SpringMVC源码解析从service到doDispatch

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：

<servlet>
	<servlet-name>spring-mvc</servlet-name>
	<servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
	<init-param>
		<param-name>contextConfigLocation</param-name>
		<param-value>classpath:learn-spring-mvc.xml</param-value>
	</init-param>
	<load-on-startup>1</load-on-startup>
</servlet>
<servlet-mapping>
	<servlet-name>spring-mvc</servlet-name>
	<url-pattern>/</url-pattern>
</servlet-mapping>

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<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(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

@Override
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
	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));
			}
		}
	}
	//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
  SpringMVC源码解析DispatcherServlet#doDispatch方法流程