SpringMVC源码分析（四）——————SpingMVC处理Http请求原理

四、SpingMVC处理Http请求原理

当一个http请求过来了首先经过的是DispatcherServlet这么一个前端控制器并调用了这个前端控制器的doService方法。这个方法最终我们发现它调用了doDispatcher这么一个方法。这就是SpringMVC处理http请求的入口了。

   protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
        if(this.logger.isDebugEnabled()) {
            String attributesSnapshot = WebAsyncUtils.getAsyncManager(request).hasConcurrentResult()?" resumed":"";
            this.logger.debug("DispatcherServlet with name \'" + this.getServletName() + "\'" + attributesSnapshot + " processing " + request.getMethod() + " request for [" + getRequestUri(request) + "]");
        }

        HashMap attributesSnapshot1 = null;
        if(WebUtils.isIncludeRequest(request)) {
            attributesSnapshot1 = new HashMap();
            Enumeration inputFlashMap = request.getAttributeNames();

            label112:
            while(true) {
                String attrName;
                do {
                    if(!inputFlashMap.hasMoreElements()) {
                        break label112;
                    }

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

                attributesSnapshot1.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 inputFlashMap1 = this.flashMapManager.retrieveAndUpdate(request, response);
            if(inputFlashMap1 != null) {
                request.setAttribute(INPUT_FLASH_MAP_ATTRIBUTE, Collections.unmodifiableMap(inputFlashMap1));
            }

            request.setAttribute(OUTPUT_FLASH_MAP_ATTRIBUTE, new FlashMap());
            request.setAttribute(FLASH_MAP_MANAGER_ATTRIBUTE, this.flashMapManager);
        }

        try {
            this.doDispatch(request, response);
        } finally {
            if(!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted() && attributesSnapshot1 != null) {
                this.restoreAttributesAfterInclude(request, attributesSnapshot1);
            }

        }

    }

这个方法里面我们发现定义了一个ModelAndView将要返回的视图与数据，并且首先检测这个请求是不是一个上传的请求，然后根据这个请求获取对应的Handler，如果Handler不为空的话就根据这个handler获取对应的HandlerAdapter。接着调用拦截器链的所有前置拦截的这么一个方法，接着调用adapter的真正的处理方法处理请求，最后调用拦截器链中的所有后置拦截方法。

  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 {
                //定义一个空的ModelAndView
                ModelAndView err = null;
                Object dispatchException = null;

                try {
                //检查是否是上传请求
                    processedRequest = this.checkMultipart(request);
                    multipartRequestParsed = processedRequest != request;
                //获取handler
                    mappedHandler = this.getHandler(processedRequest);
                    if(mappedHandler == null) {
                        this.noHandlerFound(processedRequest, response);
                        return;
                    }
                //根据handler获取handlerAdapter
                    HandlerAdapter err1 = this.getHandlerAdapter(mappedHandler.getHandler());
                    String method = request.getMethod();
                    boolean isGet = "GET".equals(method);
                    if(isGet || "HEAD".equals(method)) {
                        long lastModified = err1.getLastModified(request, mappedHandler.getHandler());
                        if(this.logger.isDebugEnabled()) {
                            this.logger.debug("Last-Modified value for [" + getRequestUri(request) + "] is: " + lastModified);
                        }

                        if((new ServletWebRequest(request, response)).checkNotModified(lastModified) && isGet) {
                            return;
                        }
                    }
                //前置拦截
                    if(!mappedHandler.applyPreHandle(processedRequest, response)) {
                        return;
                    }
                //真正处理
                    err = err1.handle(processedRequest, response, mappedHandler.getHandler());
                    if(asyncManager.isConcurrentHandlingStarted()) {
                        return;
                    }

                    this.applyDefaultViewName(processedRequest, err);
                //后置拦截
                    mappedHandler.applyPostHandle(processedRequest, response, err);
                } 
                //后面一些catch finally语句省略。。。。。

继续跟进这个getHandler方法看看具体是怎么实现的，发现就是循环遍历最开始初始化DispatcherServlet的时候初始化的那7个handlerMapping,去调用这些handlerMapping的getHandler方法；这里我们主要看这个RequestMappingHandlerMapping。

 跟进发现是调用了RequestMappingHandlerMapping的父类AbstractHandlerMapping中的getHandler方法，并且这个方法又是去调用了getHandlerInternal来获取Handler的。

继续跟进这个getHandlerInternal方法，首先它根据这个request获取它的URI，接着通过uri获取对应的HandlerMethod对象最终返回了。

    protected HandlerMethod getHandlerInternal(HttpServletRequest request) throws Exception {
        String lookupPath = this.getUrlPathHelper().getLookupPathForRequest(request);
        if(this.logger.isDebugEnabled()) {
            this.logger.debug("Looking up handler method for path " + lookupPath);
        }

        this.mappingRegistry.acquireReadLock();

        HandlerMethod var4;
        try {
            HandlerMethod handlerMethod = this.lookupHandlerMethod(lookupPath, request);
            if(this.logger.isDebugEnabled()) {
                if(handlerMethod != null) {
                    this.logger.debug("Returning handler method [" + handlerMethod + "]");
                } else {
                    this.logger.debug("Did not find handler method for [" + lookupPath + "]");
                }
            }

            var4 = handlerMethod != null?handlerMethod.createWithResolvedBean():null;
        } finally {
            this.mappingRegistry.releaseReadLock();
        }

        return var4;
    }

 跟进这个getLookupPathForRequest发现又调用了getPathWithinServletMapping方法。

 

 这里我们就知道了这个lookupPath得到的就是这个请求的URI，拿到了这个URI接着就去调用lookupHandlerMethod方法了。

这里lookupHandlerMethod方法顾名思义就是通过这个URI根据请求的URI去寻找对应的HandlerMethod。

    protected HandlerMethod lookupHandlerMethod(String lookupPath, HttpServletRequest request) throws Exception {
        ArrayList matches = new ArrayList();
        //通过这个uri去Map中查询与那个HandlerMethod映射
        List directPathMatches = this.mappingRegistry.getMappingsByUrl(lookupPath);
        if(directPathMatches != null) {
            this.addMatchingMappings(directPathMatches, matches, request);
        }

        if(matches.isEmpty()) {
            this.addMatchingMappings(this.mappingRegistry.getMappings().keySet(), matches, request);
        }
        //若查询出这个uri有对应的映射关系
        if(!matches.isEmpty()) {
            AbstractHandlerMethodMapping.MatchComparator comparator = new AbstractHandlerMethodMapping.MatchComparator(this.getMappingComparator(request));
           //对映射关系排序
            matches.sort(comparator);
            if(this.logger.isTraceEnabled()) {
                this.logger.trace("Found " + matches.size() + " matching mapping(s) for [" + lookupPath + "] : " + matches);
            }
            //获取排序后第一个HandlerMethod
            AbstractHandlerMethodMapping.Match bestMatch = (AbstractHandlerMethodMapping.Match)matches.get(0);
            if(matches.size() > 1) {
                if(CorsUtils.isPreFlightRequest(request)) {
                    return PREFLIGHT_AMBIGUOUS_MATCH;
                }

                AbstractHandlerMethodMapping.Match secondBestMatch = (AbstractHandlerMethodMapping.Match)matches.get(1);
                if(comparator.compare(bestMatch, secondBestMatch) == 0) {
                    Method m1 = bestMatch.handlerMethod.getMethod();
                    Method m2 = secondBestMatch.handlerMethod.getMethod();
                    throw new IllegalStateException("Ambiguous handler methods mapped for HTTP path \'" + request.getRequestURL() + "\': {" + m1 + ", " + m2 + "}");
                }
            }

            this.handleMatch(bestMatch.mapping, lookupPath, request);
            return bestMatch.handlerMethod;
        } else {
            return this.handleNoMatch(this.mappingRegistry.getMappings().keySet(), lookupPath, request);
        }
    }

最终得到了这么一个包含了Handler与这个URI具体处理方法的一个HandlerMethod对象。注意现在这个handler可能还没有被创建出来或者说没有得到，只是知道它的beanName。最终还要调用这么一个createWithResolvedBean方法来得到。

 获得了这个handler之后，又继续根据这个handler获得了HandlerExecutionChain。

   public final HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
        Object handler = this.getHandlerInternal(request);
        if(handler == null) {
            handler = this.getDefaultHandler();
        }

        if(handler == null) {
            return null;
        } else {
            if(handler instanceof String) {
                String executionChain = (String)handler;
                handler = this.obtainApplicationContext().getBean(executionChain);
            }

            HandlerExecutionChain executionChain1 = this.getHandlerExecutionChain(handler, request);
            if(CorsUtils.isCorsRequest(request)) {
                CorsConfiguration globalConfig = this.globalCorsConfigSource.getCorsConfiguration(request);
                CorsConfiguration handlerConfig = this.getCorsConfiguration(handler, request);
                CorsConfiguration config = globalConfig != null?globalConfig.combine(handlerConfig):handlerConfig;
                executionChain1 = this.getCorsHandlerExecutionChain(request, executionChain1, config);
            }

            return executionChain1;
        }
    }

所以得出最终结论getHandler这个方法最终所有通过这个URI返回了一个最开始注册的handler，然后通过 这个handler返回了一个包含这这个HandlerMethod以及一个拦截器链的这么一个对象。之后执行前置拦截器，handler方法，后置拦截，完成。