遇到个问题，公司是用内外网访问的，在调用CAS时要做判断
<!-- CAS Client -->
   <context-param>
        <param-name>serverName</param-name>
        <param-value>http://192.168.7.131:808</param-value>
    </context-param>

    <filter>
        <filter-name>CAS Single Sign Out Filter</filter-name>
        <filter-class>org.jasig.cas.client.session.SingleSignOutFilter</filter-class>
    </filter>

    <filter-mapping>
        <filter-name>CAS Single Sign Out Filter</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>

    <listener>
        <listener-class>org.jasig.cas.client.session.SingleSignOutHttpSessionListener</listener-class>
    </listener>
    <filter>
        <filter-name>CAS Authentication Filter</filter-name>
        <filter-class>org.jasig.cas.client.authentication.AuthenticationFilter</filter-class>
        <init-param>
            <param-name>casServerLoginUrl</param-name>
            <param-value>http://192.168.1.21:8081/sso/login</param-value>
        </init-param>
        <init-param>
            <param-name>renew</param-name>
            <param-value>false</param-value>
        </init-param>
        <init-param>
            <param-name>gateway</param-name>
            <param-value>false</param-value>
        </init-param>
        <init-param>
            <param-name>serverName</param-name>
            <param-value>http://192.168.7.131:808</param-value>
        </init-param>
    </filter>

    <filter>
        <filter-name>CAS Validation Filter</filter-name>
        <filter-class>org.jasig.cas.client.validation.Cas20ProxyReceivingTicketValidationFilter</filter-class>
        <init-param>
            <param-name>casServerUrlPrefix</param-name>
            <param-value>http://192.168.1.21:8081</param-value>
        </init-param>
        <init-param>
            <param-name>casServerUrlPrefix</param-name>
            <param-value>http://192.168.1.21:8081/sso</param-value>
        </init-param>
        <init-param>
            <param-name>useSession</param-name>
            <param-value>true</param-value>
        </init-param>
        <init-param>
            <param-name>redirectAfterValidation</param-name>
            <param-value>true</param-value>
        </init-param>
    </filter>


    <filter>
        <filter-name>CAS HttpServletRequest Wrapper Filter</filter-name>
        <filter-class>org.jasig.cas.client.util.HttpServletRequestWrapperFilter</filter-class>
    </filter>


    <filter>
        <filter-name>CAS Assertion Thread Local Filter</filter-name>
        <filter-class>org.jasig.cas.client.util.AssertionThreadLocalFilter</filter-class>
    </filter>


    <filter-mapping>
        <filter-name>CAS Authentication Filter</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>

    <filter-mapping>
        <filter-name>CAS Validation Filter</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>

    <filter-mapping>
        <filter-name>CAS HttpServletRequest Wrapper Filter</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>

    <filter-mapping>
        <filter-name>CAS Assertion Thread Local Filter</filter-name>
        <url-pattern>/*</url-pattern>
    </filter-mapping>

如内网访问时：
    <context-param>
        <param-name>serverName</param-name>
        <param-value>http://192.168.7.131:808</param-value>
    </context-param>

用外网访问是要些成
    <context-param>
        <param-name>serverName</param-name>
        <param-value>http://218.5.3.128:808</param-value>
    </context-param>