6、SpringMVC源码分析（1）：分析DispatcherServlet.doDispatch方法，了解总体流程

所有的http请求都会交给DispatcherServlet类的doDispatch方法进行处理，将DispatcherServlet.doDispatch函数的javadoc复制到下面：

/*
     * Process the actual dispatching to the handler.
     *
     * The handler will be obtained by applying the servlet's HandlerMappings in
     * order.The HandlerAdapter will be obtained by querying the servlet's
     * installed HandlerAdapters to find the first that supports the handler
     * class.
     *
     * All HTTP methods are handled by this method. It's up to HandlerAdapters
     * or handlers themselves to decide which methods are acceptable.
     */
     void org.springframework.web.servlet.DispatcherServlet.doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception;

下面分析doDispatch方法的流程，采用注释源码的方式：

 protected void doDispatch(HttpServletRequest request,
             HttpServletResponse response) throws Exception {

         // processedRequest是经过checkMultipart方法处理过的request请求
         HttpServletRequest processedRequest = request;
         /**
          * Handler execution chain, consisting of handler object and any handler
          * interceptors. Returned by HandlerMapping's HandlerMapping.getHandler
          * method. 看看HandlerExecutionChain类的属性就很清楚了：
          *
           public class HandlerExecutionChain {

                   private final Object handler; //这个就是和该请求对应的handler处理方法

                  //里面记录了所有的(any handler interceptors)和该请求相关的拦截器
                   private HandlerInterceptor[] interceptors;

                   private List<HandlerInterceptor> interceptorList; 

                   private int interceptorIndex = -1; 

                   //...
           }
          *
          */
         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.Return a handler
                 // and any interceptors for this request.
                 /*
                  * 得到的mappedHandler包含一个请求的handler处理方法以及与该请求相关的所有拦截器
                  *
                  * DispatcherServlet.getHandler方法会在底层调用HandlerMapping.getHandler方法
                  * ，这个方法中会遍 历DispatcherServlet中的private List<HandlerMapping>
                  * handlerMappings链表，找到能够处理当前 request请求的第一个HandlerMapping实例并返回：
                  *
                   protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
                       for (HandlerMapping hm : this.handlerMappings) {
                               HandlerExecutionChain handler = hm.getHandler(request);
                               if (handler != null) {
                                      return handler;
                               }
                       }
                       return null;
                   }
                  *
                  */
                 mappedHandler = getHandler(processedRequest);
                 // 如果没有找到和该请求相对应的mappedHandler，那么就会直接返回，并应答noHandlerFound异常
                 if (mappedHandler == null || mappedHandler.getHandler() == null) {
                     noHandlerFound(processedRequest, response);
                     return;
                 }

                 // Determine handler adapter for the current request.
                 /*
                  * HandlerAdapter: 它是一个接口public interface HandlerAdapter
                  * 看看源码上的说明：The DispatcherServlet accesses all installed
                  * handlers through this interface, meaning that it does not
                  * contain code specific to any handler type.
                  *
                  * 从后面的源码看出，在使用@RequestMapping注解标注handler方法的时候，获取到的是HandlerAdapter的
                  * RequestMappingHandlerAdapter实现类的一个对象。
                  *
                  * 可以看看DispatcherServlet.getHandlerAdapter方法的定义，这个对理解上回很有帮助，我们会发现
                  * ，getHandlerAdapter 方法和上面提到的getHandler方法一样都是寻找第一个可用的作为返回结果：
                  *
                  *
                   protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {
                      //this.handlerAdapters的定义是 private List<HandlerAdapter> handlerAdapters
                          for (HandlerAdapter ha : this.handlerAdapters) {
                               if (ha.supports(handler)) {
                                    return ha;
                               }
                          }
                          throw new ServletException("No adapter for handler [" + handler +
                                  "]: The DispatcherServlet configuration needs to include a HandlerAdapter that supports this handler");
                   }
                  *
                  */
                 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()) {
                         logger.debug("Last-Modified value for ["
                                 + getRequestUri(request) + "] is: "
                                 + lastModified);
                     }
                     if (new ServletWebRequest(request, response)
                             .checkNotModified(lastModified) && isGet) {
                         return;
                     }
                 }

                 // Apply preHandle methods of registered interceptors.
                 /*
                  * 会调用所有注册拦截器的preHandle方法，如果preHandle方法的返回结果为true，则会继续执行下面的程序，
                  * 否则会直接返回。
                  *
                  * 分析一下HandlerExecutionChain.applyPreHandle方法的源码 ：
                   boolean applyPreHandle(HttpServletRequest request, HttpServletResponse response) throws Exception {
                          //从上面的HandlerExecutionChain定义处可以看见有个interceptors，还有一个interceptorList。不知道有什么区别？？！
                          HandlerInterceptor[] interceptors = getInterceptors();
                          //如果已经注册有拦截器，则遍历拦截器
                          if (!ObjectUtils.isEmpty(interceptors)) {
                              for (int i = 0; i < interceptors.length; i++) {
                                  HandlerInterceptor interceptor = interceptors[i];
                                  //如果注册拦截器的preHandle方法返回一个false，则该applyPreHandle方法就会返回false，从而在doDispatcher中的代码就不会往下执行了
                                  if (!interceptor.preHandle(request, response, this.handler)) {

                                      //这个方法要注意，它会调用所有已经成功执行的拦截器的afterCompletion方法，而且是反序调用的过程，可以分析triggerAfterCompletion
                                      //的源代码，主要是利用interceptorIndex反减的方式实现的。下面是源码的英文注释：
                                      //Trigger afterCompletion callbacks on the mapped HandlerInterceptors.
                                       //Will just invoke afterCompletion for all interceptors whose preHandle invocation
                                       //has successfully completed and returned true.
                                      triggerAfterCompletion(request, response, null);
                                      return false;
                              }
                                  //没成功执行一个拦截器的preHandle方法，其interceptorIndex就会增加1；原始值为-1。
                                   this.interceptorIndex = i;
                              }
                          }
                          return true;
                      }
                  *
                  *
                  *  顺带看看triggerAfterCompletion的源代码，很容易理解为什么拦截器的afterCompletion方法是反序执行的：
                  *    void triggerAfterCompletion(HttpServletRequest request, HttpServletResponse response, Exception ex)
                              throws Exception {

                         HandlerInterceptor[] interceptors = getInterceptors();
                         if (!ObjectUtils.isEmpty(interceptors)) {
                             for (int i = this.interceptorIndex; i >= 0; i--) {
                                 HandlerInterceptor interceptor = interceptors[i];
                                 try {
                                     interceptor.afterCompletion(request, response, this.handler, ex);
                                 }
                                 catch (Throwable ex2) {
                                     logger.error("HandlerInterceptor.afterCompletion threw exception", ex2);
                                 }
                             }
                         }
                     }
                  *
                  *
                  */
                 if (!mappedHandler.applyPreHandle(processedRequest, response)) {
                     return;
                 }

                 // Actually invoke the handler.
                 /*
                  * 在这个函数里面会真正的执行request请求相对于的handler方法，可以想象：在真正调用方法之前还会有很多的
                  * 先前处理。在这里仅仅是分析出大概的代码执行流程，其细节的部分在后面的单独模块源码分析的时候做详细的讲解。
                  * 上面讲解到HandlerAdapter是一个接口：public interface HandlerAdapter，那么必然会有很多
                  * 中实现类，在采用注解@RequstMapping的方式标注handler的情况下，ha.handle方法会在底层调用具体的
                  * HandlerAdapter类实现方法RequestMappingHandlerAdapter.handleInternal
                  *
                  * 分析一下RequestMappingHandlerAdapter.handleInternal的源代码：
                       protected ModelAndView handleInternal(HttpServletRequest request,
                         HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {
                         //好像是看control的类定义处是否使用了@SessionAttributes注解，checkAndPrepare方法有什么作用？？？
                         if (getSessionAttributesHandler(handlerMethod).hasSessionAttributes()) {
                             // Always prevent caching in case of session attribute management.
                             checkAndPrepare(request, response, this.cacheSecondsForSessionAttributeHandlers, true);
                         }
                         else {
                             // Uses configured default cacheSeconds setting.
                             checkAndPrepare(request, response, true);
                         }

                         // Execute invokeHandlerMethod in synchronized block if required.
                         // 这里是个值得注意的地方，synchronizeOnSession的值默认为false，如果通过某个方法使得其为true，那么request对应的handler
                         // 将会被放在同步快中进行处理。在什么时机下，使用什么方法才能将其设置为true呢？？？
                         if (this.synchronizeOnSession) {
                             HttpSession session = request.getSession(false);
                             if (session != null) {
                                 Object mutex = WebUtils.getSessionMutex(session);
                                 // 将handler放在同步块中处理
                                 synchronized (mutex) {
                                     return invokeHandleMethod(request, response, handlerMethod);
                                 }
                             }
                         }
                         //在invokeHandleMethod中会①将所有标注有@ModelAttrib的方法都执行一遍，②调用invokeAndHandle(webRequest, mavContainer)
                         //方法，在这里面调用handler方法，③最后调用getModelAndView(mavContainer, modelFactory, webRequest)方法的到ModelAndView。
                         //invokeHandleMethod这个方法还有很多东西要分析，留在后面。
                         //从上面的③我们可以看出，无论handler采用哪种模型化处理方式，最后都是将结果转化为ModelAndView
                         return invokeHandleMethod(request, response, handlerMethod);
                     }
                  */
                 mv = ha.handle(processedRequest, response,
                         mappedHandler.getHandler());

                 if (asyncManager.isConcurrentHandlingStarted()) {
                     return;
                 }

                 applyDefaultViewName(request, mv);
                 /*
                  * 调用request相关的拦截器的postHandle方法，注意，这个也是反序调用的。看看源代码：
                  *
                   void applyPostHandle(HttpServletRequest request, HttpServletResponse response, ModelAndView mv) throws Exception {
                         HandlerInterceptor[] interceptors = getInterceptors();
                         if (!ObjectUtils.isEmpty(interceptors)) {
                             //注意，这里也是反序执行，而且是所有成功执行了的postHandle拦截器
                             for (int i = interceptors.length - 1; i >= 0; i--) {
                                 HandlerInterceptor interceptor = interceptors[i];
                                 //这里传入的参数中有mv，也就是说，我们是有办法在拦截器的postHandle方法中修改已经返回的mv
                                 interceptor.postHandle(request, response, this.handler, mv);
                             }
                         }
                     }
                  */
                 mappedHandler.applyPostHandle(processedRequest, response, mv);
             } catch (Exception ex) {
                 dispatchException = ex;
             }
             processDispatchResult(processedRequest, response, mappedHandler,
                     mv, dispatchException);
         } catch (Exception ex) {
             triggerAfterCompletion(processedRequest, response, mappedHandler,
                     ex);
         } catch (Error err) {
             triggerAfterCompletionWithError(processedRequest, response,
                     mappedHandler, 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);
                 }
             }
         }
     }

看完源代码就可以总结出doDispath方法中处理http请求的流程了：