JSP Sessions

Sessions

HttpSession interface provides the JSP with the ability to store and share information between jsp files for an ongoing interaction with a user.
Every distinct user who starts and interaction with any JSP of an application is associated with a unique sesssion object.
The session id is stored as a cookie on the user's machine. This cookie allows the server to associate a user with an active session.
Sessions have lifetimes and will expire after a period of inactivity.

The lifetime can be set and accessed by methods of the session object

setMaxInactiveInterval(int) - sets the maximum time a session can be inactive before becoming invalid (in seconds)
getMaxInactiveInterval() - retrieves this maximum inactiver interval (in seconds)

Sessions can be used to store attribues (objects) that should survive for the lifetime of the session

void setAttribute(String name, Object obj) - stores an objects as a session attribute
Object getAttribute(String name) - retrieves the attribute.
these objects are usually instances of java beans.

If you are coding in a JSP, then the session object is already available, in a Servlet, you need to get the session from the request object

HttpSession session = request.getSession();

if the session associated with the web user exists, it is returned, if not, a new one is created.
HttpSession is a "container" type class, it is used to store objects (Java beans) across requests within the same session.
you'll have to import javax.servlet.http.HttpSession to use it..
to store an object in a HttpSession, use the setAttribute method, e.g.

session.setAttribute("name","Fred Flintstone"); // sets a string object to the attribute called "name"

to get the attribute back in subsequent requests, use getAttribute method, this method returns null if the attribute is not associated with the session, e.g.

String name = (String)session.getAttribute("name");

you should always set the inactive timeout interval (in seconds) for your session, e.g.

session.setMaxInactiveInterval(600); // timeout and destroy the session and its attributes after 10 minutes

Note, the session will automatically expire if the user exits their browser if you are using the Apache tomcat server (as it uses a memory only cookie to track the sessions).

Other useful methods:

session.getLastAccessedTime(); // returns the time the client last sent a request associatied with the session (long)
session.invalidate(); // has the effect of removing all attributes objects from the session
session.removeAttribute("name"); // removes the attribute object with the specified name
session.isNew(); // returns true if the session was just created

Note, the use-bean directive can be used to create javabeans and register them with a session, e.g.

<jsp:useBean id="course"
class="registrarbeans.Course" scope ="session"
/>

    
replaces the following seqence.



registrarbeans.Course course =
(registrarbeans.Course)session.getAttribute("course");   


if(course == null)  // new sesssion, just create a course

{

  course = new registrarbeans.Course();

  session.setAttribute("course", course);

 }

Java classes and sessions

Java objects can be created with the "new" operator just like you did for desktop applications.
In Netbeans, your web project will have a "src" directory, within that is a "java" directory. To create a java class, first create a package in the Source Packages under your project, then create the class within the package.
You will need to import the package, or fully qualify the class name of the bean

<%@ page import="registrarbeans.*"%>

For example, if you had java class named "ShoppingCart" are in a package called "registrarbeans", then you could create a new ShoppingCart object using

registrarbeans.ShoppingCart cart = new registrarbeans.ShoppingCart();

To make an object exist throughout the lifetime of the session, you need to add it to the session during the request that it was created, then access it other requests., eg.

onlinecompany.ShoppingCart cart = (onlinecompany.ShoppingCart)session.getAttribute("Cart"); // retrieve object
if(cart == null) // See if object exists, if not, then create one
{
cart = new ShoppingCart();
session.setAttribute("Cart", cart);
}

if you are coding in JSP, then you could just use the use bean directive:

<jsp:useBean id="cart" class="registrarbeans.ShoppingCart" scope ="session" />

Note, it is possible for a user to have multiple browser windows open at the same time, effectively creating multiple threads on the server that are accessing the session at the same time. This can cause problems, so to prevent that, place all code that adds items to the session in a synchronized block. e.g.

synchronized(session) // lock the session object
{

} // release the lock