Re: Q: Singleton in a server environment

From:

Joshua Cranmer <Pidgeot18@epenguin.zzn.com>

Newsgroups:

comp.lang.java.help,comp.lang.java.misc

Date:

Sat, 03 Mar 2007 19:30:20 GMT

Message-ID:

<g1kGh.542$wc6.252@trndny05>

Casper wrote:

If my application server is serving multiple sessions/connections at the
same time, will my vanilla JavaSE Singleton be shared by all these, or
in what context is a singleton/static bound? (Global scope, classloader
scope, thread scope?)

I need some way of sharing an instance between all my classes serving a
client but not global wide in the VM as I suspect a classic Singleton
would behave on an application server.

Thanks,
Casper

Static variables are shared across ALL threads of ALL instances of the
VM, so your vanilla singleton would be shared by all connections.

Per your second question, I think it would be easiest to use something
like this: (Don't actually use this: it horribly leaks memory).

public class Singleton {
    private Map<Integer, Singleton> things;

    private Singleton() {}
    public static Singleton getSingleton(int client_id) {
        if (things.get(client_id) == null)
            things.put(client_id, new Singleton());
        return things.get(client_id);
    }
}