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springmvc特点

• 轻量级，简单易学

• 高效 , 基于请求响应的MVC框架

• 与Spring兼容性好，无缝结合

• 约定优于配置

• 功能强大：RESTful、数据验证、格式化、本地化、主题等

• 简洁灵活

springmvc执行流程

public class DispatcherServlet extends FrameworkServlet {
/**
	 * Process the actual dispatching to the handler.
	实际调度过程的处理器。
	 * <p>The handler will be obtained by applying the servlet's HandlerMappings in order.
	处理程序将得到应用的有序的servlet的处理映射器。
	 * The HandlerAdapter will be obtained by querying the servlet's installed HandlerAdapters
	处理调度器通过查询servlet的HandlerAdapters安装获得。 
	 * to find the first that supports the handler class.
	找到第一个处理程序类。
	 * <p>All HTTP methods are handled by this method. It's up to HandlerAdapters or handlers
	HTTP方法都是由这种方法处理。由HandlerAdapters或handlers程序
	 * themselves to decide which methods are acceptable.
	自己来决定哪些方法是可以接受的
	 * @param request current HTTP request
	请求当前的HTTP请求
	 * @param response current HTTP response
	响应当前的HTTP响应
	 * @throws Exception in case of any kind of processing failure
	抛出Exception应对任何处理失败
	 */
	@SuppressWarnings("deprecation")
	protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
		HttpServletRequest processedRequest = request;
		HandlerExecutionChain mappedHandler = null;
		boolean multipartRequestParsed = false;//是否是文件上传功能
		
		/**The central class for managing asynchronous request processing, 		  mainly intended as an SPI and not typically used directly by application classes.
		意思是主要用来管理异步请求的处理。什么时候要用到异步处理呢？就是业务逻辑复杂（或者其他原因），为了避免请求线程阻塞，需要委托给另一个线程的时候。*/
		WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);

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

			try {
				processedRequest = checkMultipart(request);// 是否是文件上传功能
				multipartRequestParsed = (processedRequest != request);

				// Determine handler for the current request.　确定当前请求的处理程序            
				// 通过handlerMappings找到对应的HandlerExecutionChain处理链
				// HandlerExecutionChain包含了拦截器handlerInterceptor和真正处理请求的handler
				mappedHandler = getHandler(processedRequest);
				if (mappedHandler == null) {
					noHandlerFound(processedRequest, response);
					return;
				}

				// Determine handler adapter for the current request.确定当前请求处理程序适配器
				// 通过Handler获取对应的适配器,主要是用来做很多预处理
				HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

				// Process last-modified header, if supported by the handler.如果处理程序支持，则处理最后修改的头文件
				// 得到当前的http方法
				String method = request.getMethod();
				boolean isGet = HttpMethod.GET.matches(method);
				if (isGet || HttpMethod.HEAD.matches(method)) {//处理http的head方法。这种方法应该很少用  
					long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
					if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
						return;
					}
				}
 				// 调用HandlerExecutionChain的interceptor 
				// Handler的预处理，还记得刚才返回的HandlerExecutionChain封装了Handler和一些拦截器，现在就是调用拦截器的时候。
				if (!mappedHandler.applyPreHandle(processedRequest, response)) {
					return;
				}

				// Actually invoke the handler.真正的调用处理程序
				// HandlerAdpater让Handler处理request与response，获取ModelAndView
				mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

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

				applyDefaultViewName(processedRequest, mv);
				// 这一步与前面的拦截器预处理类似。
				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);
			}
			// 处理请求分发的结果
			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 afterCompletion
				if (mappedHandler != null) {
					mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
				}
			}
			else {
				// Clean up any resources used by a multipart request.
				if (multipartRequestParsed) {
					cleanupMultipart(processedRequest);
				}
			}
		}
	}
}

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