SpringMvc源码-DispatcherServlet(二)

上一篇SpringMvc源码-DispatcherServlet讲了DispatcherServlet的初始化过程，本篇着重讲DispatchServlet处理一个请求的流程，即面试SpringMvc的经典面试题，SpringMvc是如何处理reqeuest的。

准备工作

我们的DispatchServlet继承于HttpServlet，HttpServlet处理请求的方法主要是httpServlet.service，该方法会根据request.method调用doXxx。FrameworkServlet自己去实现了doXxx。

整个的调用链是：接到reqeust请求后->FrameworkServlet.service()->FrameworkServlet.processRequest()->DispatchServlet.doService()（FrameworkServlet提供了抽象方法)
->DispatchServlet.doDispatch()处理请求

下面，我们根据这个调用链进行分析

FrameworkServlet

FrameworkServlet还重写了service方法，主要是支持HttpMethod.PATCH，它们和HttpMethod.PATCH一样都调用了processRequest(request, response)

所有的doXXX都是调用processRequest我们来看下processRequest方法做了什么

processRequest

protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
		throws ServletException, IOException {

	long startTime = System.currentTimeMillis();
	Throwable failureCause = null;

	//从ThreadLocal中获取上一个请求的LocaleContext
	LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();//
	//new当前请求的LocaleContext
	LocaleContext localeContext = buildLocaleContext(request);//SimpleLocaleContext

	//从ThreadLocal中获取上一个请求的RequestAttributes(ServletRequestAttributes)
	RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
	//new当前请求的ServletRequestAttributes
	ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);

	WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
	asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor());

	//保存进ThreadLocal中
	initContextHolders(request, localeContext, requestAttributes);

	try {
		//抽象方法具体实现在DeipatchServlet,重点方法
		doService(request, response);
	}
	catch (ServletException | IOException ex) {
		failureCause = ex;
		throw ex;
	}
	catch (Throwable ex) {
		failureCause = ex;
		throw new NestedServletException("Request processing failed", ex);
	}

	finally {
		//恢复previousLocaleContext，previousAttributes
		//这里是我没太看明白的地方，说下我的猜测：
		// 1、previousLocaleContext,previousAttributes大概率为空这里就重置了ThreadLocal
		// 2、previousLocaleContext,previousAttributes不为空，说明当前线程处理别的请求，所以
		// 		在doservice时候context用的是current的值，请求处理完了将值还原称之前的值。
		resetContextHolders(request, previousLocaleContext, previousAttributes);
		if (requestAttributes != null) {
			requestAttributes.requestCompleted();
		}
		logResult(request, response, failureCause, asyncManager);
		publishRequestHandledEvent(request, response, startTime, failureCause);
	}
}

doService–>doDispatch方法负责处理具体的请求逻辑

doDispatch见下面

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 {
			//判断reqeust的ContentType包含multipart/ 如果是multipart则返回StandardMultipartHttpServletRequest
			processedRequest = checkMultipart(request);
			multipartRequestParsed = (processedRequest != request);

			// Determine handler for the current request.
			//根据请求获取HandlerExecutionChain，包含对应的handlerMethod和interceptor【MappedInterceptor 和 HandlerIntercpetor等等】
			//具体介绍见下方
			mappedHandler = getHandler(processedRequest);
			if (mappedHandler == null) {
				noHandlerFound(processedRequest, response);
				return;
			}

			// Determine handler adapter for the current request.
			// 根据handler找到HandlerAdapter
			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 (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
					return;
				}
			}

			//关键代码，执行拦截器的preHandle
			if (!mappedHandler.applyPreHandle(processedRequest, response)) {
				return;
			}

			// Actually invoke the handler.
			//关键代码，执行handlerMethod，ha的类型是RequestMappingHandlerAdapter，
			// 调用路径是AbstractHandlerMethodAdapter.handle()-->AbstractHandlerMethodAdapter.handleInternal()-->ha的类型是RequestMappingHandlerAdapter.handleInternal
			mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

			/**
			 * todo 待看的组件
			 * WebDataBinderFactory
			 * ModelFactory
			 *
			 * ServletInvocableHandlerMethod
			 *
			 * HandlerMethodArgumentResolverComposite
			 * HandlerMethodReturnValueHandlerComposite
			 *
			 * ModelAndViewContainer
			 * modelAndView
			 */

			//todo asyncManager 待看
			if (asyncManager.isConcurrentHandlingStarted()) {
				return;
			}

			//todo
			applyDefaultViewName(processedRequest, mv);
			//关键代码，执行拦截器的postHandle
			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);
		}
		//todo 书签 渲染modelAndView，同时触发mappedHandler.triggerAfterCompletion
		processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
	} catch (Exception ex) {
		//触发mappedHandler.triggerAfterCompletion
		triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
	} catch (Throwable err) {
		//触发mappedHandler.triggerAfterCompletion
		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.
			//这里要注意，Multipar在这里被清理掉所以如果Multipart有异步场景，需要将copy到别的对象中。
			if (multipartRequestParsed) {
				cleanupMultipart(processedRequest);
			}
		}
	}
}

关键文章索引：

• SpringMvc源码-RequestMappingHandlerMapping
• SpringMvc源码-RequestMappingHandlerAdapter