getOutputStream()异常的原因和解决方法

tomcat5下jsp出现getOutputStream() has already been called for this response异常的原因和解决方法

在tomcat5下jsp中出现此错误一般都是在jsp中使用了输出流（如输出图片验证码，文件下载等），
没有妥善处理好的原因。

具体的原因就是
在tomcat中jsp编译成servlet之后在函数_jspService(HttpServletRequest request, HttpServletResponse response)的最后
有一段这样的代码
finally {
      if (_jspxFactory != null) _jspxFactory.releasePageContext(_jspx_page_context);
    }
这里是在释放在jsp中使用的对象，会调用response.getWriter(),因为这个方法是和
response.getOutputStream()相冲突的！所以会出现以上这个异常。

然后当然是要提出解决的办法，其实挺简单的（并不是和某些朋友说的那样--
将jsp内的所有空格和回车符号所有都删除掉），

在使用完输出流以后调用以下两行代码即可：

out.clear();
out = pageContext.pushBody();

最后这里是一个输出彩色验证码例子（这样的例子几乎随处可见）
imag.jsp

<% @ page   import = " java.awt.*,java.awt.image.*,java.util.*,javax.imageio.* "   %> 
<% @ page  import = " java.io.OutputStream "   %> 
<%! 
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);
}
%> 
<% 
try {
response.setHeader( " Pragma " , " No-cache " );
response.setHeader( " Cache-Control " , " no-cache " );
response.setDateHeader( " Expires " ,  0 );
int  width = 60 , height = 20 ;
BufferedImage image  =   new  BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
OutputStream os = response.getOutputStream();
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, 18 ));
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);
}
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, 13 * i + 6 , 16 );
}
session.setAttribute( " rand " ,sRand);
g.dispose();

ImageIO.write(image,  " JPEG " ,os);
os.flush();
os.close();
os = null ;
response.flushBuffer();
out.clear();
out  =  pageContext.pushBody();
}
catch (IllegalStateException e)
{
System.out.println(e.getMessage());
e.printStackTrace();
} %>