Semi-Singleton example

Question

Hello,
I'm probably thinking about this too much, however I'm interested in trying to implement a "Singleton" instance of a class for a unique client in that a clients subsequent calls to the servlet ensures the client recieves the exact same instance. This is synonomous to the HTTPSession object, a users interaction with the session always returns the same session object.
The below example creates 1 instance of DataAccess within the server no matter how many users accesses the servlet. HOWEVER!!!, The desired result is to have the same instance returned to the user for each request. For example, if 5 users connect then 5 DataAccess instances are loaded in memory, each unique to the user and on subsequent calls the same object is returned...a new DataAccess instance is not created.
I would like to avoid putting a created DataAccess object into the users Session.
Any ideas are greatly appreciated..
* This is an example of a Singleton
public class DataAccess(){
private static DataAccess INSTANCE = new DataAccess();
public DataAccess getInstance(){
// this returns the same instance of DataAccess no matter how many
// users call this...how do you return 1 instance for each user
// no matter how many times they call this method??
return INSTANCE;
// End of Example

Answer 1

You will need a place to store the object between client requests. Say a hashmap. You could put the hashmap as a static variable of the servlet or as an atribute of the servlet context.
Now you need to store the key to the object. This could be a cookie.
Don't forget synchronization issues. What if the original request takes a long time and the client submits a new request. How do handle access to the object.
Then you have to handle client session timeouts. You may have a logout process but clients don't always logout. You knew to know which object in the hashmap is tied to which client session and delete the object from the hashmap when the session times out.
Or you could build your own time out mechanism. Then when storing the object in the hashmap you'll need to record the date/time. Then you'll need to have a thread that periodically checks all the objects in the hashmap and removes any old objects.