JSP代码求助，这个action怎么写

bill(bid,id,b1,b2)和customer(id,c1,c2)是有联系的两张表，BillUpdateAction是一个动作，页面执行后对bill执行更新。BillUpdateAction里面怎么写？？代码如下，求完善。

package action;
import hiber.HibernateSessionFactory;
import hiber.domain.Bill;
import hiber.domain.BillDAO;
import hiber.domain.Customer;
import hiber.domain.CustomerDAO;
import hiber.domain.Room;
 import java.sql.Timestamp;
import java.util.List;
import javax.servlet.http.HttpServlet;
import org.hibernate.Transaction;
import com.opensymphony.xwork2.ActionContext;

@SuppressWarnings("unused")
public class BillUpdateAction {
 
 private Integer bid;
 private Integer id;
 private Customer customer;
 private Integer b1;
 private Integer b2;
  
 private List<Customer> customerList =null;
 private List<Bill> billList =null;

  public String execute(){
  BillDAO dao=new BillDAO();
   ？？？？？？
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   dao.save(bill);
  trans.commit();
  return "success";
 } 
    public Integer getBid() {
  return bid;
 }
    public void setBid(Integer bid) {
  this.bid = bid;
 }
    public Integer getId() {
  return id;
 }
    public void setId(Integer id) {
  this.id = id;
 }
    public Customer getCustomer() {
  return customer;
 }
    public void setCustomer(Customer customer) {
  this.customer = customer;
 }
    public Integer getB1() {
  return b1;
 }
    public void setB1(Integer b1) {
  this.b1 = b1;
 }
    public Integer getB2() {
  return b2;
 }
    public void setB2(Integer b2) {
  this.b2 = b2;
 }
    public List<Customer> getCustomerList() {
  return customerList;
 }
    public void setCustomerList(List<Customer> customerList) {
  this.customerList = customerList;
 }
  public List<Bill> getBillList() {
  return billList;
 }
   public void setBillList(List<Bill> billList) {
  this.billList = billList;
 }

}
主要是问号部分不会写，请高手帮我完善。

追问：
不对啊 这样写会报错 麻烦你帮我把整个代码写一下我再试试吧

答案：就加一句：Bill bill = new Bill(id,b1,b2);

sava()方法：public void save(Bill bill) {
  Session session = HibernateSessionFactory.getSession();
  transaction= session.beginTransaction();
  try {
   session.update(bill);
   transaction.commit();
  } catch (HibernateException e) {
   e.printStackTrace();
   if (transaction != null) {
    transaction.rollback();
   }
  } finally {
   transaction = null;
   session.close();
  }
 }

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