springMVC源码分析--DispatcherServlet请求获取及处理

在之前的博客springMVC源码分析--容器初始化（二）DispatcherServlet中我们介绍过DispatcherServlet，是在容器初始化过程中出现的，我们之前也说过DispatcherServlet其实就是一个HttpServlet，其实他是HttpServlet的子类，所以它和普通的HttpServlet有同样的配置：

	<servlet>
        <servlet-name>springmvc</servlet-name>
        <servlet-class>
            org.springframework.web.servlet.DispatcherServlet
        </servlet-class>
	    <init-param>
			<param-name>contextConfigLocation</param-name>
			<param-value>classpath:springmvc-config.xml</param-value>
		</init-param>
        <load-on-startup>1</load-on-startup>
	</servlet>

    <servlet-mapping>
        <servlet-name>springmvc</servlet-name>
        <url-pattern>*.action</url-pattern>
    </servlet-mapping>

仅仅把DispatcherServlet当做一个Servlet的话，上面配置的含义就是这个Servlet会被所有的*.action的请求所调用。

既然DispatcherServlet是一个HttpServlet那么它应该会实现HttpServlet提供的如下方法：

当然这些所有的方法的实现是DispatcherServlet的父类FrameworkServlet中实现的。

当然这些实现方法中的默认实现其实是如下的

FrameworkServlet类中

@Override
	protected final void doPost(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {

		processRequest(request, response);
	}

@Override
	protected final void doGet(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {

		processRequest(request, response);
	}

processRequest的实现是在FrameworkServlet中，此方法中最主要的操作就是调用doService方法

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(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();
			}

			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");
					}
				}
			}

			publishRequestHandledEvent(request, response, startTime, failureCause);
		}
	}

doService方法的最终实现是在DispatcherServlet中，这样所有的Http请求（GET、POST、PUT和DELETE等）的最终操作就DispatcherServlet中实现了。

DispatcherServlet中doService的实现如下，对Request设置了一些全局属性，最终接下来的操作是在doDispatcher函数中实现了。

//获取请求，设置一些request的参数，然后分发给doDispatch
	@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.
		/* 设置web应用上下文**/
		request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());
		/* 国际化本地**/
		request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
		/* 样式**/
		request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
		//设置样式资源
		request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());
		//请求刷新时保存属性
		FlashMap inputFlashMap = this.flashMapManager.retrieveAndUpdate(request, response);
		if (inputFlashMap != null) {
			request.setAttribute(INPUT_FLASH_MAP_ATTRIBUTE, Collections.unmodifiableMap(inputFlashMap));
		}
		//Flash attributes 在对请求的重定向生效之前被临时存储（通常是在session)中，并且在重定向之后被立即移除
		request.setAttribute(OUTPUT_FLASH_MAP_ATTRIBUTE, new FlashMap());
		//FlashMap 被用来管理 flash attributes 而 FlashMapManager 则被用来存储，获取和管理 FlashMap 实体.
		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);
				}
			}
		}
	}

doDispatch函数中完成了对一个请求的所有操作，包含的内容还是比较多的，我们就不做详细分解，接下来我们会一步一步的分析一个请求调用Controller的完整过程。

/**
	 *将Handler进行分发，handler会被handlerMapping有序的获得
	 *通过查询servlet安装的HandlerAdapters来获得HandlerAdapters来查找第一个支持handler的类
	 *所有的HTTP的方法都会被这个方法掌控。取决于HandlerAdapters 或者handlers 他们自己去决定哪些方法是可用
	 *@param request current HTTP request
	 *@param response current HTTP response
	 */
	protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
		/* 当前HTTP请求**/
		HttpServletRequest processedRequest = request;
		HandlerExecutionChain mappedHandler = null;
		boolean multipartRequestParsed = false;

		WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);

		try {
			ModelAndView mv = null;
			Exception dispatchException = null;

			try {
				//判断是否有文件上传
				processedRequest = checkMultipart(request);
				multipartRequestParsed = (processedRequest != request);

				// 获得HandlerExecutionChain，其包含HandlerIntercrptor和HandlerMethod
				mappedHandler = getHandler(processedRequest);
				if (mappedHandler == null || mappedHandler.getHandler() == null) {
					noHandlerFound(processedRequest, response);
					return;
				}

				//获得HandlerAdapter
				HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

				//获得HTTP请求方法
				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;
					}
				}
				//如果有拦截器的话，会执行拦截器的preHandler方法
				if (!mappedHandler.applyPreHandle(processedRequest, response)) {
					return;
				}

				//返回ModelAndView
				mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

				if (asyncManager.isConcurrentHandlingStarted()) {
					return;
				}
				//当view为空时，，根据request设置默认view
				applyDefaultViewName(processedRequest, mv);
				//执行拦截器的postHandle
				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 {
			//判断是否是异步请求
			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.
				//删除上传资源
				if (multipartRequestParsed) {
					cleanupMultipart(processedRequest);
				}
			}
		}
	}

调用完doDispatch之后就完成了一个请求的访问，其会将渲染后的页面或者数据返回给请求发起者。