HttpSession 對象簡介

public interface HttpSession

Provides a way to identify a user across more than one page request or visit to a Web site and to store information about that user.

The servlet container uses this interface to create a session between an HTTP client and an HTTP server. The session persists for a specified time period, across more than one connection or page request from the user. A session usually corresponds to one user, who may visit a site many times. The server can maintain a session in many ways such as using cookies or rewriting URLs.

該HttpSession 對象是由web容器定義的，該web容器能夠爲tomcat等。session 有如下功能：1.識別用戶，2.存儲與用戶相關的信息。session的生命週期是很長的其session中的值是存儲在 cookie 或 rewriting URLS 中的。

This interface allows servlets to

• View and manipulate information about a session, such as the session identifier, creation time, and last accessed time
• Bind objects to sessions, allowing user information to persist across multiple user connections

When an application stores an object in or removes an object from a session, the session checks whether the object implements HttpSessionBindingListener. If it does, the servlet notifies the object that it has been bound to or unbound from the session. Notifications are sent after the binding methods complete. For session that are invalidated or expire, notifications are sent after the session has been invalidated or expired.

When container migrates a session between VMs in a distributed container setting, all session attributes implementing the HttpSessionActivationListener interface are notified.

A servlet should be able to handle cases in which the client does not choose to join a session, such as when cookies are intentionally turned off.servlet應該可以處理這種狀況，就是用戶禁用cookie. Until the client joins the session, isNew returns true. If the client chooses not to join the session, getSession will return a different session on each request, and isNew will always return true.

Session information is scoped only to the current web application (ServletContext), so information stored in one context will not be directly visible in another.