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主要思路：浏览器访问CAS服务器登录，拿到凭证给后端，后端用此凭证到CAS服务器验证登录并拿到用户信息，之后基于该凭证维持用户的登录状态。

主要流程：

1.浏览器访问后端需认证登录地址（不带ticket）

2.后端向浏览器发送重定向信息到cas服务端（参数带跳转CAS服务器的地址）

3.浏览器访问CAS服务端验证登录（参数带跳转后端的地址）

4.CAS服务器向浏览器发送重定向信息到后端的地址（带ticket）

5.后端用此ticket访问CAS服务端验证用户是否登录。

步骤2后端源码片段

org.jasig.cas.client.authentication.AuthenticationFilter#doFilter

这个AuthenticationFilter过滤器专门用户浏览器第一次访问时的跳转，专门检测ticket是否存在。不存在就重定向到cas服务器，存在就不处理

第一次访问后端认证地址没有ticket,重定地址到登录cas服务器

public final void doFilter(final ServletRequest servletRequest, final ServletResponse servletResponse,final FilterChain filterChain) throws IOException, ServletException {final HttpServletRequest request = (HttpServletRequest) servletRequest;final HttpServletResponse response = (HttpServletResponse) servletResponse;if (isRequestUrlExcluded(request)) {logger.debug("Request is ignored.");filterChain.doFilter(request, response);return;}final HttpSession session = request.getSession(false);final Assertion assertion = session != null ? (Assertion) session.getAttribute(CONST_CAS_ASSERTION) : null;if (assertion != null) {filterChain.doFilter(request, response);return;}final String serviceUrl = constructServiceUrl(request, response);final String ticket = retrieveTicketFromRequest(request);final boolean wasGatewayed = this.gateway && this.gatewayStorage.hasGatewayedAlready(request, serviceUrl);if (CommonUtils.isNotBlank(ticket) || wasGatewayed) {filterChain.doFilter(request, response);return;}final String modifiedServiceUrl;logger.debug("no ticket and no assertion found");if (this.gateway) {logger.debug("setting gateway attribute in session");modifiedServiceUrl = this.gatewayStorage.storeGatewayInformation(request, serviceUrl);} else {modifiedServiceUrl = serviceUrl;}logger.debug("Constructed service url: {}", modifiedServiceUrl);final String urlToRedirectTo = CommonUtils.constructRedirectUrl(this.casServerLoginUrl,getProtocol().getServiceParameterName(), modifiedServiceUrl, this.renew, this.gateway);logger.debug("redirecting to \"{}\"", urlToRedirectTo);this.authenticationRedirectStrategy.redirect(request, response, urlToRedirectTo);}

步骤5后端源码片段

org.jasig.cas.client.validation.AbstractTicketValidationFilter#doFilter

validation.AbstractTicket类是后端 通过前端带过来的ticket请求cas服务器，认证有效性。

底层调用HttlsURLConnectionFactory。需要实现AbstractTicket自定义认证方式

public final void doFilter(final ServletRequest servletRequest, final ServletResponse servletResponse,final FilterChain filterChain) throws IOException, ServletException {if (!preFilter(servletRequest, servletResponse, filterChain)) {return;}final HttpServletRequest request = (HttpServletRequest) servletRequest;final HttpServletResponse response = (HttpServletResponse) servletResponse;final String ticket = retrieveTicketFromRequest(request);if (CommonUtils.isNotBlank(ticket)) {logger.debug("Attempting to validate ticket: {}", ticket);try {final Assertion assertion = this.ticketValidator.validate(ticket,constructServiceUrl(request, response));logger.debug("Successfully authenticated user: {}", assertion.getPrincipal().getName());request.setAttribute(CONST_CAS_ASSERTION, assertion);if (this.useSession) {request.getSession().setAttribute(CONST_CAS_ASSERTION, assertion);}onSuccessfulValidation(request, response, assertion);if (this.redirectAfterValidation) {logger.debug("Redirecting after successful ticket validation.");response.sendRedirect(constructServiceUrl(request, response));return;}} catch (final TicketValidationException e) {logger.debug(e.getMessage(), e);onFailedValidation(request, response);if (this.exceptionOnValidationFailure) {throw new ServletException(e);}response.sendError(HttpServletResponse.SC_FORBIDDEN, e.getMessage());return;}}filterChain.doFilter(request, response);}

后端需要配置cas服务器跳转地址,后端地址