使用ssm，在监听器中注入bean，获取null。求大神指导

public class LogoutListener implements HttpSessionListener
{
@Override
public void sessionCreated(HttpSessionEvent arg0)
{
}

@Override
public void sessionDestroyed(HttpSessionEvent session)
{
        ApplicationContext ctx =
            WebApplicationContextUtils.getWebApplicationContext(session.getSession().getServletContext());
    CarFactoryService cfs=(CarFactoryService) ctx.getBean("carFactoryServiceImpl");
    }
    }

http://blog.csdn.net/ft305977550/article/details/43672089