spring mvc 的工做原理（1）DispatcherServlet 请求处理流程

类关系

DispatcherServlet 继承 FrameworkServlet 继承 HttpServletBean，HttpServletBean 是一个HttpServlet和EnvironmentCapable, EnvironmentAware接口的实现。

本章关注点在DispatcherServlet对请求的分派调用流程上，这些类暂不关注。

成员变量

一、静态变量主要是一些bean名称，以及request attribute 的key。如

/** Well-known name for the MultipartResolver object in the bean factory for this namespace. */
	public static final String MULTIPART_RESOLVER_BEAN_NAME = "multipartResolver";

	/** Well-known name for the LocaleResolver object in the bean factory for this namespace. */
	public static final String LOCALE_RESOLVER_BEAN_NAME = "localeResolver";

	/** Well-known name for the ThemeResolver object in the bean factory for this namespace. */
	public static final String THEME_RESOLVER_BEAN_NAME = "themeResolver";

二、具体的特殊的bean，上一章中提到的特殊bean。如

/** MultipartResolver used by this servlet */
	private MultipartResolver multipartResolver;

	/** LocaleResolver used by this servlet */
	private LocaleResolver localeResolver;

	/** ThemeResolver used by this servlet */
	private ThemeResolver themeResolver;

	/** List of HandlerMappings used by this servlet */
	private List<HandlerMapping> handlerMappings;

方法

方法重点关注doService，doDispatch这两个方法，其余的方法是get/set 和初始化方法，不是本章的关注对象。下面的这两个方法源码和注释。

//重写HttpServlet 的请求处理方法
	@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.
		//保留一个原请求属性的备份。
		Map<String, Object> attributesSnapshot = null;
		if (WebUtils.isIncludeRequest(request)) {
		    //是内部发出的请求 forward 
			attributesSnapshot = new HashMap<String, Object>();
			Enumeration<?> attrNames = request.getAttributeNames();
			while (attrNames.hasMoreElements()) {
				String attrName = (String) attrNames.nextElement();
				//若是是DispatcherServlet cleanupAfterInclude(是include 请求就清理)属性为true，或者request的属性是DispatcherServlet附加的，保存备份
				if (this.cleanupAfterInclude || attrName.startsWith("org.springframework.web.servlet")) {
					attributesSnapshot.put(attrName, request.getAttribute(attrName));
				}
			}
		}

		//附加webApplicationContext 的特殊bean
		// Make framework objects available to handlers and view objects.
		//web application context
		request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());
		//国际化
		request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
		//主题
		request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
		//？？？
		request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());

		//闪存，重定向时能够不携带参数
		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 {
		    //分派到handler，进行业务逻辑处理
			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 {
			    //检查是否 multipart request 
				processedRequest = checkMultipart(request);
				//request 不等于 processedRequest 表示检查到multi ，并构建返回 multi request
				multipartRequestParsed = (processedRequest != request);

				// Determine handler for the current request.
				//决定、判断、获取 handler
				mappedHandler = getHandler(processedRequest);
				if (mappedHandler == null || mappedHandler.getHandler() == null) {
				    //找不到相应的handler ，返回并提示 noHandlerFound
					noHandlerFound(processedRequest, response);
					return;
				}

				// Determine handler adapter for the current request.
				//查找handler 的适配器（不一样的方法细节多样、参数等，经过adapter 封装）
				HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

				// Process last-modified header, if supported by the handler.
				//判断是否支持last-modified
				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;
					}
				}

				//执行前拦截器
				if (!mappedHandler.applyPreHandle(processedRequest, response)) {
					return;
				}

				// Actually invoke the handler.
				//调用实际业务方法逻辑
				mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

				if (asyncManager.isConcurrentHandlingStarted()) {
					return;
				}
				//解析modelview 中的名称映射对应的视图模板。
				applyDefaultViewName(processedRequest, mv);
				//执行后拦截器
				mappedHandler.applyPostHandle(processedRequest, response, mv);
			}
			catch (Exception ex) {
				dispatchException = ex;
			}
			//处理执行的结果，异常的话，跳转处理异常的代码。
			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);
				}
			}
		}
	}

注释不正确的地方，请指出。

从源码咱们能够看出，基本上请求的出来流程正如第一篇中所描述的。

第一步是绑定各类各样的web context bean。

第二步是搜索获取对应请求的处理handler，以及对应的adapter。

第三步是执行找出的handler，前拦截器，handler， 后拦截器。

第四步是将结果或异常进行处理，如将string 映射到具体的view 或者输出json等。捕获异常的调用Exception handler 处理。