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 {@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 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 {@link 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
* @author Peter Jones
* @since JDK1.1
*/
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 */
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
*/
private final int unique;
/**
* a time (as returned by {@link System#currentTimeMillis()}) at which
* the VM that this UID 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
*/
private final short count;
/**
* Generates a UID 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.
*
* @param num number for well-known UID
*/
public UID(short num) {
unique = 0;
time = 0;
count = num;
}
/**
* Constructs a UID 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.
*
* @return the hash code value for this UID
*/
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.
*
* @param obj the object to compare this UID to
*
* @return true if the given object is equivalent to
* this one, and false 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.
*
* @return a string representation of this UID
*/
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
* {@link DataOutput#writeInt(int)} method with this UID's
* unique value, then it invokes the stream's
* {@link DataOutput#writeLong(long)} method with this UID's
* time value, and then it invokes the stream's
* {@link DataOutput#writeShort(int)} method with this UID's
* count value.
*
* @param out the DataOutput instance to write
* this UID 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
* {@link DataInput#readInt()} method to read a unique value,
* then it invoke's the stream's
* {@link DataInput#readLong()} method to read a time value,
* then it invoke's the stream's
* {@link 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.
*
* @param in the DataInput instance to read
* UID from
*
* @return unmarshalled UID 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);
}
}