严重: Servlet.service() for servlet jsp threw exception java.lang.IllegalStateException: getOutputStream() has already been called

错误：

严重: Servlet.service() for servlet jsp threw exception java.lang.IllegalStateException: getOutputStream() has already been called

输出验证码的类：

package com.jxc.util;

import java.awt.Color;

import java.awt.Font;

import java.awt.Graphics;

import java.awt.Graphics2D;

import java.awt.Point;

import java.awt.image.BufferedImage;

import java.io.IOException;

import java.util.Random;

import javax.imageio.ImageIO;

import javax.servlet.ServletOutputStream;

import javax.servlet.http.HttpServletResponse;

/**

 * 验证码输出类

 * @author Pan

 *

 */

public class GraphisVerfCode {

	private HttpServletResponse response;

	private Point point;

	private int linesize;

	private String code="";

	public GraphisVerfCode(HttpServletResponse response,Point p,int linesize){

		this.response=response;

		this.point=p;

		this.linesize=linesize;

		code=RandomCode.getRandomString(4);

	}

	public String getCode() {

		return this.code;

	}

	public void Draw() throws IOException{

		//输出类型

		response.setContentType("image/jpeg");

		response.setHeader("Pragma", "No-cache");//设置响应头信息，告诉浏览器不要缓存此内容

		response.setHeader("Cache-Control", "no-cache");

		//创建对象

		BufferedImage bImage=new BufferedImage(point.x, point.y,BufferedImage.TYPE_INT_RGB);

		Graphics2D g=bImage.createGraphics();

		//前景色

		//填充矩形

		g.fillRect(0, 0, point.x, point.y);

		drowString(g);

		//干扰线

		drowLine(g,this.linesize);

		//输出图片

		 ServletOutputStream out = response.getOutputStream();

         ImageIO.write(bImage, "jpeg", out);

         out.flush();

         out.close();

	}

	//干扰线条

	 private void drowLine(Graphics g,int linesize){

		 Random random=new Random(System.currentTimeMillis());

		 for (int i = 0; i < linesize; i++) {

	        int x = random.nextInt(point.x);

	        int y = random.nextInt(point.y);

	        int xl = random.nextInt(13);

	        int yl = random.nextInt(15);

	        g.setColor(new Color(random.nextInt(255),random.nextInt(255),random.nextInt(255)));

	        g.drawLine(x, y, x+xl, y+yl);

				 }

	    }

	 private void drowString(Graphics g){

			g.setFont(new Font("微软雅黑",Font.BOLD,22));

			Random random=new Random(System.currentTimeMillis());

			//循环绘制每个字的颜色

			for(int i=0;i<code.length();i++){

		        g.setColor(new Color(random.nextInt(255),random.nextInt(255),random.nextInt(255)));

		        g.drawString(code.charAt(i)+"", 20*i, 25);

			}

	 }

}

最开始使用的时候，没有调用   out.flush();这个方法，就一直报错。加上就好了。