spring 從源碼學設計1-doDispatch（）

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;
					}
				}

				// Apply preHandle methods of registered interceptors.
				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;
					}
				}

				// Actually invoke the handler.
				mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

				// Do we need view name translation?
				if (mv != null && !mv.hasView()) {
					mv.setViewName(getDefaultViewName(request));
				}

				// Apply postHandle methods of registered interceptors.
				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);
			}
		}
	}

首先請求會通過各類filter最後進入DispatcherServlet類中的doDispatch方法。這個方法時整個請求轉發最核心的方法，請求最終匯聚到這個方法被處理。者個方法時整個spring mvc框架的精華所在。它經過高度抽象的接口描述了一個MVC設計模式的實現方案。

HandlerExecutionChain這個對象是經過請求的url得到的處理器執行鏈，這個對象裏面持有兩個屬性。一個是處理器，一個是攔截器數組。而請求將會最終經過處理器和攔截器進行處理。這兒的處理器是一個Object是java中最高的抽象，這樣的設計有高度的擴展性，這個處理器能夠是任何的實現，甚至是一個框架。

HandlerAdapter是一個處理器適配器接口，裏面有一個很核心的抽象方法 ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception;

處理器的執行流程將在這個方法中定義，並且這個方法有高度可擴展性。裏面能夠實現任何的處理流程。這個方法最後會返回一個視圖對象。

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;
					}
				}

這段代碼主要是判斷用戶請求的資源文件有沒有被修改。當用戶第二次請求同一個資源，服務器端的資源也沒有變化，那麼這個請求將不會再被執行下去。

HandlerInterceptor是攔截器，這個接口有三個方法

public interface HandlerInterceptor {

	/**
	 * Intercept the execution of a handler. Called after HandlerMapping determined
	 * an appropriate handler object, but before HandlerAdapter invokes the handler.
	 * <p>DispatcherServlet processes a handler in an execution chain, consisting
	 * of any number of interceptors, with the handler itself at the end.
	 * With this method, each interceptor can decide to abort the execution chain,
	 * typically sending a HTTP error or writing a custom response.
	 * @param request current HTTP request
	 * @param response current HTTP response
	 * @param handler chosen handler to execute, for type and/or instance evaluation
	 * @return <code>true</code> if the execution chain should proceed with the
	 * next interceptor or the handler itself. Else, DispatcherServlet assumes
	 * that this interceptor has already dealt with the response itself.
	 * @throws Exception in case of errors
	 */
	boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler)
	    throws Exception;

	/**
	 * Intercept the execution of a handler. Called after HandlerAdapter actually
	 * invoked the handler, but before the DispatcherServlet renders the view.
	 * Can expose additional model objects to the view via the given ModelAndView.
	 * <p>DispatcherServlet processes a handler in an execution chain, consisting
	 * of any number of interceptors, with the handler itself at the end.
	 * With this method, each interceptor can post-process an execution,
	 * getting applied in inverse order of the execution chain.
	 * @param request current HTTP request
	 * @param response current HTTP response
	 * @param handler chosen handler to execute, for type and/or instance examination
	 * @param modelAndView the <code>ModelAndView</code> that the handler returned
	 * (can also be <code>null</code>)
	 * @throws Exception in case of errors
	 */
	void postHandle(
			HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView)
			throws Exception;

	/**
	 * Callback after completion of request processing, that is, after rendering
	 * the view. Will be called on any outcome of handler execution, thus allows
	 * for proper resource cleanup.
	 * <p>Note: Will only be called if this interceptor's <code>preHandle</code>
	 * method has successfully completed and returned <code>true</code>!
	 * <p>As with the {@code postHandle} method, the method will be invoked on each
	 * interceptor in the chain in reverse order, so the first interceptor will be
	 * the last to be invoked.
	 * @param request current HTTP request
	 * @param response current HTTP response
	 * @param handler chosen handler to execute, for type and/or instance examination
	 * @param ex exception thrown on handler execution, if any
	 * @throws Exception in case of errors
	 */
	void afterCompletion(
			HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex)
			throws Exception;

}

preHandle是在處理器處理請求前執行，postHandle是在處理器處理請求後執行，afterCompletion是在 視圖渲染完後執行。這樣在進入處理流程的先後，視圖渲染的先後均可以同過實現一個攔截器實現不少事情。好比spring-mobile就實現了一個DeviceResolverHandlerInterceptor，在preHandle這個方法裏面實現建立了一個 Device對象，這個對象能夠用來檢查請求的來源，是手機，平板仍是桌面電腦。

public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
         Device device = deviceResolver.resolveDevice(request);
        request.setAttribute(DeviceUtils. CURRENT_DEVICE_ATTRIBUTE, device);
          return true ;
    }