String method = request.getMethod();boolean isGet = "GET".equals(method);// 判断是否需要进行视图的翻译和转换if (isGet || "HEAD".equals(method)) {long lastModified = ha.getLastModified(request, mappedHandler.getHandler());if (logger.isDebugEnabled()) {logger.debug("Last-Modified value for [" + getRequestUri(request) + "] is: " + lastModified);}if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {return;}}if (!mappedHandler.applyPreHandle(processedRequest, response)) {return;}// Actually invoke the handler.mv = ha.handle(processedRequest, response, mappedHandler.getHandler());if (asyncManager.isConcurrentHandlingStarted()) {return;}applyDefaultViewName(processedRequest, mv);mappedHandler.applyPostHandle(processedRequest, response, mv);}catch (Exception ex) {dispatchException = ex;}processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);}catch (Exception ex) {triggerAfterCompletion(processedRequest, response, mappedHandler, ex);}catch (Error err) {triggerAfterCompletionWithError(processedRequest, response, mappedHandler, err);}finally {if (asyncManager.isConcurrentHandlingStarted()) {// Instead of postHandle and afterCompletionif (mappedHandler != null) {mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);}}else {// Clean up any resources used by a multipart request.if (multipartRequestParsed) {cleanupMultipart(processedRequest);}}}
}

这里是与 MVC 框架最紧密相关的代码，得到 HTTP 请求对应的 HandlerExecutionChain ，执行 handler 并把模型数据展现到视图中去。

protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {for (HandlerMapping hm : this.handlerMappings) {if (logger.isTraceEnabled()) {logger.trace("Testing handler map [" + hm + "] in DispatcherServlet with name '" + getServletName() + "'");}HandlerExecutionChain handler = hm.getHandler(request);if (handler != null) {return handler;}}return null;
}

其实，这个方法执行结束，MVC 框架中的 Controller 和 定义的一个拦截器链已经包含在其中了。然后，需要执行的就是返回适配的结果了。

protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {// 对持有的所有 adapter 进行匹配for (HandlerAdapter ha : this.handlerAdapters) {if (logger.isTraceEnabled()) {logger.trace("Testing handler adapter [" + ha + "]");}if (ha.supports(handler)) {return ha;}}throw new ServletException("No adapter for handler [" + handler +"]: The DispatcherServlet configuration needs to include a HandlerAdapter that supports this handler");
}

这里的 handler 是不是 Controller 接口的实现，可以通过 HandlerAdapter 来了解这个适配的过程。而具体的适配过程，我们以 SimpleControllerHandlerAdapter 的实现来说明。

public class SimpleControllerHandlerAdapter implements HandlerAdapter {// 判断将要调用的 handler 是不是 Controller.@Overridepublic boolean supports(Object handler) {return (handler instanceof Controller);}@Overridepublic ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)throws Exception {return ((Controller) handler).handleRequest(request, response);}@Overridepublic long getLastModified(HttpServletRequest request, Object handler) {if (handler instanceof LastModified) {return ((LastModified) handler).getLastModified(request);}return -1L;}}

        得到 handler 对象，然后调用 handler 对象中的 HTTP 进行动作的响应。而具体的业务逻辑则被封装在 handler 中，由这些逻辑对 HTTP 请求进行相应的处理，从而生成所需要的数据，并把这些数据封装到 ModelAndView 对象中。最后，通过 handler 的 handlerRequest 方法触发完成，然后交给 MVC 的 View 部分处理。至此，Spring MVC 的主要逻辑就完成了。前面已经，分析了，MVC 的 C 和 M 部分。下面就是 View 部分的分析了。

——水门（2016年4月写于杭州）