Why can't HttpServletResponse Headers be updated AFTER getWriter() is called?

Question

I\'ve been digging around the web after fixing an issue this afternoon @ work where Cookies added to the HttpServletResponse weren\'t being properly reflected in the respons

Answer 1

Because the headers precede the body in HTTP. That's why they're called 'headers'. If you call getWriter() you are writing to the body, so afterwards it is too late to start changing response header values.