A Writer has no means of getting at the 'contents' of the Writer so there isn't a way to get it. The Writer is usually backed by some sort of Stream, sending the data it gets to some other source, so there may actually be no 'contents' of the Writer - it may pass directly through the Writer through the backing Stream to where ever the target was meant to be.
Your only hope is to see if you can have some effect on the Writer that gets returned by the controller. If you can make the controller generate a Wrapper around the the controller's intended Writer then you have a chance.
So what environment is this controller in? Is it servlets?