struts2 模型驱动 获取jsp参数

package com.shun.action;
import java.util.Map;
import com.opensymphony.xwork2.ActionContext;
import com.opensymphony.xwork2.ActionSupport;
import com.opensymphony.xwork2.ModelDriven;
import com.shun.models.User;
import com.shun.services.UserService;

/**
* Created by shun on 2015/8/6.
*/
public class UserAction extends ActionSupport implements ModelDriven<User>{
/**
*
*/
private static final long serialVersionUID = -7229113448592681894L;
private User model = new User();
//Action中多方法时 方法调用方法： ctionName!FunctionName
public String userLogin(){
UserService userService = new UserService();
int login = userService.userLogin(model.getUsername(),model.getPassword());

//获取session
Map<String,Object> session = ActionContext.getContext().getSession();
//获取session中当前用户id
String userid = (String)session.get("userid");
/**/
System.out.println("userid in session :"+userid);
System.out.println("userid pass by jsp :"+model.getUserid());

//判断是登录用户的id 是不是已经登录过
if(userid == model.getUserid()){
addActionError("大哥您已经登录了，不需要再登录啊！");
return ERROR;
}else{
if(1 == login){
session.put("userid",model.getUserid());
return SUCCESS;
}
else {
return INPUT;
}
}
}
//将页面form的值填充到javabean中
public User getModel(){
return model;
}
}
为什么获取不到呢