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9.2 The Session Tracking API

205

public void invalidate()

This method invalidates the session and unbinds all objects associated 

with it.

Associating Information with a Session

As discussed in the previous section, you read information associated with a

session by using 

getValue

 (in version 2.1 of the servlet specification) or

getAttribute

 (in version 2.2 ). To specify information in version 2.1 serv 

lets, you use 

putValue

, supplying a key and a value. Use 

setAttribute

 in

version 2.2. This is a more consistent name because it uses the 

get

/

set

 nota 

tion of JavaBeans. To let your values perform side effects when they are

stored in a session, simply have the object you are associating with the session

implement the 

HttpSessionBindingListener

 interface. Now, every time

putValue

 or 

setAttribute

 is called on one of those objects, its 

valueBound

method is called immediately afterward.

Be aware that 

putValue

 and 

setAttribute

 replace any previous values;

if you want to remove a value without supplying a replacement, use 

remove 

Value

 in version 2.1 and 

removeAttribute

 in version 2.2. These methods

trigger the 

valueUnbound

 method of any values that implement 

Http 

SessionBindingListener

. Sometimes you just want to replace previous

values; see the 

referringPage

 entry in the example below for an example.

Other times, you want to retrieve a previous value and augment it; for an

example, see the 

previousItems

 entry below. This example assumes a

ShoppingCart

 class with an 

addItem

 method to store items being ordered,

and a 

Catalog

 class with a static 

getItem

 method that returns an item, given

an item identifier. For an implementation, see Section 9.4 (An On Line Store

Using a Shopping Cart and Session Tracking).

HttpSession session = request.getSession(true);

session.putValue("referringPage", request.getHeader("Referer"));

ShoppingCart cart =

(ShoppingCart)session.getValue("previousItems");

if (cart == null) { // No cart already in session

cart = new ShoppingCart();

session.putValue("previousItems", cart);

}

String itemID = request.getParameter("itemID");

if (itemID != null) {

cart.addItem(Catalog.getItem(itemID));

}

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