引入标签：
<%@ taglib preflx="logic" uri="http://struts.apache.org/tags-logic"%>
<logic:empty name="attr1">
attr1为空
</logic:empty>
<logic:notempty name="attr1">
attr1不为空
</logic:notempty>
<logic:present name="attr1">
attr1存在
</logic:present>
<logic:notpresent name="attr1">
attr1不存在
</logic:notpresent>
//id表示的是scope中集合在迭代中的某个元素
<logic:iterate id="u" name="userlist">
<bean:write name="u" property="u.username"/>
</logic:iterate>
