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202

Chapter 9 Session Tracking

Looking Up the HttpSession Object Associated 

with the Current Request

You look up the 

HttpSession

 object by calling the 

getSession

 method of

HttpServletRequest

. Behind the scenes, the system extracts a user ID

from a cookie or attached URL data, then uses that as a key into a table of

previously created 

HttpSession

 objects. But this is all done transparently to

the programmer: you just call 

getSession

. If 

getSession

 returns 

null

, this

means that the user is not already participating in a session, so you can create

a new session. Creating a new session in this case is so commonly done that

there is an option to automatically create a new session if one doesn't already

exist. Just pass 

true

 to 

getSession

. Thus, your first step usually looks like

this: 

HttpSession session = request.getSession(true);

If you care whether the session existed previously or is newly created, you

can use 

isNew

 to check.

Looking Up Information Associated with a 

Session

HttpSession

 objects live on the server; they're just automatically associated

with the client by a behind the scenes mechanism like cookies or

URL rewriting. These session objects have a built in data structure that lets

you store any number of keys and associated values. In version 2.1 and earlier

of the servlet API, you use 

session.getValue("attribute")

 to look up a

previously stored value. The return type is 

Object

, so you have to do a type 

cast to whatever more specific type of data was associated with that attribute

name in the session. The return value is 

null

 if there is no such attribute, so

you need to check for 

null

 before calling methods on objects associated with

sessions.

In version 2.2 of the servlet API, 

getValue

 is deprecated in favor of 

get 

Attribute

 because of the better naming match with 

setAttribute

 (in ver 

sion 2.1 the match for 

getValue

 is 

putValue

, not 

setValue

). Nevertheless,

since not all commercial servlet engines yet support version 2.2, I'll use

getValue

 in my examples. 

Here's a representative example, assuming 

ShoppingCart

 is some class

you've defined to store information on items being purchased (for an imple 

mentation, see Section 9.4 (An On Line Store Using a Shopping Cart and

Session Tracking)).

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