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SpringMVC运行流程分析之核心流程

小编 2022-06-09 阅读 66
springmvc核心流程sedspringmvc编程语言

关联博客：
​SpringMVC源码分析之策略对象初始化SpringMVC运行流程分析之前置流程SpringMVC运行流程分析之核心流程 前面SpringMVC运行流程分析之前置流程

博文中我们分析了springMVC请求处理的前置流程，本文我们分析其核心流程也就是​​doDispatch(request, response);​​。

这里先放一下时序图展示整体流程：
SpringMVC运行流程分析之核心流程_springmvc核心流程
那么HandlerAdapter核心流程逻辑呢？
SpringMVC运行流程分析之核心流程_sed_02

【1】整体结构

① 源码概览

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
  HttpServletRequest processedRequest = request;
  // 处理器执行链对象
  HandlerExecutionChain mappedHandler = null;
  // 是否multipart请求
  boolean multipartRequestParsed = false;

// 异步处理管理器
  WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);

  try {
    //模型视图对象
    ModelAndView mv = null;
    // 异常对象
    Exception dispatchException = null;

    try {
    // 如果是multipart request 将会尝试使用multipartResolver解析
      processedRequest = checkMultipart(request);
      multipartRequestParsed = (processedRequest != request);

      // Determine handler for the current request.
      mappedHandler = getHandler(processedRequest);
      if (mappedHandler == null) {
        noHandlerFound(processedRequest, response);
        return;
      }

      // Determine handler adapter for the current request.
      // 获取处理器适配器
      HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

      // Process last-modified header, if supported by the handler.
      String method = request.getMethod();
      boolean isGet = "GET".equals(method);
      // 如果是get且没有修改，直接返回
      if (isGet || "HEAD".equals(method)) {
        long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
        if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
          return;
        }
      }
// 如果某个拦截器前置方法返回false，直接返回
      if (!mappedHandler.applyPreHandle(processedRequest, response)) {
        return;
      }
// 核心处理方法
      // Actually invoke the handler.
      mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

// 如果是异步请求处理，则这里就返回
      if (asyncManager.isConcurrentHandlingStarted()) {
        return;
      }
// 如果MV不为null，但是没有view，则尝试获取defaultViewName
      applyDefaultViewName(processedRequest, mv);
      // 处理执行链的后置处理--即拦截器的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);
    }
// 处理最后结果，这里可能会返回到一个页面或者抛出异常，或者返回json等    
    processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
  }
  catch (Exception ex) {
  // 拦截器的afterCompletion方法
    triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
  }
  catch (Throwable err) {
    triggerAfterCompletion(processedRequest, response, mappedHandler,
        new NestedServletException("Handler processing failed", err));
  }
  finally {
  // 如果是异步请求 且 mappedHandler 不为null，
    if (asyncManager.isConcurrentHandlingStarted()) {
      // Instead of postHandle and afterCompletion
      if (mappedHandler != null) {
// 如果拦截器是asyncInterceptor类型，则调用afterConcurrentHandlingStarted方法
        mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
      }
    }
    else {
      // Clean up any resources used by a multipart request.
      if (multipartRequestParsed) {
      // 如果是multipartRequest，则清理资源 fielItem哦
        cleanupMultipart(processedRequest);
      }
    }
  }
}

仅就这段方法来看，其流程如下所示：
SpringMVC运行流程分析之核心流程_springmvc核心流程_03
这里有三个步骤十分关键，分别是第三步​​​mappedHandler = getHandler(processedRequest);​​​获取执行器链，第六步​​mv = ha.handle(processedRequest, response, mappedHandler.getHandler());​​​获取MV以及最终的第十步​​processDispatchResult​​处理最终结果。

第三步是给请求尝试找到一个handler，比如某个controller某个方法，或者说默认的处理器DefaultServletHttpRequestHandler等。然后实例化HandlerExecutionChain 后添加合适的拦截器。

第六步则是核心的处理过程，会进行参数解析、目标方法的反射调用以及最终返回结果的处理。

第十步processDispatchResult处理最终结果通常比如渲染视图或者返回json。如果在前面处理过程中抛出了异常，那么这里也会对异常尝试处理。

② 核心对象HandlerExecutionChain

HandlerExecutionChain ，处理器执行链。其通过遍历HandlerMapping，根据其方法​​HandlerExecutionChain getHandler(HttpServletRequest request)​​获取得到。

通过如下核心属性与构造方法可知，其由handler和HandlerInterceptor组成。handler比如HandlerMethod（或其他handler类）即实际处理请求的目标类(方法)。HandlerInterceptor就是拦截器，在处理流程中分别会触发其preHandle、postHandle以及afterCompletion方法。

HandlerExecutionChain 中的handler可能如下：

实现了org.springframework.web.servlet.function.HandlerFunction，比如ResourceHandlerFunction
实现了org.springframework.web.servlet.mvc.Controller，比如ParameterizableViewController
实现了org.springframework.web.HttpRequestHandler，比如DefaultServletHttpRequestHandler
实现了javax.servlet.Servlet，比如ViewStatusMessagesServlet
实现了org.springframework.web.method.HandlerMethod，比如ServletInvocableHandlerMethod，controller里面很多方法就被包装为了该类型。这个也是最常见的

核心属性和构造方法：

private final Object handler;

private final List<HandlerInterceptor> interceptorList = new ArrayList<>();

private int interceptorIndex = -1;

/**
 * Create a new HandlerExecutionChain.
 * @param handler the handler object to execute
 */
public HandlerExecutionChain(Object handler) {
  this(handler, (HandlerInterceptor[]) null);
}

③ 核心对象ModelAndView

模型视图对象，模型这里理解为一个存放数据的map，视图view通常需要一个​​ViewResolver​​解析处理。视图解析器会获取到model然后渲染view最后返回给前台一个解析后的页面。

我们下面看下其核心属性和构造方法。

// 视图实例或者视图名称
@Nullable
private Object view;

// 存放数据 map结构
@Nullable
private ModelMap model;

// 响应状态码 可为空
@Nullable
private HttpStatus status;

// 指示是否已经调用clear方法进行清理
private boolean cleared = false;

 // 空构造方法
public ModelAndView() {
}

 // 设置viewName
public ModelAndView(String viewName) {
  this.view = viewName;
}

 // 设置view
public ModelAndView(View view) {
  this.view = view;
}


 // 设置viewName和model
public ModelAndView(String viewName, @Nullable Map<String, ?> model) {
  this.view = viewName;
  if (model != null) {
    getModelMap().addAllAttributes(model);
  }
}

 // 设置view和model
public ModelAndView(View view, @Nullable Map<String, ?> model) {
  this.view = view;
  if (model != null) {
    getModelMap().addAllAttributes(model);
  }
}
 // 设置viewName status
public ModelAndView(String viewName, HttpStatus status) {
  this.view = viewName;
  this.status = status;
}

 // 完整实例
public ModelAndView(@Nullable String viewName, @Nullable Map<String, ?> model, @Nullable HttpStatus status) {
  this.view = viewName;
  if (model != null) {
    getModelMap().addAllAttributes(model);
  }
  this.status = status;
}


 // 设置viewName 然后向model中添加属性 modelName = modelObject
public ModelAndView(String viewName, String modelName, Object modelObject) {
  this.view = viewName;
  addObject(modelName, modelObject);
}

 // 设置view然后向model中添加属性 modelName = modelObject
public ModelAndView(View view, String modelName, Object modelObject) {
  this.view = view;
  addObject(modelName, modelObject);
}

另外，关于HandlerMapping组件、HandlerAdapter组件可以参考下面博文，这里不再赘述。
​SpringMVC常见组件之HandlerMapping分析​、SpringMVC常见组件之HandlerAdapter分析

【2】方法说明

① checkMultipart

源码如下所示，其核心是判断当前请求是否为​​multipart request​​​并尝试使用​​multipartResolver​​进行解析。通常这一步应用于文件上传场景。

protected HttpServletRequest checkMultipart(HttpServletRequest request) throws MultipartException {
    if (this.multipartResolver != null && this.multipartResolver.isMultipart(request)) {
      if (WebUtils.getNativeRequest(request, MultipartHttpServletRequest.class) != null) {
        if (request.getDispatcherType().equals(DispatcherType.REQUEST)) {
          logger.trace("Request already resolved to MultipartHttpServletRequest, e.g. by MultipartFilter");
        }
      }
      else if (hasMultipartException(request)) {
        logger.debug("Multipart resolution previously failed for current request - " +
            "skipping re-resolution for undisturbed error rendering");
      }
      else {
        try {
          return this.multipartResolver.resolveMultipart(request);
        }
        catch (MultipartException ex) {
          if (request.getAttribute(WebUtils.ERROR_EXCEPTION_ATTRIBUTE) != null) {
            logger.debug("Multipart resolution failed for error dispatch", ex);
            // Keep processing error dispatch with regular request handle below
          }
          else {
            throw ex;
          }
        }
      }
    }
    // If not returned before: return original request.
    return request;
  }

② getHandler(processedRequest)

获取处理器执行链对象​​HandlerExecutionChain​​​。这里本质是遍历容器中的​​HandlerMapping​​ ，调用其getHandler方法获取HandlerExecutionChain 。

protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
  if (this.handlerMappings != null) {
    for (HandlerMapping mapping : this.handlerMappings) {
      HandlerExecutionChain handler = mapping.getHandler(request);
      if (handler != null) {
        return handler;
      }
    }
  }
  return null;
}

关于​​mapping.getHandler(request)​​​具体逻辑参考博文SpringMVC常见组件之HandlerMapping分析。

③ getHandlerAdapter(mappedHandler.getHandler())

同②一样，这里会遍历容器中的​​handlerAdapters​​​，找到一个支持当前handler的适配器。如果找不到，将会抛出​​ServletException​​异常。

protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {
  if (this.handlerAdapters != null) {
    for (HandlerAdapter adapter : this.handlerAdapters) {
      if (adapter.supports(handler)) {
        return adapter;
      }
    }
  }
  throw new ServletException("No adapter for handler [" + handler +
      "]: The DispatcherServlet configuration needs to include a HandlerAdapter that supports this handler");
}

④ 拦截器的方法

这里指的是​​HandlerExecutionChain.applyPreHandle、HandlerExecutionChain.applyPostHandle​​​以及​​triggerAfterCompletion(有两个哦)​​。

① 前置处理与后置处理

applyPreHandle与applyPostHandle本质只是遍历interceptorList，调用其preHandle方法。

需要注意的是在applyPreHandle中，如果某个拦截器的preHandle方法返回false，则会触发triggerAfterCompletion方法。applyPostHandle则不会触发triggerAfterCompletion方法。

boolean applyPreHandle(HttpServletRequest request, HttpServletResponse response) throws Exception {
  for (int i = 0; i < this.interceptorList.size(); i++) {
    HandlerInterceptor interceptor = this.interceptorList.get(i);
    if (!interceptor.preHandle(request, response, this.handler)) {
      // 这里第三个参数为null
      triggerAfterCompletion(request, response, null);
      return false;
    }
    this.interceptorIndex = i;
  }
  return true;
}

void applyPostHandle(HttpServletRequest request, HttpServletResponse response, @Nullable ModelAndView mv)
    throws Exception {

  for (int i = this.interceptorList.size() - 1; i >= 0; i--) {
    HandlerInterceptor interceptor = this.interceptorList.get(i);
    interceptor.postHandle(request, response, this.handler, mv);
  }
}

②triggerAfterCompletion

首先是​​DispatcherServlet​​​的​​triggerAfterCompletion​​​，其调用位置如下图所示(会将异常跑出去)：
SpringMVC运行流程分析之核心流程_mvc_04
方法如下所示，其会调用​​​mappedHandler(HandlerExecutionChain)​​​的​​triggerAfterCompletion​​方法，然后抛出异常。

private void triggerAfterCompletion(HttpServletRequest request, HttpServletResponse response,
    @Nullable HandlerExecutionChain mappedHandler, Exception ex) throws Exception {

  if (mappedHandler != null) {
    mappedHandler.triggerAfterCompletion(request, response, ex);
  }
  throw ex;
}

​​HandlerExecutionChain​​​的​​triggerAfterCompletion​​​方法如下所示，本质是遍历​​interceptorList​​​调用每一个拦截器的​​afterCompletion​​方法。

void triggerAfterCompletion(HttpServletRequest request, HttpServletResponse response, @Nullable Exception ex) {
  for (int i = this.interceptorIndex; i >= 0; i--) {
    HandlerInterceptor interceptor = this.interceptorList.get(i);
    try {
      interceptor.afterCompletion(request, response, this.handler, ex);
    }
    catch (Throwable ex2) {
      logger.error("HandlerInterceptor.afterCompletion threw exception", ex2);
    }
  }
}

从这里也可以看出​​HandlerExecutionChain​​​虽然维护了​​handler​​​与​​interceptorList​​​，但是本身没有业务逻辑，具体业务逻辑处理都委派给了​​HandlerInterceptor​​。

⑤ 默认视图名称处理

如果​​ha.handle(processedRequest, response, mappedHandler.getHandler())​​返回的MV不是null，但是却没有view。那么这里会尝试根据request获取一个默认viewName。

private void applyDefaultViewName(HttpServletRequest request, @Nullable ModelAndView mv) throws Exception {
  if (mv != null && !mv.hasView()) {
    String defaultViewName = getDefaultViewName(request);
    if (defaultViewName != null) {
      mv.setViewName(defaultViewName);
    }
  }
}

// 获取默认视图名称方法如下
@Nullable
protected String getDefaultViewName(HttpServletRequest request) throws Exception {
  return (this.viewNameTranslator != null ? this.viewNameTranslator.getViewName(request) : null);
}

这里根据请求获取默认视图名称步骤如下：

  • ①获取请求路径 lookupPath
  • ② 如果是以​​/​​​ 开头，则去掉头部的​​/​​;
  • ③ 如果是以​​/​​​ 结尾，则去掉尾部的​​/​​;
  • ④ 去掉文件拓展名​​e.g. "mypath/myfile.txt" -> "mypath/myfile".​​
  • ⑤ 如果SLASH不等于separator，则进行替换​​path = StringUtils.replace(path, SLASH, this.separator);​​​，默认情况下​​SLASH==separator==/​​

假设这里请求是​​http://localhost:8080/testNoView​​​，那么获取的lookupPath为 ​​/testNoView​​​,得到的默认视图名称是testNoView。如果目标位置不存在这个文件，则会抛出如下异常：
SpringMVC运行流程分析之核心流程_mvc_05

【3】结果处理

这里指的是​​processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);​​。

① processDispatchResult

代码如下所示，分为这样几步：异常处理、视图处理、异步请求判断以及最后的triggerAfterCompletion。

private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
    @Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv,
    @Nullable Exception exception) throws Exception {

  boolean errorView = false;
// 异常处理
  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);
    }
  }
// 视图处理
  if (mv != null && !mv.wasCleared()) {
  //核心步骤--视图渲染
    render(mv, request, response);
    if (errorView) { //如果是错误视图，清理request中error相关属性
      WebUtils.clearErrorRequestAttributes(request);
    }
  }
  else {
    if (logger.isTraceEnabled()) {
      logger.trace("No view rendering, null ModelAndView returned.");
    }
  }
// 异步请求判断
  if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
    // Concurrent handling started during a forward
    return;
  }
// triggerAfterCompletion 处理 这里不再赘述
  if (mappedHandler != null) {
    // Exception (if any) is already handled..
    mappedHandler.triggerAfterCompletion(request, response, null);
  }
}

② 异常处理

如下所示是doDispatch的核心逻辑 。

//doDispatch的核心逻辑
DispatcherServlet#doDispatch
  - processedRequest = checkMultipart(request);
  - mappedHandler = getHandler(processedRequest);
  - HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());
  - mappedHandler.applyPreHandle(processedRequest, response)
  - mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
  - applyDefaultViewName(processedRequest, mv);
  - mappedHandler.applyPostHandle(processedRequest, response, mv);
  - processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
  - triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
  -

如果我们的业务抛出了异常，那么是在什么位置呢？

mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
  - AbstractHandlerMethodAdapter#handle
  - RequestMappingHandlerAdapter#handleInternal
  - mav = invokeHandlerMethod(request, response, handlerMethod);
  - RequestMappingHandlerAdapter#invokeHandlerMethod
  // 假设这里抛出异常--通常就是我们的业务异常
  - invocableMethod.invokeAndHandle(webRequest, mavContainer);
  - return getModelAndView(mavContainer, modelFactory, webRequest);

如下图所示，当执行​​invokeAndHandle​​​反射调用我们的目标方法时，假设业务抛出了异常，那么这里会一路抛出到​​DispatcherServlet.doDispatch​​​中,交给​​processDispatchResult​​​去处理。无论​​RequestMappingHandlerAdapter​​​还是​​ServletInvocableHandlerMethod​​​都没有对这里的异常进行拦截。
SpringMVC运行流程分析之核心流程_sed_06

如下代码所示，如果异常是​​ModelAndViewDefiningException​​​，则直接获取其​​ModelAndView​​​属性。否则获取handler，然后根据​​processHandlerException(request, response, handler, exception)​​尝试获取MV。

​​processHandlerException​​​放入如下所示，首先从request移除属性​​HandlerMapping.class.getName() + ".producibleMediaTypes"​​​；然后尝试用​​HandlerExceptionResolver​​ 处理异常获取到exMV。获取到exMV后如果其不为null，那么将会是否有数据，是否有view；最后暴露错误属性信息给request。

@Nullable
protected ModelAndView processHandlerException(HttpServletRequest request, HttpServletResponse response,
    @Nullable Object handler, Exception ex) throws Exception {

  // Success and error responses may use different content types
  request.removeAttribute(HandlerMapping.PRODUCIBLE_MEDIA_TYPES_ATTRIBUTE);

  // 遍历异常解析器处理异常，核心步骤
  ModelAndView exMv = null;
  if (this.handlerExceptionResolvers != null) {
    for (HandlerExceptionResolver resolver : this.handlerExceptionResolvers) {
      exMv = resolver.resolveException(request, response, handler, ex);
      if (exMv != null) {
        break;
      }
    }
  }
  if (exMv != null) {
    // 判断model是否为空
    if (exMv.isEmpty()) {
      request.setAttribute(EXCEPTION_ATTRIBUTE, ex);
      return null;
    }
    // We might still need view name translation for a plain error model...
    if (!exMv.hasView()) {
    // 获取默认视图名称
      String defaultViewName = getDefaultViewName(request);
      if (defaultViewName != null) {
        exMv.setViewName(defaultViewName);
      }
    }
    if (logger.isTraceEnabled()) {
      logger.trace("Using resolved error view: " + exMv, ex);
    }
    else if (logger.isDebugEnabled()) {
      logger.debug("Using resolved error view: " + exMv);
    }
    // 暴露错误相关属性信息给request
    WebUtils.exposeErrorRequestAttributes(request, ex, getServletName());
    return exMv;
  }

  throw ex;
}

如果我们使用​​@ControllerAdvice​​​和​​@ExceptionHandler(value = {Exception.class})​​​来捕捉异常进行全局异常处理。那么上面​​exMv = resolver.resolveException(request, response, handler, ex);​​​我们会调用异常解析器去处理异常，得到的exMV呢是一个空的ModelAndView。
SpringMVC运行流程分析之核心流程_mvc_07

关于异常解析器处理异常逻辑参考博文SpringMVC常见组件之HandlerExceptionResolver分析​。这里我们看下​​WebUtils.exposeErrorRequestAttributes(request, ex, getServletName());​​方法。

public static void exposeErrorRequestAttributes(HttpServletRequest request, Throwable ex,
    @Nullable String servletName) {

  exposeRequestAttributeIfNotPresent(request, ERROR_STATUS_CODE_ATTRIBUTE, HttpServletResponse.SC_OK);
  exposeRequestAttributeIfNotPresent(request, ERROR_EXCEPTION_TYPE_ATTRIBUTE, ex.getClass());
  exposeRequestAttributeIfNotPresent(request, ERROR_MESSAGE_ATTRIBUTE, ex.getMessage());
  exposeRequestAttributeIfNotPresent(request, ERROR_EXCEPTION_ATTRIBUTE, ex);
  exposeRequestAttributeIfNotPresent(request, ERROR_REQUEST_URI_ATTRIBUTE, request.getRequestURI());
  if (servletName != null) {
    exposeRequestAttributeIfNotPresent(request, ERROR_SERVLET_NAME_ATTRIBUTE, servletName);
  }
}

错误属性key如下所示(都是​​public static final​​)：

String EXCEPTION_ATTRIBUTE = DispatcherServlet.class.getName() + ".EXCEPTION";

String ERROR_STATUS_CODE_ATTRIBUTE = "javax.servlet.error.status_code";
String ERROR_EXCEPTION_TYPE_ATTRIBUTE = "javax.servlet.error.exception_type";
String ERROR_MESSAGE_ATTRIBUTE = "javax.servlet.error.message";
String ERROR_EXCEPTION_ATTRIBUTE = "javax.servlet.error.exception";
String ERROR_REQUEST_URI_ATTRIBUTE = "javax.servlet.error.request_uri";
String ERROR_SERVLET_NAME_ATTRIBUTE = "javax.servlet.error.servlet_name";

③视图处理

视图处理这里指的是​​render(mv, request, response);​​。ModelAndView里面包含了数据和视图信息，那么这一部分通常主要逻辑就是数据渲染，给浏览器一个解析好的页面。

protected void render(ModelAndView mv, HttpServletRequest request, HttpServletResponse response) throws Exception {
  // 尝试解析获取得到locale信息
  Locale locale =
      (this.localeResolver != null ? this.localeResolver.resolveLocale(request) : request.getLocale());
  //为响应设置locale信息
  response.setLocale(locale);

  View view;
  // 获取视图名称
  String viewName = mv.getViewName();
  // 如果视图名称不为null，则解析得到View实例
  if (viewName != null) {
    //核心步骤哦，解析viewName，会遍历视图解析器处理
    view = resolveViewName(viewName, mv.getModelInternal(), locale, request);
    if (view == null) {
      throw new ServletException("Could not resolve view with name '" + mv.getViewName() +
          "' in servlet with name '" + getServletName() + "'");
    }
  }
  else {
    // 如果不存在viewName则尝试获取view实例，也不存在，则抛出异常
    view = mv.getView();
    if (view == null) {
      throw new ServletException("ModelAndView [" + mv + "] neither contains a view name nor a " +
          "View object in servlet with name '" + getServletName() + "'");
    }
  }

  // Delegate to the View object for rendering.
  if (logger.isTraceEnabled()) {
    logger.trace("Rendering view [" + view + "] ");
  }
  try {
  // 如果MV中有http status ，则给response设置status状态码
    if (mv.getStatus() != null) {
      response.setStatus(mv.getStatus().value());
    }
    // 视图渲染，核心步骤，调用具体的View实例的render方法
    view.render(mv.getModelInternal(), request, response);
  }
  catch (Exception ex) {
    if (logger.isDebugEnabled()) {
      logger.debug("Error rendering view [" + view + "]", ex);
    }
    throw ex;
  }
}

这里有两步需要注意：​​resolveViewName(viewName, mv.getModelInternal(), locale, request);​​​解析视图名称得到具体的View以及​​view.render(mv.getModelInternal(), request, response);​​调用具体的View实例的render方法进行视图渲染。

如下所示​​resolveViewName​​​方法解析视图名称得到实际的View实例，核心是遍历容器中的​​viewResolvers​​​调用其​​resolveViewName​​方法得到View实例。

@Nullable
protected View resolveViewName(String viewName, @Nullable Map<String, Object> model,
    Locale locale, HttpServletRequest request) throws Exception {

  if (this.viewResolvers != null) {
    for (ViewResolver viewResolver : this.viewResolvers) {
      View view = viewResolver.resolveViewName(viewName, locale);
      if (view != null) {
        return view;
      }
    }
  }
  return null;
}


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