本篇将涉及，通过本篇可以对整个springmvc模块有一个清晰的视角，在日常开发中可以快速定位问题和源码

1. springMVC启动初始化过程分析
2. 处理请求过程分析
3. 参数解析、返回值处理

springMVC应用启动初始化过程分析

初始化过程应该分为三步：

• 第一步，创建两个spring的ApplicationContext，配置DispatcherServlet、filter、ContextLoaderListener到servlet容器中
• 第二步，servlet容器启动后，调用ContextLoaderListener#contextInitialized初始化必要的bean(如DAO、service相关的bean)
• 第三步，调用DispatcherServlet#init方法进一步初始化必要的bean（如HandlerMappings、HandlerMappingAdapter等）

第一步

主要完成以下工作：

1. 通过配置初始化两个容器，分别为spring容器、springMVC容器。（关于为什么需要两个容器，个人认为是为了在两个DispatcherServlet中共享同一个spring容器）
2. 注册DispatcherServlet到servlet容器中，对外提供服务，处理客户端请求
3. 注册Filter到servlet容器中

第二步

主要完成以下工作：

1. 初始化父容器
2. 绑定到servletContext，在应用中全局共享

第三步

在这一步中，分为两个阶段：

1. 初始化属于springmvc的ApplicationContext、并添加SourceFilteringListener作为spring容器时间监听器
2. 在完成初始化属于springmvc的ApplicationContext的工作后，SourceFilteringListener监听到容器时间，调用onApplicationEvent方法，完成剩余的初始化工作（包括初始化HandlerMappings、HandlerMappingAdapter等属于MVC领域的bean）

SourceFilteringListener的职责

先来看一下其类关系图

SourceFilteringListener作为EventListener的实现类，在servlet的ApplicationContext初始化后，将收到事件回调完成剩余的初始化工作

protected void initStrategies(ApplicationContext context) {initMultipartResolver(context);initLocaleResolver(context);initThemeResolver(context);initHandlerMappings(context);initHandlerAdapters(context);initHandlerExceptionResolvers(context);initRequestToViewNameTranslator(context);initViewResolvers(context);initFlashMapManager(context);
}
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在这些初始化方法中，注册到spring容器中的bean由注解@EnableWebMVC导入。 导入工作委托给了DelegatingWebMvcConfiguration类

以上就是springMVC的初始化过程的整体脉络

springMVC处理请求过程分析

整体流程

整个过程相对比较复杂，整个逻辑在DispatcherServlet#doDispatcher方法中实现，该方法内部将具体工作委托给了RequestMappingHandlerMapping、HandlerExcutionChain、RequestMappingHandlerAdapter、ViewResolver、View等几个核心类来完成任务。

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {HttpServletRequest processedRequest = request;HandlerExecutionChain mappedHandler = null;boolean multipartRequestParsed = false;WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);try {ModelAndView mv = null;Exception dispatchException = null;try {processedRequest = checkMultipart(request);multipartRequestParsed = (processedRequest != request);// Determine handler for the current request.// 获取HandlerExcutionChainmappedHandler = getHandler(processedRequest);if (mappedHandler == null || mappedHandler.getHandler() == null) {noHandlerFound(processedRequest, response);return;}// 获取HandlerAdapter// Determine handler adapter for the current request.HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());// Process last-modified header, if supported by the handler.String method = request.getMethod();boolean isGet = "GET".equals(method);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;}}// 调用拦截器前置处理if (!mappedHandler.applyPreHandle(processedRequest, response)) {return;}// 真正调用HandlerMethod（即Controller方法），内部涉及了参数解析（处理@RequestParams、@PathVariables注解原理）、参数类型转换、HandlerMethod执行结果处理（@ResponseBody注解原理）这几个步骤。后面内容我们详细剖析这个函数的内部机制// Actually invoke the handler.mv = ha.handle(processedRequest, response, mappedHandler.getHandler());if (asyncManager.isConcurrentHandlingStarted()) {return;}applyDefaultViewName(processedRequest, mv);// 调用拦截器后置处理mappedHandler.applyPostHandle(processedRequest, response, mv);}catch (Exception ex) {dispatchException = ex;}catch (Throwable err) {// As of 4.3, we're processing Errors thrown from handler methods as well,// making them available for @ExceptionHandler methods and other scenarios.dispatchException = new NestedServletException("Handler dispatch failed", err);}// 处理结果，涉及到视图渲染processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);}catch (Exception ex) {triggerAfterCompletion(processedRequest, response, mappedHandler, ex);}catch (Throwable err) {triggerAfterCompletion(processedRequest, response, mappedHandler,new NestedServletException("Handler processing failed", 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);}}}
}
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根据请求寻找对应的HandlerMethod

具体寻找过程的代码细节如下：

1. 首先mappingRegistry(类MappingRegistry的实例)实例变量存放了所有的HandlerMethod（在RequestMappingHandlerMapping#afterPropertiesSet()方法中初始化完成，以RequestMappingInfo实例保存在mappingRegistry.urlLookup实例变量中），之后在请求到来时通过请求URI从mappingRegistry获取
2. 如果在使用路径参数@PathVariables的时候，需要遍历所有Controller方法（因为初始化的时候是以/user/{name}作为key保存在urlLookup变量中的，而实际的请求路径是/user/tom。无法通过this.mappingRegistry.getMappingsByUrl(lookupPath)直接获取到）。这里会涉及到Ant风格解析。O(n)的时间复杂度。效率极低

protected HandlerMethod lookupHandlerMethod(String lookupPath, HttpServletRequest request) throws Exception {List<Match> matches = new ArrayList<Match>();List<T> directPathMatches = this.mappingRegistry.getMappingsByUrl(lookupPath);if (directPathMatches != null) {addMatchingMappings(directPathMatches, matches, request);}if (matches.isEmpty()) {// No choice but to go through all mappings...// 如果没有找到，则遍历所有Controller方法。这里会涉及到Ant风格解析。O(n)的时间复杂度。效率极低// 在使用路径参数@PathVariables的时候，会进入到这段逻辑addMatchingMappings(this.mappingRegistry.getMappings().keySet(), matches, request);}if (!matches.isEmpty()) {Comparator<Match> comparator = new MatchComparator(getMappingComparator(request));Collections.sort(matches, comparator);if (logger.isTraceEnabled()) {logger.trace("Found " + matches.size() + " matching mapping(s) for [" +lookupPath + "] : " + matches);}Match bestMatch = matches.get(0);if (matches.size() > 1) {if (CorsUtils.isPreFlightRequest(request)) {return PREFLIGHT_AMBIGUOUS_MATCH;}Match secondBestMatch = matches.get(1);if (comparator.compare(bestMatch, secondBestMatch) == 0) {Method m1 = bestMatch.handlerMethod.getMethod();Method m2 = secondBestMatch.handlerMethod.getMethod();throw new IllegalStateException("Ambiguous handler methods mapped for HTTP path '" +request.getRequestURL() + "': {" + m1 + ", " + m2 + "}");}}// 设置属性到ServletHttpRequest实例中，在解析@PathVariables时使用handleMatch(bestMatch.mapping, lookupPath, request);return bestMatch.handlerMethod;}else {return handleNoMatch(this.mappingRegistry.getMappings().keySet(), lookupPath, request);}
}
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protected void handleMatch(RequestMappingInfo info, String lookupPath, HttpServletRequest request) {super.handleMatch(info, lookupPath, request);String bestPattern;Map<String, String> uriVariables;Map<String, String> decodedUriVariables;Set<String> patterns = info.getPatternsCondition().getPatterns();if (patterns.isEmpty()) {bestPattern = lookupPath;uriVariables = Collections.emptyMap();decodedUriVariables = Collections.emptyMap();}else {bestPattern = patterns.iterator().next();// 使用Ant风格解析路径参数uriVariables = getPathMatcher().extractUriTemplateVariables(bestPattern, lookupPath);decodedUriVariables = getUrlPathHelper().decodePathVariables(request, uriVariables);}request.setAttribute(BEST_MATCHING_PATTERN_ATTRIBUTE, bestPattern);request.setAttribute(HandlerMapping.URI_TEMPLATE_VARIABLES_ATTRIBUTE, decodedUriVariables);if (isMatrixVariableContentAvailable()) {Map<String, MultiValueMap<String, String>> matrixVars = extractMatrixVariables(request, uriVariables);request.setAttribute(HandlerMapping.MATRIX_VARIABLES_ATTRIBUTE, matrixVars);}if (!info.getProducesCondition().getProducibleMediaTypes().isEmpty()) {Set<MediaType> mediaTypes = info.getProducesCondition().getProducibleMediaTypes();request.setAttribute(PRODUCIBLE_MEDIA_TYPES_ATTRIBUTE, mediaTypes);}
}
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HandlerMethod执行细节

参数解析以及类型转换

主要是通过HandlerMethodArgumentResolverComposite这个特殊的HandlerMethodArgumentResolver聚合了多个HandlerMethodArgumentResolver来解析参数、其内部将调用RequestParamMethodArgumentResolver、PathVariableMapMethodArgumentResolver等来处理参数解析逻辑

返回结果处理

主要是通过HandlerMethodReturnValueHandlerComposite这个特殊的HandlerMethodReturnValueHandler聚合了多个HandlerMethodReturnValueHandler来处理返回结果，其内部将调用RequestResponseBodyMethodProcessor处理@ResponseBody、@RestController标记的方法和类来完成RESTful接口的结果返回

总结

以上就是整个springmvc的工作流程，包括了初始化、请求处理两个步骤。在请求处理的过程中又涉及到了参数解析、参数类型转换、返回结果处理这些流程。由于整个代码比较繁多，这里就不列举所有的流程（视图渲染、异常处理这些细节）通过本篇的主体流程图可以快速定位到具体的代码。后续有需要查看更多细节可参考本篇快速定位