JSP验证码报错：getOutputStream() has already been called for this response异常

getOutputStream() has already been called for this response异常
原因，如异常法医，getOutputStream()已经调用。。。

解决方案：

OutputStream os=response.getOutputStream();

把response.getOutputStream()实例化，然后

os.flush();
os.close();
os=null;
response.flushBuffer();
out.clear();
out = pageContext.pushBody();

参考实例

<%@ page contentType="image/jpeg" import="java.awt.*,java.awt.image.*,java.util.*,javax.imageio.*" pageEncoding="UTF-8" %>
<%@ page import="java.io.OutputStream" %>
<%!Color getRandColor(int fc, int bc) {//给定范围获得随机颜色
Random random = new Random();
if (fc > 255)
fc = 255;
if (bc > 255)
bc = 255;
int r = fc + random.nextInt(bc - fc);
int g = fc + random.nextInt(bc - fc);
int b = fc + random.nextInt(bc - fc);
return new Color(r, g, b);
}%>
<%
try{
//设置页面不缓存
response.setHeader("Pragma", "No-cache");
response.setHeader("Cache-Control", "no-cache");
response.setDateHeader("Expires", 0);

// 在内存中创建图象
int width = 80, height = 20;
BufferedImage image = new BufferedImage(width, height,
BufferedImage.TYPE_INT_RGB);
OutputStream os=response.getOutputStream();

// 获取图形上下文
Graphics g = image.getGraphics();

//生成随机类
Random random = new Random();

// 设定背景色
g.setColor(getRandColor(200, 250));
g.fillRect(0, 0, width, height);

//设定字体
g.setFont(new Font("Times New Roman", Font.BOLD, 18));

//画边框
g.setColor(new Color(0, 0, 0));
g.drawRect(0, 0, width - 1, height - 1);

// 随机产生155条干扰线，使图象中的认证码不易被其它程序探测到
g.setColor(getRandColor(160, 200));
for (int i = 0; i < 155; i++) {
int x = random.nextInt(width);
int y = random.nextInt(height);
int xl = random.nextInt(12);
int yl = random.nextInt(12);
g.drawLine(x, y, x + xl, y + yl);
}

// 取随机产生的认证码(4位数字)
String sRand = "";
for (int i = 0; i < 4; i++) {
String rand = null;
//随机生成数字或者字母
if (random.nextInt(10) > 5) {
rand = String.valueOf((char)(random
.nextInt(10) + 48));
} else {
rand = String.valueOf((char)(random
.nextInt(26) + 65));
}
sRand += rand;
// 将认证码显示到图象中
g.setColor(new Color(random.nextInt(80), random
.nextInt(80), random.nextInt(80)));
//调用函数出来的颜色相同，可能是因为种子太接近，所以只能直接生成
g.drawString(rand, 15 * i + 10, 16);
}

// 将认证码存入SESSION
session.setAttribute("rand", sRand);

// 图象生效
g.dispose();

// 输出图象到页面
ImageIO.write(image, "JPEG", os);

//注意看以下几句的使用
//防止 getOutputStream() has already been called for this response错误
os.flush();
os.close();
os=null;
response.flushBuffer();
out.clear();
out = pageContext.pushBody();
}
catch(IllegalStateException e)
{
System.out.println(e.getMessage());
e.printStackTrace();
}
%>

这样，在后台就不会再出现这样的错误。
（上面代码直接保存到一个jsp页面中，即可正常使用）