SpringMVC执行流程_attributesnapshot==null-优快云博客

本文链接：https://blog.youkuaiyun.com/qq_42671904/article/details/126966046

SpringMVC常用组件

1 DispatcherServlet：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，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，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，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，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
作用：将模型数据通过页面展示给用户

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初始化WebApplicationContext

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 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
    // 创建WebApplicationContext
    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) {
      // 刷新WebApplicationContext
      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 = getServletContextAttributeName();
    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 = 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;
}

③DispatcherServlet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) {
 initMultipartResolver(context);
 initLocaleResolver(context);
 initThemeResolver(context);
 initHandlerMappings(context);
 initHandlerAdapters(context);
 initHandlerExceptionResolvers(context);
 initRequestToViewNameTranslator(context);
 initViewResolvers(context);
 initFlashMapManager(context);
}

DispatcherServlet调用组件处理请求

①processRequest()

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);
 }
}

②doService()

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 requestPath = null;
  if (this.parseRequestPath &&
!ServletRequestPathUtils.hasParsedRequestPath(request)) {
    requestPath = 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 (requestPath != null) {
      ServletRequestPathUtils.clearParsedRequestPath(request);
   }
 }
}

③doDispatch()

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.
      /*
     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 = 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 = "GET".equals(method);
      if (isGet || "HEAD".equals(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 afterCompletion
      if (mappedHandler != null) {
      
 mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
     }
   }
    else {
      // Clean up any resources used by a multipart request.
      if (multipartRequestParsed) {
        cleanupMultipart(processedRequest);
     }
   }
 }
}

④processDispatchResult()

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 forward
    return;
 }
  if (mappedHandler != null) {
    // Exception (if any) is already handled..
    // 调用拦截器的afterCompletion()
    mappedHandler.triggerAfterCompletion(request, response, null);
 }
}

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：
a) 不存在
i. 再判断是否配置了mvc:default-servlet-handler
ii. 如果没配置，则控制台报映射查找不到，客户端展示404错误

iii. 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

b) 存在则执行下面的流程
3) 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。
4) DispatcherServlet 根据获得的Handler，选择一个合适的HandlerAdapter。
5) 如果成功获得HandlerAdapter，此时将开始执行拦截器的preHandler(…)方法【正向】
6) 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：
a) 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
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
7) Handler执行完成后，向DispatcherServlet 返回一个ModelAndView对象。
8) 此时将开始执行拦截器的postHandle(...)方法【逆向】。
9) 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。
10) 渲染视图完毕执行拦截器的afterCompletion(…)方法【逆向】。
11) 将渲染结果返回给客户端。