一文讀懂Spring MVC執行流程

時間 2019-11-07

標籤一文讀懂 spring mvc 執行流程欄目 Spring 简体版

原文原文鏈接

說到Spring MVC執行流程，網上有不少這方面的文章介紹，可是都不太詳細，做爲一個初學者去讀會有許多不理解的地方，今天這篇文章記錄一下我學習Spring MVC的心得體會前端

話很少說，先上圖：java

Spring MVC執行流程圖程序員

下面對Spring MVC處理請求的過程進行詳解：

（1）整個過程始於客戶端發出一個HTTP請求，Web應用服務器接收到這個請求。若是匹配DispatcherServlet的請求映射路徑（在web.xml中指定），則Web容器將該請求轉交給DispatcherServlet處理。web

（2）DispatcherServlet接收到這個請求後，將根據請求的信息（包括 URL、HTTP方法、請求報文頭、請求參數、Cookie等）調用處理器映射器HandlerMapping。spring

（3）處理器映射器根據請求url找到具體的處理器，生成處理器執行鏈HandlerExecutionChain(包括處理器對象和處理器攔截器)一併返回給DispatcherServlet。可將HandlerMapping看做路由控制器，將Handler看做目標主機後端

（4）當DispatcherServlet 根據HandlerMapping獲得對應當前請求的Handler後，經過HandlerAdapter對Handler進行封裝，再以統一的適配器接口調用Handler。HandlerAdapter 是Spring MVC的框架級接口，顧名思義，HandlerAdapter是一個適配器，它用統一的接口對各類Handler方法進行調用。服務器

（5）處理器完成業務邏輯的處理後將返回一個ModelAndView給DispatcherServlet，ModelAndView包含了視圖邏輯名和模型數據信息。mvc

（6）ModelAndView中包含的是「邏輯視圖名」而非真正的視圖對象，DispatcherServlet藉由ViewResolver完成邏輯視圖名到真實視圖對象的解析工做。app

（7）當獲得真實的視圖對象View後，DispatcherServlet就使用這個View對象對ModelAndView中的模型數據進行視圖渲染。框架

（8）最終客戶端獲得的響應消息多是一個普通的HTML頁面，也多是一個XML或JSON串，甚至是一張圖片或一個PDF文檔等不一樣的媒體形式。

組件詳解：

1.DispatcherServlet：前端控制器。用戶請求到達前端控制器，它就至關於mvc模式中的c，dispatcherServlet是整個流程控制的中心，由它調用其它組件處理用戶的請求，dispatcherServlet的存在下降了組件之間的耦合性,系統擴展性提升。由框架實現

2.HandlerMapping：處理器映射器。HandlerMapping負責根據用戶請求的url找到Handler即處理器，springmvc提供了不一樣的映射器實現不一樣的映射方式，根據必定的規則去查找,例如：xml配置方式，實現接口方式，註解方式等。由框架實現

3.Handler：處理器。Handler 是繼DispatcherServlet前端控制器的後端控制器，在DispatcherServlet的控制下Handler對具體的用戶請求進行處理。因爲Handler涉及到具體的用戶業務請求，因此通常狀況須要程序員根據業務需求開發Handler。

4.HandlAdapter：處理器適配器。經過HandlerAdapter對處理器進行執行，這是適配器模式的應用，經過擴展適配器能夠對更多類型的處理器進行執行。由框架實現。

5.ModelAndView：是springmvc的封裝對象，將model和view封裝在一塊兒。

6.ViewResolver：視圖解析器。ViewResolver負責將處理結果生成View視圖，ViewResolver首先根據邏輯視圖名解析成物理視圖名即具體的頁面地址，再生成View視圖對象，最後對View進行渲染將處理結果經過頁面展現給用戶。

7.View:是springmvc的封裝對象，是一個接口, springmvc框架提供了不少的View視圖類型，包括：jspview，pdfview,jstlView、freemarkerView、pdfView等。通常狀況下須要經過頁面標籤或頁面模版技術將模型數據經過頁面展現給用戶，須要由程序員根據業務需求開發具體的頁面。

源碼詳解：

1.請求到達DispatcherServlet：

/**
 * Exposes the DispatcherServlet-specific request attributes and delegates to {@link #doDispatch}
 * for the actual dispatching.
 */
@Override
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
   if (logger.isDebugEnabled()) {
      String requestUri = urlPathHelper.getRequestUri(request);
      logger.debug("DispatcherServlet with name '" + getServletName() + "' processing " + request.getMethod() +
            " request for [" + requestUri + "]");
   }

    //保護現場
   // 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)) {
      logger.debug("Taking snapshot of request attributes before include");
      attributesSnapshot = new HashMap<String, Object>();
      Enumeration<?> attrNames = request.getAttributeNames();
      while (attrNames.hasMoreElements()) {
         String attrName = (String) attrNames.nextElement();
         if (this.cleanupAfterInclude || attrName.startsWith("org.springframework.web.servlet")) {
            attributesSnapshot.put(attrName, request.getAttribute(attrName));
         }
      }
   }

    //將框架相關信息存儲至request，方便後面的處理器和視圖用到
   // 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());

   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);

    //請求分發
   try {
      doDispatch(request, response);
   }
   finally {
      // Restore the original attribute snapshot, in case of an include.
      if (attributesSnapshot != null) {
         restoreAttributesAfterInclude(request, attributesSnapshot);
      }
   }
}

2.開始處理請求：
（1）經過url查找HandlerMap中的key(url)，而後由key獲取HandlerMapping對象；
（2）經過處理器映射器獲取處理器；
（3）經過查詢處理器適配器得到。

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
   HttpServletRequest processedRequest = request;
   HandlerExecutionChain mappedHandler = null;
   int interceptorIndex = -1;

   try {
      ModelAndView mv;
      boolean errorView = false;

      try {
         processedRequest = checkMultipart(request);

         // Determine handler for the current request
        //步驟3.1~3.4用於獲取包含處理器Handler和攔截器AdapterIntercepters的處理器執行鏈HandlerExecutionChain
         mappedHandler = getHandler(processedRequest, false);
         if (mappedHandler == null || mappedHandler.getHandler() == null) {
            noHandlerFound(processedRequest, response);
            return;
         }

         // Determine handler adapter for the current request.
        //步驟4.1~4.2,根據HandlerExecutionChain中的處理器Handler獲取處理器適配器
         HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

               // Process last-modified header, if supported by the handler.
         String method = request.getMethod();
         boolean isGet = "GET".equals(method);
         if (isGet || "HEAD".equals(method)) {
            long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
            if (logger.isDebugEnabled()) {
               String requestUri = urlPathHelper.getRequestUri(request);
               logger.debug("Last-Modified value for [" + requestUri + "] is: " + lastModified);
            }
            if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
               return;
            }
         }

         // Apply preHandle methods of registered interceptors.
         HandlerInterceptor[] interceptors = mappedHandler.getInterceptors();
         if (interceptors != null) {
            for (int i = 0; i < interceptors.length; i++) {
               HandlerInterceptor interceptor = interceptors[i];
               if (!interceptor.preHandle(processedRequest, response, mappedHandler.getHandler())) {
                  triggerAfterCompletion(mappedHandler, interceptorIndex, processedRequest, response, null);
                  return;
               }
               interceptorIndex = i;
            }
         }

         // Actually invoke the handler.
        //5.1~5.3經過處理器適配器HandlerApapter來調用處理器完成對請求的處理
         mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

         // Do we need view name translation?
         if (mv != null && !mv.hasView()) {
            mv.setViewName(getDefaultViewName(request));
         }

         // Apply postHandle methods of registered interceptors.
         if (interceptors != null) {
            for (int i = interceptors.length - 1; i >= 0; i--) {
               HandlerInterceptor interceptor = interceptors[i];
               interceptor.postHandle(processedRequest, response, mappedHandler.getHandler(), mv);
            }
         }
      }
      catch (ModelAndViewDefiningException ex) {
         logger.debug("ModelAndViewDefiningException encountered", ex);
         mv = ex.getModelAndView();
      }
      catch (Exception ex) {
         Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);
         mv = processHandlerException(processedRequest, response, handler, ex);
         errorView = (mv != null);
      }

      // Did the handler return a view to render?
      if (mv != null && !mv.wasCleared()) {
         render(mv, processedRequest, response);
         if (errorView) {
            WebUtils.clearErrorRequestAttributes(request);
         }
      }
      else {
         if (logger.isDebugEnabled()) {
            logger.debug("Null ModelAndView returned to DispatcherServlet with name '" + getServletName() +
                  "': assuming HandlerAdapter completed request handling");
         }
      }

      // Trigger after-completion for successful outcome.
      triggerAfterCompletion(mappedHandler, interceptorIndex, processedRequest, response, null);
   }

   catch (Exception ex) {
      // Trigger after-completion for thrown exception.
      triggerAfterCompletion(mappedHandler, interceptorIndex, processedRequest, response, ex);
      throw ex;
   }
   catch (Error err) {
      ServletException ex = new NestedServletException("Handler processing failed", err);
      // Trigger after-completion for thrown exception.
      triggerAfterCompletion(mappedHandler, interceptorIndex, processedRequest, response, ex);
      throw ex;
   }

   finally {
      // Clean up any resources used by a multipart request.
      if (processedRequest != request) {
         cleanupMultipart(processedRequest);
      }
   }
}

3.1 getHandler(HttpServletRequest request)，經由HandlerMapping對象獲取HandlerExecutionChain(處理器和攔截器)：

protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
   for (HandlerMapping hm : this.handlerMappings) {
      if (logger.isTraceEnabled()) {
         logger.trace(
               "Testing handler map [" + hm + "] in DispatcherServlet with name '" + getServletName() + "'");
      }
      HandlerExecutionChain handler = hm.getHandler(request);
      if (handler != null) {
         return handler;
      }
   }
   return null;
}

3.2.1 getHandler(HttpServletRequest request)，經由request獲取處理器，獲取處理器Handler後，再獲取攔截器，最後組成HandlerExecutionChain：

public final HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
   Object handler = getHandlerInternal(request);
   if (handler == null) {
      handler = getDefaultHandler();
   }
   if (handler == null) {
      return null;
   }
   // Bean name or resolved handler?
   if (handler instanceof String) {
      String handlerName = (String) handler;
      handler = getApplicationContext().getBean(handlerName);
   }
   return getHandlerExecutionChain(handler, request);
}

3.2.2 根據查找到的處理器Handler和request獲取包含Handler和AdaptedInterceptors的HandlerExecutionChain：

protected HandlerExecutionChain getHandlerExecutionChain(Object handler, HttpServletRequest request) {
   HandlerExecutionChain chain = 
      (handler instanceof HandlerExecutionChain) ?
         (HandlerExecutionChain) handler : new HandlerExecutionChain(handler);

   chain.addInterceptors(getAdaptedInterceptors());

   String lookupPath = urlPathHelper.getLookupPathForRequest(request);
   for (MappedInterceptor mappedInterceptor : mappedInterceptors) {
      if (mappedInterceptor.matches(lookupPath, pathMatcher)) {
         chain.addInterceptor(mappedInterceptor.getInterceptor());
      }
   }

   return chain;
}
/**
 * Return the adapted interceptors as HandlerInterceptor array.
 * @return the array of HandlerInterceptors, or <code>null</code> if none
 */
protected final HandlerInterceptor[] getAdaptedInterceptors() {
   int count = adaptedInterceptors.size();
   return (count > 0) ? adaptedInterceptors.toArray(new HandlerInterceptor[count]) : null;
}

3.3.getHandlerInternal(HttpServletRequest request)獲取Handler：

@Override
protected Object getHandlerInternal(HttpServletRequest request) throws Exception {
   String lookupPath = getUrlPathHelper().getLookupPathForRequest(request);
   Object handler = lookupHandler(lookupPath, request);
   if (handler == null) {
      // We need to care for the default handler directly, since we need to
      // expose the PATH_WITHIN_HANDLER_MAPPING_ATTRIBUTE for it as well.
      Object rawHandler = null;
      if ("/".equals(lookupPath)) {
         rawHandler = getRootHandler();
      }
      if (rawHandler == null) {
         rawHandler = getDefaultHandler();
      }
      if (rawHandler != null) {
         // Bean name or resolved handler?
         if (rawHandler instanceof String) {
            String handlerName = (String) rawHandler;
            rawHandler = getApplicationContext().getBean(handlerName);
         }
         validateHandler(rawHandler, request);
         handler = buildPathExposingHandler(rawHandler, lookupPath, lookupPath, null);
      }
   }
   if (handler != null && logger.isDebugEnabled()) {
      logger.debug("Mapping [" + lookupPath + "] to " + handler);
   }
   else if (handler == null && logger.isTraceEnabled()) {
      logger.trace("No handler mapping found for [" + lookupPath + "]");
   }
   return handler;
}

3.4 lookupHandler(lookupPath, request)根據給定url path和request獲取Handler：

protected Object lookupHandler(String urlPath, HttpServletRequest request) throws Exception {
   // Direct match?
   Object handler = this.handlerMap.get(urlPath);
   if (handler != null) {
      // Bean name or resolved handler?
      if (handler instanceof String) {
         String handlerName = (String) handler;
         handler = getApplicationContext().getBean(handlerName);
      }
      validateHandler(handler, request);
      return buildPathExposingHandler(handler, urlPath, urlPath, null);
   }
   // Pattern match?
   List<String> matchingPatterns = new ArrayList<String>();
   for (String registeredPattern : this.handlerMap.keySet()) {
      if (getPathMatcher().match(registeredPattern, urlPath)) {
         matchingPatterns.add(registeredPattern);
      }
   }
   String bestPatternMatch = null;
   Comparator<String> patternComparator = getPathMatcher().getPatternComparator(urlPath);
   if (!matchingPatterns.isEmpty()) {
      Collections.sort(matchingPatterns, patternComparator);
      if (logger.isDebugEnabled()) {
         logger.debug("Matching patterns for request [" + urlPath + "] are " + matchingPatterns);
      }
      bestPatternMatch = matchingPatterns.get(0);
   }
   if (bestPatternMatch != null) {
      handler = this.handlerMap.get(bestPatternMatch);
      // Bean name or resolved handler?
      if (handler instanceof String) {
         String handlerName = (String) handler;
         handler = getApplicationContext().getBean(handlerName);
      }
      validateHandler(handler, request);
      String pathWithinMapping = getPathMatcher().extractPathWithinPattern(bestPatternMatch, urlPath);

      // There might be multiple 'best patterns', let's make sure we have the correct URI template variables
      // for all of them
      Map<String, String> uriTemplateVariables = new LinkedHashMap<String, String>();
      for (String matchingPattern : matchingPatterns) {
         if (patternComparator.compare(bestPatternMatch, matchingPattern) == 0) {
            uriTemplateVariables
                  .putAll(getPathMatcher().extractUriTemplateVariables(matchingPattern, urlPath));
         }
      }
      if (logger.isDebugEnabled()) {
         logger.debug("URI Template variables for request [" + urlPath + "] are " + uriTemplateVariables);
      }
      return buildPathExposingHandler(handler, bestPatternMatch, pathWithinMapping, uriTemplateVariables);
   }
   // No handler found...
   return null;
}

4 HandlerAdapter getHandlerAdapter(Object handler)，根據Handler獲取HandlerAdapter：

protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {
   for (HandlerAdapter ha : this.handlerAdapters) {
      if (logger.isTraceEnabled()) {
         logger.trace("Testing handler adapter [" + ha + "]");
      }
      if (ha.supports(handler)) {
         return ha;
      }
   }
   throw new ServletException("No adapter for handler [" + handler +
         "]: Does your handler implement a supported interface like Controller?");
}

5.1 使用處理器完成對請求的處理：

public ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler)
      throws Exception {

   ((Servlet) handler).service(request, response);
   return null;
}
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);
}

5.2 service(HttpServletRequest req, HttpServletResponse resp)：

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);
    }
}

5.3 doGet(HttpServletRequest req, HttpServletResponse resp)：

protected void doGet(HttpServletRequest req, HttpServletResponse resp)
    throws ServletException, IOException
{
    String protocol = req.getProtocol();
    String msg = lStrings.getString("http.method_get_not_supported");
    if (protocol.endsWith("1.1")) {
        resp.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED, msg);
    } else {
        resp.sendError(HttpServletResponse.SC_BAD_REQUEST, msg);
    }
}

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