DispatcherServlet的逻辑处理（下篇）

MultipartContent类型的request处理

对于请求的处理，Spring首先考虑的是对于Multipart的处理，如果是MultipartContent类型的request，则转换为MultipartHttpServletRequest类型的Request。

protected HttpServletRequest checkMultipart(HttpServletRequest request) throws MultipartException {
        if (this.multipartResolver != null && this.multipartResolver.isMultipart(request)) {
            if (WebUtils.getNativeRequest(request, MultipartHttpServletRequest.class) != null) {
                if (request.getDispatcherType().equals(DispatcherType.REQUEST)) {
                    logger.trace("Request already resolved to MultipartHttpServletRequest, e.g. by MultipartFilter");
                }
            }
            else if (hasMultipartException(request) ) {
                logger.debug("Multipart resolution previously failed for current request - " +
                        "skipping re-resolution for undisturbed error rendering");
            }
            else {
                try {
                    return this.multipartResolver.resolveMultipart(request);
                }
                catch (MultipartException ex) {
                    if (request.getAttribute(WebUtils.ERROR_EXCEPTION_ATTRIBUTE) != null) {
                        logger.debug("Multipart resolution failed for error dispatch", ex);
                        // Keep processing error dispatch with regular request handle below
                    }
                    else {
                        throw ex;
                    }
                }
            }
        }
        // If not returned before: return original request.
        return request;
    }

根据request信息寻找对应的Handler

在Spring加载的过程中，Spring会将类型为SimpleUrlHandlerMapping的实例加载到this.HandlerMappings中，按照常理推断，根据request提取对应的Handler，但是该Handler继承自AbstractController类型实例，与HandlerExecutionChain并无任何关联，那么这一步是如何封装的呢？

protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
        if (this.handlerMappings != null) {
            for (HandlerMapping mapping : this.handlerMappings) {
                HandlerExecutionChain handler = mapping.getHandler(request);
                if (handler != null) {
                    return handler;
                }
            }
        }
        return null;
    }

在系统启动的时候Spring会将所有的映射类型的bean注册到this.handlerMappings变量中，所以此函数的目的就是遍历所有的HandlerMapping，并调用其getHandler方法进行封装处理。以SimpleURLHandlerMapping为例查看其getHandler方法如下：

public final HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
        //根据request获取对应的handler
        Object handler = getHandlerInternal(request);
        if (handler == null) {
            handler = getDefaultHandler();
        }
        if (handler == null) {
            //如果没有对应request的handler则使用默认的handler
            return null;
        }
        // 如果也没有提供默认的handler则无法继续处理返回null
        if (handler instanceof String) {
            String handlerName = (String) handler;
            handler = obtainApplicationContext().getBean(handlerName);
        }

        HandlerExecutionChain executionChain = getHandlerExecutionChain(handler, request);

        if (logger.isTraceEnabled()) {
            logger.trace("Mapped to " + handler);
        }
        else if (logger.isDebugEnabled() && !request.getDispatcherType().equals(DispatcherType.ASYNC)) {
            logger.debug("Mapped to " + executionChain.getHandler());
        }

        if (CorsUtils.isCorsRequest(request)) {
            CorsConfiguration globalConfig = this.corsConfigurationSource.getCorsConfiguration(request);
            CorsConfiguration handlerConfig = getCorsConfiguration(handler, request);
            CorsConfiguration config = (globalConfig != null ? globalConfig.combine(handlerConfig) : handlerConfig);
            executionChain = getCorsHandlerExecutionChain(request, executionChain, config);
        }

        return executionChain;
    }

上述函数首先使用getHandlerInternal方法根据request信息获取对应的Handler，如果以SimpleUrlHandlerMapping为例分析，那么我们推断此步骤提供的功能很可能就是根据URL找到匹配的Controller并返回，当然如果没有找到对应的Controller处理器那么程序会尝试去查找配置中的默认处理器，当然，当查找的Controller为String类型时，那就意味着返回的是配置的bean名称，需要根据bean名称查找对应的bean，最后，还要通过getHandlerExecutionChain方法返回的Handler进行封装，以保证满足返回类型的匹配。下面详细分析这个过程：

1.根据request查找对应的Handler

首先从根据request查找对应的Handler开始分析：

protected Object getHandlerInternal(HttpServletRequest request) throws Exception {
        //截取用于匹配的URL有效路径
        String lookupPath = getUrlPathHelper().getLookupPathForRequest(request);
        //根据路径寻找Handler
        Object handler = lookupHandler(lookupPath, request);
        if (handler == null) {
            Object rawHandler = null;
            if ("/".equals(lookupPath)) {
                //如果请求的路径仅仅是“/"，那么使用RootHandler进行处理
                rawHandler = getRootHandler();
            }
            if (rawHandler == null) {
                //无法找到Handler则使用默认的Handler
                rawHandler = getDefaultHandler();
            }
            if (rawHandler != null) {
                // 根据beanName获取对应的bean
                if (rawHandler instanceof String) {
                    String handlerName = (String) rawHandler;
                    rawHandler = obtainApplicationContext().getBean(handlerName);
                }
                //模板方法
                validateHandler(rawHandler, request);
                handler = buildPathExposingHandler(rawHandler, lookupPath, lookupPath, null);
            }
        }
        return handler;
    }

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 = obtainApplicationContext().getBean(handlerName);
            }
            validateHandler(handler, request);
            return buildPathExposingHandler(handler, urlPath, urlPath, null);
        }

        // Pattern match?
        List<String> matchingPatterns = new ArrayList<>();
        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 bestMatch = null;
        Comparator<String> patternComparator = getPathMatcher().getPatternComparator(urlPath);
        //直接匹配的情况处理
        if (!matchingPatterns.isEmpty()) {
            matchingPatterns.sort(patternComparator);
            if (logger.isTraceEnabled() && matchingPatterns.size() > 1) {
                logger.trace("Matching patterns " + matchingPatterns);
            }
            bestMatch = matchingPatterns.get(0);
        }
        //通配符匹配的处理
        if (bestMatch != null) {
            handler = this.handlerMap.get(bestMatch);
            if (handler == null) {
                if (bestMatch.endsWith("/")) {
                    handler = this.handlerMap.get(bestMatch.substring(0, bestMatch.length() - 1));
                }
                if (handler == null) {
                    throw new IllegalStateException(
                            "Could not find handler for best pattern match [" + bestMatch + "]");
                }
            }
            // Bean name or resolved handler?
            if (handler instanceof String) {
                String handlerName = (String) handler;
                handler = obtainApplicationContext().getBean(handlerName);
            }
            validateHandler(handler, request);
            String pathWithinMapping = getPathMatcher().extractPathWithinPattern(bestMatch, urlPath);

            // There might be multiple 'best patterns', let's make sure we have the correct URI template variables
            // for all of them
            Map<String, String> uriTemplateVariables = new LinkedHashMap<>();
            for (String matchingPattern : matchingPatterns) {
                if (patternComparator.compare(bestMatch, matchingPattern) == 0) {
                    Map<String, String> vars = getPathMatcher().extractUriTemplateVariables(matchingPattern, urlPath);
                    Map<String, String> decodedVars = getUrlPathHelper().decodePathVariables(request, vars);
                    uriTemplateVariables.putAll(decodedVars);
                }
            }
            if (logger.isTraceEnabled() && uriTemplateVariables.size() > 0) {
                logger.trace("URI variables " + uriTemplateVariables);
            }
            return buildPathExposingHandler(handler, bestMatch, pathWithinMapping, uriTemplateVariables);
        }

        // No handler found...
        return null;
    }

lookupHandler