code: @Override
  	public ServletOutputStream getOutputStream() throws IOException {
  		if (printWriter != null) {
  			throw new IOException("getWriter method has been called on this response");
  		}
  
  		if (outputStream == null) {
  			outputStream = new MockServletOutputStream();
  		}
  
  		return outputStream;
  	}
docs: Returns a ServletOutputStream that would normally be used to write content back to the client. Instead, the
  content is held internally and may be retrieved by calling {@link #getOutput()}.
  
  @return a ServletOutputStream to write content to.
  @throws IOException if {@link #getWriter()} has been called.