购物车中只能添加一个商品，第二次添加就会把第一次给覆盖掉了，怎么办？

Question

action中的代码如下:
[code="java"]
import java.util.Date;
import java.util.HashMap;
import java.util.Map;
import javax.servlet.http.HttpServletRequest;
import org.apache.struts2.ServletActionContext;
import org.apache.struts2.interceptor.SessionAware;
import com.opensymphony.xwork2.ActionSupport;
import ez.store.pojo.Book;
import ez.store.pojo.User;
import ez.store.service.UserService;
import ez.store.util.CartItem;
public class OrderCartItemAction extends ActionSupport implements SessionAware{
private UserService userService;
private Map session;
private CartItem ci;

public Map getSession() {
    return session;
}

public void setSession(Map session) {
    this.session = session;
}

public UserService getUserService() {
    return userService;
}

public void setUserService(UserService userService) {
    this.userService = userService;
}

// 用于将商品放到购物车里面
public String addBookToCart() {
    HttpServletRequest request = ServletActionContext.getRequest();
    Book book = (Book) session.get("book");
    int score = user.getScore();
    String bname = book.getBname();
    double price = book.getPrice();
    int amount = Integer.parseInt(request.getParameter("amount"));
    ci=new CartItem();
    Date date = new Date();
    Map map = new HashMap();
    ci.setBname(bname);
    ci.setPrice(price);
    ci.setAmount(amount);
    map.put(date,ci);
    session.put("list",map);
    return "success";
}

}
[/code]
然后就在jp页面接受"list"的值用c:if迭代，但是现在每次只能添加一个商品信息进购物车，且第二次添加的会把第一次添加的给覆盖掉。问题好像是因为每次调用这个action都会重新去new一次，但是就是不知道该如何解决?求指点。

Answer 1

List list = null;
Object obj = session.get("list");
if(obj != null){
list = (List) obj;
}else{
list = new ArrayList();
session.put("list",list);
}
//list.add 商品
......