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">

<bean id="beforeAdvice" class="com.spring.advice.BeforeAdvice"  />
<bean id="afterAdvice" class="com.spring.advice.AfterAdvice"  /> 
<bean id="compareInterceptor" class="com.spring.advice.CompareInterceptor"  />
<bean id="studentTarget" class="com.spring.aop.StudentImpl"  />


<bean id="student" class="org.springframework.aop.framework.ProxyFactoryBean"  >
  <property name="proxyInterfaces">
      <value>com.spring.aop.Istudent</value>
  </property>
  <property name="interceptorNames">
       <list>
           <value>beforeAdvice</value>
           <value>afterAdvice</value>
           <value>compareInterceptor</value>
       </list>
  </property>
  <property name="target">
       <value>studentTarget</value>
  </property>
</bean>     

只要不是参数不是forever，一切都正常。。。求帮助！

你这段代码有问题！