摘要：概述根據(jù)規(guī)范，當(dāng)用戶請求到達(dá)應(yīng)用時(shí)，由的方法進(jìn)行處理，對于而言，處理用戶請求的入口為，通過其父類的攔截處理。最后創(chuàng)建了群方便大家交流，可掃描加入，同時(shí)也可加我，共同學(xué)習(xí)共同進(jìn)步，謝謝

根據(jù)Servlet規(guī)范，當(dāng)用戶請求到達(dá)應(yīng)用時(shí)，由Servlet的service()方法進(jìn)行處理，對于Spring MVC而言，處理用戶請求的入口為DispatcherServlet，通過其父類FrameworkServlet的service()攔截處理。

用戶請求的入口方法為service(request,response)，源碼如下：

/**
 * Override the parent class implementation in order to intercept PATCH requests.
 */
@Override
protected void service(HttpServletRequest request, HttpServletResponse response)
        throws ServletException, IOException {
    HttpMethod httpMethod = HttpMethod.resolve(request.getMethod());
    // Spring3.0增加了PATCH方法攔截處理
    if (httpMethod == HttpMethod.PATCH || httpMethod == null) {
        processRequest(request, response);
    } else { // GET, HEAD, POST, PUT, DELETE, OPTIONS, TRACE
        super.service(request, response);
    }
}

該方法重寫了HttpServlet的service()方法，并增加了PATCH請求的攔截處理，其他諸如GET、POST、PUT等請求則調(diào)用父類的service方法，HttpServlet類的service()方法就是根據(jù)請求類型進(jìn)行不同的路由處理，如為GET請求，則路由至doGet()方法處理，以此類推，此處省略源碼。

假如用戶請求為GET，則路由到子類（FrameworkServlet）的doGet()方法進(jìn)行處理，doGet()源碼如下：

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

其實(shí)，所有類型的請求最終都由processRequest()方法進(jìn)行處理，processRequest方法源碼如下：

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

    long startTime = System.currentTimeMillis();
    Throwable failureCause = null;
    // 
    LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
    // 構(gòu)建請求(request)的LocalContext上下文，提供基本的作為當(dāng)前地區(qū)的主要語言環(huán)境
    LocaleContext localeContext = buildLocaleContext(request);

    RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
    // 構(gòu)建請求(request)的ServletRequestAttributes對象，保存本次請求的request和response
    ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);
    // 異步請求處理的管理核心類
    WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
    asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor());
    // 綁定request context上線到當(dāng)前線程
    initContextHolders(request, localeContext, requestAttributes);

    try {
        // 模板方法，調(diào)用子類（DispatcherServlet）的doService方法進(jìn)行處理
        doService(request, response);
    }
    catch (ServletException | IOException ex) {
        failureCause = ex;
        throw ex;
    }
    catch (Throwable ex) {
        failureCause = ex;
        throw new NestedServletException("Request processing failed", ex);
    }

    finally {
        // 接觸請求線程與LocalContext和RequestAttributes的綁定
        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");
                }
            }
        }
        // 發(fā)布ApplicationEvent事件，可由ApplicationListener進(jìn)行監(jiān)聽
        // 繼承ApplicationListener接口，實(shí)現(xiàn)onApplicationEvent()接口，并注冊到spring容器，即可捕獲該事件
        publishRequestHandledEvent(request, response, startTime, failureCause);
    }
}

@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) + "]");
    }

    // 如果是include請求，保存request attribute快照數(shù)據(jù)，并在finally中進(jìn)行還原
    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));
            }
        }
    }

    // 把環(huán)境上下文設(shè)置到請求域中
    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);
    }

    try {
        // 調(diào)用請求處理方法
        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);
            }
        }
    }
}

// 請求處理關(guān)鍵方法
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) {
                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;
        }
        catch (Throwable err) {
            // As of 4.3, we"re processing Errors thrown from handler methods as well,
            // making them available for @ExceptionHandler methods and other scenarios.
            dispatchException = new NestedServletException("Handler dispatch failed", err);
        }
        processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
    }
    catch (Exception ex) {
        triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
    }
    catch (Throwable err) {
        triggerAfterCompletion(processedRequest, response, mappedHandler,
                new NestedServletException("Handler processing failed", 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);
            }
        }
    }
}

doDispatch()是Spring MVC中重要的方法，用戶處理用戶請求，其主要經(jīng)過過程如下：

獲取當(dāng)前請求的Handler

獲取當(dāng)前請求的Handler Adapter

執(zhí)行preHandle方法

執(zhí)行Handle方法，即Controller中的方法

執(zhí)行postHandle方法

處理返回結(jié)果

本文簡要概述了Spring MVC處理用戶請求過程中所涉及的主要方法，對請求處理的流程做了概述，后續(xù)將對內(nèi)部細(xì)節(jié)進(jìn)行分析，以加深理解。

最后創(chuàng)建了qq群方便大家交流，可掃描加入，同時(shí)也可加我qq：276420284，共同學(xué)習(xí)、共同進(jìn)步，謝謝！