DispatcherServlet 源码阅读（1）

有时间还是应该多看看源码。

DispatcherServlet 是一个实实在在的 Servlet，所以 Spring MVC 引入后不会改变 Servlet 容器的行为，仍然是解析 web.xml 部署文件，只需要在里面配置这个 Servlet 即可。
比如下面配置 dispatcher Servlet 处理所有的请求，也体现了 DispatcherServlet 是前端控制器（Front Controller）。contextConfigLocation 上下文参数用于配置路径的指定，如果没有的话就使用默认的值。

<?xml version="1.0" encoding="UTF-8"?><web-app xmlns="http://java.sun.com/xml/ns/javaee"         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"         xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"         version="3.0">    <servlet>        <servlet-name>dispatcher</servlet-name>        <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>        <load-on-startup>1</load-on-startup>    </servlet>    <servlet-mapping>        <servlet-name>dispatcher</servlet-name>        <url-pattern>/</url-pattern>    </servlet-mapping>    <listener>        <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>    </listener>    <context-param>        <param-name>contextConfigLocation</param-name>        <param-value>            /WEB-INF/dispatcher-servlet.xml            classpath:service-context.xml        </param-value>    </context-param></web-app>

DispatcherServlet 初始化

DispatcherServlet 的父类 HttpServletBean 覆盖了 HttpServlet 的 init 方法，实现该 servlet 的初始化。

/**     * Map config parameters onto bean properties of this servlet, and     * invoke subclass initialization.     * @throws ServletException if bean properties are invalid (or required     * properties are missing), or if subclass initialization fails.     */    @Override    public final void init() throws ServletException {        if (logger.isDebugEnabled()) {            logger.debug("Initializing servlet '" + getServletName() + "'");        }        // 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();        if (logger.isDebugEnabled()) {            logger.debug("Servlet '" + getServletName() + "' configured successfully");        }    }

正如注释所说 initServletBean() 留由子类实现，体现了模板方法模式，当上述bean属性设置完成后，进入这里 FrameworkServlet#init() 创建 Servlet 的上下文 WebApplicationContext，initWebApplicationContext 首先会获得该 Web 应用的 root WebApplicationContext （通常是由 org.springframework.web.context.ContextLoaderListener 加载的），然后根据这个根上下文得到我们这个 Servlet 的 WebApplicationContext。initFrameworkServlet 方法是空的，而且子类 DispatcherServlet 也没有覆盖。

    /**     * Overridden method of {@link HttpServletBean}, invoked after any bean properties     * have been set. Creates this servlet's WebApplicationContext.     */    @Override    protected final void initServletBean() throws ServletException {        getServletContext().log("Initializing Spring FrameworkServlet '" + getServletName() + "'");        if (this.logger.isInfoEnabled()) {            this.logger.info("FrameworkServlet '" + getServletName() + "': initialization started");        }        long startTime = System.currentTimeMillis();        try {            this.webApplicationContext = initWebApplicationContext();            initFrameworkServlet();        }        catch (ServletException ex) {            this.logger.error("Context initialization failed", ex);            throw ex;        }        catch (RuntimeException ex) {            this.logger.error("Context initialization failed", ex);            throw ex;        }        if (this.logger.isInfoEnabled()) {            long elapsedTime = System.currentTimeMillis() - startTime;            this.logger.info("FrameworkServlet '" + getServletName() + "': initialization completed in " +                    elapsedTime + " ms");        }    }    /**     * Initialize and publish the WebApplicationContext for this servlet.     * <p>Delegates to {@link #createWebApplicationContext} for actual creation     * of the context. Can be overridden in subclasses.     * @return the WebApplicationContext instance     * @see #FrameworkServlet(WebApplicationContext)     * @see #setContextClass     * @see #setContextConfigLocation     */    protected WebApplicationContext initWebApplicationContext() {        WebApplicationContext rootContext =                WebApplicationContextUtils.getWebApplicationContext(getServletContext());        WebApplicationContext wac = null;        if (this.webApplicationContext != null) {            // A context instance was injected at construction time -> use it            wac = this.webApplicationContext;            if (wac instanceof ConfigurableWebApplicationContext) {                ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) wac;                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 -> set                        // the root application context (if any; may be null) as the parent                        cwac.setParent(rootContext);                    }                    configureAndRefreshWebApplicationContext(cwac);                }            }        }        if (wac == null) {            // No context instance was injected at construction time -> see if one            // has been registered in the servlet context. If one exists, it is assumed            // that the parent context (if any) has already been set and that the            // user has performed any initialization such as setting the context id            wac = findWebApplicationContext();        }        if (wac == null) {            // No context instance is defined for this servlet -> create a local one            wac = createWebApplicationContext(rootContext);        }        if (!this.refreshEventReceived) {            // Either the context is not a ConfigurableApplicationContext with refresh            // support or the context injected at construction time had already been            // refreshed -> trigger initial onRefresh manually here.            onRefresh(wac);        }        if (this.publishContext) {            // Publish the context as a servlet context attribute.            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;    }

DispatcherServlet 处理请求流程

FrameworkServlet 中覆盖了 HttpServlet 的 doGet(),doPost()等方法，而 doGet(),doPost()等又直接调用方法 processRequest 来处理请求，代码如下。

    /**     * Delegate GET requests to processRequest/doService.     * <p>Will also be invoked by HttpServlet's default implementation of {@code doHead},     * with a {@code NoBodyResponse} that just captures the content length.     * @see #doService     * @see #doHead     */    @Override    protected final void doGet(HttpServletRequest request, HttpServletResponse response)            throws ServletException, IOException {        processRequest(request, response);    }    /**     * Delegate POST requests to {@link #processRequest}.     * @see #doService     */    @Override    protected final void doPost(HttpServletRequest request, HttpServletResponse response)            throws ServletException, IOException {        processRequest(request, response);    }

然后我们进入 processRequest 方法，实际的请求处理是调用其抽象方法 doService。

/**     * Process this request, publishing an event regardless of the outcome.     * <p>The actual event handling is performed by the abstract     * {@link #doService} template method.     */    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);        }    }    /**     * Subclasses must implement this method to do the work of request handling,     * receiving a centralized callback for GET, POST, PUT and DELETE.     * <p>The contract is essentially the same as that for the commonly overridden     * {@code doGet} or {@code doPost} methods of HttpServlet.     * <p>This class intercepts calls to ensure that exception handling and     * event publication takes place.     * @param request current HTTP request     * @param response current HTTP response     * @throws Exception in case of any kind of processing failure     * @see javax.servlet.http.HttpServlet#doGet     * @see javax.servlet.http.HttpServlet#doPost     */    protected abstract void doService(HttpServletRequest request, HttpServletResponse response)            throws Exception;

然后在 DispatcherServlet 中具体实现请求的处理分发，先是把一些资源放到请求属性中，然后调用 doDispatch 实现请求分发到控制器的 handler。doDispatch 中首先会判断是否是文件传输流的请求（利用MultipartResolver），如果是的话就会转为 MultipartHttpServletRequest。接下来 getHandler(processedRequest) 根据请求获得对应的handler，最后调用 handle() 处理请求，会反射到在控制器中实现的方法。

/**     * Exposes the DispatcherServlet-specific request attributes and delegates to {@link #doDispatch}     * for the actual dispatching.     */    @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()) {                // Restore the original attribute snapshot, in case of an include.                if (attributesSnapshot != null) {                    restoreAttributesAfterInclude(request, attributesSnapshot);                }            }        }    }    /**     * 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;                }                // 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;            }            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                if (mappedHandler != null) {                    mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);                }            }            else {                // Clean up any resources used by a multipart request.                if (multipartRequestParsed) {                    cleanupMultipart(processedRequest);                }            }        }    }

HandlerAdapter接口的handle方法抽象的是一个handler如何处理一个请求，该接口实现有下面几个。

AnnotationMethodHandlerAdapter 基于HTTP请求的路径，方法，请求参数，使用 RequestMapping 注解来映handler，从 Spring 3.2建议使用RequestMappingHandlerAdapter。这里看看AnnotationMethodHandlerAdapter是如何实现handle方法的, 使用了反射，最后通过 invokeHandlerMethod 执行了对应的handler方法。

@Override    public ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)            throws Exception {        Class<?> clazz = ClassUtils.getUserClass(handler);        Boolean annotatedWithSessionAttributes = this.sessionAnnotatedClassesCache.get(clazz);        if (annotatedWithSessionAttributes == null) {            annotatedWithSessionAttributes = (AnnotationUtils.findAnnotation(clazz, SessionAttributes.class) != null);            this.sessionAnnotatedClassesCache.put(clazz, annotatedWithSessionAttributes);        }        if (annotatedWithSessionAttributes) {            checkAndPrepare(request, response, this.cacheSecondsForSessionAttributeHandlers, true);        }        else {            checkAndPrepare(request, response, true);        }        // Execute invokeHandlerMethod in synchronized block if required.        if (this.synchronizeOnSession) {            HttpSession session = request.getSession(false);            if (session != null) {                Object mutex = WebUtils.getSessionMutex(session);                synchronized (mutex) {                    return invokeHandlerMethod(request, response, handler);                }            }        }        return invokeHandlerMethod(request, response, handler);    }    protected ModelAndView invokeHandlerMethod(HttpServletRequest request, HttpServletResponse response, Object handler)            throws Exception {        ServletHandlerMethodResolver methodResolver = getMethodResolver(handler);        Method handlerMethod = methodResolver.resolveHandlerMethod(request);        ServletHandlerMethodInvoker methodInvoker = new ServletHandlerMethodInvoker(methodResolver);        ServletWebRequest webRequest = new ServletWebRequest(request, response);        ExtendedModelMap implicitModel = new BindingAwareModelMap();        Object result = methodInvoker.invokeHandlerMethod(handlerMethod, handler, webRequest, implicitModel);        ModelAndView mav =                methodInvoker.getModelAndView(handlerMethod, handler.getClass(), result, implicitModel, webRequest);        methodInvoker.updateModelAttributes(handler, (mav != null ? mav.getModel() : null), implicitModel, webRequest);        return mav;    }

小结

不要满足于得心应手的事，要不断的做触及能力之上的东西，才会成长。