我运行以下代码在注销时清除sessionscope:
val = context.getSubmittedValue();
if (val == "logout") {
facesContext.getExternalContext().getSession(false).invalidate();
href=facesContext.getExternalContext().getRequest().getContextPath() + "?Logout&redirectTo=" + facesContext.getExternalContext().getRequest().getContextPath() ;
facesContext.getExternalContext().redirect(href);
}