DTO传值有关问题

DTO传值问题
大家帮忙看一下System.out.println(regvo.getUsername());能够打印出来jsp页面传的值
但是当把值传给dto的时候出现空指针异常，为何？

Java code

package com.action;import com.dto.RegDTO;import com.opensymphony.xwork2.ActionSupport;import com.service.RegService;import com.vo.RegVO;import freemarker.template.utility.Execute;public class RegAction extends ActionSupport {    private RegVO regvo;    private RegDTO regdto;    private RegService regservice;    public String execute() throws Exception {        // 首先将VO的值传递给DTO        System.out.println(regvo.getUsername());        this.populateDTO();        regservice.RegUser();        return SUCCESS;    }    public void populateDTO() {        regdto.setUsername(regvo.getUsername());        regdto.setPassword(regvo.getPassword());    }    public RegVO getRegvo() {        return regvo;    }    public void setRegvo(RegVO regvo) {        this.regvo = regvo;    }    public RegDTO getRegdto() {        return regdto;    }    public void setRegdto(RegDTO regdto) {        this.regdto = regdto;    }    public RegService getRegservice() {        return regservice;    }    public void setRegservice(RegService regservice) {        this.regservice = regservice;    }    public static void main(String[] args) throws Exception {        RegAction sa = new RegAction();        sa.execute();    }}

Java code

java.lang.NullPointerException    com.action.RegAction.populateDTO(RegAction.java:25)    com.action.RegAction.execute(RegAction.java:19)    sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)    sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)    sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)    java.lang.reflect.Method.invoke(Method.java:597)