How to keep track of client in rmi server

how can I keep track of a disconnected client in a rmi server ?
And what logic do I use to disconnect a client fro mthe server ?

When a RMI client receives a stub to the remote object, RMI client runtime sends a dirty() call to the RMI server runtime which in turn helps the RMI server runtime in updating the client references to the remote object. Client reference is nothing but a unique identification generated by client RMI runtime for that instance of JVM. RMI uses a reference counting algorithm for destroying unreferenced remote objects. Once all references (all clients who are holding the stub to this remote object) to a remote object drops to zero, local garbage collector takes care of freeing this remote object.
RMI client runtime initiates a TCP connection with the remote object only when a method is invoked on the stub. RMI transport layer uses a simple connection pooling mechanism to use already established TCP connections with server objects in further remote method invocations. Let's say the client is invoking a remote method on the remote object's stub fetched from the registry service. Now RMI transport layer initiates a TCP connection with the remote object (exported at ("host1", 4567)). After completion of the remote method, RMI client's transport layer keeps the connection open for some time (some configurable value) to take the advantage of the already established TCP connection in future remote method invocations on remote objects exported on the same host and port ("Host1", 4567"). RMI transport layer also takes care of closing these connections if the connections are idle more than configured value of connection open time.
From the RMI server's perspective, once the RMI server runtime receives a remote method invocation request from the RMI client, it creates a new thread and try to dispatch the method on the designated remote object. After returning the results of the remote method to the client, RMI client's transport layer may not end this conservation with this remote object to take the advantage of this TCP connection in future remote method invocations. So the same thread may be used for dispatching another remote method in future. This is actually the side effect of re-using the established TCP connections on the client side.
RMI server maintains client's reference in the client's reference set of the remote object as long as the client holds a normal reference to remote object's stub in its JVM. Once the client relinquishes the reference to the stub (goes out of the scope or setting it to the null), RMI client automatically sends clean() call to the RMI server runtime to update client reference set of the remote object. There is no public API to lookup or manipulate the client references for a given remote object, but you can always unexport the remote object even some clients are holding stubs to the remote object. Next time when the client invokes the remote method using this stub, client will receive a "connection refused exception" from the server because the remote object is already unexported. Let's say if the RMI server is started again (remote object is exported and registered with registry service), the client once again obtains the stub to the remote object from the registry service instead of using earlier stale stub. (This time remote object may have been exported on different port, so the port details in the old stub may not be valid any more).
At any point of time there can be more than one established TCP connection with the RMI client. This may be the temporary condition; RMI client's transport layer automatically closes these connections if it does not receive any remote method invocation requests with in a pre-determined interval of time (default: 2 min). You can always find out RMI client's host in a given remote invocation by using getClientHost(), but how can you semantically define a "disconnected client in a rmi server" in your application context ? You can always force all the clients to fetch the stub again in future remote method invocations by unexporting the remote object in RMI server. I hope this information helps ...
-- Srinath Mandalapu

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