【Spring MVC】MVC如何浏览器请求（service方法）

背景：平时我们学习 MVC 重点关注的时DispatcherServlet 的 doDispatcher 方法，但是在 doDispatcher 方法之前 还有请求处理的前置过程，这个过程作为一个高级程序员是必须要了解的。

1. DispatcherServlet 的 service 方法

DispatcherServlet�的继承关系如下图：

因为是 Servlet，自然看它的 service 方法

    public void service(ServletRequest req, ServletResponse res)
        throws ServletException, IOException
    {
        HttpServletRequest  request;
        HttpServletResponse response;
        
        if (!(req instanceof HttpServletRequest &&
                res instanceof HttpServletResponse)) {
            throw new ServletException("non-HTTP request or response");
        }

        request = (HttpServletRequest) req;
        response = (HttpServletResponse) res;

        service(request, response);
    }

为了处理 patch 请求

	protected void service(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {

		HttpMethod httpMethod = HttpMethod.resolve(request.getMethod());
		if (httpMethod == HttpMethod.PATCH || httpMethod == null) {
			processRequest(request, response);
		}
		else {
			super.service(request, response);
		}
	}

各种请求类型分发：

    protected void service(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException
    {
        String method = req.getMethod();

        if (method.equals(METHOD_GET)) {
            long lastModified = getLastModified(req);
            if (lastModified == -1) {
                // servlet doesn't support if-modified-since, no reason
                // to go through further expensive logic
                doGet(req, resp);
            } else {
                long ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);
                if (ifModifiedSince < lastModified) {
                    // 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
                    maybeSetLastModified(resp, lastModified);
                    doGet(req, resp);
                } else {
                    resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
                }
            }

        } else if (method.equals(METHOD_HEAD)) {
            long lastModified = getLastModified(req);
            maybeSetLastModified(resp, lastModified);
            doHead(req, resp);

        } else if (method.equals(METHOD_POST)) {
            doPost(req, resp);
            
        } else if (method.equals(METHOD_PUT)) {
            doPut(req, resp);
            
        } else if (method.equals(METHOD_DELETE)) {
            doDelete(req, resp);
            
        } else if (method.equals(METHOD_OPTIONS)) {
            doOptions(req,resp);
            
        } else if (method.equals(METHOD_TRACE)) {
            doTrace(req,resp);
            
        } else {
            //
            // Note that this means NO servlet supports whatever
            // method was requested, anywhere on this server.
            //

            String errMsg = lStrings.getString("http.method_not_implemented");
            Object[] errArgs = new Object[1];
        rArgs[0] = method;
            errMsg = MessageFormat.format(errMsg, errArgs);
            
            resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg);
        }
    }

各种 doXxx 方法会流转到，以下以 doGet 为例子，其它类似。特殊的请求方法有特殊处理，我们不用管。

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

		processRequest(request, response);
	}

继续看 processRequest 方法

1.1. processRequest 方法

	protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {

		long startTime = System.currentTimeMillis();
		Throwable failureCause = null;

        // <1> 
		LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
		LocaleContext localeContext = buildLocaleContext(request);

        // <2> 
		RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
		ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);

        // <3> 
		WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
		asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor());


        // <4> 
		initContextHolders(request, localeContext, requestAttributes);

		try {
            // <5> 
			doService(request, response);
		}
		catch (ServletException | IOException ex) {
			failureCause = ex;
			throw ex;
		}
		catch (Throwable ex) {
			failureCause = ex;
			throw new NestedServletException("Request processing failed", ex);
		}
            
    	// <6> 
		finally {
			resetContextHolders(request, previousLocaleContext, previousAttributes);
			if (requestAttributes != null) {
				requestAttributes.requestCompleted();
			}
			logResult(request, response, failureCause, asyncManager);
			publishRequestHandledEvent(request, response, startTime, failureCause);
		}
	}

• 在<1>处，获取之前的语言上下文。然后构建本次的语言上下文。LocaleContext

语言上下文是可以继承的。把之前的保存下来，在请求处理完之后要“还原”。

• 在<2>处，获取之前的“请求属性上下文”RequestAttributes，具体实现类是：ServletRequestAttributes��

这个上下文不能继承，必须是当次请求的。

• 在<3>处，获取“Web 的异步管理器”

此处与异步请求相关：RequestBindingInterceptor�

• 在<4>处，绑定 2 个上下文。把 LocaleContext� 绑定到LocaleContextHolder�中，把 ServletRequestAttributes� 绑定到RequestContextHolder�中

这个上下文非常有用，Web 的很多工具类使用到这个上下文来简化编程。不需要传参数request，直接在任意位置获取request。

• 在<5>处，具体如何处理请求。后续会详细分析
• 在<6>处，清尾操作。

1、把之前的2个上下文还原。
2、记录请求的日志（此处是否就是框架提供的打印响应日志的地方？）
3、发布事件（请求处理完成事件ServletRequestHandledEvent�）

1.2. doService 方法

	protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
		logRequest(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<>();
			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());

		if (this.flashMapManager != null) {
			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);
		}

		RequestPath previousRequestPath = null;
		if (this.parseRequestPath) {
			previousRequestPath = (RequestPath) request.getAttribute(ServletRequestPathUtils.PATH_ATTRIBUTE);
			ServletRequestPathUtils.parseAndCache(request);
		}

		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);
				}
			}
			if (this.parseRequestPath) {
				ServletRequestPathUtils.setParsedRequestPath(previousRequestPath, request);
			}
		}
	}

不详细解释了，主要是把 Spring 框架的一些组件放入到request�的请求属性中。在后续可以根据 request 拿出 Spring 框架的组件。比如设置了如下的内容：

• Web 的上下文
• 本地语言解析器
• 主题解析器
• …