js css鼠标悬停显示下拉菜单

<div class="swss_xx">
            <ul class="clearfix">
                <li id="xxwh"><a href="/go.html?url=#" onclick="javascript:studentInfoUpdate()">个人信息维护</a>
                    <div id="show_xxwh" style="display: none">
                        <ul>
                            <li><a href="/go.html?url=#" onclick="javascript:studentInfoUpdate()">学生信息修改</a></li>
                            <%--<li><a href="/go.html?url=#">修改密码</a></li>--%>
                            <%--<li><a href="/go.html?url=#">修改密码</a></li>--%>
                        </ul>
                    </div>
                </li>
                <%--<li><a href="/go.html?url=javascript:pwdCon();">修改密码</a></li>--%>
                <li><a href="/go.html?url=<%=ctx %>/logout">退出</a></li>
            </ul>
            <p><span id="uname">登录用户：</span>            
                <span id="cid">所在班级： </span>
            </p>
</div><div class="swss_xx">
            <ul cla