servlet和Spring的DispatcherServlet详解

Servlet是什么

1. Servlet是服务器端运行的一个程序，是一个被编译好的Java类。它不是框架等。

2. Web容器的启动，需要依赖Servlet。当web服务器开始执行时，servlet类就被初始化

3. 当用户通过浏览器访问的时候，会输入URI，这个时候，web服务器就通过Servlet来分发请求执行不同的内容。

Servlet是有一个Jar包，里面包含了一些Servlet的相关设计和细节。

Spring的DispatcherServlet

Spring主要通过DispatcherServlet实现了Servlet。DispatcherServlet最上层也是继承的是HttpServlet这个类。

@Override

	protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {

		if (logger.isDebugEnabled()) {

			String requestUri = urlPathHelper.getRequestUri(request);

			logger.debug("DispatcherServlet with name '" + getServletName() + "' processing " + request.getMethod() +

					" request for [" + requestUri + "]");

		}

		// Keep a snapshot of the request attributes in case of an include,

		// to be able to restore the original attributes after the include.

		Map<String, Object> attributesSnapshot = null;

		if (WebUtils.isIncludeRequest(request)) {

			logger.debug("Taking snapshot of request attributes before include");

			attributesSnapshot = new HashMap<String, Object>();

			Enumeration<?> attrNames = request.getAttributeNames();

			while (attrNames.hasMoreElements()) {

				String attrName = (String) attrNames.nextElement();

				if (this.cleanupAfterInclude || attrName.startsWith("org.springframework.web.servlet")) {

					attributesSnapshot.put(attrName, request.getAttribute(attrName));

				}

			}

		}

		// Make framework objects available to handlers and view objects.

		request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());

		request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);

		request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);

		request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());

		FlashMap inputFlashMap = this.flashMapManager.retrieveAndUpdate(request, response);

		if (inputFlashMap != null) {

			request.setAttribute(INPUT_FLASH_MAP_ATTRIBUTE, Collections.unmodifiableMap(inputFlashMap));

		}

		request.setAttribute(OUTPUT_FLASH_MAP_ATTRIBUTE, new FlashMap());

		request.setAttribute(FLASH_MAP_MANAGER_ATTRIBUTE, this.flashMapManager);

		try {

			doDispatch(request, response); //这边最终也是调用了doDispatch方法，该方法主要用来处理SPring框架的具体业务分发逻辑。

		}

		finally {

			// Restore the original attribute snapshot, in case of an include.

			if (attributesSnapshot != null) {

				restoreAttributesAfterInclude(request, attributesSnapshot);

			}

		}

	}

        //Spring框架最终的分发都是通过该方法的

	protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {

		HttpServletRequest processedRequest = request;

		HandlerExecutionChain mappedHandler = null;

		int interceptorIndex = -1;

		try {

			ModelAndView mv;

			boolean errorView = false;

			try {

				processedRequest = checkMultipart(request);

				// Determine handler for the current request.

				mappedHandler = getHandler(processedRequest, false);

				if (mappedHandler == null || mappedHandler.getHandler() == 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()) {

						String requestUri = urlPathHelper.getRequestUri(request);

						logger.debug("Last-Modified value for [" + requestUri + "] is: " + lastModified);

					}

					if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {

						return;

					}

				}

				// 这里是处理前置拦截器

				HandlerInterceptor[] interceptors = mappedHandler.getInterceptors();

				if (interceptors != null) {

					for (int i = 0; i < interceptors.length; i++) {

						HandlerInterceptor interceptor = interceptors[i];

						if (!interceptor.preHandle(processedRequest, response, mappedHandler.getHandler())) {

							triggerAfterCompletion(mappedHandler, interceptorIndex, processedRequest, response, null);

							return;

						}

						interceptorIndex = i;

					}

				}

				//处理最终的Action逻辑

				mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

				// Do we need view name translation?

				if (mv != null && !mv.hasView()) {

					mv.setViewName(getDefaultViewName(request));

				}

                                //处理后置拦截器

				if (interceptors != null) {

					for (int i = interceptors.length - 1; i >= 0; i--) {

						HandlerInterceptor interceptor = interceptors[i];

						interceptor.postHandle(processedRequest, response, mappedHandler.getHandler(), mv);

					}

				}

			}

			catch (ModelAndViewDefiningException ex) {

				logger.debug("ModelAndViewDefiningException encountered", ex);

				mv = ex.getModelAndView();

			}

			catch (Exception ex) {

				Object handler = (mappedHandler != null ? mappedHandler.getHandler() : null);

				mv = processHandlerException(processedRequest, response, handler, ex);

				errorView = (mv != null);

			}

			// Did the handler return a view to render?

			if (mv != null && !mv.wasCleared()) {

				render(mv, processedRequest, response);

				if (errorView) {

					WebUtils.clearErrorRequestAttributes(request);

				}

			}

			else {

				if (logger.isDebugEnabled()) {

					logger.debug("Null ModelAndView returned to DispatcherServlet with name '" + getServletName() +

							"': assuming HandlerAdapter completed request handling");

				}

			}

			// Trigger after-completion for successful outcome.

			triggerAfterCompletion(mappedHandler, interceptorIndex, processedRequest, response, null);

		}

		catch (Exception ex) {

			// Trigger after-completion for thrown exception.

			triggerAfterCompletion(mappedHandler, interceptorIndex, processedRequest, response, ex);

			throw ex;

		}

		catch (Error err) {

			ServletException ex = new NestedServletException("Handler processing failed", err);

			// Trigger after-completion for thrown exception.

			triggerAfterCompletion(mappedHandler, interceptorIndex, processedRequest, response, ex);

			throw ex;

		}

		finally {

			// Clean up any resources used by a multipart request.

			if (processedRequest != request) {

				cleanupMultipart(processedRequest);

			}

		}

	}

巴特西

servlet和Spring的DispatcherServlet详解

Servlet是什么

Spring的DispatcherServlet

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