springmvc的核心的代码就是DispatcherServlet，作用就是负责接收请求和转发请求
先看一下DispatcherServlet的类图

从图中可以看到，最终继承的是HttpServlet, 既然是servlet，那么就必然后走HttpServlet的service方法，
这个service方法里会判断，请求是get,还是post等等，如果是get请求，就会走this.doGet，这时候就会走子类FrameworkServlet的doGet方法，执行processRequest方法，在processRequest方法里，走到doService(request, response)， 这个方法又会由FrameworkServlet子类DispatcherServlet的doService方法重写，doSerivce方法里的doDispatch(request, response)，才是SpringMVC的核心代码，

刚讲的有点绕，参考下面的流程图

我们现在再来看一下SpringMVC的核心代码doDispatch()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 {ModelAndView mv = null;Exception dispatchException = null;try {processedRequest = checkMultipart(request);multipartRequestParsed = (processedRequest != request);// Determine handler for the current request.mappedHandler = getHandler(processedRequest);if (mappedHandler == null) {noHandlerFound(processedRequest, response);return;}// Determine handler adapter for the current request.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;}}if (!mappedHandler.applyPreHandle(processedRequest, response)) {return;}// Actually invoke the handler.mv = ha.handle(processedRequest, response, mappedHandler.getHandler());if (asyncManager.isConcurrentHandlingStarted()) {return;}applyDefaultViewName(processedRequest, mv);mappedHandler.applyPostHandle(processedRequest, response, mv);}catch (Exception ex) {dispatchException = ex;}catch (Throwable err) {// As of 4.3, we're processing Errors thrown from handler methods as well,// making them available for @ExceptionHandler methods and other scenarios.dispatchException = new NestedServletException("Handler dispatch failed", err);}processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);}catch (Exception ex) {triggerAfterCompletion(processedRequest, response, mappedHandler, ex);}catch (Throwable err) {triggerAfterCompletion(processedRequest, response, mappedHandler,new NestedServletException("Handler processing failed", err));}finally {if (asyncManager.isConcurrentHandlingStarted()) {// Instead of postHandle and afterCompletionif (mappedHandler != null) {mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);}}else {// Clean up any resources used by a multipart request.if (multipartRequestParsed) {cleanupMultipart(processedRequest);}}}}

由源码可得：他首先主要是创建一个视图对象 ModelAndView mv = null; 检查当前请求是否是二进制的请求processedRequest = this.checkMultipart(request);然后就是只要代码

mappedHandler = this.getHandler(processedRequest);

就是根据当前的请求去拿一个Handler，这个Handler其实就是我们的控制器。进入这个方法源码如下：

	@Nullableprotected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {if (this.handlerMappings != null) {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;}

由图可知他取Handle。最终要去找HandlerMapping里拿一个Handler。那么为什么要去找HandlerMapping里拿一个Handler呢？首先我们在配置控制器的时候有两种方式1.xml方式，2.注解的方式。因此spring源码他给我们不止一种控制器 。因为两种方式控制器 。因此spring并不知道我们使用的事哪一种控制器。因为两种控制器，spring去底层去找的控制的实现方式是不一样的。因此这就是为什么第二步他要去找Handler（控制器）的了。但是Handler怎么找的到呢?就是通过HandlerMapping这样一个处理器映射器。如代码可知他首先是判断当前HandlerMappers是否为空：this.handlerMappings

这个时候hm类型是RequestMappingHandlerMapping, 也就是基于xml方法

这里可以看到返回的是一个HandlerExecutionChain类型的Handler，这就是对Handler进行了一次包装，
里面封装了此次请求的IndexController,也就是我们自己的创建controller和拦截器

如果Handler为空，没有找到的话，那会抛出我们经常见的错误，404

总结：

前面一顿调用，最终来到了SpringMVC核心的代码DispatcherServlet的doDispatch方法，第一个主要的方法就是获取handler，一共有2个handler, 我们获取handler的目的就是因为spring并不知道我们使用的事哪一种控制器。因为这两种控制器，spring底层实现方式是不一样的，这就是为什么第一步就是要找到对应的handler了