uid.java

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/*
 * @(#)UID.java	1.20 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.io.DataInput;
import java.io.DataOutput;
import java.io.EOFException;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;

/**
 * A <code>UID</code> represents an identifier that is unique over time
 * with respect to the host it is generated on, or one of 2<sup>16</sup>
 * "well-known" identifiers.
 *
 * <p>The {@link #UID()} constructor can be used to generate an
 * identifier that is unique over time with respect to the host it is
 * generated on.  The {@link #UID(short)} constructor can be used to
 * create one of 2<sup>16</sup> well-known identifiers.
 *
 * <p>A <code>UID</code> instance contains three primitive values:
 * <ul>
 * <li><code>unique</code>, an <code>int</code> that uniquely identifies
 * the VM that this <code>UID</code> was generated in, with respect to its
 * host and at the time represented by the <code>time</code> value (an
 * example implementation of the <code>unique</code> value would be a
 * process identifier),
 *  or zero for a well-known <code>UID</code>
 * <li><code>time</code>, a <code>long</code> equal to a time (as returned
 * by {@link System#currentTimeMillis()}) at which the VM that this
 * <code>UID</code> was generated in was alive,
 * or zero for a well-known <code>UID</code>
 * <li><code>count</code>, a <code>short</code> to distinguish
 * <code>UID</code>s generated in the same VM with the same
 * <code>time</code> value
 * </ul>
 *
 * <p>An independently generated <code>UID</code> instance is unique
 * over time with respect to the host it is generated on as long as
 * the host requires more than one millisecond to reboot and its system
 * clock is never set backward.  A globally unique identifier can be
 * constructed by pairing a <code>UID</code> instance with a unique host
 * identifier, such as an IP address.
 *
 * @author	Ann Wollrath
 * @author	Peter Jones
 * @version	1.20, 03/01/23
 * @since	JDK1.1
 */
public final class UID implements java.io.Serializable {

    private static long  ONE_SECOND = 1000; // in milliseconds
    private static final int hostUnique = getHostUniqueNum();

    private static final Object lock = new Object();
    private static long lastTime = System.currentTimeMillis();
    private static short lastCount = Short.MIN_VALUE;

    /** indicate compatibility with JDK 1.1.x version of class */
    private static final long serialVersionUID = 1086053664494604050L;

    /**
     * number that uniquely identifies the VM that this <code>UID</code>
     * was generated in with respect to its host and at the given time
     * @serial
     */
    private final int unique;

    /**
     * a time (as returned by {@link System#currentTimeMillis()}) at which
     * the VM that this <code>UID</code> was generated in was alive
     * @serial
     */
    private final long time;

    /**
     * 16-bit number to distinguish <code>UID</code> instances created
     * in the same VM with the same time value
     * @serial
     */
    private final short count;

    /**
     * In the absence of an actual pid, just do something somewhat
     * random.
     */
    private static int getHostUniqueNum() {
	return (new Object()).hashCode();
    }

    /**
     * Generates a <code>UID</code> that is unique over time with
     * respect to the host that it was generated on.
     */
    public UID() {
	unique = hostUnique;

	synchronized (lock) {
	    if (lastCount == Short.MAX_VALUE) {
		boolean done = false;
		while (!done) {
		    long now = System.currentTimeMillis();
		    if (now < lastTime + ONE_SECOND) {
			// pause for a second to wait for time to change
			try {
			    Thread.currentThread().sleep(ONE_SECOND);
			} catch (java.lang.InterruptedException e) {
			}	// ignore exception
			continue;
		    } else {
			lastTime = now;
			lastCount = Short.MIN_VALUE;
			done = true;
		    }
		}
	    }
	    time = lastTime;
	    count = lastCount++;
	}
    }

    /**
     * Creates a "well-known" <code>UID</code>.
     *
     * There are 2<sup>16</sup> possible such well-known ids.
     *
     * <p>A <code>UID</code> created via this constructor will not
     * clash with any <code>UID</code>s generated via the no-arg
     * constructor.
     *
     * @param	num number for well-known <code>UID</code>
     */
    public UID(short num) {
	unique = 0;
	time = 0;
	count = num;
    }

    /**
     * Constructs a <code>UID</code> given data read from a stream.
     */
    private UID(int unique, long time, short count) {
	this.unique = unique;
	this.time = time;
	this.count = count;
    }

    /**
     * Returns the hash code value for this <code>UID</code>.
     *
     * @return	the hash code value for this <code>UID</code>
     */
    public int hashCode() {
	return (int) time + (int) count;
    }

    /**
     * Compares the specified object with this <code>UID</code> for
     * equality.
     *
     * This method returns <code>true</code> if and only if the
     * specified object is a <code>UID</code> instance with the same
     * <code>unique</code>, <code>time</code>, and <code>count</code>
     * values as this one.
     *
     * @param	obj the object to compare this <code>UID</code> to
     *
     * @return	<code>true</code> if the given object is equivalent to
     * this one, and <code>false</code> otherwise
     */
    public boolean equals(Object obj) {
	if ((obj != null) && (obj instanceof UID)) {
	    UID uid = (UID)obj;
	    return (unique == uid.unique &&
		    count == uid.count &&
		    time == uid.time);
	} else {
	    return false;
	}
    }

    /**
     * Returns a string representation of this <code>UID</code>.
     *
     * @return	a string representation of this <code>UID</code>
     */
    public String toString() {
	return Integer.toString(unique,16) + ":" +
	    Long.toString(time,16) + ":" +
	    Integer.toString(count,16);
    }

    /**
     * Marshals a binary representation of this <code>UID</code> to
     * a <code>DataOutput</code> instance.
     *
     * <p>Specifically, this method first invokes the given stream's
     * {@link DataOutput#writeInt(int)} method with this <code>UID</code>'s
     * <code>unique</code> value, then it invokes the stream's
     * {@link DataOutput#writeLong(long)} method with this <code>UID</code>'s
     * <code>time</code> value, and then it invokes the stream's
     * {@link DataOutput#writeShort(int)} method with this <code>UID</code>'s
     * <code>count</code> value.
     *
     * @param	out the <code>DataOutput</code> instance to write
     * this <code>UID</code> to
     *
     * @throws	IOException if an I/O error occurs while performing
     * this operation
     */
    public void write(DataOutput out) throws IOException {
	out.writeInt(unique);
	out.writeLong(time);
	out.writeShort(count);
    }

    /**
     * Constructs and returns a new <code>UID</code> instance by
     * unmarshalling a binary representation from an
     * <code>DataInput</code> instance.
     *
     * <p>Specifically, this method first invokes the given stream's
     * {@link DataInput#readInt()} method to read a <code>unique</code> value,
     * then it invoke's the stream's
     * {@link DataInput#readLong()} method to read a <code>time</code> value,
     * then it invoke's the stream's
     * {@link DataInput#readShort()} method to read a <code>count</code> value,
     * and then it creates and returns a new <code>UID</code> instance
     * that contains the <code>unique</code>, <code>time</code>, and
     * <code>count</code> values that were read from the stream.
     *
     * @param	in the <code>DataInput</code> instance to read
     * <code>UID</code> from
     *
     * @return	unmarshalled <code>UID</code> instance
     *
     * @throws	IOException if an I/O error occurs while performing
     * this operation
     */
    public static UID read(DataInput in) throws IOException {
	int unique = in.readInt();
	long time = in.readLong();
	short count = in.readShort();
	return new UID(unique, time, count);
    }
}