Spring MVC原理（二）请求处理流程源码分析

「爱情、让人受尽委屈。」 2022-07-12 02:55 211阅读 0赞

请求的大概处理流程请参考这篇文章，这里摘来一张图。  
[http://www.jianshu.com/p/85a30095245e][http_www.jianshu.com_p_85a30095245e]

![这里写图片描述][SouthEast]

DispatcherServlet作为流程控制的中心，整个请求的处理都是围绕这个servlet来进行。由名称可以看出这是一个servlet，具备普通servlet的所有的性质，既然是servlet肯定要在web.xml中配置，具体配置参考上面的文章。下面直接分析源码：

一个请求过来，只要是符合DispatcherServlet的拦截规则，web容器就会调用其doGet/doPost方法。

DispatcherServlet的继承体系和处理时序如下图所示：  
![这里写图片描述][SouthEast 1]

DispatcherServlet的doGet/doPost方法的实现在其父类FrameworkServlet里面，

/** * Delegate GET requests to processRequest/doService. * <p>Will also be invoked by HttpServlet's default implementation of {@code doHead}, * with a {@code NoBodyResponse} that just captures the content length. * @see #doService * @see #doHead */
        @Override
        protected final void doGet(HttpServletRequest request, HttpServletResponse response)
                throws ServletException, IOException {
    
            processRequest(request, response);
        }
    
        /** * Delegate POST requests to {@link #processRequest}. * @see #doService */
        @Override
        protected final void doPost(HttpServletRequest request, HttpServletResponse response)
                throws ServletException, IOException {
    
            processRequest(request, response);
        }

这里以Get方法为例分析整个处理过程：  
(源码中省略了一些注释或者不重要的片段)

protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
                throws ServletException, IOException {
    
            long startTime = System.currentTimeMillis();
            Throwable failureCause = null;
    
            LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
            LocaleContext localeContext = buildLocaleContext(request);
    
            RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
            ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);
    
            WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
            asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor());
    
            initContextHolders(request, localeContext, requestAttributes);
    
            try {
                //doService的实现在DispatchServlet中
                doService(request, response);
            }
            catch (ServletException ex) {
                failureCause = ex;
                throw ex;
            }
            catch (IOException ex) {
                failureCause = ex;
                throw ex;
            }
            catch (Throwable ex) {
                failureCause = ex;
                throw new NestedServletException("Request processing failed", ex);
            }
    
            finally {
                resetContextHolders(request, previousLocaleContext, previousAttributes);
                if (requestAttributes != null) {
                    requestAttributes.requestCompleted();
                }
    
                publishRequestHandledEvent(request, response, startTime, failureCause);
            }
        }

protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
            // 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));
                    }
                }
            }
    
            // Make framework objects available to handlers and view objects.
            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 {
                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);
                    }
                }
            }
        }

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.
                    // 根据当前请求查找合适的处理器Handler
                    mappedHandler = getHandler(processedRequest);
                    if (mappedHandler == null || mappedHandler.getHandler() == null) {
                        noHandlerFound(processedRequest, response);
                        return;
                    }
    
                    // 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;
                        }
                    }
    
                    if (!mappedHandler.applyPreHandle(processedRequest, response)) {
                        return;
                    }
    
                    // Actually invoke the handler.
                    mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
    
                    if (asyncManager.isConcurrentHandlingStarted()) {
                        return;
                    }
    
                    applyDefaultViewName(request, 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 afterCompletion
                    if (mappedHandler != null) {
                        mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
                    }
                }
                else {
                    // Clean up any resources used by a multipart request.
                    if (multipartRequestParsed) {
                        cleanupMultipart(processedRequest);
                    }
                }
            }
        }

protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
            /* * HandlerMapping的作用是找到URL匹配的Handler对象，这里注册的HandlerMapping（handlerMappings中的元素）有三个： * 1)BeanNameUrlHandlerMapping: bean的name和url匹配 * 要求任何有可能处理请求的Bean都要起一个以反斜杠(/)字符开头的名称或者别名，这个名称或者别名可以是符合URL Path匹配原则中的任何名字。 * 它不能影射一个Prototype的Bean.换句话说，当使用 BeanNameUrlHandlerMapping时，所有的请求处理类只能是单例的(singletons) . 一般来 * 说,Controllers 都是按照单例建立的，所以这个并不是一个很严重的问题。 * 2)SimpleUrlHandlerMapping :直接匹配url和对应的bean（在mvc配置文件中配置，url和bean一一对应） * 由于BeanNameUrlHandlerMapping机制中URL和Handler是绑定在一起的，所以无法配置更为复杂的Mapping关系，SimpleUrlHandlerMapping * 可以看做是其的高级版，能够配置复杂的路径匹配关系，可以配置多组映射关系，把不同的URL映射到同一个handler对象。 * 3)RequestMappingHandlerMapping:专门处理注解@RequestMapping配置的类,和RequestMappingHandlerAdapter 对应起来使用 */
            for (HandlerMapping hm : this.handlerMappings) {
                 /* * 遍历所有的HandlerMapping，找到能够处理当前request的一个 */
                HandlerExecutionChain handler = hm.getHandler(request);
                if (handler != null) {
                    return handler;
                }
            }
            return null;
        }

public final HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
            /* * */
            Object handler = getHandlerInternal(request);
            if (handler == null) { handler = getDefaultHandler();
            }
            if (handler == null) { return null;
            }
            // Bean name or resolved handler? if (handler instanceof String) { String handlerName = (String) handler;
                handler = getApplicationContext().getBean(handlerName);
            }
            return getHandlerExecutionChain(handler, request);
        }

protected HandlerMethod getHandlerInternal(HttpServletRequest request) throws Exception {
            //拿到请求的url
            String lookupPath = getUrlPathHelper().getLookupPathForRequest(request);
            if (logger.isDebugEnabled()) {
                logger.debug("Looking up handler method for path " + lookupPath);
            }
            //根据请求的路径URL找到对应的处理方法（controller里的方法）
            HandlerMethod handlerMethod = lookupHandlerMethod(lookupPath, request);
            if (logger.isDebugEnabled()) {
                if (handlerMethod != null) {
                    logger.debug("Returning handler method [" + handlerMethod + "]");
                }
                else {
                    logger.debug("Did not find handler method for [" + lookupPath + "]");
                }
            }
            return (handlerMethod != null ? handlerMethod.createWithResolvedBean() : null);
        }

protected HandlerMethod lookupHandlerMethod(String lookupPath, HttpServletRequest request) throws Exception {
            List<Match> matches = new ArrayList<Match>();
            /* * this.urlMap放的是当前这个HandlerMapping对象类型处理的URL和handler * 在Spring容器启动阶段，就已经把mvc配置文件或者controller类注解的映射关系添加到了 * urlMap里面，以匹配的URL作为键值。 */
            List<T> directPathMatches = this.urlMap.get(lookupPath);
            if (directPathMatches != null) {
                addMatchingMappings(directPathMatches, matches, request);
            }
            if (matches.isEmpty()) {
                // No choice but to go through all mappings...
                addMatchingMappings(this.handlerMethods.keySet(), matches, request);
            }
    
            if (!matches.isEmpty()) {
                Comparator<Match> comparator = new MatchComparator(getMappingComparator(request));
                Collections.sort(matches, comparator);
                Match bestMatch = matches.get(0);
                if (matches.size() > 1) {
                    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 + "}");
                    }
                }
                handleMatch(bestMatch.mapping, lookupPath, request);
                return bestMatch.handlerMethod;
            }
            else {
                return handleNoMatch(handlerMethods.keySet(), lookupPath, request);
            }
        }

protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {
             /* * HandlerAdapter的作用是确定Handler对象中的哪个方法来处理当前URL，这里注册的HandlerAdapter有三种： * 1)HttpRequestHandlerAdapter：要求handler（controller）实现HttpRequestHandler接口 * 2)SimpleControllerHandlerAdapter：要求handler实现Controller接口 * 3)RequestMappingHandlerAdapter：和RequestMappingHandlerMapping配对使用，专门处理@RequestMapping 注解的类和方法 */
            for (HandlerAdapter ha : this.handlerAdapters) {
                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");
        }

[http_www.jianshu.com_p_85a30095245e]: http://www.jianshu.com/p/85a30095245e
[SouthEast]: /images/20220711/fc2e83e0819140b6a59614754ef4e3cc.png
[SouthEast 1]: /images/20220711/b78c8250ff87473dab52447925449a23.png