一文讲述Spring MVC的执行流程

来自官网：

在网上找到的更加详细的图：

我们搜索下DispatchServlet类看看有没有，果然有一个

看下DispatchServlet的类关系图

1 研究DispatchServlet

看下这个类的全部方法，不用想一定会有一个类似于Servlet中的service方法，果然如此

1.1 DispatchServlet的doService方法

在doService方法中一定是主要处理业务请求的

@Override
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
   // 记录日志
   logRequest(request);
   // 保留请求属性的快照，以便在包含之后能够恢复原始属性  
   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));
         }
      }
   }
   // 设置一些request的值，使框架对象对处理程序和视图对象可用
   request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());  
   ......
   try {
      //最主要的doDispatch方法
      doDispatch(request, response);
   }
   ......
}

1.2 DispatchServlet的doDispatch方法

doDispatch是被doService方法调用，是处理请求流程的一个主要方法，主要分为一下几个流程

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
   ......
   try {
      ModelAndView mv = null;
      Exception dispatchException = null;
      try {
         //检查处理Multipart请求
         processedRequest = checkMultipart(request);
         multipartRequestParsed = (processedRequest != request);
         // 确定当前请求的处理，请求Handler
         mappedHandler = getHandler(processedRequest);
         if (mappedHandler == null) {
            noHandlerFound(processedRequest, response);
            return;
         }
         // 获取适配器
         HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());
         // 判断请求方法
         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;
            }
         }
         if (!mappedHandler.applyPreHandle(processedRequest, response)) {
            return;
         }
         // 实际的Handler调用
         mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
         if (asyncManager.isConcurrentHandlingStarted()) {
            return;
         }
         // 解析ModelAndView
         applyDefaultViewName(processedRequest, mv);
         mappedHandler.applyPostHandle(processedRequest, response, mv);
      }
      ......
      // 渲染视图
      processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
   }
   ......
   finally {
      ......
   }
}

2 深入源码

2.1 怎样找到Handler的？

先写一个RequestMapping

@ResponseBody
@RequestMapping("/helloMvc")
public String helloMvc() {
    return "Hello I am Spring MVC";
}

打断点，debug

请求

开始debug
点进方法内部再看看

protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
   if (this.handlerMappings != null) {
       // 遍历所有的request，直到最后都没有的话返回null
       for (HandlerMapping mapping : this.handlerMappings) {
         HandlerExecutionChain handler = mapping.getHandler(request);
         if (handler != null) {
            return handler;
         }
      }
   }
   return null;
}

在这之后，请求就会找到对应的Handler进行处理。

2.2 Spring MVC视图解析机制

首先定义一个ViewResolver类型的List

/** List of ViewResolvers used by this servlet. */
@Nullable
private List<ViewResolver> viewResolvers;

初始化ViewResolvers

private void initViewResolvers(ApplicationContext context) {
   this.viewResolvers = null;
   if (this.detectAllViewResolvers) {
      // 在ApplicationContext中找到所有的viewresolver，包括全部上下文
      Map<String, ViewResolver> matchingBeans =
            BeanFactoryUtils.beansOfTypeIncludingAncestors(context, ViewResolver.class, true, false);
      if (!matchingBeans.isEmpty()) {
         this.viewResolvers = new ArrayList<>(matchingBeans.values());
         // 保持viewresolver的排序。
         AnnotationAwareOrderComparator.sort(this.viewResolvers);
      }
   }
   ......
}

然后进行视图解析

private void processDispatchResult(HttpServletRequest request, HttpServletResponse response,
      @Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv,
      @Nullable Exception exception) throws Exception {
   boolean errorView = false;
   // 判断有无异常
   if (exception != null) {
      // 如果异常不为空，则返回一个错误的页面
      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);
      }
   }
   if (mv != null && !mv.wasCleared()) {
      // 视图渲染
      render(mv, request, response);
      if (errorView) {
         WebUtils.clearErrorRequestAttributes(request);
      }
   }
   ......
}

视图渲染

protected void render(ModelAndView mv, HttpServletRequest request, HttpServletResponse response) throws Exception {
   // 确定请求的区域设置并将其应用于响应
   Locale locale =
         (this.localeResolver != null ? this.localeResolver.resolveLocale(request) : request.getLocale());
   response.setLocale(locale);
   View view;
   String viewName = mv.getViewName();
   if (viewName != null) {
      // 获取视图名称
      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 {
      // No need to lookup: the ModelAndView object contains the actual View object.
      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() + "'");
      }
   }
   ......
   try {
      if (mv.getStatus() != null) {
         response.setStatus(mv.getStatus().value());
      }
      view.render(mv.getModelInternal(), request, response);
   }
   catch (Exception ex) {
      if (logger.isDebugEnabled()) {
         logger.debug("Error rendering view [" + view + "]", ex);
      }
      throw ex;
   }
}