The content of element type "struts" must match "((package|include|bean|constant)*,unknown-handler-s

<struts>
<!-- 配置为开发模式 -->
    <constant name="struts.devMode" value="true" />
<!-- 扩展名配置为action -->
<content name="struts.action.extension" value="action"/>
    <!-- 把主题配置simple -->
    <content name="struts.ui.theme" value="simple"/>


    <!-- Add packages here -->

</struts>

以上提示了：The content of element type "struts" must match "((package|include|bean|constant)*,unknown-handler-s

在配置struts.xml文件时不小心把红色部分的字写错了，应改为constant