Spring MVC工作原理及源码解析（二）DispatcherServlet实现原理及源码解析

1、DispatcherServlet 处理流程

从上一篇文章中Spring MVC原理图中我们可以看出：DispatcherServlet 在 Spring MVC框架 中处于核心位置，它负责协调和组织不同组件完成请求处理并返回响应的工作。在分析 DispatcherServlet 源码之前，我们先来看一下请求处理的大致流程：

1. Tomcat 上的项目（采用了Spring MVC框架）启动，对 DispatcherServlet 进行实例化，然后调用它的 init() 方法进行初始化，在这个初始化过程中完成了：对 web.xml 中初始化参数的加载；建立 WebApplicationContext (SpringMVC的IOC容器)；进行组件的初始化；
2. 用户在浏览器发出请求，由 Tomcat 接收到这个请求，如果匹配到 DispatcherServlet 在 web.xml 中配置的映射路径，Tomcat 就将请求转交给 DispatcherServlet 处理；
3. DispatcherServlet 从容器中取出所有 HandlerMapping 实例（每个实例对应一个 HandlerMapping 接口的实现类）并遍历，每个 HandlerMapping 会根据请求信息，通过自己实现类中的方式去找到处理该请求的 Handler (执行程序，如Controller中的方法)，并且将这个 Handler 与一堆 HandlerInterceptor (拦截器) 封装成一个 HandlerExecutionChain 对象，一旦有一个 HandlerMapping 可以找到 Handler 则退出循环；
4. DispatcherServlet 取出 HandlerAdapter 组件，根据已经找到的 Handler，再从所有 HandlerAdapter 中找到可以处理该 Handler 的 HandlerAdapter 对象；
5. 执行 HandlerExecutionChain 中所有拦截器的 preHandler() 方法，然后再利用 HandlerAdapter 执行 Handler ，执行完成得到 ModelAndView，再依次调用拦截器的 postHandler() 方法；
6. 利用 ViewResolver 将 ModelAndView 或是 Exception（可解析成 ModelAndView）解析成 View，然后 View 会调用 render() 方法，再根据 ModelAndView 中的数据渲染出页面；
7. 最后再依次调用拦截器的 afterCompletion() 方法，这一次请求就结束了。

2、DispatcherServlet 源码分析

DispatcherServlet 继承自 HttpServlet，它遵循 Servlet 里的“init-service-destroy”三个阶段，首先我们先来看一下它的 init() 阶段。 

1、初始化

1.1、HttpServletBean 的 init() 方法

首先我们来看一下DispatcherServlet的继承关系，它继承自FrameworkServlet，而FrameworkServlet又继承自HttpServletBean。

DispatcherServlet 的 init() 方法在其父类 HttpServletBean 中实现的，它覆盖了 GenericServlet 的 init() 方法，主要作用是加载 web.xml 中 DispatcherServlet 的 <init-param> 配置，并调用子类的初始化。下面是 init() 方法的具体代码：

    /**
     * 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 {

        // Set bean properties from init parameters.（ServletConfigPropertyValues 是静态内部类，它使用 ServletConfig 获取 web.xml 中配置的参数）
        PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
        if (!pvs.isEmpty()) {
            try {
                // 使用 BeanWrapper 来构造 DispatcherServlet
                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) {
                if (logger.isErrorEnabled()) {
                    logger.error("Failed to set bean properties on servlet '" + getServletName() + "'", ex);
                }
                throw ex;
            }
        }

        // Let subclasses do whatever initialization they like.（让子类实现的方法，这种在父类定义在子类实现的方式叫做模版方法模式）
        initServletBean();
    }

如上所述，web.xml 中配置的参数是 ServletConfigPropertyValues 使用 ServletConfig 获取到的，ServletConfigPropertyValues 的代码如下所示：

　　/**
     * PropertyValues implementation created from ServletConfig init parameters.
     */
    private static class ServletConfigPropertyValues extends MutablePropertyValues {

        /**
         * Create new ServletConfigPropertyValues.
         * @param config the ServletConfig we'll use to take PropertyValues from
         * @param requiredProperties set of property names we need, where
         * we can't accept default values
         * @throws ServletException if any required properties are missing
         */
        public ServletConfigPropertyValues(ServletConfig config, Set<String> requiredProperties)
                throws ServletException {

            Set<String> missingProps = (!CollectionUtils.isEmpty(requiredProperties) ?
                    new HashSet<>(requiredProperties) : null);

            Enumeration<String> paramNames = config.getInitParameterNames();
            while (paramNames.hasMoreElements()) {
                String property = paramNames.nextElement();
                Object value = config.getInitParameter(property);
                addPropertyValue(new PropertyValue(property, value));
                if (missingProps != null) {
                    missingProps.remove(property);
                }
            }

            // Fail if we are still missing properties.
            if (!CollectionUtils.isEmpty(missingProps)) {
                throw new ServletException(
                        "Initialization from ServletConfig for servlet '" + config.getServletName() +
                        "' failed; the following required properties were missing: " +
                        StringUtils.collectionToDelimitedString(missingProps, ", "));
            }
        }
    }

1.2、FrameworkServlet 的 initServletBean() 方法

在 HttpServletBean 的 init() 方法中调用了 initServletBean() 这个方法，它是在 FrameworkServlet 类中实现的，主要作用是建立 WebApplicationContext 容器（有时也称上下文），并加载 SpringMVC 配置文件中定义的 Bean 到改容器中，最后将该容器添加到 ServletContext 中。下面是 initServletBean() 方法的具体代码：

    /**
     * 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 " + getClass().getSimpleName() + " '" + getServletName() + "'");
        if (logger.isInfoEnabled()) {
            logger.info("Initializing Servlet '" + getServletName() + "'");
        }
        long startTime = System.currentTimeMillis();

        try {
            // 初始化 WebApplicationContext (即SpringMVC的IOC容器)
            this.webApplicationContext = initWebApplicationContext();
            initFrameworkServlet();
        }
        catch (ServletException | RuntimeException ex) {
            logger.error("Context initialization failed", ex);
            throw ex;
        }

        if (logger.isDebugEnabled()) {
            String value = this.enableLoggingRequestDetails ?
                    "shown which may lead to unsafe logging of potentially sensitive data" :
                    "masked to prevent unsafe logging of potentially sensitive data";
            logger.debug("enableLoggingRequestDetails='" + this.enableLoggingRequestDetails +
                    "': request parameters and headers will be " + value);
        }

        if (logger.isInfoEnabled()) {
            logger.info("Completed initialization in " + (System.currentTimeMillis() - startTime) + " ms");
        }
    }

 WebApplicationContext 继承于 ApplicationContext 接口，从容器中可以获取当前应用程序环境信息，它也是 SpringMVC 的 IOC 容器。下面是 initWebApplicationContext() 方法的具体代码：

    /**
     * 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() {
        // 获取 ContextLoaderListener 初始化并注册在 ServletContext 中的根容器，即 Spring 的容器
        WebApplicationContext rootContext =
                WebApplicationContextUtils.getWebApplicationContext(getServletContext());
        WebApplicationContext wac = null;

        if (this.webApplicationContext != null) {
            // A context instance was injected at construction time -> use it（因为 WebApplicationContext 不为空，说明该类在构造时已经将其注入，可以直接使用它）
            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（将 Spring 的容器设为 SpringMVC 容器的父容器）
                        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（如果 WebApplicationContext 为空，则进行查找，能找到说明上下文已经在别处初始化。）
            wac = findWebApplicationContext();
        }
        if (wac == null) {
            // No context instance is defined for this servlet -> create a local one（如果 WebApplicationContext 仍为空，则以 Spring 的容器为父上下文建立一个新的。）
            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.
            synchronized (this.onRefreshMonitor) {
                // 模版方法，由 DispatcherServlet 实现
                onRefresh(wac);
            }
        }

        if (this.publishContext) {
            // Publish the context as a servlet context attribute.（发布这个 WebApplicationContext 容器到 ServletContext 中）
            String attrName = getServletContextAttributeName();
            getServletContext().setAttribute(attrName, wac);
        }

        return wac;
    }

下面是查找 WebApplicationContext 的 findWebApplicationContext() 方法代码：

    /**
     * Retrieve a {@code WebApplicationContext} from the {@code ServletContext}
     * attribute with the {@link #setContextAttribute configured name}. The
     * {@code WebApplicationContext} must have already been loaded and stored in the
     * {@code ServletContext} before this servlet gets initialized (or invoked).
     * <p>Subclasses may override this method to provide a different
     * {@code WebApplicationContext} retrieval strategy.
     * @return the WebApplicationContext for this servlet, or {@code null} if not found
     * @see #getContextAttribute()
     */
    @Nullable
    protected WebApplicationContext findWebApplicationContext() {
        String attrName = getContextAttribute();
        if (attrName == null) {
            return null;
        }
        // 从 ServletContext 中查找已经发布的 WebApplicationContext 容器
        WebApplicationContext wac =
                WebApplicationContextUtils.getWebApplicationContext(getServletContext(), attrName);
        if (wac == null) {
            throw new IllegalStateException("No WebApplicationContext found: initializer not registered?");
        }
        return wac;
    }

1.3、DispatcherServlet 的 onRefresh() 方法

建立好 WebApplicationContext(上下文) 后，通过 onRefresh(ApplicationContext context) 方法回调，进入 DispatcherServlet 类中。onRefresh() 方法，提供 SpringMVC 的初始化，具体代码如下：

    /**
     * This implementation calls {@link #initStrategies}.
     */
    @Override
    protected void onRefresh(ApplicationContext context) {
        initStrategies(context);
    }

    /**
     * Initialize the strategy objects that this servlet uses.
     * <p>May be overridden in subclasses in order to initialize further strategy objects.
     */
    protected void initStrategies(ApplicationContext context) {
        initMultipartResolver(context);
        initLocaleResolver(context);
        initThemeResolver(context);
        initHandlerMappings(context);
        initHandlerAdapters(context);
        initHandlerExceptionResolvers(context);
        initRequestToViewNameTranslator(context);
        initViewResolvers(context);
        initFlashMapManager(context);
    }

在 initStrategies() 方法中进行了各个组件的初始化，先来看一下这些组件的初始化方法，稍后再来详细分析这些组件。

1.3.1、initHandlerMappings 方法

initHandlerMappings() 方法从 SpringMVC 的容器及 Spring 的容器中查找所有的 HandlerMapping 实例，并把它们放入到 handlerMappings 这个 list 中。这个方法并不是对 HandlerMapping 实例的创建，HandlerMapping 实例是在上面 WebApplicationContext 容器初始化，即 SpringMVC 容器初始化的时候创建的。 

    /**
     * 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.(从 SpringMVC 的 IOC 容器及 Spring 的 IOC 容器中查找 HandlerMapping 实例)
            Map<String, HandlerMapping> matchingBeans =
                    BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerMapping.class, true, false);
            if (!matchingBeans.isEmpty()) {
                this.handlerMappings = new ArrayList<>(matchingBeans.values());
                // We keep HandlerMappings in sorted order.（按一定顺序放置 HandlerMapping 对象）
                AnnotationAwareOrderComparator.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.（如果没有找到 HandlerMapping，则加载默认的）
        if (this.handlerMappings == null) {
            this.handlerMappings = getDefaultStrategies(context, HandlerMapping.class);
            if (logger.isTraceEnabled()) {
                logger.trace("No HandlerMappings declared for servlet '" + getServletName() +
                        "': using default strategies from DispatcherServlet.properties");
            }
        }
    }

1.3.2、initHandlerAdapters 方法

    /**
     * Initialize the HandlerAdapters used by this class.
     * <p>If no HandlerAdapter beans are defined in the BeanFactory for this namespace,
     * we default to SimpleControllerHandlerAdapter.
     */
    private void initHandlerAdapters(ApplicationContext context) {
        this.handlerAdapters = null;

        if (this.detectAllHandlerAdapters) {
            // Find all HandlerAdapters in the ApplicationContext, including ancestor contexts.(从 SpringMVC 的 IOC 容器及 Spring 的 IOC 容器中查找 HandlerAdapters 实例)
            Map<String, HandlerAdapter> matchingBeans =
                    BeanFactoryUtils.beansOfTypeIncludingAncestors(context, HandlerAdapter.class, true, false);
            if (!matchingBeans.isEmpty()) {
                this.handlerAdapters = new ArrayList<>(matchingBeans.values());
                // We keep HandlerAdapters in sorted order.（按一定顺序放置 HandlerAdapters 对象）
                AnnotationAwareOrderComparator.sort(this.handlerAdapters);
            }
        }
        else {
            try {
                HandlerAdapter ha = context.getBean(HANDLER_ADAPTER_BEAN_NAME, HandlerAdapter.class);
                this.handlerAdapters = Collections.singletonList(ha);
            }
            catch (NoSuchBeanDefinitionException ex) {
                // Ignore, we'll add a default HandlerAdapter later.
            }
        }

        // Ensure we have at least some HandlerAdapters, by registering
        // default HandlerAdapters if no other adapters are found.（如果没有找到 HandlerAdapters，则加载默认的）
        if (this.handlerAdapters == null) {
            this.handlerAdapters = getDefaultStrategies(context, HandlerAdapter.class);
            if (logger.isTraceEnabled()) {
                logger.trace("No HandlerAdapters declared for servlet '" + getServletName() +
                        "': using default strategies from DispatcherServlet.properties");
            }
        }
    }

2、处理请求

HttpServlet 提供了 doGet()、doPost() 等方法，DispatcherServlet 中这些方法是在其父类 FrameworkServlet 中实现的，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);
    }

这些方法又都调用了 processRequest() 方法，我们来看一下它的代码：

    /**
     * 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;

        // 返回与当前线程相关联的 LocaleContext
        LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
        // 根据请求构建 LocaleContext，公开请求的语言环境为当前语言环境
        LocaleContext localeContext = buildLocaleContext(request);

        // 返回当前绑定到线程的 RequestAttributes
        RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
        // 根据请求构建ServletRequestAttributes
        ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);

        // 获取当前请求的 WebAsyncManager，如果没有找到则创建
        WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
        asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor());

        // 使 LocaleContext 和 requestAttributes 关联
        initContextHolders(request, localeContext, requestAttributes);

        try {
            // 由 DispatcherServlet 实现
            doService(request, response);
        }
        catch (ServletException | IOException ex) {
            failureCause = ex;
            throw ex;
        }
        catch (Throwable ex) {
            failureCause = ex;
            throw new NestedServletException("Request processing failed", ex);
        }

        finally {
            // 重置 LocaleContext 和 requestAttributes，解除关联
            resetContextHolders(request, previousLocaleContext, previousAttributes);
            if (requestAttributes != null) {
                requestAttributes.requestCompleted();
            }
            logResult(request, response, failureCause, asyncManager);
            // 发布 ServletRequestHandlerEvent 事件
            publishRequestHandledEvent(request, response, startTime, failureCause);
        }
    }

DispatcherServlet 的 doService() 方法主要是设置一些 request 属性，并调用 doDispatch() 方法进行请求分发处理，doDispatch() 方法的主要过程是通过 HandlerMapping 获取 Handler，再找到用于执行它的 HandlerAdapter，执行 Handler 后得到 ModelAndView ，ModelAndView 是连接“业务逻辑层”与“视图展示层”的桥梁，接下来就要通过 ModelAndView 获得 View，再通过它的 Model 对 View 进行渲染。doDispatch() 方法如下：

    /**
     * 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;

        // 获取当前请求的WebAsyncManager，如果没找到则创建并与请求关联
        WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);

        try {
            ModelAndView mv = null;
            Exception dispatchException = null;

            try {
                // 检查是否有 Multipart，有则将请求转换为 Multipart 请求
                processedRequest = checkMultipart(request);
                multipartRequestParsed = (processedRequest != request);

                // Determine handler for the current request.（遍历所有的 HandlerMapping 找到与请求对应的 Handler，并将其与一堆拦截器封装到 HandlerExecution 对象中。）
                mappedHandler = getHandler(processedRequest);
                if (mappedHandler == null) {
                    noHandlerFound(processedRequest, response);
                    return;
                }

                // Determine handler adapter for the current request.（遍历所有的 HandlerAdapter，找到可以处理该 Handler 的 HandlerAdapter）
                HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

                // Process last-modified header, if supported by the handler.（处理 last-modified 请求头）
                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;
                    }
                }

                // 遍历拦截器，执行它们的 preHandle() 方法
                if (!mappedHandler.applyPreHandle(processedRequest, response)) {
                    return;
                }

                // Actually invoke the handler.（执行实际的处理程序）
                mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

                if (asyncManager.isConcurrentHandlingStarted()) {
                    return;
                }

                applyDefaultViewName(processedRequest, mv);
                // 遍历拦截器，执行它们的 postHandle() 方法
                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);
            }
            // 处理执行结果，是一个 ModelAndView 或 Exception，然后进行渲染
            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) {
                    // 遍历拦截器，执行它们的 afterCompletion() 方法
                    mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
                }
            }
            else {
                // Clean up any resources used by a multipart request.
                if (multipartRequestParsed) {
                    cleanupMultipart(processedRequest);
                }
            }
        }
    }