struts2配置错误

```出现红色叉叉。
>The errors below were detected when validating the file "struts-2.3.dtd" via the file "struts.xml".  In most cases these errors can be detected by validating "struts-2.3.dtd" directly.  However it is possible that errors will only occur when struts-2.3.dtd is validated in the context of struts.xml.

>Description:the markup declarations contained or pointed to by the document type declara

解决了，原因是机器没上网，找不到指定的dtd文件！
可以这样做：在eclipse的菜单栏window中单击preference，依次找到XML-DTD Files，选择XML Catalog，选中右上方的User Specified Entries，单击add按钮，找到该dtd文件在本地的路径：
struts-2.1.8\src\core\src\main\resources\struts-2.3.dtd,Key Type 中选择uri，key中输入以下路径：http://struts.apache.org/dtds/struts-2.3.dtd,点击确定。回到struts.xml后在任意位置输入空格保存就行了