Capturing OutputStream from a StrutsAction

In my StrutsAction classes I write a JSON string to the Response 
outputStream:
                 OutputStream os = response.getOutputStream();
                 os.write(result.getBytes());
                 os.flush();
                 os.close();
                 return null;
I'm trying to capture the response in my StrutsTest class, but I can't 
find it. I mean I can find the Response object, but I don't know where 
to look for the bytes that I wrote to the Response outputStream.

thanks