Spring-SpringMVC的DispatcherServlet解析

SpringMVC 执行流程

首先，一图以概之。

根据上图，先给出总结：

1. 前端控制器DispatcherServlet 接收请求，进行请求转发。
2. HandlerMapping来寻找Handler，并且返回执行链（包含拦截器方法）。
3. HandlerAdapter来执行这个执行链。执行的过程中，先执行拦截器的preHandle方法，然后执行Controller方法。此时返回ModelAndView对象。
4. 如果有 postHandle方法，就执行。
5. dispatcherServlet调用processDispatchResult方法。
6. 此方法中，先resolve ViewName获得view对象，然后又将model中的数据render进view里。
7. 最后把view返回给客户端。

源码

首先进入doservice方法

protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
        this.logRequest(request);
        Map<String, Object> attributesSnapshot = null;
        if (WebUtils.isIncludeRequest(request)) {
            attributesSnapshot = new HashMap();
            Enumeration attrNames = request.getAttributeNames();
            label95:
            while(true) {
                String attrName;
                do {
                    if (!attrNames.hasMoreElements()) {
                        break label95;
                    }

                    attrName = (String)attrNames.nextElement();
                } while(!this.cleanupAfterInclude && !attrName.startsWith("org.springframework.web.servlet"));

                attributesSnapshot.put(attrName, request.getAttribute(attrName));
            }
        }

        request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.getWebApplicationContext());
        request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
        request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
        request.setAttribute(THEME_SOURCE_ATTRIBUTE, this.getThemeSource());
        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 {
            this.doDispatch(request, response);// 此处进入！！！
        } finally {
            if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted() && attributesSnapshot != null) {
                this.restoreAttributesAfterInclude(request, attributesSnapshot);
            }

        }

    }

进入doDisPatch，开始执行上图中的流程

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
        HttpServletRequest processedRequest = request;
        HandlerExecutionChain mappedHandler = null;
        boolean multipartRequestParsed = false;
        WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);

        try {
            try {
                ModelAndView mv = null;
                Object dispatchException = null;

                try {
                    processedRequest = this.checkMultipart(request);
                    multipartRequestParsed = processedRequest != request;
                    mappedHandler = this.getHandler(processedRequest);//步骤2，3。获取执行链
                    if (mappedHandler == null) {
                        this.noHandlerFound(processedRequest, response);
                        return;
                    }
										//4.获取HandlerAdapter
                    HandlerAdapter ha = this.getHandlerAdapter(mappedHandler.getHandler());
                    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;
                        }
                    }
										//5.执行拦截器的preHandle
                    if (!mappedHandler.applyPreHandle(processedRequest, response)) {
                        return;
                    }
										//5.执行Controller， 6.返回MV
                    mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
                    if (asyncManager.isConcurrentHandlingStarted()) {
                        return;
                    }
										
                    this.applyDefaultViewName(processedRequest, mv);
                    //注意，此时才发生postHandle
                    mappedHandler.applyPostHandle(processedRequest, response, mv);
                } catch (Exception var20) {
                    dispatchException = var20;
                } catch (Throwable var21) {
                    dispatchException = new NestedServletException("Handler dispatch failed", var21);
                }
								
                //进入步骤8-11
                this.processDispatchResult(processedRequest, response, mappedHandler, mv, (Exception)dispatchException);
            } catch (Exception var22) {
                this.triggerAfterCompletion(processedRequest, response, mappedHandler, var22);
            } catch (Throwable var23) {
                this.triggerAfterCompletion(processedRequest, response, mappedHandler, new NestedServletException("Handler processing failed", var23));
            }

        } finally {
            if (asyncManager.isConcurrentHandlingStarted()) {
                if (mappedHandler != null) {
                    mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
                }
            } else if (multipartRequestParsed) {
                this.cleanupMultipart(processedRequest);
            }

        }
    }

最后processDispatchResult

private void processDispatchResult(HttpServletRequest request, HttpServletResponse response, @Nullable HandlerExecutionChain mappedHandler, @Nullable ModelAndView mv, @Nullable Exception exception) throws Exception {
      //···省略一些
        if (mv != null && !mv.wasCleared()) {
            this.render(mv, request, response);//核心代码就这一句，进去看看是怎么render的！
            if (errorView) {
                WebUtils.clearErrorRequestAttributes(request);
            }
        } else if (this.logger.isTraceEnabled()) {
            this.logger.trace("No view rendering, null ModelAndView returned.");
        }

        if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
            if (mappedHandler != null) {
                mappedHandler.triggerAfterCompletion(request, response, (Exception)null);
            }

        }
    }

protected void render(ModelAndView mv, HttpServletRequest request, HttpServletResponse response) throws Exception {
        Locale locale = this.localeResolver != null ? this.localeResolver.resolveLocale(request) : request.getLocale();
        response.setLocale(locale);
        String viewName = mv.getViewName();
        View view;
        if (viewName != null) {
          //8. resolve ViewName!
            view = this.resolveViewName(viewName, mv.getModelInternal(), locale, request);
            if (view == null) {
                throw new ServletException("Could not resolve view with name '" + mv.getViewName() + "' in servlet with name '" + this.getServletName() + "'");
            }
        } else {
            view = mv.getView();
            if (view == null) {
                throw new ServletException("ModelAndView [" + mv + "] neither contains a view name nor a View object in servlet with name '" + this.getServletName() + "'");
            }
        }

        if (this.logger.isTraceEnabled()) {
            this.logger.trace("Rendering view [" + view + "] ");
        }

        try {
            if (mv.getStatus() != null) {
                response.setStatus(mv.getStatus().value());
            }

          	//10. 把model数据render进view里
            view.render(mv.getModelInternal(), request, response);
        } catch (Exception var8) {
            if (this.logger.isDebugEnabled()) {
                this.logger.debug("Error rendering view [" + view + "]", var8);
            }

            throw var8;
        }
    }