关于 struts2 Unable to load configuration. - action

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE struts PUBLIC "-//Apache Software Foundation//DTD Struts Configuration 2.1//EN" "http://struts.apache.org/dtds/struts-2.1.dtd">
<struts>

	<package name="student" namespace="/student" extends="struts-default,json-default">
		<action name="student_*" class="cn.edu.nwsuaf.action.StudentAction"
			method="{1}">
			<result name="init">/pages/student/student-edit.jsp</result>
			<result name="update">/pages/student/student-list.jsp</result>
			<result name="find">/pages/student/student-list.jsp</result>
			<result name="delete">/pages/student/student-list.jsp</result>
			<result name="add" type="json">
				<param name="root">studentAdd</param>
			</result>
		</action>
	</package>

</struts>    

因为没有继承json-default，导致再加入result type="json"属性时，启动tomcat控制台报错：Exception starting filter struts2 Unable to load configuration. - action

解决方法：

package extends要继承json-default