Java Cookbook

Cookies

Problem

You want the client (the browser) to remember some bit of information for you.

Solution

Bake a cookie, and serve it to the client along with your response.

Discussion

Cookies were invented by Netscape as a debugging technique, but have since become ubiquitous: all modern browsers, including MSIE, and text browsers such as Lynx accept and store them. A cookie is, at heart, a small piece of text -- a name and value pair -- that the server side generates and sends to the client. The browser remembers them (nontransient cookies are stored to your hard disk; Netscape creates a file called cookies or cookies.txt, for example). The browser then sends them back to the server on any subsequent visit to a page from the same site. The Cookie class is part of the javax.servlet.http package, so any servlet implementation will include it. The constructor is passed a name and value, but there are other parameters you can set. Most important is the expiry time, which is in seconds from the time you first send it. The default is -1; if the value is negative, the cookie is not saved to disk; it becomes a "transient cookie” that exists only until the browser exits and is then forgotten. For cookies that are stored to disk, the expiry time is converted to a base of January 1, 1970, the beginning of Unix time and of the modern computing era.

When the browser visits a site that has sent it a cookie or cookies, it returns all of them as part of the HTTP headers. You retrieve them all (as ...