14、SpringMVC执行流程

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【尚硅谷】SSM框架全套教程-讲师：杨博超

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14、SpringMVC执行流程

14.1、SpringMVC常用组件

1 DispatcherServlet（前端控制器）

介绍：前端控制器，不需要工程师开发，由框架提供

作用：统一处理请求和响应，整个流程控制的中心，由它调用其它组件处理用户的请求

2 HandlerMapping（处理器映射器）

介绍：处理器映射器，不需要工程师开发，由框架提供

作用：根据请求的url、method等信息查找Handler，即控制器方法

3 Handler（处理器）

介绍：处理器，需要工程师开发

作用：在DispatcherServlet的控制下Handler对具体的用户请求进行处理

4 HandlerAdapter（处理器适配器）

介绍：处理器适配器，不需要工程师开发，由框架提供

作用：通过HandlerAdapter对处理器（控制器方法）进行执行

5 ViewResolver（视图解析器）

介绍：视图解析器，不需要工程师开发，由框架提供

作用：进行视图解析，得到相应的视图，例如：ThymeleafView、InternalResourceView、RedirectView

6 View（视图）

介绍：视图

作用：将模型数据通过页面展示给用户

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。

继承关系

DispatcherServlet

public class DispatcherServlet extends FrameworkServlet {}

FrameworkServlet

public abstract class FrameworkServlet extends HttpServletBean implements ApplicationContextAware {}

HttpServletBean

public abstract class HttpServletBean extends HttpServlet implements EnvironmentCapable, EnvironmentAware {}

HttpServlet

public abstract class HttpServlet extends GenericServlet{}

GenericServlet

public abstract class GenericServlet implements Servlet, ServletConfig, java.io.Serializable{}

Servlet

public interface Servlet {}

1 初始化WebApplicationContext

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

@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 WebApplicationContextthis.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");}
}

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

protected WebApplicationContext initWebApplicationContext() {WebApplicationContext rootContext =WebApplicationContextUtils.getWebApplicationContext(getServletContext());WebApplicationContext wac = null;if (this.webApplicationContext != null) {// A context instance was injected at construction time -> use itwac = 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, etcif (cwac.getParent() == null) {// The context instance was injected without an explicit parent -> set// the root application context (if any; may be null) as the parentcwac.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 idwac = findWebApplicationContext();}if (wac == null) {// No context instance is defined for this servlet -> create a local one// 创建WebApplicationContextwac = 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) {// 刷新WebApplicationContextonRefresh(wac);}}if (this.publishContext) {// Publish the context as a servlet context attribute.// 将IOC容器在应用域共享String attrName = getServletContextAttributeName();getServletContext().setAttribute(attrName, wac);}return wac;
}

2 创建WebApplicationContext

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

protected WebApplicationContext createWebApplicationContext(@Nullable WebApplicationContext parent) {return createWebApplicationContext((ApplicationContext) parent);
}

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

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

protected void onRefresh(ApplicationContext context) {// For subclasses: do nothing by default.
}

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

@Override
protected void onRefresh(ApplicationContext context) {initStrategies(context);
}

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

protected void initStrategies(ApplicationContext context) {initMultipartResolver(context); // 初始化文件上传解析器initLocaleResolver(context); // 初始化本地解析器initThemeResolver(context); // initHandlerMappings(context); // 初始化处理器映射器initHandlerAdapters(context); // 初始化处理器适配器initHandlerExceptionResolvers(context); // 初始化异常解析器initRequestToViewNameTranslator(context); // initViewResolvers(context); // 初始化视图解析器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 = 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 {// 执行服务，doService()是一个抽象方法，在DispatcherServlet中进行了重写doService(request, response);}catch (ServletException | 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();}logResult(request, response, failureCause, asyncManager);publishRequestHandledEvent(request, response, startTime, failureCause);}
}

2 doService()

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

@Override
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {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();while (attrNames.hasMoreElements()) {String attrName = (String) attrNames.nextElement();if (this.cleanupAfterInclude || attrName.startsWith(DEFAULT_STRATEGIES_PREFIX)) {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());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);}}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 {ModelAndView mv = null;Exception dispatchException = null;try {processedRequest = checkMultipart(request);multipartRequestParsed = (processedRequest != request);// Determine handler for the current request.// Determine handler for the current request./*mappedHandler：调用链包含handler、interceptorList、interceptorIndexhandler：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 = getHandler(processedRequest);if (mappedHandler == null) {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 = 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;}applyDefaultViewName(processedRequest, mv);// 调用拦截器的postHandle()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);}// 后续处理：处理模型数据和渲染视图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 afterCompletionif (mappedHandler != null) {mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);}}else {// Clean up any resources used by a multipart request.if (multipartRequestParsed) {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) {logger.debug("ModelAndViewDefiningException encountered", exception);mv = ((ModelAndViewDefiningException) exception).getModelAndView();}else {Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);mv = 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render(mv, request, response);if (errorView) {WebUtils.clearErrorRequestAttributes(request);}}else {if (logger.isTraceEnabled()) {logger.trace("No view rendering, null ModelAndView returned.");}}if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {// Concurrent handling started during a forwardreturn;}if (mappedHandler != null) {// Exception (if any) is already handled..// 调用拦截器的afterCompletion()mappedHandler.triggerAfterCompletion(request, response, null);}
}

14.4、SpringMVC的执行流程

1 DispatcherServlet

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

DispatcherServlet（前端控制器）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：

假设请求的url为 : http://localhost:8080/SpringMVC/hello
如上url拆分成三部分：
http://localhost:8080 ------> 服务器域名
SpringMVC ------> 部署在服务器上的web站点
hello ------> 表示控制器方法
通过分析，如上url表示为：请求位于服务器localhost:8080上的SpringMVC站点的hello控制器方法

2 控制器方法不存在

再判断是否配置了

<!--默认servlet处理-->
<mvc:default-servlet-handler></mvc:default-servlet-handler>
<!--开启mvc注解驱动-->
<mvc:annotation-driven/>

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，则先交给 DispatcherServlet（前端控制器） 处理，处理不了，再交给默认servlet处理，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

3 控制器方法存在则执行下面的流程

根据该URI，调用HandlerMapping（处理器映射器）获得该Handler（处理器：controller中的方法）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。

DispatcherServlet 根据获得的 Handler，选择一个合适的HandlerAdapter（处理器适配器），用于执行控制器方法。

如果成功获得HandlerAdapter（处理器适配器），此时将开始执行拦截器的 preHandler(…)方法【正向】

提取Request中的模型数据，填充Handler入参，开始执行Handler（处理器)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：

4 额外的工作

a HttpMessageConveter（报文信息转换器@RequestBody注解）： 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

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

c 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

d 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 余下步骤

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

此时将开始执行拦截器的 postHandle(…)方法【逆向】

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（视图解析器)进行视图解析，根据Model和View，来渲染视图

渲染视图完毕执行拦截器的afterCompletion(…)方法【逆向】

将渲染结果返回给客户端

14.5、画图

图为SpringMVC的一个较完整的流程图，实线表示SpringMVC框架提供的技术，不需要开发者实现，虚线表示需要开发者实现。