public void validate() {
        // TODO Auto-generated method stub
        if(null == user.getUserid() || "".equals(user.getUserid().trim()));
        {
            addFieldError("userid","用户不为空");
        }
        if(null == user.getPassword() || "".equals(user.getPassword().trim()));
        {
            addFieldError("password","密码不为空");
        }
    }

<s:form action="Login">
              <s:textfield name="userid" label="用户名"></s:textfield>
              <s:password name="password" label="密码"></s:password>
            <s:submit value="登录" />
          </s:form>

package com.ty.actions;

import com.opensymphony.xwork2.ActionSupport;
import com.ty.beans.User;

public class LoginAction extends ActionSupport {

    private User user;

    public User getUser() {
        return user;
    }

    public LoginAction() {

        System.out.println("我是Action我在初始化");
    }

    public void setUser(User user) {
        this.user = user;
    }

    public String execute() throws Exception {

        return "success";
    }

    @Override
    public void validate() {

        User user = getUser();
        String name = null;
        String pass = null;
        if (user != null) {

            name = user.getName();
            pass = user.getPassword();

            if (name != null && name.equals("")) {
                addFieldError("error", "name.error");
            }
        }

        addFieldError("error", "error");

    }

}