code: private Authentication getAuthentication(SecurityContext securityContext) {
  		// 用户未登录
  		Authentication authentication = securityContext.getAuthentication();
  		if (authentication == null) {
  			authentication = new AnonymousAuthenticationToken(UUID.randomUUID().toString(), "anonymous",
  					Collections.<GrantedAuthority> singletonList(new SimpleGrantedAuthority("ROLE_ANONYMOUS")));
  		}
  		return authentication;
  	}
docs: 获取到当前登录用户凭证，如果不存在，返回一个匿名凭证
  
  @param securityContext
  当前安全上下文
  @return 当前登录用户凭证，如果不存在，返回一个匿名凭证