SpringMVC解析4-DispatcherServlet逻辑脉络
HttpServlet提供了不同的服务方法，它们是doDelete(),doGet(),doOptions(),doPost(),doPut(),和doTrace(),它会根据不同的请求形式将程序引导至对应的函数进行处理。这几个函数中最常用的函数无非就是doGet()和doPost(),我们看看DispatcherServlet对着两个函数的逻辑实现。
@Override protected final void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { processRequest(request, response); } @Override protected final void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { processRequest(request, response); } protected final void processRequest(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { //记录当前时间，用于计算web请求的处理时间 long startTime = System.currentTimeMillis(); Throwable failureCause = null; //根据当前request创建对应的LocaleContext和RequestAttributes,并绑定到当前线程 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 { //委托给doService方法进一步处理 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 { //请求处理结束后回复线程到原始状态 resetContextHolders(request, previousLocaleContext, previousAttributes); if (requestAttributes != null) { requestAttributes.requestCompleted(); } if (logger.isDebugEnabled()) { if (failureCause != null) { this.logger.debug("Could not complete request", failureCause); } else { if (asyncManager.isConcurrentHandlingStarted()) { logger.debug("Leaving response open for concurrent processing"); } else { this.logger.debug("Successfully completed request"); } } } //请求处理结束后无论成功与否发布事件通知 publishRequestHandledEvent(request, startTime, failureCause); } }
函数中已经开始了对请求的处理，虽然把细节转移到了doService函数中实现，但也看得出处理请求前后所做的准备工作。
（1）为了保证当前线程的LocaleContext以及RequestAttributes可以在当前请求后还能恢复，提取当前线程的两个属性。
（2）根据当前request创建对应的LocaleContext和RequestAttributes，并绑定到当前线程。
（3）委托给doService方法进一步处理。
（4）请求处理结束后恢复线程到原始状态。
（5）请求处理结束后无论成功与否发布事件通知
继续查看doService方法。
@Override 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)); } } } // 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); } } } }
我们猜想对请求处理至少应该包括一些诸如寻找handler并页面跳转之类的逻辑处理，但是，在doService中我们并没有看到想看到的逻辑，相反却同样是一些准备工作，但是这些准备工作是必不可少的。Spring将已经初始化的功能辅助工具变量，比如localeResolver，themeResolver等设置在request属性中，而这些属性会在接下来派上用场。doDispatch函数中展示了Spring请求处理所涉及的主要逻辑，而我们之前设置在request中的各种辅助属性也都有被派上了用场。
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 { //如果是MultipartContent类型的request则转换request为MultipartHttpServletRequest类型的request processedRequest = checkMultipart(request); multipartRequestParsed = (processedRequest != request); // Determine handler for the current request. //根据request信息寻找对应的handler mappedHandler = getHandler(processedRequest, false); if (mappedHandler == null || mappedHandler.getHandler() == null) { //如果没有找到对应的handler则通过response反馈错误信息 noHandlerFound(processedRequest, response); return; } // Determine handler adapter for the current request. //根据当前的handler寻找对应的handlerAdapter HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler()); // Process last-modified header, if supported by the handler. String method = request.getMethod(); //如果handler支持last-modified头处理 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; } } //拦截器的preHandler方法的调用 if (!mappedHandler.applyPreHandle(processedRequest, response)) { return; } // Actually invoke the handler. //真正的激活handler并返回视图 mv = ha.handle(processedRequest, response, mappedHandler.getHandler()); if (asyncManager.isConcurrentHandlingStarted()) { return; } //视图名称转换应用于需要添加前缀后缀的情况 applyDefaultViewName(request, mv); //调用所有拦截器的postHandle方法 mappedHandler.applyPostHandle(processedRequest, response, mv); } catch (Exception ex) { dispatchException = ex; } //页面处理 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); } } } } private void processDispatchResult(HttpServletRequest request, HttpServletResponse response, HandlerExecutionChain mappedHandler, ModelAndView mv, Exception exception) throws Exception { boolean errorView = false; if (exception != null) { if (exception instanceof ModelAndViewDefiningException) { logger.debug("ModelAndViewDefiningException encountered", exception); mv = ((ModelAndViewDefiningException) exception).getModelAndView(); } else { Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null); mv = processHandlerException(request, response, handler, exception); errorView = (mv != null); } } // Did the handler return a view to render? if (mv != null && !mv.wasCleared()) { //处理页面跳转 render(mv, request, response); if (errorView) { WebUtils.clearErrorRequestAttributes(request); } } else { if (logger.isDebugEnabled()) { logger.debug("Null ModelAndView returned to DispatcherServlet with name '" + getServletName() + "': assuming HandlerAdapter completed request handling"); } } if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) { // Concurrent handling started during a forward return; } if (mappedHandler != null) { //完成激活处理触发器 mappedHandler.triggerAfterCompletion(request, response, null); } }
Spring每一步是怎么处理dispatcher中的方法的，在下面一节分析。
