getOutputStream() has already been called for this response

Question

getOutputStream() has already been called for this response

this error comes in line where I have forwarded the response to some other page..

rd.forward(req,res);

Answer 1

As per the comments:

no it is simply jsp/servlet and using the same I download csv file

It sounds like as if you're trying to forward the response after sending the CSV file to the response. That is not possible. You cannot send two different responses to a single request. When you're finished with sending the CSV, you should stop touching the request/response. It has already done its job and you should wait on the next request.

If your actual intent is to change the parent page after your download the CSV, then it has to be solved differently, but that would require JavaScript code.