spring 从源码学设计1-doDispatch（）

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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);}}}

首先请求会经过各种filter最后进入DispatcherServlet类中的doDispatch方法。这个方法时整个请求转发最核心的方法，请求最终汇聚到这个方法被处理。者个方法时整个spring mvc框架的精华所在。它通过高度抽象的接口描述了一个MVC设计模式的实现方案。

HandlerExecutionChain这个对象是通过请求的url获得的处理器执行链，这个对象里面持有两个属性。一个是处理器，一个是拦截器数组。而请求将会最终通过处理器和拦截器进行处理。这儿的处理器是一个Object是java中最高的抽象，这样的设计有高度的扩展性，这个处理器可以是任何的实现，甚至是一个框架。

HandlerAdapter是一个处理器适配器接口，里面有一个很核心的抽象方法 ModelAndView handle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception;

处理器的执行流程将在这个方法中定义，而且这个方法有高度可扩展性。里面可以实现任何的处理流程。这个方法最后会返回一个视图对象。

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;}}

这段代码主要是判断用户请求的资源文件有没有被修改。当用户第二次请求同一个资源，服务器端的资源也没有变化，那么这个请求将不会再被执行下去。

HandlerInterceptor是拦截器，这个接口有三个方法

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是在处理器处理请求前执行，postHandle是在处理器处理请求后执行，afterCompletion是在视图渲染完后执行。这样在进入处理流程的前后，视图渲染的前后都可以同过实现一个拦截器实现很多事情。比如spring-mobile就实现了一个DeviceResolverHandlerInterceptor，在preHandle这个方法里面实现创建了一个 Device对象，这个对象可以用来检查请求的来源，是手机，平板还是桌面电脑。

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 ;}

转载于:https://my.oschina.net/u/1583086/blog/342104

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