(Quick Reference)
                response
Purpose
The 
response object is an instance of the Servlet API's 
HttpServletResponse class
Examples
class BookController {
    def downloadFile() {
        byte[] bytes = // read bytes
        response.outputStream << bytes
    }
}Description
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 provides the following additional methods and/or properties on the 
response object
Grails also overrides the left shift operator on the 
response object making it easier to write to the response writer:
response << "Hello World"