remoteobject.java

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/*
 * @(#)RemoteObject.java	1.28 03/01/23
 *
 * Copyright 2003 Sun Microsystems, Inc. All rights reserved.
 * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */

package java.rmi.server;

import java.rmi.Remote;
import java.rmi.UnmarshalException;
import java.rmi.NoSuchObjectException;

/**
 * The <code>RemoteObject</code> class implements the
 * <code>java.lang.Object</code> behavior for remote objects.
 * <code>RemoteObject</code> provides the remote semantics of Object by
 * implementing methods for hashCode, equals, and toString.
 *
 * @author	Ann Wollrath
 * @author	Laird Dornin
 * @author	Peter Jones
 * @version	1.28, 03/01/23
 * @since	JDK1.1
 */
public abstract class RemoteObject implements Remote, java.io.Serializable {

    /** The object's remote reference. */
    transient protected RemoteRef ref;
    
    /** indicate compatibility with JDK 1.1.x version of class */
    private static final long serialVersionUID = -3215090123894869218L;

    /**
     * Creates a remote object.
     */
    protected RemoteObject() {
	ref = null;
    }
    
    /**
     * Creates a remote object, initialized with the specified remote
     * reference.
     * @param newref remote reference
     */
    protected RemoteObject(RemoteRef newref) {
	ref = newref;
    }

    /**
     * Returns the remote reference for the remote object.
     *
     * <p>Note: The object returned from this method may be an instance of
     * an implementation-specific class.  The <code>RemoteObject</code>
     * class ensures serialization portability of its instances' remote
     * references through the behavior of its custom
     * <code>writeObject</code> and <code>readObject</code> methods.  An
     * instance of <code>RemoteRef</code> should not be serialized outside
     * of its <code>RemoteObject</code> wrapper instance or the result may
     * be unportable.
     *
     * @return remote reference for the remote object
     * @since 1.2
     */
    public RemoteRef getRef() {
	return ref;
    }
    
    /**
     * Returns the stub for the remote object <code>obj</code> passed
     * as a parameter. This operation is only valid <i>after</i>
     * the object has been exported.
     * @param obj the remote object whose stub is neede
     * @return the stub for the remote object, <code>obj</code>.
     * @exception NoSuchObjectException if the stub for the
     * remote object could not be found.
     * @since 1.2
     */
    public static Remote toStub(Remote obj) throws NoSuchObjectException {
	if (obj instanceof RemoteStub) {
	    return (RemoteStub)obj; 
	} else {
	    return sun.rmi.transport.ObjectTable.getStub(obj);
	}
    }

    /**
     * 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 hash tables).
     *
     * @see		java.util.Hashtable
     */
    public int hashCode() {
	return (ref == null) ? super.hashCode() : ref.remoteHashCode();
    }

    /**
     * Compares two remote objects for equality.
     * Returns a boolean that indicates whether this remote object is
     * equivalent to the specified Object. This method is used when a
     * remote object is stored in a hashtable.
     * If the specified Object is not itself an instance of RemoteObject,
     * then this method delegates by returning the result of invoking the
     * <code>equals</code> method of its parameter with this remote object
     * as the argument.
     * @param	obj	the Object to compare with
     * @return	true if these Objects are equal; false otherwise.
     * @see		java.util.Hashtable
     */
    public boolean equals(Object obj) {
	if (obj instanceof RemoteObject) {
	    if (ref == null) {
		return obj == this;
	    } else {
		return ref.remoteEquals(((RemoteObject)obj).ref);
	    }
	} else if (obj != null) {
	    /*
	     * Fix for 4099660: if object is not an instance of RemoteObject,
	     * use the result of its equals method, to support symmetry is a
	     * remote object implementation class that does not extend
	     * RemoteObject wishes to support equality with its stub objects.
	     */
	    return obj.equals(this);
	} else {
	    return false;
	}
    }

    /**
     * Returns a String that represents the value of this remote object.
     */
    public String toString() {
	String classname = this.getClass().getName();
	return (ref == null) ? classname :
	    classname + "[" +ref.remoteToString() + "]";
    }

    /**
     * <code>writeObject</code> for custom serialization.
     *
     * <p>This method writes this object's serialized form for this class
     * as follows:
     *
     * <p>The {@link RemoteRef#getRefClass(java.io.ObjectOutput) getRefClass}
     * method is invoked on this object's <code>ref</code> field
     * to obtain its external ref type name.
     * If the value returned by <code>getRefClass</code> was
     * a non-<code>null</code> string of length greater than zero,
     * the <code>writeUTF</code> method is invoked on <code>out</code>
     * with the value returned by <code>getRefClass</code>, and then
     * the <code>writeExternal</code> method is invoked on
     * this object's <code>ref</code> field passing <code>out</code>
     * as the argument; otherwise,
     * the <code>writeUTF</code> method is invoked on <code>out</code>
     * with a zero-length string (<code>""</code>), and then
     * the <code>writeObject</code> method is invoked on <code>out</code>
     * passing this object's <code>ref</code> field as the argument.
     * 
     * @serialData
     *
     * The serialized data for this class comprises a string (written with
     * <code>ObjectOutput.writeUTF</code>) that is either the external
     * ref type name of the contained <code>RemoteRef</code> instance
     * (the <code>ref</code> field) or a zero-length string, followed by
     * either the external form of the <code>ref</code> field as written by
     * its <code>writeExternal</code> method if the string was of non-zero
     * length, or the serialized form of the <code>ref</code> field as
     * written by passing it to the serialization stream's
     * <code>writeObject</code> if the string was of zero length.
     *
     * <p>If this object is an instance of
     * <code>java.rmi.server.RemoteStub</code> that was returned from any of
     * the <code>UnicastRemoteObject.exportObject</code> methods
     * and custom socket factories are not used,
     * the external ref type name is <code>"UnicastRef"</code>.
     *
     * If this object is an instance of
     * <code>RemoteStub</code> that was returned from any of
     * the <code>UnicastRemoteObject.exportObject</code> methods
     * and custom socket factories are used,
     * the external ref type name is <code>"UnicastRef2"</code>.
     *
     * If this object is an instance of
     * <code>RemoteStub</code> that was returned from any of
     * the <code>java.rmi.activation.Activatable.exportObject</code> methods,
     * the external ref type name is <code>"ActivatableRef"</code>.
     *
     * If this object is an instance of
     * <code>RemoteStub</code> that was returned from
     * the <code>RemoteObject.toStub</code> method (and the argument passed
     * to <code>toStub</code> was not itself a <code>RemoteStub</code>),
     * the external ref type name is a function of how the remote object
     * passed to <code>toStub</code> was exported, as described above.
     *
     * If this object is an instance of
     * <code>RemoteStub</code> that was originally
     * created via deserialization,
     * the external ref type name is the same as that which was read
     * when this object was deserialized.
     *
     * <p>If this object is an instance of
     * <code>java.rmi.server.UnicastRemoteObject</code> that does not
     * use custom socket factories,
     * the external ref type name is <code>"UnicastServerRef"</code>.
     *
     * If this object is an instance of
     * <code>UnicastRemoteObject</code> that does
     * use custom socket factories,
     * the external ref type name is <code>"UnicastServerRef2"</code>.
     *
     * <p>If this object is an instance of
     * <code>java.rmi.activation.Activatable</code>,
     * the external ref type name is <code>"ActivatableServerRef"</code>.