用户访问Web服务器时，服务器在用户硬盘上存放的信息，好像是服务器送给用户的Cookie一样。
添加Cookie到客户端：响应方式；从客户端获取Cookie：请求方式。
一个Cookie包含一对键值对信息：
//生成Cookie
Cookie cookie = new Cookie("username","arthinking");
//通过服务器响应将Cookie添加到用户硬盘上
resp.addCookie(cookie);
setMaxAge
public void setMaxAge(int expiry)
Sets the maximum age of the cookie in seconds.
A positive value indicates that the cookie will expire after that many seconds have passed. Note that the value is the maximum age when the cookie will expire, not the cookie's current age.
A negative value means that the cookie is not stored persistently and will be deleted when the Web browser exits. A zero value causes the cookie to be deleted.
Parameters:
expiry - an integer specifying the maximum age of the cookie in seconds; if negative, means the cookie is not stored; if zero, deletes the cookie
See Also:
getMaxAge()
从客户端获取Cookie：
//服务器从客户端获取Cookie
Cookie[] cookies = req.getCookies();
for(Cookie c : cookies){
System.out.println(c.getName()+c.getValue());
}
