@Audit(
            action="TICKET_GRANTING_TICKET_DESTROYED",
            actionResolverName="DESTROY_TICKET_GRANTING_TICKET_RESOLVER",
            resourceResolverName="DESTROY_TICKET_GRANTING_TICKET_RESOURCE_RESOLVER")
    @Profiled(tag = "DESTROY_TICKET_GRANTING_TICKET", logFailuresSeparately = false)
    @Transactional(readOnly = false)
    @Override
    public List<LogoutRequest> destroyTicketGrantingTicket(final String ticketGrantingTicketId) {
        Assert.notNull(ticketGrantingTicketId);

        logger.debug("Removing ticket [{}] from registry.", ticketGrantingTicketId);
        final TicketGrantingTicket ticket = this.ticketRegistry.getTicket(ticketGrantingTicketId,
                TicketGrantingTicket.class);

        if (ticket == null) {
            logger.debug("TicketGrantingTicket [{}] cannot be found in the ticket registry.", ticketGrantingTicketId);
            return Collections.emptyList();
        }

        logger.debug("Ticket found. Processing logout requests and then deleting the ticket...");
　　　　//在这里cas server会去根据客户端带过来的ticket找到所有在cas server服务注册过的cas client server，让后对这些客户端服务发送一个http请求，客户端接受请求，删除//本身的ticket及注销session，cas server发送请求看下一段代码
        final List<LogoutRequest> logoutRequests = logoutManager.performLogout(ticket);
        this.ticketRegistry.deleteTicket(ticketGrantingTicketId);

        return logoutRequests;
    }
 @Override

public List<LogoutRequest> performLogout(final TicketGrantingTicket ticket) {
        final Map<String, Service> services;
        // synchronize the retrieval of the services and their cleaning for the TGT
        // to avoid concurrent logout mess ups
        synchronized (ticket) {
            services = ticket.getServices();
            ticket.removeAllServices();
        }
        ticket.markTicketExpired();

        final List<LogoutRequest> logoutRequests = new ArrayList<LogoutRequest>();
        // if SLO is not disabled
        if (!disableSingleSignOut) {
            // through all services

　　　　　　　　//循环遍历客户端的server
            for (final String ticketId : services.keySet()) {
                final Service service = services.get(ticketId);
                // it's a SingleLogoutService, else ignore
                if (service instanceof SingleLogoutService) {
                    final SingleLogoutService singleLogoutService = (SingleLogoutService) service;
                    // the logout has not performed already
                    if (!singleLogoutService.isLoggedOutAlready()) {
                        final LogoutRequest logoutRequest = new LogoutRequest(ticketId, singleLogoutService);
                        // always add the logout request
                        logoutRequests.add(logoutRequest);
                        final RegisteredService registeredService = servicesManager.findServiceBy(service);
                        // the service is no more defined, or the logout type is not defined or is back channel
                        if (registeredService == null || registeredService.getLogoutType() == null
                                || registeredService.getLogoutType() == LogoutType.BACK_CHANNEL) {
                            // perform back channel logout
　　　　　　　　　　　　　　　　
　　　　　　　　　　　　　　　　//向客户端发送一个请求，
                            if (performBackChannelLogout(logoutRequest)) {
                                logoutRequest.setStatus(LogoutRequestStatus.SUCCESS);
                            } else {
                                logoutRequest.setStatus(LogoutRequestStatus.FAILURE);
                                LOGGER.warn("Logout message not sent to [{}]; Continuing processing...",
                                        singleLogoutService.getId());
                            }
                        }
                    }
                }
            }
        }

        return logoutRequests;
    }

public void doFilter(final ServletRequest servletRequest, final ServletResponse servletResponse, final FilterChain filterChain) throws IOException, ServletException {
        final HttpServletRequest request = (HttpServletRequest) servletRequest;
        //重点的两个IF判断，同时也是逻辑处理，第一是判断是不是第一次登录的时候会进入方法往cas SessionMappingStorage的添加ticket,sessionId键值对
        if (handler.isTokenRequest(request)) {
            LOG.warn("第{}次进来映射，sessionId={}", index++, request.getSession().getId());
            handler.recordSession(request);
　　　　　　//判断是否是注销请求，如果是注销请求进入逻辑，注销session同时删除SessionMappingStorage的键值对（就是这里实现了统一注销）
        } else if (handler.isLogoutRequest(request)) {
            LOG.warn("第{}进来删除映射，sessionId={}", removeIndex++, request.getSession().getId());
            handler.destroySession(request);
            // Do not continue up filter chain
            return;
        } else {
            log.trace("Ignoring URI " + request.getRequestURI());
        }

        filterChain.doFilter(servletRequest, servletResponse);
    }