前面我们分析了SpringMVC源码分析之策略对象初始化，那么接下来我们看一下SpringMVC的执行流程。
SpringMVC的核心流程主要是由DispatcherServlet处理的，DispatcherServlet也是springMVC的灵魂处理器。其本质是一个servlet，那么我们就按照servlet的观点去看一下。
① XXXServlet的请求确定
如下是HttpServlet的service方法，其唯一作用就是对请求和响应对象进行了类型转换。
@Override public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException { HttpServletRequest request; HttpServletResponse response; if (!(req instanceof HttpServletRequest && res instanceof HttpServletResponse)) { throw new ServletException("non-HTTP request or response"); } request = (HttpServletRequest) req; response = (HttpServletResponse) res; service(request, response); }
而FrameworkServlet的service方法覆盖了父类的 protected void service(HttpServletRequest req, HttpServletResponse resp)方法对PATCH 请求类型进行了拦截处理(因为其父类HttpServlet并没有拦截PATCH类型的请求，将会直接resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg);)。
@Override protected void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { HttpMethod httpMethod = HttpMethod.resolve(request.getMethod()); if (httpMethod == HttpMethod.PATCH || httpMethod == null) { processRequest(request, response); } else { // 这里会调用父类HttpServlet的service方法 super.service(request, response); } }
HttpServlet的service(HttpServletRequest req, HttpServletResponse resp)方法。
protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { String method = req.getMethod(); if (method.equals(METHOD_GET)) { long lastModified = getLastModified(req); if (lastModified == -1) { // servlet doesn't support if-modified-since, no reason // to go through further expensive logic doGet(req, resp); } else { long ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE); if (ifModifiedSince < lastModified) { // If the servlet mod time is later, call doGet() // Round down to the nearest second for a proper compare // A ifModifiedSince of -1 will always be less maybeSetLastModified(resp, lastModified); doGet(req, resp); } else { resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED); } } } else if (method.equals(METHOD_HEAD)) { long lastModified = getLastModified(req); maybeSetLastModified(resp, lastModified); doHead(req, resp); } else if (method.equals(METHOD_POST)) { doPost(req, resp); } else if (method.equals(METHOD_PUT)) { doPut(req, resp); } else if (method.equals(METHOD_DELETE)) { doDelete(req, resp); } else if (method.equals(METHOD_OPTIONS)) { doOptions(req,resp); } else if (method.equals(METHOD_TRACE)) { doTrace(req,resp); } else { // // Note that this means NO servlet supports whatever // method was requested, anywhere on this server. // String errMsg = lStrings.getString("http.method_not_implemented"); Object[] errArgs = new Object[1]; errArgs[0] = method; errMsg = MessageFormat.format(errMsg, errArgs); resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg); } }
基本就是根据请求类型转向不同的方法，如下所示FrameworkServlet重写了一系列doXXX方法。也就是说HttpServlet的service方法判断请求后，会转发到FrameworkServlet的doXXX处理。
而重写后doXXX方法核心本质是转发给了processRequest(request, response);。
② FrameworkServlet的processRequest
这里其实就是为当前线程绑定local、request、response等对象。
protected final void processRequest(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { long startTime = System.currentTimeMillis(); Throwable failureCause = null; // 获取当前LocaleContext LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext(); // 创建新的localeContext -new SimpleLocaleContext(request.getLocale()) LocaleContext localeContext = buildLocaleContext(request); // 获取RequestAttributes RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes(); // 如果previousAttributes 为null或者其是ServletRequestAttributes实例，则返回 new ServletRequestAttributes(request, response); // 否则返回null ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes); // 异步请求管理器 WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request); // 注册请求回调拦截器 asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor()); // 向LocaleContextHolder RequestContextHolder的ThreadLocal中 //放入 localeContext requestAttributes initContextHolders(request, localeContext, requestAttributes); try { doService(request, response); } catch (ServletException | IOException ex) { failureCause = ex; throw ex; } catch (Throwable ex) { failureCause = ex; throw new NestedServletException("Request processing failed", ex); } finally { // 恢复LocaleContext和RequestAttributes为原先的值，默认为null resetContextHolders(request, previousLocaleContext, previousAttributes); if (requestAttributes != null) { requestAttributes.requestCompleted(); } logResult(request, response, failureCause, asyncManager); // 发布事件 publishRequestHandledEvent(request, response, startTime, failureCause); } }
获取的localeContext 如下：
获取的requestAttributes 如下：
③ FrameworkServlet的doService
如下所示其doService方法是个抽象方法由子类DispatcherServlet实现
protected abstract void doService(HttpServletRequest request, HttpServletResponse response) throws Exception;
子类DispatcherServlet的doService方法。
@Override protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception { logRequest(request); // Keep a snapshot of the request attributes in case of an include, // to be able to restore the original attributes after the include. // 在include请求存在的的情况下保留请求属性的快照， //以便能够在include请求之后恢复原始属性。 Map<String, Object> attributesSnapshot = null; if (WebUtils.isIncludeRequest(request)) { attributesSnapshot = new HashMap<>(); Enumeration<?> attrNames = request.getAttributeNames(); while (attrNames.hasMoreElements()) { String attrName = (String) attrNames.nextElement(); // DEFAULT_STRATEGIES_PREFIX = org.springframework.web.servlet if (this.cleanupAfterInclude || attrName.startsWith(DEFAULT_STRATEGIES_PREFIX)) { attributesSnapshot.put(attrName, request.getAttribute(attrName)); } } } // Make framework objects available to handlers and view objects. // 设置一些核心属性对象 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()); // 闪存属性处理 if (this.flashMapManager != null) { 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); } // 解析请求路径 RequestPath requestPath = null; if (this.parseRequestPath && !ServletRequestPathUtils.hasParsedRequestPath(request)) { requestPath = ServletRequestPathUtils.parseAndCache(request); } try { // 核心分派方法 doDispatch(request, response); } finally { // 非异步请求，且attributesSnapshot != null，那么将attributesSnapshot 恢复为请求属性 if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) { // Restore the original attribute snapshot, in case of an include. if (attributesSnapshot != null) { restoreAttributesAfterInclude(request, attributesSnapshot); } } if (requestPath != null) { //request.removeAttribute(PATH_ATTRIBUTE) ServletRequestPathUtils.clearParsedRequestPath(request); } } }
最终这里走到了核心入口方法doDispatch(request, response);，这也是很多资料中分析springmvc流程的第一步。
这里首先对include请求(也就是请求包含)进行了判断， 在include请求存在的的情况下保留请求属性的快照，以便能够在include请求之后恢复原始属性。。那么什么是include请求呢？如下方法所示，即请求属性里面包含javax.servlet.include.request_uri，换而言之即并非外部浏览器或其他客户端发出的顶级请求。
public static boolean isIncludeRequest(ServletRequest request) { return (request.getAttribute(INCLUDE_REQUEST_URI_ATTRIBUTE) != null); }
然后设置了一些核心请求属性对象，比如WebApplicationContext、localeResolver、themeResolver等。
闪存属性处理，本质是针对重定向请求参数传递。
