获取IP的方法出错，急，求解！

String ip =request.getHeader("x-forwarded-for");
为什么我把浏览器关闭了，然后让session自动销毁，这句话出错了，
Hibernate: delete from MCLONDBA.PUB_LOCK where EMP_ID=1301 and BELONG_SYSTEM=1
2010-10-20 11:27:52 org.apache.catalina.session.StandardSession expire
严重: Session event listener threw exception
java.lang.NullPointerException
at org.apache.struts2.ServletActionContext.getRequest(ServletActionContext.java:112)
at cn.mclon.is.util.common.Common.getRequest(Common.java:273)
at cn.mclon.is.util.online.OnlineCounter.getIpAddr(OnlineCounter.java:114)
at cn.mclon.is.util.listener.WLSessionListener.sessionDestroyed(WLSessionListener.java:54)
at org.apache.catalina.session.StandardSession.expire(StandardSession.java:702)
at org.apache.catalina.session.StandardSession.isValid(StandardSession.java:592)
at org.apache.catalina.session.ManagerBase.processExpires(ManagerBase.java:682)
at org.apache.catalina.session.ManagerBase.backgroundProcess(ManagerBase.java:667)
at org.apache.catalina.core.ContainerBase.backgroundProcess(ContainerBase.java:1316)
at org.apache.catalina.core.ContainerBase$ContainerBackgroundProcessor.processChildren(ContainerBase.java:1601)
at org.apache.catalina.core.ContainerBase$ContainerBackgroundProcessor.processChildren(ContainerBase.java:1610)
at org.apache.catalina.core.ContainerBase$ContainerBackgroundProcessor.processChildren(ContainerBase.java:1610)
at org.apache.catalina.core.ContainerBase$ContainerBackgroundProcessor.run(ContainerBase.java:1590)
at java.lang.Thread.run(Thread.java:619)