SSH Struts2 中获取service失效

<bean id="gradeDao_msi" class="com.msi.dao.impl.GradeDaoImpl">
  <property name="sessionFactory">
   <ref local="sessionFactory" />
  </property>
 </bean>

<bean id="gradeService" class="com.msi.service.impl.GradeServiceImpl">
  <property name="gradeService_msi">
   <ref local="gradeDao_msi" />
  </property>
 </bean>

 <bean name="GradeAction" id="GradeAction" class="com.msi.action.GradeAction"> 
  <property name="service"> 
   <ref bean="gradeService" /> 
  </property> 
 </bean>
-------------------------------------

Action 中
GradeService service;
public void setService(GradeService service) {
  this.service = service;
 }

然後使用service 調用GradeService中的方法，總是提示java.lang.NullPointerException
請教如何解決