1 /*
   2  * Copyright (c) 1996, 2006, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.  Oracle designates this
   8  * particular file as subject to the "Classpath" exception as provided
   9  * by Oracle in the LICENSE file that accompanied this code.
  10  *
  11  * This code is distributed in the hope that it will be useful, but WITHOUT
  12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14  * version 2 for more details (a copy is included in the LICENSE file that
  15  * accompanied this code).
  16  *
  17  * You should have received a copy of the GNU General Public License version
  18  * 2 along with this work; if not, write to the Free Software Foundation,
  19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  20  *
  21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  22  * or visit www.oracle.com if you need additional information or have any
  23  * questions.
  24  */
  25 package java.rmi.server;
  26 
  27 import java.io.DataInput;
  28 import java.io.DataOutput;
  29 import java.io.IOException;
  30 import java.io.Serializable;
  31 import java.security.SecureRandom;
  32 
  33 /**
  34  * A <code>UID</code> represents an identifier that is unique over time
  35  * with respect to the host it is generated on, or one of 2<sup>16</sup>
  36  * "well-known" identifiers.
  37  *
  38  * <p>The {@link #UID()} constructor can be used to generate an
  39  * identifier that is unique over time with respect to the host it is
  40  * generated on.  The {@link #UID(short)} constructor can be used to
  41  * create one of 2<sup>16</sup> well-known identifiers.
  42  *
  43  * <p>A <code>UID</code> instance contains three primitive values:
  44  * <ul>
  45  * <li><code>unique</code>, an <code>int</code> that uniquely identifies
  46  * the VM that this <code>UID</code> was generated in, with respect to its
  47  * host and at the time represented by the <code>time</code> value (an
  48  * example implementation of the <code>unique</code> value would be a
  49  * process identifier),
  50  *  or zero for a well-known <code>UID</code>
  51  * <li><code>time</code>, a <code>long</code> equal to a time (as returned
  52  * by {@link System#currentTimeMillis()}) at which the VM that this
  53  * <code>UID</code> was generated in was alive,
  54  * or zero for a well-known <code>UID</code>
  55  * <li><code>count</code>, a <code>short</code> to distinguish
  56  * <code>UID</code>s generated in the same VM with the same
  57  * <code>time</code> value
  58  * </ul>
  59  *
  60  * <p>An independently generated <code>UID</code> instance is unique
  61  * over time with respect to the host it is generated on as long as
  62  * the host requires more than one millisecond to reboot and its system
  63  * clock is never set backward.  A globally unique identifier can be
  64  * constructed by pairing a <code>UID</code> instance with a unique host
  65  * identifier, such as an IP address.
  66  *
  67  * @author      Ann Wollrath
  68  * @author      Peter Jones
  69  * @since       JDK1.1
  70  */
  71 public final class UID implements Serializable {
  72 
  73     private static int hostUnique;
  74     private static boolean hostUniqueSet = false;
  75 
  76     private static final Object lock = new Object();
  77     private static long lastTime = System.currentTimeMillis();
  78     private static short lastCount = Short.MIN_VALUE;
  79 
  80     /** indicate compatibility with JDK 1.1.x version of class */
  81     private static final long serialVersionUID = 1086053664494604050L;
  82 
  83     /**
  84      * number that uniquely identifies the VM that this <code>UID</code>
  85      * was generated in with respect to its host and at the given time
  86      * @serial
  87      */
  88     private final int unique;
  89 
  90     /**
  91      * a time (as returned by {@link System#currentTimeMillis()}) at which
  92      * the VM that this <code>UID</code> was generated in was alive
  93      * @serial
  94      */
  95     private final long time;
  96 
  97     /**
  98      * 16-bit number to distinguish <code>UID</code> instances created
  99      * in the same VM with the same time value
 100      * @serial
 101      */
 102     private final short count;
 103 
 104     /**
 105      * Generates a <code>UID</code> that is unique over time with
 106      * respect to the host that it was generated on.
 107      */
 108     public UID() {
 109 
 110         synchronized (lock) {
 111             if (!hostUniqueSet) {
 112                 hostUnique = (new SecureRandom()).nextInt();
 113                 hostUniqueSet = true;
 114             }
 115             unique = hostUnique;
 116             if (lastCount == Short.MAX_VALUE) {
 117                 boolean interrupted = Thread.interrupted();
 118                 boolean done = false;
 119                 while (!done) {
 120                     long now = System.currentTimeMillis();
 121                     if (now <= lastTime) {
 122                         // wait for time to change
 123                         try {
 124                             Thread.currentThread().sleep(1);
 125                         } catch (InterruptedException e) {
 126                             interrupted = true;
 127                         }
 128                     } else {
 129                         lastTime = now;


 130                         lastCount = Short.MIN_VALUE;
 131                         done = true;
 132                     }
 133                 }
 134                 if (interrupted) {
 135                     Thread.currentThread().interrupt();
 136                 }
 137             }
 138             time = lastTime;
 139             count = lastCount++;
 140         }
 141     }
 142 
 143     /**
 144      * Creates a "well-known" <code>UID</code>.
 145      *
 146      * There are 2<sup>16</sup> possible such well-known ids.
 147      *
 148      * <p>A <code>UID</code> created via this constructor will not
 149      * clash with any <code>UID</code>s generated via the no-arg
 150      * constructor.
 151      *
 152      * @param   num number for well-known <code>UID</code>
 153      */
 154     public UID(short num) {
 155         unique = 0;
 156         time = 0;
 157         count = num;
 158     }
 159 
 160     /**
 161      * Constructs a <code>UID</code> given data read from a stream.
 162      */
 163     private UID(int unique, long time, short count) {
 164         this.unique = unique;
 165         this.time = time;
 166         this.count = count;
 167     }
 168 
 169     /**
 170      * Returns the hash code value for this <code>UID</code>.
 171      *
 172      * @return  the hash code value for this <code>UID</code>
 173      */
 174     public int hashCode() {
 175         return (int) time + (int) count;
 176     }
 177 
 178     /**
 179      * Compares the specified object with this <code>UID</code> for
 180      * equality.
 181      *
 182      * This method returns <code>true</code> if and only if the
 183      * specified object is a <code>UID</code> instance with the same
 184      * <code>unique</code>, <code>time</code>, and <code>count</code>
 185      * values as this one.
 186      *
 187      * @param   obj the object to compare this <code>UID</code> to
 188      *
 189      * @return  <code>true</code> if the given object is equivalent to
 190      * this one, and <code>false</code> otherwise
 191      */
 192     public boolean equals(Object obj) {
 193         if (obj instanceof UID) {
 194             UID uid = (UID) obj;
 195             return (unique == uid.unique &&
 196                     count == uid.count &&
 197                     time == uid.time);
 198         } else {
 199             return false;
 200         }
 201     }
 202 
 203     /**
 204      * Returns a string representation of this <code>UID</code>.
 205      *
 206      * @return  a string representation of this <code>UID</code>
 207      */
 208     public String toString() {
 209         return Integer.toString(unique,16) + ":" +
 210             Long.toString(time,16) + ":" +
 211             Integer.toString(count,16);
 212     }
 213 
 214     /**
 215      * Marshals a binary representation of this <code>UID</code> to
 216      * a <code>DataOutput</code> instance.
 217      *
 218      * <p>Specifically, this method first invokes the given stream's
 219      * {@link DataOutput#writeInt(int)} method with this <code>UID</code>'s
 220      * <code>unique</code> value, then it invokes the stream's
 221      * {@link DataOutput#writeLong(long)} method with this <code>UID</code>'s
 222      * <code>time</code> value, and then it invokes the stream's
 223      * {@link DataOutput#writeShort(int)} method with this <code>UID</code>'s
 224      * <code>count</code> value.
 225      *
 226      * @param   out the <code>DataOutput</code> instance to write
 227      * this <code>UID</code> to
 228      *
 229      * @throws  IOException if an I/O error occurs while performing
 230      * this operation
 231      */
 232     public void write(DataOutput out) throws IOException {
 233         out.writeInt(unique);
 234         out.writeLong(time);
 235         out.writeShort(count);
 236     }
 237 
 238     /**
 239      * Constructs and returns a new <code>UID</code> instance by
 240      * unmarshalling a binary representation from an
 241      * <code>DataInput</code> instance.
 242      *
 243      * <p>Specifically, this method first invokes the given stream's
 244      * {@link DataInput#readInt()} method to read a <code>unique</code> value,
 245      * then it invoke's the stream's
 246      * {@link DataInput#readLong()} method to read a <code>time</code> value,
 247      * then it invoke's the stream's
 248      * {@link DataInput#readShort()} method to read a <code>count</code> value,
 249      * and then it creates and returns a new <code>UID</code> instance
 250      * that contains the <code>unique</code>, <code>time</code>, and
 251      * <code>count</code> values that were read from the stream.
 252      *
 253      * @param   in the <code>DataInput</code> instance to read
 254      * <code>UID</code> from
 255      *
 256      * @return  unmarshalled <code>UID</code> instance
 257      *
 258      * @throws  IOException if an I/O error occurs while performing
 259      * this operation
 260      */
 261     public static UID read(DataInput in) throws IOException {
 262         int unique = in.readInt();
 263         long time = in.readLong();
 264         short count = in.readShort();
 265         return new UID(unique, time, count);
 266     }
 267 }
--- EOF ---