what is RMI registry

Solution 1:

Essentially the RMI registry is a place for the server to register services it offers and a place for clients to query for those services. See Introduction to Java RMI. Excerpt:

Figure 1 shows the connections made by the client when using RMI. Firstly, the client must contact an RMI registry, and request the name of the service. Developer B won't know the exact location of the RMI service, but he knows enough to contact Developer A's registry. This will point him in the direction of the service he wants to call..

Solution 2:

RMI Registry acts a broker between RMI servers and the clients. The server "registers" its services in the registry - hence a RMI Registry can act as a "directory" for many servers/services. The client does not need to know the location of individual servers, and does a lookup on the RMI Registry for the service it needs. The registry, being a naming directory returns the appropriate handle to the client to invoke methods on.

Google around, there is plenty of info on RMI available.