随机验证码出错

随机验证码出错:

<%@ page contentType= "image/jpeg "%>
<%@ page
import= "java.awt.*,java.awt.image.*,java.util.*,javax.imageio.* "%>
<%!Color getRandColor(int fc, int bc) {
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);
}%>
<%
response.setHeader( "Pragma ", "No-cache ");
response.setHeader( "Cache-Control ", "no-cache ");
response.setDateHeader( "Expires ", 0);

int width = 50, height = 18;
BufferedImage image = new BufferedImage(width, height,
BufferedImage.TYPE_INT_RGB);

Graphics g = image.getGraphics();

Random random = new Random();

g.setColor(getRandColor(200, 250));
g.fillRect(0, 0, width, height);

g.setFont(new Font( "Times New Roman ", Font.PLAIN, 16));

g.setColor(getRandColor(160, 200));
for (int i = 0; i < 50; i++) {
int x = random.nextInt(width);
int y = random.nextInt(height);
int xl = random.nextInt(width);
int yl = random.nextInt(height);
g.drawLine(x, y, x + xl, y + yl);
}

String sRand = " ";
for (int i = 0; i < 4; i++) {
String rand = String.valueOf(random.nextInt(10));
sRand += rand;

g.setColor(new Color(20 + random.nextInt(110), 20
+ random.nextInt(110), 20 + random.nextInt(110)));

g.drawString(rand, 11 * i + 6, 16);
}

session.setAttribute( "rand ", sRand);

g.dispose();

try {
ImageIO.write(image, "JPEG ", response.getOutputStream());

//就是这里的getOutputStream抛出异常，内
// 容：java.lang.IllegalStateException:
// getOutputStream() has already been called for this response
//程序能运行，不受异常影响，不过还是想知道为什么
} catch (Exception e) {
System.out.println( "catch ");
}
%>

------解决方法--------------------------------------------------------
write( "... ")和response.getOutputStream()产生冲突.
Servlet规范说明，不能既调用 response.getOutputStream()，又调用response.getWriter()，无论先调用哪一个，在调用第二个时候应会抛出 IllegalStateException.
------解决方法--------------------------------------------------------
因为Application Server在处理编译jsp时对于％> 和 <％之间的内容通过PrintWriter输出(包括HTML标签,空格,回车等通过浏览器生成的标识符)，而在上面的例子又要进行流输出：ServletOutputStream，这样做相当于试图在Servlet中使用两种输出机制，
就会发生：getOutputStream() has already been called for this response的错误
你把空格这些删除了就好了，jsp页面只写成这样
<%@ page language= "java " import= "java.awt.*,java.awt.image.*,javax.imageio.*,java.util.* " pageEncoding= "GBK "%> <%@ page contentType= "image/jpeg "%> <%!Color getRandColor(int fc, int bc) {
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);