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); }这里是袭亩枝在释放在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（并不是和拍敏某些朋友说的那样--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","耐扒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 j=0;j<4;j++){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*j+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();}%>如有不足之处，欢迎斧正！2getOutputStream() has already been called for this response问题的解决在jsp向页面输出图片的时候,使用response.getOutputStream()会有这样的提示：java.lang.IllegalStateException:getOutputStream() has already been called for this response,会抛出Exception原因一:JSP默认的输出流为PrintWriter ,即<% %>以外的东西所默认的输出方式,如果你尝试在JSP中使用ServletOutputStream就会引起错误.要嘛直接改用Servlet输出(复写service方法),要嘛删除除%><%中的任何东西(包括HTML标签,空格,回车等东西)应该就可以。对于这样的情况应该这样来解决，删除%><%之间的所有内容包括空格和换行符，最后也要消除空格和换行符，最好再加上一句response.reset()。原因二: 在J2EE的API参考里有这么个:ServletResponse的getWriter()方法里会抛出这个异常,IllegalStateException - if the getOutputStream method has already been called for this response object而它的getOutputStream()方法里会抛出这个异常.IllegalStateException - if the getOutputStream method has already been called for this response object并且两者的函数申明里都有这么样的一句Either this method or getOutputStream() may be called to write the body, not both.Either this method or getWriter() may be called to write the body, not both.以上说明也解释了为什么在往页面中写入图片的时候要使用如下循环格式OutputStream output=response.getOutputStream();while((len=in.read(b)) >0) {output.write(b,0,len); }output.flush();而不是把response.getOutputStream().write()放到循环体内在页面中直接写：
