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Java Remote Method Invocation

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8.1 The RemoteStub Class

The java.rmi.server.RemoteStub class is the common superclass for stubs of remote objects. Stub objects are surrogates that support exactly the same set of remote interfaces defined by the actual implementation of a remote object.

package java.rmi.server;

public abstract class RemoteStub extends java.rmi.RemoteObject {
    protected RemoteStub() {...}
    protected RemoteStub(RemoteRef ref) {...}

    protected static void setRef(RemoteStub stub, RemoteRef ref) {...}
}

The first constructor of RemoteStub creates a stub with a null remote reference. The second constructor creates a stub with the given remote reference, ref.

The setRef method is deprecated (and unsupported) as of the Java 2 SDK, Standard Edition, v1.2.

8.1.1 Type Equivalency of Remote Objects with a Stub Class

Clients interact with stub (surrogate) objects that have exactly the same set of remote interfaces defined by the remote object's class; the stub class does not include the non-remote portions of the class hierarchy that constitutes the object's type graph. This is because the stub class is generated from the most refined implementation class that implements one or more remote interfaces. For example, if C extends B and B extends A, but only B implements a remote interface, then a stub is generated from B, not C.

Because the stub implements the same set of remote interfaces as the remote object's class, the stub has the same type as the remote portions of the server object's type graph. A client, therefore, can make use of the built-in Java programming language operations to check a remote object's type and to cast from one remote interface to another.

Stubs are generated using the rmic compiler.

8.1.2 The Semantics of Object Methods Declared final

The following methods are declared final in the java.lang.Object class and therefore cannot be overridden by any implementation:

• getClass
• notify
• notifyAll
• wait

The default implementation for getClass is appropriate for all objects written in the Java programming language, local or remote; so, the method needs no special implementation for remote objects. When used on a remote stub, the getClass method reports the exact type of the stub object, generated by rmic. Note that stub type reflects only the remote interfaces implemented by the remote object, not that object's local interfaces.

The wait and notify methods of java.lang.Object deal with waiting and notification in the context of the Java programming language's threading model. While use of these methods for remote stubs does not break the threading model, these methods do not have the same semantics as they do for local objects written in the Java programming language. Specifically, these methods operate on the client's local reference to the remote object (the stub), not the actual object at the remote site.

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