為什么我在struts.xml文件配置了<constant name="struts.action.extension" value="do"></constant>之后會報錯，刪除后恢復原來正常的樣子

<?xml?version="1.0"?encoding="UTF-8"?>
<!DOCTYPE?struts?PUBLIC
"-//Apache?Software?Foundation//DTD?Struts?Configuration?2.3//EN"
"http://struts.apache.org/dtds/struts-2.3.dtd">

<struts>
<package?name="default"?namespace="/"?extends="struts-default">
		<default-action-ref?name="index"></default-action-ref>
		<action?name="index">
		<result>error.jsp</result>
			</action>
			
<action?name="helloworld_*"??method="{1}"?class="com.imooc.action.HelloWorldAction">
<result>/result.jsp</result>
<result?name="dada">/{1}.jsp</result>
<result?name="update">/{1}.jsp</result>
</action>
</package>
<!--?把網址的后綴.action修改為.do?-->
<constant?name="struts.action.extension"?value="do"></constant>
<!--?感嘆號方式設置動態方法調用?-->
<!--?
<constant?name="struts.enable.DynamicMethodInvocation"?value="true"></constant>
?-->
</struts>