今天写了一个自定义拦截器.却遇到了以下问题

package dgut.ke.interceptors;

import java.util.ArrayList;
import java.util.List;

import javax.servlet.http.HttpServletRequest;

import org.apache.struts2.ServletActionContext;

import com.opensymphony.xwork2.Action;
import com.opensymphony.xwork2.ActionContext;
import com.opensymphony.xwork2.ActionInvocation;
import com.opensymphony.xwork2.interceptor.MethodFilterInterceptor;

import dgut.ke.model.Catalog;
import dgut.ke.model.Publish;
import dgut.ke.model.Subject;
import dgut.ke.service.ICatalogService;
import dgut.ke.service.IPublishService;
import dgut.ke.service.ISubjectService;

public class AddBookInterceptor extends MethodFilterInterceptor {

 /**
  *
  */
 private static final long serialVersionUID = 1135497226250835266L;

 private ISubjectService subjectService ;
 private ICatalogService catalogService ;
 private IPublishService publishService ;

 public ICatalogService getCatalogService() {
  return catalogService;
 }

 public IPublishService getPublishService() {
  return publishService;
 }

 public ISubjectService getSubjectService() {
  return subjectService;
 }

 public void setCatalogService(ICatalogService catalogService) {
  this.catalogService = catalogService;
 }

 public void setPublishService(IPublishService publishService) {
  this.publishService = publishService;
 }

 public void setSubjectService(ISubjectService subjectService) {
  this.subjectService = subjectService;
 }

 @Override
 protected String doIntercept(ActionInvocation invoation) throws Exception {
  // TODO 自动生成方法存根
  System.out.println("-------- 拦截器开始执行 ----------");
  List<Subject> subject_list = (ArrayList<Subject>)subjectService.findAll();
  List<Catalog> catalog_list = (ArrayList<Catalog>)catalogService.findAll();
  List<Publish> pubish_list =  (ArrayList<Publish>)publishService.findAll();

  //ActionContext act = ActionContext.getContext() ;
  ActionContext act = invocation.getInvocationContext() ;
  HttpServletRequest request = (HttpServletRequest) act.get(ServletActionContext.HTTP_REQUEST) ;
  request.setAttribute("subject_list", subject_list) ;
  request.setAttribute("catalog_list", catalog_list) ;
  request.setAttribute("publish_list", pubish_list) ;
  System.out.println("-------- 拦截器执行结束 ----------");
  return Action.INPUT;
 }

}
为了实现request.setAttribute(String str, Object obj) ;.最先是让上面的类实现ServletRequestAware 接口.
但是在运行的时候却出现了空指针异常。后来改成以上的代码就可以正常运行，但是还是
不太明白
曾经在一本书上看到一段话：构建interceptor最重要的原则：interceptor必须是无状态的，并且不能
够使用任何ActionInvocation提供范围以外的API