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

曾经终败给现在 2022-07-14 11:59 231阅读 0赞

在之前的博客[ springMVC源码分析--容器初始化（二）DispatcherServlet][springMVC_--_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：

![Image 1][]

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

![Image 1][]

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。

[springMVC_--_DispatcherServlet]: http://blog.csdn.net/qq924862077/article/details/52809312
[Image 1]: