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package java.rmi.server;

import java.io.DataInput;
import java.io.DataOutput;
import java.io.IOException;
import java.io.Serializable;
import java.security.SecureRandom;


A UID represents an identifier that is unique over time
with respect to the host it is generated on, or one of 216
"well-known" identifiers.

The UID() constructor can be used to generate an identifier that is unique over time with respect to the host it is generated on. The UID(short) constructor can be used to create one of 216 well-known identifiers.

A UID instance contains three primitive values:

    

  • unique, an int that uniquely identifies
the VM that this UID was generated in, with respect to its
host and at the time represented by the time value (an
example implementation of the unique value would be a
process identifier),
 or zero for a well-known UID

  • time, a long equal to a time (as returned by System.currentTimeMillis()) at which the VM that this UID was generated in was alive,
or zero for a well-known UID

  • count, a short to distinguish
UIDs generated in the same VM with the same
time value



An independently generated UID 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 UID instance with a unique host
identifier, such as an IP address.

Author:     Ann Wollrath,      Peter Jones
Since:      1.1
/**
 * 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
 * @since       1.1
 */
public final class UID implements Serializable {

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

    
number that uniquely identifies the VM that this UID
was generated in with respect to its host and at the given time

@serial
/**
     * 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 System.currentTimeMillis()) at which the VM that this UID was generated in was alive

@serial
/**
     * 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 UID instances created
in the same VM with the same time value

@serial
/**
     * 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 UID that is unique over time with
respect to the host that it was generated on.

/**
     * 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 interrupted = Thread.interrupted();
                boolean done = false;
                while (!done) {
                    long now = System.currentTimeMillis();
                    if (now == lastTime) {
                        // wait for time to change
                        try {
                            Thread.sleep(1);
                        } catch (InterruptedException e) {
                            interrupted = true;
                        }
                    } else {
                        // If system time has gone backwards increase
                        // original by 1ms to maintain uniqueness
                        lastTime = (now < lastTime) ? lastTime+1 : now;
                        lastCount = Short.MIN_VALUE;
                        done = true;
                    }
                }
                if (interrupted) {
                    Thread.currentThread().interrupt();
                }
            }
            time = lastTime;
            count = lastCount++;
        }
    }

    
Creates a "well-known" UID.
There are 216 possible such well-known ids.

A UID created via this constructor will not
clash with any UIDs generated via the no-arg
constructor.

Params:
  • num – number for well-known UID
/**
     * 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 UID given data read from a stream.

/**
     * 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 UID.

Returns: the hash code value for this UID
/**
     * 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 UID for
equality.
This method returns true if and only if the
specified object is a UID instance with the same
unique, time, and count
values as this one.

Params:
  • obj – the object to compare this UID to
Returns: true if the given object is equivalent to
this one, and false otherwise
/**
     * 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 UID.

Returns: a string representation of this UID
/**
     * 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 UID to
a DataOutput instance.

Specifically, this method first invokes the given stream's DataOutput.writeInt(int) method with this UID's
unique value, then it invokes the stream's DataOutput.writeLong(long) method with this UID's
time value, and then it invokes the stream's DataOutput.writeShort(int) method with this UID's
count value.

Params:
  • out – the DataOutput instance to write
this UID to
Throws:
  • IOException – if an I/O error occurs while performing
this operation
/**
     * 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 UID instance by
unmarshalling a binary representation from an
DataInput instance.

Specifically, this method first invokes the given stream's DataInput.readInt() method to read a unique value, then it invoke's the stream's DataInput.readLong() method to read a time value, then it invoke's the stream's DataInput.readShort() method to read a count value,
and then it creates and returns a new UID instance
that contains the unique, time, and
count values that were read from the stream.

Params:
  • in – the DataInput instance to read
UID from
Throws:
  • IOException – if an I/O error occurs while performing
this operation
Returns: unmarshalled UID instance
/**
     * 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);
    }
}