java.lang.NullPointerException的问题

package com.zxcs.action;

import javax.servlet.http.HttpServletRequest;

import org.apache.struts2.ServletActionContext;

import com.zxcs.interceptor.IsLoginedInterceptor;
import com.opensymphony.xwork2.ActionSupport;
import com.zxcs.bean.*;
import com.zxcs.service.UserService;

public class LoginAction extends ActionSupport{

/**
*
*/
private static final long serialVersionUID = 1L;
private User user;
private UserService service;

public User getUser() {
return user;
}

public void setUser(User user) {
this.user = user;
}
public UserService getService() {
return service;
}

public void setService(UserService service) {
this.service = service;
}

public void validate() {
if(this.getUser().getUsername()==null||" ".equals(this.getUser().getUsername())){
this.addFieldError(this.getUser().getUsername(), "用户名不能为空!");
}
if(this.getUser().getUserpassword()==null||" ".equals(this.getUser().getUserpassword())){
this.addFieldError(this.getUser().getUsername(),"不能为空!");
}
}

@Override
public String execute() throws Exception
{
User user1 = null;
HttpServletRequest req=ServletActionContext.getRequest();
user1 = this.service.findLogin(this.getUser().getUsername(),this.getUser().getUserpassword());

if(user1!=null){

req.getSession(true).setAttribute(IsLoginedInterceptor.USER_SESSION_KEY, user1);//把用户信息保存到Session里,方便获取
return SUCCESS;

}else{
System.out.println("用户名和密码错误！");
this.addFieldError(this.getUser().getUsername(), "户名和密码错误!");
//ActionError("用户名和密码错误!");

return INPUT;

}

}
}

java.lang.NullPointerException
com.zxcs.action.LoginAction.validate(LoginAction.java:38)
com.opensymphony.xwork2.validator.ValidationInterceptor.doBeforeInvocation(ValidationInterceptor.java:249)
com.opensymphony.xwork2.validator.ValidationInterceptor.doIntercept(ValidationInterceptor.java:261)
org.apache.struts2.interceptor.validation.AnnotationValidationInterceptor.doIntercept(AnnotationValidationInterceptor.java:68)
com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
com.opensymphony.xwork2.interceptor.ConversionErrorInterceptor.intercept(ConversionErrorInterceptor.java:133)
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:207)
com.opensymphony.xwork2.interceptor.MethodFilterInterceptor.intercept(MethodFilterInterceptor.java:98)
com.opensymphony.xwork2.DefaultActionInvocation.invoke(DefaultActionInvocation.java:248)
com.opensymphony.xwork2.interceptor.ParametersInterceptor.doIntercept(ParametersInterceptor.java:207)

相关了解……