struts+hibernate登录问题

我随意做了个登录界面。可是错误的用户跟密码都能进去。请大家为我看看我这代码该怎么改：
package com.action;

import java.util.Iterator;
import java.util.List;

import org.hibernate.HibernateException;

import com.opensymphony.xwork2.ActionContext;
import com.opensymphony.xwork2.ActionSupport;

public class UserLoginAction extends ActionSupport
{
private Integer userid;
private String username;
private String userpassword;
public Integer getUserid() {
return userid;
}
public void setUserid(Integer userid) {
this.userid = userid;
}
public String getUsername() {
return username;
}
public void setUsername(String username) {
this.username = username;
}
public String getUserpassword() {
return userpassword;
}
public void setUserpassword(String userpassword) {
this.userpassword = userpassword;
}
public String execute() throws Exception