thymeleaf中双下划线问题

<div th:fragment="foot_menu(index)">
    <div class="weui-tabbar wy-foot-menu">
        <a href="index.html" th:class="${index}==0?'weui-bar_'+'_item--on weui-tabbar_'+'_item':'weui-tabbar_'+'_item'">
            <div class="weui-tabbar__icon foot-menu-home"></div>
            <p class="weui-tabbar__label">首页</p>
        </a>
        <a href="classify.html" class="weui-tabbar__item">
            <div class="weui-tabbar__icon foot-menu-list"></div>
            <p class="weui-tabbar__label">分类</p>
        </a>
    </div>
</div>

　weui中　要求菜单未选中为 class="weui-tabbar__item"，选中后为 class="weui-tabbar__item weui-bar__item--on"

错误写法

th:class="${index}==0?'weui-bar__item--on weui-tabbar__item':'weui-tabbar__item'"

正确写法

th:class="${index}==0?'weui-bar_'+'_item--on weui-tabbar_'+'_item':'weui-tabbar_'+'_item'"