Java SE 24 & JDK 24

Java Remote Method Invocation: 8 - Stub/Skeleton Interfaces

This section contains the interfaces and classes used by the stubs and skeletons generated by the rmic stub compiler.

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:

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.

8.2 The RemoteCall Interface

The interface RemoteCall is an abstraction used by the stubs and skeletons of remote objects to carry out a call to a remote object.

Note: The RemoteCall interface is deprecated as of the Java 2 SDK, Standard Edition, v1.2. The 1.2 stub protocol does not make use of this interface anymore. As of the Java 2 SDK, Standard Edition, v1.2, stubs now use the new invoke method which does not require RemoteCall as a parameter.

package java.rmi.server;

import java.io.*;

public interface RemoteCall {
        ObjectOutput getOutputStream() throws IOException;
        void releaseOutputStream() throws IOException;
        ObjectInput getInputStream() throws IOException;
        void releaseInputStream() throws IOException;
        ObjectOutput getResultStream(boolean success)
                throws IOException, StreamCorruptedException;
        void executeCall() throws Exception;
        void done() throws IOException;
}

The method getOutputStream returns the output stream into which either the stub marshals arguments or the skeleton marshals results.

The method releaseOutputStream releases the output stream; in some transports this will release the stream.

The method getInputStream returns the InputStream from which the stub unmarshals results or the skeleton unmarshals parameters.

The method releaseInputStream releases the input stream. This will allow some transports to release the input side of a connection early.

The method getResultStream returns an output stream (after writing out header information relating to the success of the call). Obtaining a result stream should only succeed once per remote call. If success is true, then the result to be marshaled is a normal return; otherwise the result is an exception. StreamCorruptedException is thrown if the result stream has already been obtained for this remote call.

The method executeCall does whatever it takes to execute the call.

The method done allows cleanup after the remote call has completed.

8.3 The RemoteRef Interface

The interface RemoteRef represents the handle for a remote object. Each stub contains an instance of RemoteRef that contains the concrete representation of a reference. This remote reference is used to carry out remote calls on the remote object for which it is a reference.

package java.rmi.server;

public interface RemoteRef extends java.io.Externalizable {
        Object invoke(Remote obj,
                      java.lang.reflect.Method method,
                      Object[] params,
                      long opnum)
                throws Exception;

        RemoteCall newCall(RemoteObject obj, Operation[] op, int opnum,
                           long hash) throws RemoteException;
        void invoke(RemoteCall call) throws Exception;
        void done(RemoteCall call) throws RemoteException;
        String getRefClass(java.io.ObjectOutput out);
        int remoteHashCode();
        boolean remoteEquals(RemoteRef obj);
        String remoteToString();
}

The invoke(Remote,Method,Object[],long) method delegates method invocation to the stub's (obj) remote reference and allows the reference to take care of setting up the connection to the remote host, marshaling some representation for the method and parameters, params, then communicating the method invocation to the remote host. This method either returns the result of the method invocation on the remote object which resides on the remote host or throws a RemoteException if the call failed or an application-level exception if the remote invocation throws an exception. Note that the operation number, opnum, represents a hash of the method signature and may be used to encode the method for transmission.

The method hash to be used for the opnum parameter is a 64-bit (long) integer computed from the first two 32-bit values of the message digest of a particular byte stream using the National Institute of Standards and Technology (NIST) Secure Hash Algorithm (SHA-1). This byte stream contains a string as if it was written using the java.io.DataOutput.writeUTF method, consisting of the remote method's name followed by its method descriptor (see The Java Virtual Machine Specification (JVMS) for a description of method descriptors).

For example, if a method of a remote interface has the following name and signature:

void myRemoteMethod(int count, Object obj, boolean flag)

the string containing the remote method's name and descriptor would be the following:

myRemoteMethod(ILjava/lang/Object;Z)V

The 64-bit hash value is the little-endian composition of an eight byte sequence where the first four bytes are the first 32-bit value of the message digest in big-endian byte order and the last four bytes are the second 32-bit value of the message digest in big-endian byte order. For example, if the first two 32-bit values of the message digest are 0xB0B1B2B3 and 0xB4B5B6B7, then the hash value would be 0xB7B6B5B4B3B2B1B0.

Note: The methods newCall(RemoteObject,Operation[],int,long), invoke(RemoteCall), and done(RemoteCall) are deprecated as of the Java 2 SDK, Standard Edition, v1.2. The stubs generated by rmic using the 1.2 stub protocol version do not use these methods any longer. The sequence of calls consisting of newCall, invoke, and done have been replaced by a new invoke method that takes a Method object as one of its parameters.

The method newCall creates an appropriate call object for a new remote method invocation on the remote object obj. The operation array, op, contains the available operations on the remote object. The operation number, opnum, is an index into the operation array which specifies the particular operation for this remote call. The interface hash is a 64-bit value used to enforce compatibility between a stub and skeleton using the v1.1 stub protocol. The interface hash is computed from the first two 32-bit values of the message digest of a particular byte stream using SHA-1. This byte stream contains data as if it was written using the writeInt and writeUTF methods of the interface java.io.DataOutput, consisting of the following items:

The interface hash value is composed from the message digest in the same manner as described above for the method hash used in the invoke method.

The method invoke(RemoteCall) executes the remote call. invoke will raise any "user" exceptions which should pass through and not be caught by the stub. If any exception is raised during the remote invocation, invoke should take care of cleaning up the connection before raising the "user exception" or RemoteException.

The method done allows the remote reference to clean up (or reuse) the connection. done should only be called if the invoke call returns successfully (non-exceptionally) to the stub.

The method getRefClass returns the nonpackage-qualified class name of the reference type to be serialized onto the stream out.

The method remoteHashCode returns a hashcode for a remote object. Two remote object stubs that refer to the same remote object will have the same hash code (in order to support remote objects as keys in hashtables). A RemoteObject forwards a call to its hashCode method to the remoteHashCode method of the remote reference.

The method remoteEquals compares two remote objects for equality. Two remote objects are equal if they refer to the same remote object. For example, two stubs are equal if they refer to the same remote object. A RemoteObject forwards a call to its equals method to the remoteEquals method of the remote reference.

The method remoteToString returns a String that represents the reference of this remote object.

8.4 The ServerRef Interface

The interface ServerRef represents the server-side handle for a remote object implementation.

package java.rmi.server;

public interface ServerRef extends RemoteRef {
        RemoteStub exportObject(java.rmi.Remote obj, Object data)
                throws java.rmi.RemoteException;
        String getClientHost() throws ServerNotActiveException;
}

The method exportObject finds or creates a client stub object for the supplied Remote object implementation obj.The parameter data contains information necessary to export the object (such as port number).

The method getClientHost returns the host name of the current client. When called from a thread actively handling a remote method invocation, the host name of the client invoking the call is returned. If a remote method call is not currently being service, then ServerNotActiveException is called.

8.5 The Skeleton Interface

The interface Skeleton is used solely by the implementation of skeletons generated by the rmic compiler. A skeleton for a remote object is a server-side entity that dispatches calls to the actual remote object implementation.

Note: The Skeleton interface was deprecated as of the Java 2 SDK, Standard Edition, v 1.2. Every 1.1 (and version 1.1 compatible skeletons generated in 1.2 using rmic -vcompat, the default) skeleton class generated by the rmic stub compiler implements this interface. Skeletons are no longer required for remote method call dispatch as of Java 2 SDK, Standard Edition, v1.2-compatible versions. To generate stubs that are compatible with 1.2 or later versions, use the command rmic with the option -v1.2.

package java.rmi.server;

public interface Skeleton {
        void dispatch(Remote obj, RemoteCall call, int opnum, long hash)
                throws Exception;
        Operation[] getOperations();
}

The dispatch method unmarshals any arguments from the input stream obtained from the call object, invokes the method (indicated by the operation number opnum) on the actual remote object implementation obj, and marshals the return value or throws an exception if one occurs during the invocation.

The getOperations method returns an array containing the operation descriptors for the remote object's methods.

8.6 The Operation Class

The class Operation holds a description of a method in the Java programming language for a remote object.

Note: The Operation interface is deprecated as of the Java 2 SDK, Standard Edition, v1.2. The 1.2 stub protocol no longer uses the old RemoteRef.invoke method which takes an Operation as one of its arguments. As of the Java 2 SDK, Standard Edition, v1.2, stubs now use the new invoke method which does not require Operation as a parameter.

package java.rmi.server;

public class Operation {
        public Operation(String op) {...}
        public String getOperation() {...}
        public String toString() {...}
}

An Operation object is typically constructed with the method signature.

The method getOperation returns the contents of the operation descriptor (the value with which it was initialized).

The method toString also returns the string representation of the operation descriptor (typically the method signature).