Java Enterprise in a Nutshell, Second Edition

Remote Object Activation

Automatic activation of remote objects was added to RMI as of Java 1.2. The activation subsystem in RMI provides you with two basic features: the ability to have remote objects instantiated (activated) on-demand by client requests, and the ability for remote object references to remain valid across server crashes, making the references persistent. These features can be quite useful in certain types of distributed applications.

For example, think back to the AccountManager class we discussed when we talked about factory objects. We might not want to keep the AccountManager running on our server 24 hours a day; perhaps it consumes lots of server resources (memory, database connections, etc.), so we don’t want it running unless it is being used. Using the RMI activation service, we can set up the AccountManager so that it doesn’t start running until the first client requests an Account. In addition, after some period of inactivity, we can have the AccountManager shut down to conserve server resources and then reactivated the next time a client asks for an Account.

If a remote object is made activatable, it can be registered with the RMI registry without actually being instantiated. Normally, RMI remote objects (based on the UnicastRemoteObject interface) provide only nonpersistent references to themselves. Such a reference can be created for a client only if the referenced object already exists in a remote Java VM. In addition, the remote reference is ...