RMI Notes

	Sign In Sign-Up

RMI Notes

Garbage Collection of Remote Objects
A remote object needing unreferenced notification must implement the java.rmi.server.Unreferenced interface. When those references no longer exist, the unreferenced method will be invoked. unreferenced is called when the set of references is found to be empty so it might be called more than once. Remote objects are only collected when no more references, either local or remote, still exist.
The RMISecurityManager Class
In RMI applications, if no security manager has been set, stubs and classes can only be loaded from the local classpath. This ensures that the application is protected from code that is downloaded as a result of remote method invocations.
Thread Usage in Remote Method Invocations
A method dispatched by the RMI runtime to a remote object implementation may or may not execute in a separate thread. The RMI runtime makes no guarantees with respect to mapping remote object invocations to threads. Since remote method invocation on the same remote object may execute concurrently, a remote object implementation needs to make sure its implementation is thread-safe.
UnicastRemoteObject
The class java.rmi.server.UnicastRemoteObject provides support for creating and exporting remote objects. The class implements a remote server object with the following characteristics:

References to such objects are valid only for, at most, the life of the process that creates the remote object.
Communication with the remote object uses a TCP transport.
Invocations, parameters, and results use a stream protocol for communicating between client and server.

A remote object implementation (one that implements one or more remote interfaces) must be created and exported. Exporting a remote object makes that object available to accept incoming calls from clients. For a remote object implementation that is exported as a UnicastRemoteObject, the exporting involves listening on a TCP port (note that more than one remote object can accept incoming calls on the same port, so listening on a new port is not always necessary). A remote object implemention can extend the class UnicastRemoteObject to make use of its constructors that export the object, or it can extend some other class (or none at all) and export the object via UnicastRemoteObject's exportObject methods. The object must be exported prior to the first time it is passed in an RMI call as either a parameter or return value, otherwise, a java.rmi.server.StubNotFoundException is thrown when a remote call is attempted in which an "unexported" remote object is passed as an argument or return value.

Once exported, the object can be passed as an argument in an RMI call or returned as the result of an RMI call.

The exportObject method returns a Remote stub which is the stub object for the remote object, obj, that is passed in place of the remote object in an RMI call.