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请求在被Servlet处理之前会先被过滤器处理，之后调用Servlet的service方法来对相应的请求进行处理响应。所以我们这里分析的入口是Servlet的service方法。

我们在用SpringMVC的时候，通常都会在web.xml中进行这样的配置：

   	
    
     spring-mvc
    	
    
     org.springframework.web.servlet.DispatcherServlet
    	
    		
     
      contextConfigLocation
     		
     
      classpath:learn-spring-mvc.xml
     	
    	
    
     1
    
   
   	
    
     spring-mvc
    	
    
     /
    
   

所有的请求(除静态资源)将由DispatcherServlet处理。

DispatcherServlet继承了FrameworkServlet，FrameworkServlet继承了HttpServletBean，HttpServletBean继承了HttpServlet，HttpServlet继承了GenericServlet，GenericServlet则实现了我们最顶级的接口Servlet和ServletConfig。

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
   
     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));				}			}		}		// Make framework objects available to handlers and view objects.		request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());		request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);		request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);		request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());		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()) {
   				// Restore the original attribute snapshot, in case of an include.				if (attributesSnapshot != null) {
   					restoreAttributesAfterInclude(request, attributesSnapshot);				}			}		}	}
    
   

从DispatcherServlet的源码中我们没有找到service(ServletRequest req, ServletResponse res)这个方法，但是我们在DispatcherServlet的父类HttpServlet中找到了这个方法，我们去HttpServlet中看看这个方法的内容：

HttpServlet#service

将ServletRequest和ServletResponse转换为HttpServletRequest和HttpServletResponse

因为web开发，用HTTP协议，所以需要HttpServletRequest和HttpServletResponse

接下来就是调用service(HttpServletRequest request, HttpServletResponse response)，

HttpServlet和FrameworkServlet中都找到了这个方法，但是HttpServlet是FrameworkServlet的父类，即FrameworkServlet中重写了service这个方法，所以我们这里取FrameworkServlet中去看看这个方法的内容：

FrameworkServlet#service

根据请求的方法类型转换对应的枚举类

HttpMethod这个定义了这样的几种枚举类型：GET, HEAD, POST, PUT, PATCH, DELETE, OPTIONS, TRACE;而这些也是RFC标准中几种请求类型。如果请求类型为PATCH或者没有找到相应的请求类型的话，则直接调用processRequest这个方法。但是这种情况我们很少很少会遇到。

所以这里会执行super.service这个方法。即调用HttpServlet中的service方法。我们看一下HttpServlet中这个service方法的内容：

HttpServlet#service

protected void service(HttpServletRequest req, HttpServletResponse resp)    throws ServletException, IOException {
   	//获取请求类型    String method = req.getMethod();	//如果是get请求    if (method.equals(METHOD_GET)) {
   		//检查是不是开启了页面缓存 通过header头的 Last-Modified/If-Modified-Since		//获取Last-Modified的值        long lastModified = getLastModified(req);        if (lastModified == -1) {
               // servlet doesn't support if-modified-since, no reason            // to go through further expensive logic			//没有开启页面缓存调用doGet方法            doGet(req, resp);        } else {
               long ifModifiedSince;            try {
   				//获取If-Modified-Since的值                ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);            } catch (IllegalArgumentException iae) {
                   ifModifiedSince = -1;            }            if (ifModifiedSince < (lastModified / 1000 * 1000)) {
                   // If the servlet mod time is later, call doGet()                // Round down to the nearest second for a proper compare                // A ifModifiedSince of -1 will always be less				//更新Last-Modified                maybeSetLastModified(resp, lastModified);				//调用doGet方法                doGet(req, resp);            } else {
   				//设置304状态码 在HttpServletResponse中定义了很多常用的状态码                resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED);            }        }    } else if (method.equals(METHOD_HEAD)) {
   		//调用doHead方法        long lastModified = getLastModified(req);        maybeSetLastModified(resp, lastModified);        doHead(req, resp);    } else if (method.equals(METHOD_POST)) {
   		//调用doPost方法        doPost(req, resp);    } else if (method.equals(METHOD_PUT)) {
   	   //调用doPost方法        doPut(req, resp);    } else if (method.equals(METHOD_DELETE)) {
   	    //调用doPost方法        doDelete(req, resp);    } else if (method.equals(METHOD_OPTIONS)) {
   		//调用doPost方法        doOptions(req,resp);    } else if (method.equals(METHOD_TRACE)) {
   		//调用doPost方法        doTrace(req,resp);    } else {
   		//服务器不支持的方法 直接返回错误信息        String errMsg = lStrings.getString("http.method_not_implemented");        Object[] errArgs = new Object[1];        errArgs[0] = method;        errMsg = MessageFormat.format(errMsg, errArgs);        resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg);    }}

根据请求类型调用响应的请求方法如果GET类型，调用doGet方法

POST类型，调用doPost方法。 这些方法都是在HttpServlet中定义的，平时我们做web开发的时候主要是继承HttpServlet这个类，然后重写它的doPost或者doGet方法。 我们的FrameworkServlet这个子类就重写了这些方法中的一部分：doGet、doPost、doPut、doDelete、doOption、doTrace。

这里我们只说我们最常用的doGet和doPost这两个方法。通过翻开源码我们发现，这两个方法体的内容是一样的，都是调用了processRequest

FrameworkServlet#processRequest

protected final void processRequest(HttpServletRequest request, HttpServletResponse response)		throws ServletException, IOException {
    	long startTime = System.currentTimeMillis();	Throwable failureCause = null;	LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();	//国际化	LocaleContext localeContext = buildLocaleContext(request);	RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();	//构建ServletRequestAttributes对象	ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);	//异步管理	WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);	asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor());    //初始化ContextHolders	initContextHolders(request, localeContext, requestAttributes);	//执行doService	try {
   		doService(request, response);	}	finally {
   		//重新设置ContextHolders		resetContextHolders(request, previousLocaleContext, previousAttributes);		if (requestAttributes != null) {
   			requestAttributes.requestCompleted();		}		//发布请求处理事件		publishRequestHandledEvent(request, response, startTime, failureCause);	}}

国际化的设置，创建ServletRequestAttributes对象，初始化上下文holders(即将Request对象放入到线程上下文中)，调用doService方法。

国际化的设置

DispatcherServlet#buildLocaleContext这个方法中完成的，其源码如下：

protected LocaleContext buildLocaleContext(final HttpServletRequest request) {
   	if (this.localeResolver instanceof LocaleContextResolver) {
   		return ((LocaleContextResolver) this.localeResolver).resolveLocaleContext(request);	}	else {
   		return new LocaleContext() {
   			@Override			public Locale getLocale() {
   				return localeResolver.resolveLocale(request);			}		};	}}

没有配置国际化解析器的话，那么它会使用默认的解析器：AcceptHeaderLocaleResolver，即从Header中获取国际化的信息。

除了AcceptHeaderLocaleResolver之外，SpringMVC中还提供了这样的几种解析器：CookieLocaleResolver、SessionLocaleResolver、FixedLocaleResolver。分别从cookie、session中去国际化信息和JVM默认的国际化信息(Local.getDefault())。

initContextHolders这个方法主要是将Request请求、ServletRequestAttribute对象和国际化对象放入到上下文中。其源码如下：

private void initContextHolders(		HttpServletRequest request, LocaleContext localeContext, RequestAttributes requestAttributes) {
   	if (localeContext != null) {
   		LocaleContextHolder.setLocaleContext(localeContext, this.threadContextInheritable);//threadContextInheritable默认为false	}	if (requestAttributes != null) {
   //threadContextInheritable默认为false		RequestContextHolder.setRequestAttributes(requestAttributes, this.threadContextInheritable);	}}

RequestContextHolder这个类有什么用呢？有时候我们想在某些类中获取HttpServletRequest对象，比如在AOP拦截的类中，那么我们就可以这样来获取Request的对象了，

HttpServletRequest request = (HttpServletRequest) RequestContextHolder.getRequestAttributes().resolveReference(RequestAttributes.REFERENCE_REQUEST);

DispatcherServlet#doService

@Overrideprotected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
   	Map
   
     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("org.springframework.web.servlet")) {
   				attributesSnapshot.put(attrName, request.getAttribute(attrName));			}		}	}	//Spring上下文	request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());	//国际化解析器	request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);	//主题解析器	request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);	//主题	request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());	//重定向的数据	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 {
   		//调用doDispatch方法-核心方法		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);			}		}	}}
    
   

处理include标签的请求，将上下文放到request的属性中，将国际化解析器放到request的属性中，将主题解析器放到request属性中，将主题放到request的属性中，处理重定向的请求数据最后调用doDispatch这个核心的方法对请求进行处理

后续流程

• 参见

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