DispatcherServlet的逻辑处理（上篇）

在HttpServlet类中分别提供了相应的服务方法，它们是doDelete、doGet、doOptions、doPost、doPut和doTrace，它会根据不同的请求形式将程序引导至对应的函数处理，这几个函数中最常用的就是doGet和doPost，那么我们就直接查看DispatcherServlet中对于这两个函数的逻辑实现。

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

        processRequest(request, response);
    }

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

        processRequest(request, response);
    }

对于不同的方法，Spring并没有做特殊的处理，而是统一将程序再一次的引导至processRequest(request, response)。

protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        //记录当前时间，用于计算web请求的处理时间
        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 | 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();
            }
            logResult(request, response, failureCause, asyncManager);
            publishRequestHandledEvent(request, response, startTime, failureCause);
        }
    }

上述函数已经开始了对请求的处理，虽然把细节转移到了doService函数中实现，但是我们不难看出请求前后所做的准备与处理工作。

（1）为了保证当前线程的LocaleContext以及RequestAttributes可以在当前请求后还能恢复，提取了当前线程的两个属性。

（2）根据当前request创建对应的LocaleContext和RequestAttributes，并绑定到当前线程。

（3）委托给doService方法进行进一步处理。

（4）请求处理结束后恢复线程到原始状态。

（5）请求处理结束后无论成功与否发布事件通知。

继续查看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);
        }

        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);
                }
            }
        }
    }

我们猜想对请求处理至少应该包括一些诸如寻找Handler并页面跳转之类的逻辑处理，但是，在doService中我们并没有看到相应的逻辑，相反同样是一些准备工作，但是这些准备工作却是必不可少的。Spring将已经初始化的功能辅助工具变量，比如localeResolver、themeResolver等设置在Request属性中，而这些属性会在接下来的处理中派上用场。

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 {
                //如果是MultipartContext类型的request则转换request为MultipartHttpServletRequest类型的request
                processedRequest = checkMultipart(request);
                multipartRequestParsed = (processedRequest != request);

                //根据request信息寻找对应的handler
                mappedHandler = getHandler(processedRequest);
                if (mappedHandler == null) {
                    noHandlerFound(processedRequest, response);
                    return;
                }

                // 根据当前的handler寻找对应的HandlerAdapter
                HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

                // 如果是当前的handler支持last-modified头处理
                String method = request.getMethod();
                boolean isGet = "GET".equals(method);
                if (isGet || "HEAD".equals(method)) {
                    long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
                    if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
                        return;
                    }
                }

                if (!mappedHandler.applyPreHandle(processedRequest, response)) {
                    return;
                }

                //实际调用处理程序。
                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);
                }
            }
        }
    }

private void applyDefaultViewName(HttpServletRequest request, @Nullable ModelAndView mv) throws Exception {
        if (mv != null && !mv.hasView()) {
            String defaultViewName = getDefaultViewName(request);
            if (defaultViewName != null) {
                mv.setViewName(defaultViewName);
            }
        }
    }

void applyPostHandle(HttpServletRequest request, HttpServletResponse response, @Nullable ModelAndView mv)
            throws Exception {

        HandlerInterceptor[] interceptors = getInterceptors();
        if (!ObjectUtils.isEmpty(interceptors)) {
            for (int i = interceptors.length - 1; i >= 0; i--) {
                HandlerInterceptor interceptor = interceptors[i];
                interceptor.postHandle(request, response, this.handler, mv);
            }
        }
    }

doDispatch函数中展示了Spring请求处理所涉及的主要逻辑，而我们之前设置在request中的各种辅助属性也都有被派上了用场。

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下面的文章将回顾逻辑处理的全过程。

参考：《Spring源码深度解析》郝佳编著：

DispatcherServlet的逻辑处理（上篇）

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