Understanding distributed garbage collection

The RMI subsystem implements reference counting based Distributed Garbage Collection (DGC) to provide automatic memory management facilities for remote server objects.

When the client creates (unmarshalls) a remote reference, it calls dirty() on the server-side DGC. After the client has finished with the remote reference, it calls the corresponding clean() method.

A reference to a remote object is leased for a time by the client holding the reference. The lease period starts when the dirty() call is received. The client must renew the leases by making additional dirty() calls on the remote references it holds before such leases expire. If the client does not renew the lease before it expires, the distributed garbage collector assumes that the remote object is no longer referenced by that client.

DGCClient implements the client side of the RMI distributed garbage collection system. The external interface to DGCClient is the registerRefs() method. When a LiveRef to a remote object enters the JVM, it must be registered with the DGCClient to participate in distributed garbage collection. When the first LiveRef to a particular remote object is registered, a dirty() call is made to the server-side DGC for the remote object. The call returns a lease guaranteeing that the server-side DGC will not collect the remote object for a certain time. While LiveRef instances to remote objects on a particular server exist, the DGCClient periodically sends more dirty calls to renew its lease. The DGCClient tracks the local availability of registered LiveRef instances using phantom references. When the LiveRef instance for a particular remote object is garbage collected locally, a clean() call is made to the server-side DGC. The call indicates that the server does not need to keep the remote object alive for this client. The RenewCleanThread handles the asynchronous client-side DGC activity by renewing the leases and making clean calls. So this thread waits until the next lease renewal or until any phantom reference is queued for generating clean requests as necessary.