Re: Q: Singleton in a server environment

Joshua Cranmer <>
Sat, 03 Mar 2007 20:53:50 GMT
Lew wrote:

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.

Joshua Cranmer wrote:

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

Aren't there games you can play with different ClassLoaders that can
achieve "scoped singletons [so to speak]"? There are a couple of class
loaders involved in web apps already, aren't there?

-- Lew

After basic testing, it would seem that ClassLoaders would probably not
work (easily). Piping the class loader to print out each class it loads
shows that the function loadClass() is called twice on each class.
findClass() and resolveClass() might be called more times, but I doubt
it. Anyways, using a ClassLoader to solve this problem would be quite
difficult, IMHO.

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