SSM 之 SpringMVC（下）-阿里云开发者社区

14.2、DispatcherServlet初始化过程

DispatcherServlet 本质上是一个 Servlet，所以天然的遵循 Servlet 的生命周期。所以宏观上是 Servlet 生命周期来进行调度。

+1.png ① 初始化WebApplicationContext

所在类：org.springframework.web.servlet.FrameworkServlet

protected WebApplicationContext initWebApplicationContext() {
    WebApplicationContext rootContext = WebApplicationContextUtils.getWebApplicationContext(this.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);
                }
                this.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 = this.findWebApplicationContext();
    }
    if (wac == null) {
        // No context instance is defined for this servlet -> create a local one
    // 创建WebApplicationContext
        wac = this.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) {
            this.onRefresh(wac);
        }
    }
    if (this.publishContext) {
        // Publish the context as a servlet context attribute.
    // 将IOC容器在应用域共享
        String attrName = this.getServletContextAttributeName();
        this.getServletContext().setAttribute(attrName, wac);
    }
    return wac;
}

② 创建WebApplicationContext

所在类：org.springframework.web.servlet.FrameworkServlet

protected WebApplicationContext createWebApplicationContext(@Nullable ApplicationContext parent) {
    Class<?> contextClass = this.getContextClass();
    if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
        throw new ApplicationContextException("Fatal initialization error in servlet with name '" + this.getServletName() + "': custom WebApplicationContext class [" + contextClass.getName() + "] is not of type ConfigurableWebApplicationContext");
    } else {
        // 通过反射创建 IOC 容器对象
        ConfigurableWebApplicationContext wac = (ConfigurableWebApplicationContext)BeanUtils.instantiateClass(contextClass);
        wac.setEnvironment(this.getEnvironment());
        // 设置父容器
        wac.setParent(parent);
        String configLocation = this.getContextConfigLocation();
        if (configLocation != null) {
            wac.setConfigLocation(configLocation);
        }
        this.configureAndRefreshWebApplicationContext(wac);
        return wac;
    }
}

③ DispatcherServlet初始化策略

FrameworkServlet创建WebApplicationContext后，刷新容器，调用onRefresh(wac)，此方法在 DispatcherServlet中进行了重写，调用了initStrategies(context)方法，初始化策略，即初始化 DispatcherServlet的各个组件

所在类：org.springframework.web.servlet.DispatcherServlet

protected void initStrategies(ApplicationContext context) {
    this.initMultipartResolver(context);
    this.initLocaleResolver(context);
    this.initThemeResolver(context);
    this.initHandlerMappings(context);
    this.initHandlerAdapters(context);
    this.initHandlerExceptionResolvers(context);
    this.initRequestToViewNameTranslator(context);
    this.initViewResolvers(context);
    this.initFlashMapManager(context);
}

14.3、DispatcherServlet调用组件处理请求

① processRequest()

FrameworkServlet重写HttpServlet中的service()和doXxx()，这些方法中调用了 processRequest(request, response)

所在类：org.springframework.web.servlet.FrameworkServlet

protected final void processRequest(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    long startTime = System.currentTimeMillis();
    Throwable failureCause = null;
    LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
    LocaleContext localeContext = this.buildLocaleContext(request);
    RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
    ServletRequestAttributes requestAttributes = this.buildRequestAttributes(request, response, previousAttributes);
    WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
    asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor());
    this.initContextHolders(request, localeContext, requestAttributes);
    try {
        // 执行服务，doService()是一个抽象方法，在DispatcherServlet中进行了重写
        this.doService(request, response);
    } catch (IOException | ServletException var16) {
        failureCause = var16;
        throw var16;
    } catch (Throwable var17) {
        failureCause = var17;
        throw new NestedServletException("Request processing failed", var17);
    } finally {
        this.resetContextHolders(request, previousLocaleContext, previousAttributes);
        if (requestAttributes != null) {
            requestAttributes.requestCompleted();
        }
        this.logResult(request, response, (Throwable)failureCause, asyncManager);
        this.publishRequestHandledEvent(request, response, startTime, (Throwable)failureCause);
    }
}

② doService()

所在类：org.springframework.web.servlet.DispatcherServlet

protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
    this.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();
        label116:
        while(true) {
            String attrName;
            do {
                if (!attrNames.hasMoreElements()) {
                    break label116;
                }
                attrName = (String)attrNames.nextElement();
            } while(!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, this.getWebApplicationContext());
    request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
    request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
    request.setAttribute(THEME_SOURCE_ATTRIBUTE, this.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);
    }
    RequestPath previousRequestPath = null;
    if (this.parseRequestPath) {
        previousRequestPath = (RequestPath)request.getAttribute(ServletRequestPathUtils.PATH_ATTRIBUTE);
        ServletRequestPathUtils.parseAndCache(request);
    }
    try {
        // 处理请求和响应
        this.doDispatch(request, response);
    } finally {
        // Restore the original attribute snapshot, in case of an include.
        if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted() && attributesSnapshot != null) {
            this.restoreAttributesAfterInclude(request, attributesSnapshot);
        }
        if (this.parseRequestPath) {
            ServletRequestPathUtils.setParsedRequestPath(previousRequestPath, request);
        }
    }
}

③ doDispatch()

所在类：org.springframework.web.servlet.DispatcherServlet

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
    HttpServletRequest processedRequest = request;
    HandlerExecutionChain mappedHandler = null;
    boolean multipartRequestParsed = false;
    WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
    try {
        try {
            ModelAndView mv = null;
            Exception dispatchException = null;
            try {
                processedRequest = this.checkMultipart(request);
                multipartRequestParsed = processedRequest != request;
                // Determine handler for the current request.
                /*
                    mappedHandler：调用链
                    包含handler、interceptorList、interceptorIndex
                    handler：浏览器发送的请求所匹配的控制器方法
                    interceptorList：处理控制器方法的所有拦截器集合
                    interceptorIndex：拦截器索引，控制拦截器afterCompletion()的执行
                */
                mappedHandler = this.getHandler(processedRequest);
                if (mappedHandler == null) {
                    this.noHandlerFound(processedRequest, response);
                    return;
                }
        // Determine handler adapter for the current request.
        // 通过控制器方法创建相应的处理器适配器，调用所对应的控制器方法
                HandlerAdapter ha = this.getHandlerAdapter(mappedHandler.getHandler());
                // Process last-modified header, if supported by the handler.
                String method = request.getMethod();
                boolean isGet = HttpMethod.GET.matches(method);
                if (isGet || HttpMethod.HEAD.matches(method)) {
                    long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
                    if ((new ServletWebRequest(request, response)).checkNotModified(lastModified) && isGet) {
                        return;
                    }
                }
        // 调用拦截器的preHandle()
                if (!mappedHandler.applyPreHandle(processedRequest, response)) {
                    return;
                }
        // Actually invoke the handler.
        // 由处理器适配器调用具体的控制器方法，最终获得ModelAndView对象
                mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
                if (asyncManager.isConcurrentHandlingStarted()) {
                    return;
                }
                this.applyDefaultViewName(processedRequest, mv);
                // 调用拦截器的postHandle()
                mappedHandler.applyPostHandle(processedRequest, response, mv);
            } catch (Exception var20) {
                dispatchException = var20;
            } catch (Throwable var21) {
                // 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", var21);
            }
      // 后续处理：处理模型数据和渲染视图
            this.processDispatchResult(processedRequest, response, mappedHandler, mv, (Exception)dispatchException);
        } catch (Exception var22) {
            this.triggerAfterCompletion(processedRequest, response, mappedHandler, var22);
        } catch (Throwable var23) {
            this.triggerAfterCompletion(processedRequest, response, mappedHandler, new NestedServletException("Handler processing failed", var23));
        }
    } finally {
        if (asyncManager.isConcurrentHandlingStarted()) {
            // Instead of postHandle and afterCompletion
            if (mappedHandler != null) {
                mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
            }
        } else if (multipartRequestParsed) {
            // Clean up any resources used by a multipart request.
            this.cleanupMultipart(processedRequest);
        }
    }
}

④ processDispatchResult()

private void processDispatchResult(HttpServletRequest request, HttpServletResponse response, @Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv, @Nullable Exception exception) throws Exception {
    boolean errorView = false;
    if (exception != null) {
        if (exception instanceof ModelAndViewDefiningException) {
            this.logger.debug("ModelAndViewDefiningException encountered", exception);
            mv = ((ModelAndViewDefiningException)exception).getModelAndView();
        } else {
            Object handler = mappedHandler != null ? mappedHandler.getHandler() : null;
            mv = this.processHandlerException(request, response, handler, exception);
            errorView = mv != null;
        }
    }
  // Did the handler return a view to render?
    if (mv != null && !mv.wasCleared()) {
        // 处理模型数据和渲染视图
        this.render(mv, request, response);
        if (errorView) {
            WebUtils.clearErrorRequestAttributes(request);
        }
    } else if (this.logger.isTraceEnabled()) {
        this.logger.trace("No view rendering, null ModelAndView returned.");
    }
  // Concurrent handling started during a forward
    if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
        if (mappedHandler != null) {
            // Exception (if any) is already handled..
      // 调用拦截器的afterCompletion()
            mappedHandler.triggerAfterCompletion(request, response, (Exception)null);
        }
    }
}

14.4、SpringMVC的执行流程

1.用户向服务器发送请求，请求被SpringMVC 前端控制器 DispatcherServlet捕获。

2.DispatcherServlet对请求URL进行解析，得到请求资源标识符（URI），判断请求URI对应的映射：

不存在

再判断是否配置了 mvc:default-servlet-handler

如果没配置，则控制台报映射查找不到，客户端展示404错误

如果有配置，则访问目标资源（一般为静态资源，如：JS,CSS,HTML），找不到客户端也会展示404错误

存在

1.根据该URI，调用HandlerMapping获得该Handler配置的所有相关的对象（包括Handler对象以及 Handler对象对应的拦截器），最后以HandlerExecutionChain执行链对象的形式返回。

2.DispatcherServlet 根据获得的Handler，选择一个合适的HandlerAdapter。

3.如果成功获得HandlerAdapter，此时将开始执行拦截器的preHandler(…)方法【正向】

4.提取Request中的模型数据，填充Handler入参，开始执行Handler（Controller)方法，处理请求。在填充Handler的入参过程中，根据你的配置，Spring将帮你做一些额外的工作：

HttpMessageConveter：将请求消息（如Json、xml等数据）转换成一个对象，将对象转换为指定的响应信息

数据转换：对请求消息进行数据转换。如String转换成Integer、Double等

数据格式化：对请求消息进行数据格式化。如将字符串转换成格式化数字或格式化日期等

数据验证：验证数据的有效性（长度、格式等），验证结果存储到BindingResult或Error中

5.Handler执行完成后，向DispatcherServlet 返回一个ModelAndView对象。

6.此时将开始执行拦截器的postHandle(…)方法【逆向】。

7.根据返回的ModelAndView（此时会判断是否存在异常：如果存在异常，则执行 HandlerExceptionResolver进行异常处理）选择一个适合的ViewResolver进行视图解析，根据Model 和View，来渲染视图。

8.渲染视图完毕执行拦截器的afterCompletion(…)方法【逆向】。

9.将渲染结果返回给客户端。

SSM 之 SpringMVC（下）

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