/*
* @(#)UID.java 1.22 03/12/19
*
* Copyright 2004 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;
import java.security.SecureRandom;
/**
* 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 1.22, 03/12/19
* @since JDK1.1
*/
public final class UID implements java.io.Serializable {
private static final long ONE_SECOND = 1000; // in milliseconds
private static int hostUnique;
private static boolean hostUniqueSet = false;
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;
/**
* Generates a <code>UID</code> that is unique over time with
* respect to the host that it was generated on.
*/
public UID() {
synchronized (lock) {
if (!hostUniqueSet) {
hostUnique = (new SecureRandom()).nextInt();
hostUniqueSet = true;
}
unique = hostUnique;
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 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);
}
}
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