jsp下载压缩文件

<%@ page language="java" import="java.util.*,java.io.*" pageEncoding="GB2312"%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html> 
  <body>
    <%OutputStream o = response.getOutputStream();%>
    <% 
    	byte b[] = new byte[1024*2];
    	File fileload = new File("c:/12.rar");
    	response.setHeader("Content-disposition","attachment;filename=12.rar");
    	response.setContentType("application/x-tar");
    	long fileLength = fileload.length();
    	String length = String.valueOf(fileLength);
    	response.setHeader("Content_Length",length);
    	FileInputStream in = new FileInputStream(fileload);
    	int n = 0;
    	while((n=in.read(b))!=-1){
    		//o.write(b,0,n);
    	}
    	in.close();
    	o.flush();
    	o.close();
    	out.clear();
 	out  =  pageContext.pushBody();
	%>
  </body>
</html>

 在没加下面两条语句的时候,程序会抛出下面的错误:

java.lang.IllegalStateException: getOutputStream() has already been called for this response

 产生这样的异常原因:是web容器生成的servlet代码中有out.write(""),这个和JSP中调用的response.getOutputStream()产生冲突.即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,因为在jsp中,out变量实际上是通过response.getWriter得到的,你的程序中既用了response.getOutputStream,又用了out变量,故出现以上错误。
下面是解决办法:

 

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

 

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