请求由tomcat转到servlet的临界点

 >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>StandardWrapperValve--->Servlet>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
    请求转出，都由valve来调用servlet的service方法
    [1] 直接转出
    StandardWrapperValve
    void java.org.apache.catalina.core.StandardWrapperValve.invoke(Request request, Response response) throws IOException, ServletException
    --->filterChain.doFilter
    ------------>servlet.service(request, response);

    [2] 内部循环再转出
    -------->((AsyncContextImpl)request.getAsyncContext()).doInternalDispatch();
           ----> void java.org.apache.catalina.core.AsyncContextImpl.doInternalDispatch() throws ServletException, IOException
                ---->
                         Runnable runnable = dispatch;
                         dispatch = null;
                         runnable.run();

                         ------------->
                          void java.org.apache.catalina.core.AsyncContextImpl.dispatch(ServletContext context, String path)
                                 ---->applicationDispatcher.dispatch(servletRequest, servletResponse);

                                 AsyncDispatcher=applicationDispatcher

                                 final class ApplicationDispatcher implements AsyncDispatcher, RequestDispatcher

                                 ------>final class ApplicationDispatcher implements AsyncDispatcher, RequestDispatcher
                                         ---------> void java.org.apache.catalina.core.ApplicationDispatcher.doDispatch(ServletRequest request, ServletResponse response) throws ServletException, IOException
                                                    -------------> void java.org.apache.catalina.core.ApplicationDispatcher.doDispatch(ServletRequest request, ServletResponse response) throws ServletException, IOException
                                                                 //Servlet Service Method is called by the FilterChain
                                                                   ----------->void java.org.apache.catalina.core.ApplicationFilterChain.doFilter(ServletRequest request, ServletResponse response) throws IOException, ServletException
                                                                      --> void java.org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ServletRequest request, ServletResponse response) throws IOException, ServletException
                                                                             ------------>servlet.service(request, response);
     最终都是由servlet.service(request, response);
 >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>StandardWrapperValve--->Servlet>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>