https://www.cnblogs.com/Java-Starter/p/10310565.html

初始化过程

https://blog.csdn.net/dhaiuda/article/details/80026354

总结下HttpServletBean,FrameworkServlet和DispatcherServlet初始化过程

1.HttpServletBean

初始化web.xml中的参数

2.FrameworkServlet

将上下文赋予当前Servlet

3.DispatcherServlet

初始化HandlerMapping(请求映射),HandlerExceptionResolver(异常处理),ViewResolver(视图解析)等功能实现类

DispatcherServlet处理请求

在浏览器输入http://localhost:8080/springmvcdemo/employee，触发DispatcherServlet的processRequest方法

先看父类FrameworkServlet的processRequest方法:

previousLocaleContext获取和当前线程相关的LocaleContext

根据已有请求构造一个新的和当前线程相关的LocaleContext

previousAttributes获取和当前线程绑定的RequestAttributes

为已有请求构造新的ServletRequestAttributes，加入预绑定属性

initContextHolders让新构造的RequestAttributes和ServletRequestAttributes和当前线程绑定，加入到ThreadLocal，完成绑定

抽象方法doService由FrameworkServlet子类DispatcherServlet重写

resetContextHolders方法解除RequestAttributes,ServletRequestAttributes和当前线程的绑定

注册监听事件ServletRequestHandledEvent,在调用上下文的时候产生Event

现在我们看下DispatcherServlet的doService方法

attributesSnapshot用来保存request域中的数据，可以叫做“快照”

进入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);

				
                //通过当前request获取到 HandlerExecutionChain
				mappedHandler = getHandler(processedRequest);
				if (mappedHandler == null) {
					noHandlerFound(processedRequest, response);
					return;
				}

				
                //确定当前请求的处理程序适配器。
				HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

				// 如果处理程序支持，则处理最后修改的标头
				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;
					}
				}

				if (!mappedHandler.applyPreHandle(processedRequest, response)) {
					return;
				}

				// 实际调用处理程序。
				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 afterCompletion
				if (mappedHandler != null) {
					mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
				}
			}
			else {
				// Clean up any resources used by a multipart request.
				if (multipartRequestParsed) {
					cleanupMultipart(processedRequest);
				}
			}
		}
	}

protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {
   if (this.handlerAdapters != null) {
      for (HandlerAdapter adapter : this.handlerAdapters) {
         if (adapter.supports(handler)) {
            return adapter;
         }
      }
   }
   throw new ServletException("No adapter for handler [" + handler +
         "]: The DispatcherServlet configuration needs to include a HandlerAdapter that supports this handler");
}