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 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 */
+ 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);
+ }
+}