(生成excel)解决getOutputStream() has already been called for this response java getOutputStream()的问题

在jsp中用java语言生成excel时，虽然对功能没有影响，但在后台会报个小错误：

getOutputStream() has already been called for this response java getOutputStream()

在网上搜了一下，找到了解决办法：

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

加上这两行代码就可以了。

OutputStream os = response.getOutputStream(); 

WritableWorkbook wwb = Workbook.createWorkbook(os);

要加在wwb.close()之后，os.close()之前.....

问题产生原因：未能找到，希望高手告诉一下。