java.lang.IllegalStateException: Response has already been committed, be sure not to write to the OutputStream or to trigger a commit due to any other action before calling this method.
The error message says it all...
Is there a way to redirect from a page to another when I used response.getOutputStream().
No. You've started writing a response; telling a browser to go somewhere else in the middle of it is a completely different response.
The answer to your problem is either to redirect the user to the file or to write the entire file back to the output stream yourself.
Also, if any errors occured, how do I show it. Actually in my code I am trying to get Writer object to show error messages. This will not work. Any ideas how to show error messages when response.getOutputStream() is used.
It's much more elegant from both the developers' and users' viewpoints to redirect to an error page and write the errors to a log. You can either forward or redirect the request/response using the RequestDispatcher.