A query about Cookies in Servlets

Cookie usernameCookie = new Cookie ("username", attributeUsername);
usernameCookie.setMaxAge(24*60*60);开发者_开发知识库
response.addCookie(usernameCookie);

A cookie is created in the server in this way and it is added to the HttpServletResponse .

Then why is it that , a HttpServletRequest Header contain cookies ??

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Cookies are transmitted per request, example use case:

1. The first client request does not contain cookies
2. Server sets cookie A, it is transmitted back to the client in HTTP header Set-Cookie; this is what response.addCookie does.
3. The second client request sends HTTP header Cookie containing a list of all matching cookies (including cookie A)

Cookies are part of the HTTP protocol. Wikipedia provides a fine overview: http://en.wikipedia.org/wiki/HTTP_cookie