spring mvc DispatcherServlet详解之一---处理请求深入解析

要深入理解spring mvc的工作流程，就需要先了解spring mvc的架构：

从上图可以看到 前端控制器DispatcherServlet在其中起着主导作用，理解了DispatcherServlet 就完全可以说弄清楚了spring mvc。

为了加深对spring mvc的整个工作流程的理解，本文从分析DispatcherServlet的工作过程来一窥spring mvc的整个面貌。

1. 初始化

protected void initStrategies(ApplicationContext context) {
        initMultipartResolver(context); //文件上传解析，如果请求类型是multipart将通过MultipartResolver进行文件上传解析；
        initLocaleResolver(context);    //本地化解析
        initThemeResolver(context);　　　//主题解析
        initHandlerMappings(context);   //通过HandlerMapping，将请求映射到处理器
        initHandlerAdapters(context);   //通过HandlerAdapter支持多种类型的处理器
        initHandlerExceptionResolvers(context); //如果执行过程中遇到异常将交给HandlerExceptionResolver来解析
        initRequestToViewNameTranslator(context); //直接解析请求到视图名
        initViewResolvers(context);      //通过ViewResolver解析逻辑视图名到具体视图实现
        initFlashMapManager(context);   //flash映射管理器
    }

单个resolver
initMultipartResolver，initLocaleResolver，initThemeResolver，initRequestToViewNameTranslator，initFlashMapManager 这五个初始化方法流程相同，都是使用
context.getBean(String name, Class<FlashMapManager> requiredType)的方式获取到相应的Resolver。以initMultipartResolver为例，见如下：

    /**
     * Initialize the MultipartResolver used by this class.
     * <p>If no bean is defined with the given name in the BeanFactory for this namespace,
     * no multipart handling is provided.
     */
    private void initMultipartResolver(ApplicationContext context) {
        try {
            this.multipartResolver = context.getBean(MULTIPART_RESOLVER_BEAN_NAME, MultipartResolver.class);
            if (logger.isDebugEnabled()) {
                logger.debug("Using MultipartResolver [" + this.multipartResolver + "]");
            }
        }
        catch (NoSuchBeanDefinitionException ex) {
            // Default is no multipart resolver.
            this.multipartResolver = null;
            if (logger.isDebugEnabled()) {
                logger.debug("Unable to locate MultipartResolver with name '" + MULTIPART_RESOLVER_BEAN_NAME +
                        "': no multipart request handling provided");
            }
        }
    }

多个resolver

initHandlerMappings，initHandlerAdapters，initHandlerExceptionResolvers，initViewResolvers 获取方式相同，使用：
BeanFactoryUtils.beansOfTypeIncludingAncestors(ListableBeanFactory lbf, Class<HandlerMapping> type, boolean includeNonSingletons, boolean allowEagerInit)

的方式获取到相应的Resolver。以initHandlerMappings为例，见如下：

/**
     * Initialize the HandlerMappings used by this class.
     * <p>If no HandlerMapping beans are defined in the BeanFactory for this namespace,
     * we default to BeanNameUrlHandlerMapping.
     */
    private void initHandlerMappings(ApplicationContext context) {
        this.handlerMappings = null;

        if (this.detectAllHandlerMappings) {
            // Find all HandlerMappings in the ApplicationContext, including ancestor contexts.
            Map<String, HandlerMapping> matchingBeans =
                    BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerMapping.class, true, false);
            if (!matchingBeans.isEmpty()) {
                this.handlerMappings = new ArrayList<HandlerMapping>(matchingBeans.values());
                // We keep HandlerMappings in sorted order.
                OrderComparator.sort(this.handlerMappings);
            }
        }
        else {
            try {
                HandlerMapping hm = context.getBean(HANDLER_MAPPING_BEAN_NAME, HandlerMapping.class);
                this.handlerMappings = Collections.singletonList(hm);
            }
            catch (NoSuchBeanDefinitionException ex) {
                // Ignore, we'll add a default HandlerMapping later.
            }
        }

        // Ensure we have at least one HandlerMapping, by registering
        // a default HandlerMapping if no other mappings are found.
        if (this.handlerMappings == null) {
            this.handlerMappings = getDefaultStrategies(context, HandlerMapping.class);
            if (logger.isDebugEnabled()) {
                logger.debug("No HandlerMappings found in servlet '" + getServletName() + "': using default");
            }
        }
    }

那么深入看一下BeanFactoryUtils.beansOfTypeIncludingAncestors 究竟做了什么？返回指定类型和子类型的所有bean，若该bean factory 是一个继承类型的beanFactory，这个方法也会获取祖宗factory中定义的指定类型的bean。

/**
     * Return all beans of the given type or subtypes, also picking up beans defined in
     * ancestor bean factories if the current bean factory is a HierarchicalBeanFactory.
     * The returned Map will only contain beans of this type.
     * <p>Does consider objects created by FactoryBeans if the "allowEagerInit" flag is set,
     * which means that FactoryBeans will get initialized. If the object created by the
     * FactoryBean doesn't match, the raw FactoryBean itself will be matched against the
     * type. If "allowEagerInit" is not set, only raw FactoryBeans will be checked
     * (which doesn't require initialization of each FactoryBean).
     * <p><b>Note: Beans of the same name will take precedence at the 'lowest' factory level,
     * i.e. such beans will be returned from the lowest factory that they are being found in,
     * hiding corresponding beans in ancestor factories.</b> This feature allows for
     * 'replacing' beans by explicitly choosing the same bean name in a child factory;
     * the bean in the ancestor factory won't be visible then, not even for by-type lookups.
     * @param lbf the bean factory
     * @param type type of bean to match
     * @param includeNonSingletons whether to include prototype or scoped beans too
     * or just singletons (also applies to FactoryBeans)
     * @param allowEagerInit whether to initialize <i>lazy-init singletons</i> and
     * <i>objects created by FactoryBeans</i> (or by factory methods with a
     * "factory-bean" reference) for the type check. Note that FactoryBeans need to be
     * eagerly initialized to determine their type: So be aware that passing in "true"
     * for this flag will initialize FactoryBeans and "factory-bean" references.
     * @return the Map of matching bean instances, or an empty Map if none
     * @throws BeansException if a bean could not be created
     */
    public static <T> Map<String, T> beansOfTypeIncludingAncestors(
            ListableBeanFactory lbf, Class<T> type, boolean includeNonSingletons, boolean allowEagerInit)
            throws BeansException {

        Assert.notNull(lbf, "ListableBeanFactory must not be null");
        Map<String, T> result = new LinkedHashMap<String, T>(4);
        result.putAll(lbf.getBeansOfType(type, includeNonSingletons, allowEagerInit));
        if (lbf instanceof HierarchicalBeanFactory) {
            HierarchicalBeanFactory hbf = (HierarchicalBeanFactory) lbf;
            if (hbf.getParentBeanFactory() instanceof ListableBeanFactory) {
                Map<String, T> parentResult = beansOfTypeIncludingAncestors(
                        (ListableBeanFactory) hbf.getParentBeanFactory(), type, includeNonSingletons, allowEagerInit);
                for (Map.Entry<String, T> entry : parentResult.entrySet()) {
                    String beanName = entry.getKey();
                    if (!result.containsKey(beanName) && !hbf.containsLocalBean(beanName)) {
                        result.put(beanName, entry.getValue());
                    }
                }
            }
        }
        return result;
    }

2. 提供服务

我们来看看这个servlet是如何提供服务的？

    @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) + "]");
        }

        // 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("org.springframework.web.servlet")) {
                    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()) {
                return;
            }
            // Restore the original attribute snapshot, in case of an include.
            if (attributesSnapshot != null) {
                restoreAttributesAfterInclude(request, attributesSnapshot);
            }
        }
    }

从上面我们可以看到，提供服务只要分4步：

1. 保存现场。保存request 熟悉的快照，以便能在必要时恢复。

2. 将框架需要的对象放入request中，以便view和handler使用。

3. 请求分发服务.

4. 恢复现场。

其中最重要的是请求分发服务：

    /**
     * Process the actual dispatching to the handler.
     * <p>The handler will be obtained by applying the servlet's HandlerMappings in order.
     * The HandlerAdapter will be obtained by querying the servlet's installed HandlerAdapters
     * to find the first that supports the handler class.
     * <p>All HTTP methods are handled by this method. It's up to HandlerAdapters or handlers
     * themselves to decide which methods are acceptable.
     * @param request current HTTP request
     * @param response current HTTP response
     * @throws Exception in case of any kind of processing failure
     */
    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 || mappedHandler.getHandler() == 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;
                }

                try {
                    // Actually invoke the handler.
                    mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
                }
                finally {
                    if (asyncManager.isConcurrentHandlingStarted()) {
                        return;
                    }
                }

                applyDefaultViewName(request, mv);
                mappedHandler.applyPostHandle(processedRequest, response, mv);
            }
            catch (Exception ex) {
                dispatchException = ex;
            }
            processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
        }
        catch (Exception ex) {
            triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
        }
        catch (Error err) {
            triggerAfterCompletionWithError(processedRequest, response, mappedHandler, err);
        }
        finally {
            if (asyncManager.isConcurrentHandlingStarted()) {
                // Instead of postHandle and afterCompletion
                mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
                return;
            }
            // Clean up any resources used by a multipart request.
            if (multipartRequestParsed) {
                cleanupMultipart(processedRequest);
            }
        }
    }

分发过程如下：

1. 判断是否设置了multipart resolver，设置的话转换为multipart request，没有的话则继续下面的步骤。

2. 根据当前request，获取hangdler。

3. 根据当前request，获取HandlerAdapter。

4. 如果支持http请求头，处理 last-modified header请求头。

5. 应用已注册interceptor的preHandle方法

6. HandlerAdapter处理请求。

7. 设置默认视图。

8. 应用已注册interceptor的postHandle方法。

9. 处理异常或者视图渲染。

小结：

DispatherServlet整个过程的细节一章之内很难描述的面面俱到，只能分析部分流程，想了解更具体的实现需要从源代码中去寻找。