Use the Unreferenced Interface to Clean Up Allocated Server State
The distributed garbage collector is a wonderful piece of code. It works in a very straightforward manner: a client gets a lease on a particular server object. The lease has a specific duration, and the client is responsible for renewing the lease before it expires. If the lease expires, and the client hasn’t renewed the lease, the server JVM is allowed to garbage-collect the server object (as long as no other clients have leases against that particular object).
If a server implements the
Unreferenced
interface—which contains a single method,
unreferenced( )—the server will be notified via a call to
unreferenced( ) that there are no valid leases
against the server.
It’s important to note that any active instance of a stub, in any JVM, will automatically try to connect to the server and maintain a lease. This means that, for example, if the server is bound into the RMI registry, the registry will keep the server alive. (The RMI registry basically stores the stub in a hashtable. The stub keeps renewing its lease.)
In turn, this means that if you’re using a naming
service to get instances of stubs, no other process can actually get
a stub to a server if unreferenced has been called
(unreferenced will be called only if the server is
no longer bound into any naming services).
All of this makes the unreferenced method an ideal
place to release server-side resources and shut down the server
object gracefully.