第一步：在web.xml中找到DispatcherServlet，然后ctrl+鼠标左键进入这个类中

第二步：ctrl+f找一个叫doDispatch的方法

//入参进来一个requestprotected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {        HttpServletRequest processedRequest = request;       处理器执行链        HandlerExecutionChain mappedHandler = null;       多部分请求 解析器  （文件上传）        boolean multipartRequestParsed = false;        WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);        try {            try {           模型（数据）和视图                ModelAndView err = null;        异常                Exception dispatchException = null;                try {           //指的是 DispatcherServlet 的实例  看看请求是不是多部分请求            
     
        表单有文件域 上传文件了                    processedRequest = this.checkMultipart(request);                    multipartRequestParsed = processedRequest != request; 　　　　　　　　　　　　 　　　　　　　　　　　　mappedHandler = this.getHandler(processedRequest);
     

第三步：ctrl+鼠标左键进入getHandler方法中

protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {        handlerMappings单列集合 。Set   List          Iterator var2 = this.handlerMappings.iterator();        HandlerExecutionChain handler;            if(!var2.hasNext())    return null;           每next()拿到一个HandlerMapping实例            HandlerMapping hm = (HandlerMapping)var2.next();            handler = hm.getHandler(request);           return handler;    }

第四步：接着ctrl+鼠标左键找getHandler方法，我们找到的是一个接口，然后ctrl+h找接口的实现类 AbstractHandlerMapping（抽象类）

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 handlerName = (String)handler;  //  /hello                handler = this.getApplicationContext().getBean(handlerName);            }            return this.getHandlerExecutionChain(handler, request);        }    }

第五步：找getHandlerExecutionChain方法

protected HandlerExecutionChain getHandlerExecutionChain(Object handler, HttpServletRequest request) {        HandlerExecutionChain chain = handler instanceof HandlerExecutionChain?(HandlerExecutionChain)handler:new HandlerExecutionChain(handler);        String lookupPath = this.urlPathHelper.getLookupPathForRequest(request);        Iterator var5 = this.adaptedInterceptors.iterator();        while(var5.hasNext()) {            HandlerInterceptor interceptor = (HandlerInterceptor)var5.next();            if(interceptor instanceof MappedInterceptor) {                MappedInterceptor mappedInterceptor = (MappedInterceptor)interceptor;                if(mappedInterceptor.matches(lookupPath, this.pathMatcher)) {                    chain.addInterceptor(mappedInterceptor.getInterceptor());                }            } else {                chain.addInterceptor(interceptor);            }        }        return chain;    }