解决getOutputStream() has already been called for this response

getOutputStream() has already been called for this response异常出现的原因和解决方法：

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：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

1. finally {  
2. if (_jspxFactory != null)   
3.           _jspxFactory.releasePageContext(_jspx_page_context);  
4. }

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，在使用完输出流以后调用以下两行代码即可：

1. out.clear();  
2. 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）。

1. <%@ page  import="java.awt.*,java.awt.image.*,java.util.*,javax.imageio.*" %>  
2. <%@ page import="java.io.OutputStream" %>  
3. <%!  
4. Color getRandColor(int fc,int bc){  
5. Random random = new Random();  
6. if(fc>255) fc=255;  
7. if(bc>255) bc=255;  
8. int r=fc+random.nextInt(bc-fc);  
9. int g=fc+random.nextInt(bc-fc);  
10. int b=fc+random.nextInt(bc-fc);  
11. return new Color(r,g,b);  
12. }  
13. %>  
14. <%  
15. try{  
16. response.setHeader("Pragma","No-cache");  
17. response.setHeader("Cache-Control","no-cache");  
18. response.setDateHeader("Expires", 0);  
19. int width=60, height=20;  
20. BufferedImage image = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);  
21. OutputStream os=response.getOutputStream();  
22. Graphics g = image.getGraphics();  
23. Random random = new Random();  
24. g.setColor(getRandColor(200,250));  
25. g.fillRect(0, 0, width, height);  
26.   
27. g.setFont(new Font("Times New Roman",Font.PLAIN,18));  
28. g.setColor(getRandColor(160,200));  
29. for (int i=0;i<155;i++)  
30. {  
31. int x = random.nextInt(width);  
32. int y = random.nextInt(height);  
33. int xl = random.nextInt(12);  
34. int yl = random.nextInt(12);  
35. g.drawLine(x,y,x+xl,y+yl);  
36. }  
37. String sRand="";  
38. for (int i=0;i<4;i++){  
39. String rand=String.valueOf(random.nextInt(10));  
40. sRand+=rand;  
41. g.setColor(new Color(20+random.nextInt(110),20+random.nextInt(110),20+random.nextInt(110)));  
42. g.drawString(rand,13*i+6,16);  
43. }  
44. session.setAttribute("rand",sRand);  
45. g.dispose();  
46.   
47. ImageIO.write(image, "JPEG",os);  
48.   
49. //注意看以下几句的使用  
50. os.flush();  
51. os.close();  
52. os=null;  
53. response.flushBuffer();  
54. out.clear();  
55. out = pageContext.pushBody();  
56. }  
57. catch(IllegalStateException e)  
58. {  
59. System.out.println(e.getMessage());  
60. e.printStackTrace();  
61. }%>