SpringMVC源码笔记(三) DispatcherServlet.service

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概述

DispatcherServlet的真正请求处理，也就是Servlet的service()方法逻辑

// org.springframework.web.servlet.FrameworkServlet#processRequest
protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {

		long startTime = System.currentTimeMillis();
		Throwable failureCause = null;
        
        // 备份当前线程的国际化Context，再为当前请求重新创建一个
		LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
		LocaleContext localeContext = buildLocaleContext(request);

        // 备份当前线程的RequestAttributes，再为当前请求创建一个
		RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
		ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);

        // 异步请求管理 TODO
		WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
		asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor());

        // 将新创建的LocaleContext  和RequestAttributes 绑定到线程变量
		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doService前的上下文对象，发布ServletRequestHandledEvent事件
			resetContextHolders(request, previousLocaleContext, previousAttributes);
			if (requestAttributes != null) {
				requestAttributes.requestCompleted();
			}
			logResult(request, response, failureCause, asyncManager);
			publishRequestHandledEvent(request, response, startTime, failureCause);
		}
	}

LocaleContextHolder用于做本地化、国际化的上下文容器

RequestContextHolder持有当前request上下文的容器

两个Holder内部都是使用ThreadLocal变量存放与当前请求相关的属性、对象等，例如我们在业务处理程序中可以如下获取当前的HttpServletRequest对象，而不用一层一层传参数进来

HttpServletRequest request = ((ServletRequestAttributes) RequestContextHolder.getRequestAttributes()).getRequest();

doService

设置请求request属性，调用doDispatch

	protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
		logRequest(request);

		// 如果是include的请求（RequestDispatcher.include），备份attributes
		Map<String, Object> attributesSnapshot = null;
		if (WebUtils.isIncludeRequest(request)) {
			attributesSnapshot = new HashMap<>();
			Enumeration<?> attrNames = request.getAttributeNames();
			while (attrNames.hasMoreElements()) {
				String attrName = (String) attrNames.nextElement();
				if (this.cleanupAfterInclude || attrName.startsWith(DEFAULT_STRATEGIES_PREFIX)) {
					attributesSnapshot.put(attrName, request.getAttribute(attrName));
				}
			}
		}

		// 对request设置一些属性
		request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());// 容器对象
		request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
		request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
		request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());

        // 重定向请求设置FlashMap
		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);
		}

		try {
			doDispatch(request, response);
		}
		finally {
			if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
				// 如果是include请求，进行还原
				if (attributesSnapshot != null) {
					restoreAttributesAfterInclude(request, attributesSnapshot);
				}
			}
		}
	}

doDispatch

1、解析文件请求

2、获取请求对应的handler

3、获取handler对应的适配器adapter

4、执行handler拦截器preHandle

5、适配器调用handler方法执行逻辑处理，返回mav

6、执行handler拦截器postHandle

7、处理完成渲染View、执行拦截器afterCompletion；如果是异常结束那就是调用异常解析器处理异常

	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 {
                // 判断是否是文件请求，如果是解析请求返回MultipartHttpServletRequest
				processedRequest = checkMultipart(request);
				multipartRequestParsed = (processedRequest != request);

				// 从所有handlerMappings中获取请求对应的处理器handler
				mappedHandler = getHandler(processedRequest);
				if (mappedHandler == null) {
					noHandlerFound(processedRequest, response);
					return;
				}

				// 获取handler类型对应的处理器适配器
				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)) {
                    // LastModified缓存机制，适配器调用handler.getLastModified()的获取资源的最后更新时间
                    // 如果时间与客户端请求头的缓存时间一致，则直接返回HTTP状态码304，表示使用缓存资源
                    // 继承Controller的handler方式实现LastModified接口即可以支持这种缓存机制
                    // 而@RequestMapping注解形式的需要自己去调用webRequest.checkNotModified
					long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
					if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
						return;
					}
				}

                // 执行handler的拦截器的PreHandle方法，拦截器顺序从前到后
				if (!mappedHandler.applyPreHandle(processedRequest, response)) {
					return;
				}

				// 适配器调用handler方法，返回mav
				mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

				if (asyncManager.isConcurrentHandlingStarted()) {
					return;
				}

                // 返回的mav中不带view，给一个默认的view
				applyDefaultViewName(processedRequest, mv);

                // 执行handler拦截器的PostHandle方法，拦截器顺序从后到前
				mappedHandler.applyPostHandle(processedRequest, response, mv);
			}
			catch (Exception ex) {
				dispatchException = ex;
			}
			catch (Throwable err) {
				dispatchException = new NestedServletException("Handler dispatch failed", err);
			}

            // 调用结束，异常解析器解析异常、渲染视图、执行拦截器AfterCompletion方法
			processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
		}
		catch (Exception ex) {
            // 执行handler拦截器的AfterCompletion方法，顺序从后到前
			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 afterCompletion
				if (mappedHandler != null) {
					mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
				}
			}
			else {
				// 如果是文件请求，清理资源
				if (multipartRequestParsed) {
					cleanupMultipart(processedRequest);
				}
			}
		}
	}