SpringMvc之请求执行流程

MVC思想

模型-视图-控制器（MVC）是一个众所周知的以设计界面应用程序为基础的设计思想。它主要通过分离模型、视图及控制器在应用程序中的角色将业务逻辑从界面中解耦。通常，模型负责封装应用程序数据在视图层展示。视图仅仅只是展示这些数据，不包含任何业务逻辑。控制器负责接收来自用户的请求，并调用后台服务（service或者dao）来处理业务逻辑。处理后，后台业务层可能会返回了一些数据在视图层展示。控制器收集这些数据及准备模型在视图层展示。MVC模式的核心思想是将业务逻辑从界面中分离出来，允许它们单独改变而不会相互影响。

SpringMVC请求流程

Spring MVC框架也是一个基于请求驱动的Web框架，并且使用了前端控制器模式（是用来提供一个集中的请求处理机制，所有的请求都将由一个单一的处理程序处理来进行设计，再根据请求映射规则分发给相应的页面控制器（动作/处理器）进行处理。首先让我们整体看一下Spring MVC处理请求的流程：

首先用户发送请求,请求被SpringMvc前端控制器（DispatherServlet）捕获；
前端控制器(DispatherServlet)对请求URL解析获取请求URI,根据URI, 调用HandlerMapping；
前端控制器(DispatherServlet)获得返回的HandlerExecutionChain（包括Handler对象以及Handler对象对应的拦截器）；
DispatcherServlet 根据获得的HandlerExecutionChain，选择一个合适的HandlerAdapter。（附注：如果成功获得HandlerAdapter后，此时将开始执行拦截器的preHandler(...)方法）；
HandlerAdapter根据请求的Handler适配并执行对应的Handler；HandlerAdapter(提取Request中的模型数据，填充Handler入参，开始执行Handler（Controller)。在填充Handler的入参过程中，根据配置，Spring将做一些额外的工作：

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：

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

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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）
Handler执行完毕，返回一个ModelAndView(即模型和视图)给HandlerAdaptor
HandlerAdaptor适配器将执行结果ModelAndView返回给前端控制器。
前端控制器接收到ModelAndView后，请求对应的视图解析器。
视图解析器解析ModelAndView后返回对应View;
渲染视图并返回渲染后的视图给前端控制器。
最终前端控制器将渲染后的页面响应给用户或客户端

SpringMVC请求执行源码解读

对于SpringMvc 项目所有的请求入口(静态资源除外)这里都是从web.xml文件配置的前端控制器DispatcherServlet开始，

<!-- servlet请求分发器 -->
<servlet>
  <servlet-name>springMvc</servlet-name>
  <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
  <init-param>
    <param-name>contextConfigLocation</param-name>
    <param-value>classpath:servlet-context.xml</param-value>
  </init-param>
  <!-- 表示启动容器时初始化该Servlet -->
  <load-on-startup>1</load-on-startup>
</servlet>
<servlet-mapping>
  <servlet-name>springMvc</servlet-name>
  <!-- 这是拦截请求, /代表拦截所有请求,拦截所有.do请求 -->
  <url-pattern>/</url-pattern>
</servlet-mapping>

DispatcherServlet UML继承关系图如下:

这里关注蓝线部分继承结构:DispatcherServlet-->FrameworkServlet-->HttpServletBean-->HttpServlet-->GenericServlet-->Servlet,对于请求核心时序图如下

对于web 请求的处理，大家都知道是通过继承HttpServlet重写其service方法，这里打开DispatcherServlet源码发现这里并没有看到我们要找的service方法，此时到父类FrameworkServlet 查找如下:可以看到父类重写HttpServlet service方法。

FrameworkServlet #service

/**
 * Override the parent class implementation in order to intercept PATCH requests.
 */
@Override
protected void service(HttpServletRequest request, HttpServletResponse response)
      throws ServletException, IOException {
   HttpMethod httpMethod = HttpMethod.resolve(request.getMethod());
   if (httpMethod == HttpMethod.PATCH || httpMethod == null) {
      processRequest(request, response);
   }
   else {
      super.service(request, response);
   }
}
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从源码分析来看当请求方法为patch请求或者为null时执行processRequest 方法，其他情况则调用父类service 方法，大家都知道SpringMvc 请求大多请求是get|post请求为主，此时继续向上查看FrameworkServlet 父类HttpServletBean（抽象类继承HttpServlet 并未重写service方法所以向上继续寻找)-->HttpServlet service 方法

HttpServlet#service

    @Override
    public void service(ServletRequest req, ServletResponse res)
        throws ServletException, IOException
    {
        HttpServletRequest  request;
        HttpServletResponse response;
        
        if (!(req instanceof HttpServletRequest &&
                res instanceof HttpServletResponse)) {
            throw new ServletException("non-HTTP request or response");
        }

        request = (HttpServletRequest) req;
        response = (HttpServletResponse) res;

        service(request, response);
    }
}


protected void service(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException
    {
        String method = req.getMethod();

        if (method.equals(METHOD_GET)) {
            long lastModified = getLastModified(req);
            if (lastModified == -1) {
                // servlet doesn't support if-modified-since, no reason
                // to go through further expensive logic
                doGet(req, resp);
            } else {
                long ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);
                if (ifModifiedSince < lastModified) {
                    // If the servlet mod time is later, call doGet()
                    // Round down to the nearest second for a proper compare
                    // A ifModifiedSince of -1 will always be less
                    maybeSetLastModified(resp, lastModified);
                    doGet(req, resp);
                } else {
                    resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
                }
            }

        } else if (method.equals(METHOD_HEAD)) {
            long lastModified = getLastModified(req);
            maybeSetLastModified(resp, lastModified);
            doHead(req, resp);

        } else if (method.equals(METHOD_POST)) {
            doPost(req, resp);
            
        } else if (method.equals(METHOD_PUT)) {
            doPut(req, resp);
            
        } else if (method.equals(METHOD_DELETE)) {
            doDelete(req, resp);
            
        } else if (method.equals(METHOD_OPTIONS)) {
            doOptions(req,resp);
            
        } else if (method.equals(METHOD_TRACE)) {
            doTrace(req,resp);
            
        } else {
            //
            // Note that this means NO servlet supports whatever
            // method was requested, anywhere on this server.
            //

            String errMsg = lStrings.getString("http.method_not_implemented");
            Object[] errArgs = new Object[1];
            errArgs[0] = method;
            errMsg = MessageFormat.format(errMsg, errArgs);
            
            resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg);
        }
    }
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可以看到HttpServlet service 进行了重载，根据不同的请求类型然后调用不同处理方法，这里以get请求为例，当请求方法为get 请求时在重载service 方法中调用doGet 方法进行处理，这里需要特别注意的是:HttpServlet 存在doGet方法实现，然而在继承的子类中也存在doGet方法实现，到底调用哪个方法？很明显调用子类的doGet方法(面向对象多态思想!!!),从继承UML关系图上看，最外层子类实现doGet方法的为FrameworkServlet

FrameworkServlet #doGet&processRequest

@Override
protected final void doGet(HttpServletRequest request, HttpServletResponse response)
      throws ServletException, IOException {

   processRequest(request, response);
}

	protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {

        // 系统计时开始时间
		long startTime = System.currentTimeMillis();
		Throwable failureCause = null;
        // 国际化
		LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
		LocaleContext localeContext = buildLocaleContext(request);
        //构建ServletRequestAttributes对象
		RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
		ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);
		//异步管理
		WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
		asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor());
		//初始化ContextHolders
		initContextHolders(request, localeContext, requestAttributes);

		try {
			doService(request, response);
		}
		catch (ServletException | IOException ex) {
			failureCause = ex;
			throw ex;
		}
		catch (Throwable ex) {
			failureCause = ex;
			throw new NestedServletException("Request processing failed", ex);
		}

		finally {
             //恢复原来的LocaleContext和ServiceRequestAttributes到LocaleContextHolder和RequestContextHolder，避免影响Servlet以外的处理，如Filter
			resetContextHolders(request, previousLocaleContext, previousAttributes);
			if (requestAttributes != null) {
				requestAttributes.requestCompleted();
			}
			logResult(request, response, failureCause, asyncManager);
            //发布ServletRequestHandlerEvent消息，这个请求是否执行成功都会发布消息的
			publishRequestHandledEvent(request, response, startTime, failureCause);
		}
	}




// initContextHolders(request, localeContext, requestAttributes);
	private void initContextHolders(HttpServletRequest request,
			@Nullable LocaleContext localeContext, @Nullable RequestAttributes requestAttributes) {

		if (localeContext != null) {
			LocaleContextHolder.setLocaleContext(localeContext, this.threadContextInheritable);
		}
		if (requestAttributes != null) {
			RequestContextHolder.setRequestAttributes(requestAttributes, this.threadContextInheritable);
		}
	}
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该方法大概做了这几件事：国际化的设置，创建ServletRequestAttributes对象，初始化上下文holders(即将Request对象放入到线程上下文中，如后续想要在方法中获取request,response对象此时可以通过调用LocaleContextHolder对应方法即可)，然后调用doService方法。对于doService方法，FrameworkServlet 类并未提供实现，该方法由DispatcherServlet子类实现

DispatcherServlet#doService

DispatcherServlet里面执行处理的入口方法是doService，由于这个类继承于FrameworkServlet类，重写了doService()方法

@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));
         }
      }
   }

    //Spring上下文
   request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());
    //国际化解析器
   request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
    //主题解析器
   request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
    //主题
   request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());

    //重定向的数据  
   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);
   }

   try {
      //request设置完相关的属性做真正的请求处理
      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);
         }
      }
   }
}
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整个方法看下来处理的操作有:处理include标签的请求，将上下文放到request的属性中，将国际化解析器放到request的属性中，将主题解析器放到request属性中，将主题放到request的属性中，处理重定向的请求数据最后调用doDispatch这个核心的方法对请求进行处理。

DispatcherServlet#doDispatch

该方法是在doService方法中调用的，从底层设计了整个请求的处理流程:

根据request找到Handler
根据Handler找到对应的HandlerAdapter
用HandlerAdapter处理Handler
调用 processDispatchResult方法处理上面之后的结果（包含View渲染并输出给用户）

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 {
          // 校验是否为上传请求 是上传请求执行解析 否则返回request
         processedRequest = checkMultipart(request);
         multipartRequestParsed = (processedRequest != request);

         // 根据访问的Handler 返回指定对应的HandlerExecutionChain对象 这里从HandlerMapping 集合中查找 HandlerExecutionChain 对象包含Handler与拦截器HandlerInterceptor列表
         mappedHandler = getHandler(processedRequest);
         if (mappedHandler == null) {
            noHandlerFound(processedRequest, response);
            return;
         }

         // 根据得到的Handler 获取对应的HandlerAdaptor对象
         HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

         // 处理GET、HEAD请求的Last-Modified
         String method = request.getMethod();
         boolean isGet = "GET".equals(method);
         if (isGet || "HEAD".equals(method)) {
            long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
            //当数据没有更改时，就直接返回上次的数据，提高效率
             if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
               return;
            }
         }

         //执行Interceptor的preHandle 
         if (!mappedHandler.applyPreHandle(processedRequest, response)) {
            return;
         }

         // 执行Handler 返回ModelAndView
         mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

          //如果需要异步处理，直接返回
         if (asyncManager.isConcurrentHandlingStarted()) {
            return;
         }

         //当view为空时，根据request设置默认view，如Handler返回值为void
         applyDefaultViewName(processedRequest, mv);
         //执行相应Interceptor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);
      }
       //处理返回结果，包括处理异常、渲染页面，发出完成通知触发Interceptor的afterCompletion
      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);
         }
      }
   }
}
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