The response object is an instance of the Servlet API's HttpServletResponse class


class BookController {
	def downloadFile = {
		byte[] bytes = // read bytes
		response.outputStream << bytes


The Servlet API's HttpServletResponse class can be used within Grails to perform all typical activities such as writing out binary data, writing directly to the response and sending error response codes to name but a few. Refer to the documentation on the HttpServletResponse class in the Servlet API for more information.

Grails also overrides the left shift operator on the response object making it easier to write to the response writer:

response << "Hello World"