编了一个Servlet，出现java.lang.IllegalStateException: getWriter() has already been called for this

package com.lai;

import java.io.*;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class Servlet3 extends HttpServlet {

public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {

response.setContentType("text/html;charset=utf-8");
PrintWriter out = response.getWriter();
//演示下载文件
response.setHeader("Content-Disposition", "attachment;filename=girl.jpg");
//打开文件。说明一下web站点下载文件的原理
//1.获取到要下载文件的全路径
String path=this.getServletContext().getRealPath("/image/girl.jpg");
//System.out.println("path="+path);
//2.创建一个文件输入流
FileInputStream fis=new FileInputStream(path);
//做一个缓冲字节数组
byte buff[]=new byte[1024];
int len=0;//表示实际每次读取了多少个字节
OutputStream os=response.getOutputStream();

while((len=fis.read(buff))>0){

os.write(buff, 0, len);
}
//关闭
os.close();
fis.close();

}

public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
this.doGet(request, response);
}

}
刷新页面的时候就出错，好像是服务器出错了吗？

相关了解……