初学spring aop 遇到了object is not an instance of declaring class的问题

CompareInterceptor类

package com.spring.advice;
import org.aopalliance.intercept.MethodInterceptor;
import org.aopalliance.intercept.MethodInvocation;
public class CompareInterceptor implements MethodInterceptor {
@Override
public Object invoke(MethodInvocation invocation) throws Throwable {
// TODO Auto-generated method stub
Object result = null;
String sname = invocation.getArguments()[0].toString();

if(sname.equals("forever")){
result = invocation.proceed();
}
else{
System.out.println("此学生是："+sname+"，而不是forever");
}
return result;
}

}

Test 类
package com.spring.test;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

import com.spring.aop.Istudent;

public class Test {
public static void main(String args[]){
ApplicationContext context = new ClassPathXmlApplicationContext("applicationContext.xml");
Istudent st = (Istudent)context.getBean("student");
st.addStudent("forever");
}
}

applicationContext.xml

<?xml version="1.0" encoding="UTF-8"?>
xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:p="http://www.springframework.org/schema/p"
xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.0.xsd">

com.spring.aop.Istudent



beforeAdvice
afterAdvice
compareInterceptor



studentTarget

只要不是参数不是forever，一切都正常。。。求帮助！