SpringMVC框架是Spring框架中web模塊死遭，時(shí)下常用來構(gòu)建web應(yīng)用裙盾。在應(yīng)用之余绞愚，也一直想要搞明白SpringMVC中是如何接受處理請(qǐng)求的？

SpingMVC 初始化

Spring框架和其他框架類似史翘，都是配置元素集中于xml配置文件中枉长，在框架初始化的時(shí)候，加載配置文件琼讽，解析文件必峰，生成對(duì)應(yīng)的配置。SpringMVC框架是依托于Spring容器钻蹬。Spring初始化的過程其實(shí)就是IoC容器啟動(dòng)的過程吼蚁，也就是上下文建立的過程。

ServletContext

每一個(gè)web應(yīng)用中都有一個(gè)Servlet上下文脉让。servlet容器提供一個(gè)全局上下文的環(huán)境桂敛，這個(gè)上下文環(huán)境將成為其他IoC容器的宿主環(huán)境，例如：WebApplicationContext就是作為ServletContext的一個(gè)屬性存在溅潜。

WebApplicationContext

在使用SpringMVC的時(shí)候，通常需要在web.xml文件中配置：

  <listener>
    <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
  </listener>

ContextLoaderListener 實(shí)現(xiàn)了ServletContextListener接口薪伏，在SpringMVC中作為監(jiān)聽器的存在滚澜，當(dāng)servlet容器啟動(dòng)時(shí)候，會(huì)調(diào)用contextInitialized進(jìn)行一些初始化的工作嫁怀。而ContextLoaderListener中contextInitialized的具體實(shí)現(xiàn)在ContextLoader類中设捐。

        try {
            // Store context in local instance variable, to guarantee that
            // it is available on ServletContext shutdown.
            if (this.context == null) {
                this.context = createWebApplicationContext(servletContext);
            }
            if (this.context instanceof ConfigurableWebApplicationContext) {
                ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) this.context;
                if (!cwac.isActive()) {
                    // The context has not yet been refreshed -> provide services such as
                    // setting the parent context, setting the application context id, etc
                    if (cwac.getParent() == null) {
                        // The context instance was injected without an explicit parent ->
                        // determine parent for root web application context, if any.
                        ApplicationContext parent = loadParentContext(servletContext);
                        cwac.setParent(parent);
                    }
                    configureAndRefreshWebApplicationContext(cwac, servletContext);
                }
            }
            servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.context);

上面的部分代碼可以看出，初始化時(shí)候通過createWebApplicationContext(servletContext);聲明一個(gè)WebApplicationContext并賦值給ServletContext的org.springframework.web.context.WebApplicationContext.ROOT屬性塘淑，作為WebApplicationContext的根上下文（root context）萝招。

DispatcherServlet

在加載完<context-param>和<listener>之后，容器將加載配置了load-on-startup的servlet存捺。

    <servlet>
        <servlet-name>example</servlet-name>
        <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
        <load-on-startup>1</load-on-startup>
    </servlet>

    <servlet-mapping>
        <servlet-name>example</servlet-name>
        <url-pattern>/example/*</url-pattern>
    </servlet-mapping>

DispatcherServlet在初始化的過程中槐沼，會(huì)建立一個(gè)自己的IoC容器上下文Servlet WebApplicationContext曙蒸，會(huì)以ContextLoaderListener建立的根上下文作為自己的父級(jí)上下文。DispatcherServlet持有的上下文默認(rèn)的實(shí)現(xiàn)類是XmlWebApplicationContext岗钩。Servlet有自己獨(dú)有的Bean空間纽窟，也可以共享父級(jí)上下文的共享Bean，當(dāng)然也存在配置有含有一個(gè)root WebApplicationContext配置兼吓。其關(guān)系如下圖所示臂港，后面也還會(huì)詳細(xì)介紹DispatcherServlet這個(gè)類。

DispatcherServlet類

DispatcherServlet最為SpringMVC核心類视搏，起到了前端控制器（Front controller）的作用审孽，負(fù)責(zé)請(qǐng)求分發(fā)等工作。

從類圖中可以看出浑娜，DispatcherServlet的繼承關(guān)系大致如此：

DispatcherServlet -> FrameworkServlet -> HttpServletBean -> HttpServlet -> GenericServlet

從繼承關(guān)系上可以得出結(jié)論佑力，DispatcherServlet本質(zhì)上還是一個(gè)Servlet。Servlet的生命周期大致分為三個(gè)階段：

• 初始化階段 init方法
• 處理請(qǐng)求階段 service方法
• 結(jié)束階段 destroy方法

這里就重點(diǎn)關(guān)注DispatcherServlet在這三個(gè)階段具體做了那些工作棚愤。

DispatcherServlet初始化

DispatcherServlet的init()的實(shí)現(xiàn)在其父類HttpServletBean中搓萧。

    public final void init() throws ServletException {
        ...
        // Set bean properties from init parameters.
        try {
            PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
            BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);
            ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext());
            bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, getEnvironment()));
            initBeanWrapper(bw);
            bw.setPropertyValues(pvs, true);
        }
        catch (BeansException ex) {
            logger.error("Failed to set bean properties on servlet '" + getServletName() + "'", ex);
            throw ex;
        }

        // Let subclasses do whatever initialization they like.
        initServletBean();

        ...
    }

以上部分源碼描述的過程是通過讀取<init-param>的配置元素，讀取到DispatcherServlet中宛畦，配置相關(guān)bean的配置瘸洛。完成配置后調(diào)用initServletBean方法來創(chuàng)建Servlet WebApplicationContext。

initServletBean方法在FrameworkServlet類中重寫了：

    protected final void initServletBean() throws ServletException {
        ...

        try {
            this.webApplicationContext = initWebApplicationContext();
            initFrameworkServlet();
        }
        ...
    }
    
    protected WebApplicationContext initWebApplicationContext() {
        WebApplicationContext rootContext =
                WebApplicationContextUtils.getWebApplicationContext(getServletContext());
        WebApplicationContext wac = null;

        if (this.webApplicationContext != null) {
            wac = this.webApplicationContext;
            if (wac instanceof ConfigurableWebApplicationContext) {
                ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) wac;
                if (!cwac.isActive()) {
                    if (cwac.getParent() == null) {
                        cwac.setParent(rootContext);
                    }
                    configureAndRefreshWebApplicationContext(cwac);
                }
            }
        }
        if (wac == null) {
            wac = findWebApplicationContext();
        }
        if (wac == null) {
            wac = createWebApplicationContext(rootContext);
        }

        if (!this.refreshEventReceived) {
            onRefresh(wac);
        }

        if (this.publishContext) {
            String attrName = getServletContextAttributeName();
            getServletContext().setAttribute(attrName, wac);
            if (this.logger.isDebugEnabled()) {
                this.logger.debug("Published WebApplicationContext of servlet '" + getServletName() +
                        "' as ServletContext attribute with name [" + attrName + "]");
            }
        }

        return wac;
    }

上文提到Servlet容器在啟動(dòng)的時(shí)候次和，通過ContextLoaderListener創(chuàng)建一個(gè)根上下文反肋，并配置到ServletContext中√な可以看出FrameworkServlet這個(gè)類做的作用是用來創(chuàng)建WebApplicationContext上下文的石蔗。大致過程如下：

• 首先檢查webApplicationContext是否通過構(gòu)造函數(shù)注入，如果有的話畅形，直接使用养距，并將根上下文設(shè)置為父上下文。
• 如果webApplicationContext沒有注入日熬，則檢查是否在ServletContext已經(jīng)注冊(cè)過棍厌，如果已經(jīng)注冊(cè)過，直接返回使用竖席。
• 如果沒有注冊(cè)過耘纱，將重新新建一個(gè)webApplicationContext。將根上下文設(shè)置為父級(jí)上下文毕荐。
• 不管是何種策略獲取的webApplicationContext束析，都將會(huì)調(diào)用onRefresh方法，onRefresh方法會(huì)調(diào)用initStrategies方法憎亚，通過上下文初始化HandlerMappings员寇、HandlerAdapters弄慰、ViewResolvers等等。
• 最后丁恭，同樣會(huì)將所得webApplicationContext注冊(cè)到ServletContext中曹动。

而initFrameworkServlet()默認(rèn)的實(shí)現(xiàn)是空的。這也可算是SpingMVC留的一個(gè)擴(kuò)展點(diǎn)牲览。

DispatcherServlet處理請(qǐng)求

縱觀SpringMVC的源碼墓陈，大量運(yùn)用模板方法的設(shè)計(jì)模式。Servlet的service方法也不例外第献。FrameworkServlet類重寫service方法：

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

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

如果請(qǐng)求的方法是PATCH或者空贡必，直接調(diào)用processRequest方法（后面會(huì)詳細(xì)解釋）；否則庸毫，將調(diào)用父類的service的方法仔拟，即HttpServlet的service方法, 而這里會(huì)根據(jù)請(qǐng)求方法，去調(diào)用相應(yīng)的doGet飒赃、doPost利花、doPut......

而doXXX系列方法的實(shí)現(xiàn)并不是HttpServlet類中，而是在FrameworkServlet類中载佳。在FrameworkServlet中doXXX系列實(shí)現(xiàn)中炒事，都調(diào)用了上面提到的processRequest方法：

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

        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 ex) {
            failureCause = ex;
            throw ex;
        }
        catch (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();
            }

            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");
                    }
                }
            }

            publishRequestHandledEvent(request, response, startTime, failureCause);
        }
    }

為了避免子類重寫它，該方法用final修飾蔫慧。

• 首先調(diào)用initContextHolders方法,將獲取到的localeContext挠乳、requestAttributes、request綁定到線程上姑躲。
• 然后調(diào)用doService方法睡扬，doService具體是由DispatcherServlet類實(shí)現(xiàn)的。
• doService執(zhí)行完成后黍析，調(diào)用resetContextHolders卖怜，解除localeContext等信息與線程的綁定。
• 最終調(diào)用publishRequestHandledEvent發(fā)布一個(gè)處理完成的事件阐枣。

DispatcherServlet類中的doService方法實(shí)現(xiàn)會(huì)調(diào)用doDispatch方法韧涨，這里請(qǐng)求分發(fā)處理的主要執(zhí)行邏輯。

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;
                }

                // 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主要流程是：

• 先判斷是否Multipart類型的請(qǐng)求侮繁。如果是則通過multipartResolver解析request
• 通過getHandler方法找到從HandlerMapping找到該請(qǐng)求對(duì)應(yīng)的handler,如果沒有找到對(duì)應(yīng)的handler則拋出異常。
• 通過getHandlerAdapter方法找到handler對(duì)應(yīng)的HandlerAdapter
• 如果有攔截器如孝，執(zhí)行攔截器preHandler方法
• HandlerAdapter執(zhí)行handle方法處理請(qǐng)求宪哩，返回ModelAndView。
• 如果有攔截器第晰，執(zhí)行攔截器postHandle方法
• 然后調(diào)用processDispatchResult方法處理請(qǐng)求結(jié)果锁孟，封裝到response中彬祖。

SpingMVC 請(qǐng)求處理流程

SpringMVC框架是圍繞DispatcherServlet設(shè)計(jì)的。DispatcherServlet負(fù)責(zé)將請(qǐng)求分發(fā)給對(duì)應(yīng)的處理程序品抽。從網(wǎng)上找了兩個(gè)圖储笑，可以大致了解SpringMVC的框架對(duì)請(qǐng)求的處理流程。

• 用戶發(fā)送請(qǐng)求圆恤，Front Controller（DispatcherServlet）根據(jù)請(qǐng)求信息將請(qǐng)求委托給對(duì)應(yīng)的Controller進(jìn)行處理突倍。
• DispatcherServlet接收到請(qǐng)求后，HandlerMapping將會(huì)把請(qǐng)求封裝為HandlerExecutionChain盆昙，而HandlerExecutionChain包含請(qǐng)求的所有信息羽历，包括攔截器、Handler處理器等淡喜。
• DispatcherServlet會(huì)找到對(duì)應(yīng)的HandlerAdapter秕磷，并調(diào)用對(duì)應(yīng)的處理方法，并返回一個(gè)ModelAndView對(duì)象炼团。
• DispatcherServlet會(huì)將ModelAndView對(duì)象傳入View層進(jìn)行渲染澎嚣。
• 最終DispatcherServlet將渲染好的response返回給用戶。

總結(jié)

本文主要分析SpringMVC中DispatcherServlet的初始化瘟芝、請(qǐng)求流傳過程等易桃。

發(fā)現(xiàn)了SpringMVC中在DispatcherServlet的實(shí)現(xiàn)過程中運(yùn)用了模板方法設(shè)計(jì)模式，看到SpringMVC中留給用戶可擴(kuò)展的點(diǎn)也有很多模狭，體會(huì)到Open for extension, closed for modification的設(shè)計(jì)原則颈抚。

本文只關(guān)注了DispatcherServlet主流程，忽略了很多寶貴的細(xì)枝末節(jié)嚼鹉，如：HandlerMapping贩汉、HandlerExecutionChain、HandlerAdapter等锚赤。后面有機(jī)會(huì)定會(huì)追本溯源匹舞。