关于request.getRequestDispatcher().forward()的妙用及DispatcherType对Filter配置的影响

<servlet-mapping>
    <servlet-name>RedServlet</servlet-name>
    <url-pattern>/red/*</url-pattern>
</servlet-mapping>
<servlet-mapping>
    <servlet-name>RedServlet</servlet-name>
    <url-pattern>/red/red/*</url-pattern>
</servlet-mapping>

@Bean
    public ServletRegistrationBean dispatcherServletWithNewPrefix(DispatcherServlet dispatcherServlet) {
        ServletRegistrationBean registration = new ServletRegistrationBean(dispatcherServlet,
                "/ceshi/*");
        registration.setName("ceshi");
        return registration;
    }

    private ServletRegistration.Dynamic addServlet(String servletName, String servletClass,
            Servlet servlet, Map<String,String> initParams) throws IllegalStateException {
        # 此处参数校验等不重要的逻辑
        
        # 通过本次要注册的servletName获取之前注册过的
        Wrapper wrapper = (Wrapper) context.findChild(servletName);


        if (wrapper == null) {
            wrapper = context.createWrapper();
            wrapper.setName(servletName);
            context.addChild(wrapper);
        } else {
            # wrapper不为空说明之前有同名servlet，此次的servlet不在进行注册
            if (wrapper.getName() != null &&
                    wrapper.getServletClass() != null) {
                if (wrapper.isOverridable()) {
                    wrapper.setOverridable(false);
                } else {
                    return null;
                }
            }
        }
        ... ...
 }

 @RequestMapping(value = "/xx/upload", method = RequestMethod.POST)
 public handleFormUpload(@RequestParam("file") MultipartFile file)

    public Collection<Part> getParts() throws IOException, ServletException;
    public Part getPart(String name) throws IOException, ServletException;

	private void parseRequest(HttpServletRequest request) {
		try {
		    # 使用servlet原生的API对附件进行处理
			Collection<Part> parts = request.getParts();
			this.multipartParameterNames = new LinkedHashSet<>(parts.size());
			MultiValueMap<String, MultipartFile> files = new LinkedMultiValueMap<>(parts.size());
			for (Part part : parts) {
				... 
			}
			setMultipartFiles(files);
		}
	}

    private void parseParts(boolean explicit) {
        ... 
        Context context = getContext();
        MultipartConfigElement mce = getWrapper().getMultipartConfigElement();

        if (mce == null) {
            if(context.getAllowCasualMultipartParsing()) {
                mce = new MultipartConfigElement(null, connector.getMaxPostSize(),
                        connector.getMaxPostSize(), connector.getMaxPostSize());
            } else {
                if (explicit) {
                    # 异常在这里抛出
                    partsParseException = new IllegalStateException(
                            sm.getString("coyoteRequest.noMultipartConfig"));
                    return;
                } else {
                    parts = Collections.emptyList();
                    return;
                }
            }
    }

		public DispatcherServletRegistrationBean dispatcherServletRegistration(DispatcherServlet dispatcherServlet,
				WebMvcProperties webMvcProperties, ObjectProvider<MultipartConfigElement> multipartConfig) {
			DispatcherServletRegistrationBean registration = new DispatcherServletRegistrationBean(dispatcherServlet,
					webMvcProperties.getServlet().getPath());
			registration.setName(DEFAULT_DISPATCHER_SERVLET_BEAN_NAME);
			registration.setLoadOnStartup(webMvcProperties.getServlet().getLoadOnStartup());
			# 对servlet的MultipartConfig进行配置
			multipartConfig.ifAvailable(registration::setMultipartConfig);
			return registration;
		}

@WebFilter("/ceshi/*")
public class TestFilter implements Filter {
    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
            throws IOException, ServletException {
        HttpServletRequest httpServletRequest = (HttpServletRequest) request;
        String dispatcher = httpServletRequest.getPathInfo();
        request.getRequestDispatcher(dispatcher).forward(request, response);
    }
}

    public static ApplicationFilterChain createFilterChain(ServletRequest request,
            Wrapper wrapper, Servlet servlet) {
        # 去掉多余的参数校验，保留主逻辑方便代码阅读
        
        ApplicationFilterChain filterChain = new ApplicationFilterChain();
        
        filterChain.setServlet(servlet);
        filterChain.setServletSupportsAsync(wrapper.isAsyncSupported());

        StandardContext context = (StandardContext) wrapper.getParent();
        FilterMap filterMaps[] = context.findFilterMaps();

        # 重定向后DispatcherType为FORWARD
        DispatcherType dispatcher =
                (DispatcherType) request.getAttribute(Globals.DISPATCHER_TYPE_ATTR);

        String requestPath = null;
        Object attribute = request.getAttribute(Globals.DISPATCHER_REQUEST_PATH_ATTR);
        if (attribute != null){
            requestPath = attribute.toString();
        }

        String servletName = wrapper.getName();

        // Add the relevant path-mapped filters to this filter chain
        for (FilterMap filterMap : filterMaps) {
            # 判断Filter的DispatcherType是否匹配
            if (!matchDispatcher(filterMap, dispatcher)) {
                continue;
            }
            # 判断Filter的url是否匹配
            if (!matchFiltersURL(filterMap, requestPath))
                continue;
            ApplicationFilterConfig filterConfig = (ApplicationFilterConfig)
                    context.findFilterConfig(filterMap.getFilterName());
            filterChain.addFilter(filterConfig);
        }

        // Add filters that match on servlet name second
        for (FilterMap filterMap : filterMaps) {
            if (!matchDispatcher(filterMap, dispatcher)) {
                continue;
            }
            # 判断Filter是否匹配该servletName
            if (!matchFiltersServlet(filterMap, servletName))
                continue;
            ApplicationFilterConfig filterConfig = (ApplicationFilterConfig)
                    context.findFilterConfig(filterMap.getFilterName());
            filterChain.addFilter(filterConfig);
        }
        return filterChain;
    }
    private static boolean matchDispatcher(FilterMap filterMap, DispatcherType type) {
        switch (type) {
            case FORWARD :
                if ((filterMap.getDispatcherMapping() & FilterMap.FORWARD) != 0) {
                    return true;
                }
                break;
            case INCLUDE :
                if ((filterMap.getDispatcherMapping() & FilterMap.INCLUDE) != 0) {
                    return true;
                }
                break;
            case REQUEST :
                if ((filterMap.getDispatcherMapping() & FilterMap.REQUEST) != 0) {
                    return true;
                }
                break;
            case ERROR :
                if ((filterMap.getDispatcherMapping() & FilterMap.ERROR) != 0) {
                    return true;
                }
                break;
            case ASYNC :
                if ((filterMap.getDispatcherMapping() & FilterMap.ASYNC) != 0) {
                    return true;
                }
                break;
        }
        return false;
    }

for (FilterMap filterMap : filterMaps) {
    if (!matchDispatcher(filterMap, dispatcher)) {
        continue;
    }
    if (!matchFiltersURL(filterMap, requestPath) && !matchFiltersServlet(filterMap, servletName))
        continue;
    ApplicationFilterConfig filterConfig = (ApplicationFilterConfig)
                    context.findFilterConfig(filterMap.getFilterName());
    filterChain.addFilter(filterConfig);
}