SpringMVC流程源码分析及DispatcherServlet核心源码

一、源码分析前还是需要一张流程图作为指导，如下：

二、简单介绍以及源码定位

DispatcherServlet其实就是一个HttpServlet，他是HttpServlet的子类，所以它和普通的HttpServlet在web.xml里同样的配置。

这个Servlet的doPost和doGet方法的实现是DispatcherServlet的父类FrameworkServlet中实现的，两个方法里都是调用processRequest方法。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。

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。

[java] view plain copy

//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()方法源码如下：

/**

	 * Process the actual dispatching to the handler.

	 * <p>The handler will be obtained by applying the servlet's HandlerMappings in order.

	 * The HandlerAdapter will be obtained by querying the servlet's installed HandlerAdapters

	 * to find the first that supports the handler class.

	 * <p>All HTTP methods are handled by this method. It's up to HandlerAdapters or handlers

	 * themselves to decide which methods are acceptable.

	 * @param request current HTTP request

	 * @param response current HTTP response

	 * @throws Exception in case of any kind of processing failure

	 */

	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 {

				processedRequest = checkMultipart(request);

				multipartRequestParsed = (processedRequest != request);

				// Determine handler for the current request.

				mappedHandler = getHandler(processedRequest);

				if (mappedHandler == null || mappedHandler.getHandler() == null) {

					noHandlerFound(processedRequest, response);

					return;

				}

				// Determine handler adapter for the current request.

				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)) {

					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;

					}

				}

				if (!mappedHandler.applyPreHandle(processedRequest, response)) {

					return;

				}

				// Actually invoke the handler.

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

				if (asyncManager.isConcurrentHandlingStarted()) {

					return;

				}

				applyDefaultViewName(processedRequest, mv);

				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 {

			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);

				}

			}

		}

	}

三、通过阅读源码总结分析流程如下：

1. web应用服务器接收到一个新请求是，读取web.xml中的配置，如果匹配DispatcherServlet的请求映射路径，web容器将该请求转发给DispatcherServlet进行处理

2. DispatcherServlet接收到请求后，执行doDispatch方法，此方法遍历DispatcherServlet中的HandlerMapping（处理器映射器）集合，根据请求的HttpServletRequest信息通过HandlerMapping对象方法找到HandlerExecutionChain(执行链，内含拦截器和处理器)。

3. DispatcherServlet继续执行doDispatch方法，根据得到的HandlerExecutionChain(执行链)中的handler遍历自己的HandlerAdapter(处理器适配器)集合，找到支持这个Handler的HandlerAdapter并返回。

4.继续doDispatch方法，把HandlerExecutionChain(执行链)内部的那些前置拦截器逻辑都执行完，然后再再通过得到的HandlerAdapter执行HandlerExecutionChain内部的处理器，会返回一个ModelAndView包含了视图逻辑名和模型数据信息

5.调用执行链的方法，执行拦截器的后置拦截器

6. ModelAndView中包含的是“逻辑视图名”，而非真正的视图对象，DispatcherServlet借助ViewResolver完成逻辑视图名到真实视图名对象的解析工作

7. 当得到真实的视图对象View后，DispatcherServlet就使用这个View对象对ModelAndView中的模型数据进行视图渲染

8. 最终客户端得到HTML页面什么的

巴特西

SpringMVC流程源码分析及DispatcherServlet核心源码

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