最近会写一篇分布式的ID生成器的文章，先占位。借鉴Mongodb的ObjectId的生成：
4byte时间戳 + 3byte机器标识 + 2byte PID + 3byte自增id
简单代码：
import com.google.common.base.Objects;
import java.net.NetworkInterface;
import java.nio.ByteBuffer;
import java.util.Date;
import java.util.Enumeration;
import java.util.Random;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.logging.Level;
import java.util.logging.Logger;
/**
* <p>A globally unique identifier for objects.</p>
* <p/>
* <p>Consists of 12 bytes, divided as follows:</p>
* <table border="1">
* <caption>ObjectID layout</caption>
* <tr>
* <td>0</td><td>1</td><td>2</td><td>3</td><td>4</td><td>5</td><td>6</td><td>7</td><td>8</td><td>9</td><td>10</td><td>11</td>
* </tr>
* <tr>
* <td colspan="4">time</td><td colspan="3">machine</td> <td colspan="2">pid</td><td colspan="3">inc</td>
* </tr>
* </table>
* <p/>
* <p>Instances of this class are immutable.</p>
*/
public class ObjectId implements Comparable<ObjectId>, java.io.Serializable {
private final int _time;
private final int _machine;
private final int _inc;
private boolean _new;
private static final int _genmachine;
private static AtomicInteger _nextInc = new AtomicInteger((new java.util.Random()).nextInt());
private static final long serialVersionUID = -4415279469780082174L;
private static final Logger LOGGER = Logger.getLogger("org.bson.ObjectId");
/**
* Create a new object id.
*/
public ObjectId() {
_time = (int) (System.currentTimeMillis() / 1000);
_machine = _genmachine;
_inc = _nextInc.getAndIncrement();
_new = true;
}
/**
* Gets a new object id.
*
* @return the new id
*/
public static ObjectId get() {
return new ObjectId();
}
/**
* Checks if a string could be an {@code ObjectId}.
*
* @param s a potential ObjectId as a String.
* @return whether the string could be an object id
* @throws IllegalArgumentException if hexString is null
*/
public static boolean isValid(String s) {
if (s == null)
return false;
final int len = s.length();
if (len != 24)
return false;
for (int i = 0; i < len; i++) {
char c = s.charAt(i);
if (c >= '0' && c <= '9')
continue;
if (c >= 'a' && c <= 'f')
continue;
if (c >= 'A' && c <= 'F')
continue;
return false;
}
return true;
}
/**
* Converts this instance into a 24-byte hexadecimal string representation.
*
* @return a string representation of the ObjectId in hexadecimal format
*/
public String toHexString() {
final StringBuilder buf = new StringBuilder(24);
for (final byte b : toByteArray()) {
buf.append(String.format("%02x", b & 0xff));
}
return buf.toString();
}
/**
* Convert to a byte array. Note that the numbers are stored in big-endian order.
*
* @return the byte array
*/
public byte[] toByteArray() {
byte b[] = new byte[12];
ByteBuffer bb = ByteBuffer.wrap(b);
// by default BB is big endian like we need
bb.putInt(_time);
bb.putInt(_machine);
bb.putInt(_inc);
return b;
}
private int _compareUnsigned(int i, int j) {
long li = 0xFFFFFFFFL;
li = i & li;
long lj = 0xFFFFFFFFL;
lj = j & lj;
long diff = li - lj;
if (diff < Integer.MIN_VALUE)
return Integer.MIN_VALUE;
if (diff > Integer.MAX_VALUE)
return Integer.MAX_VALUE;
return (int) diff;
}
public int compareTo(ObjectId id) {
if (id == null)
return -1;
int x = _compareUnsigned(_time, id._time);
if (x != 0)
return x;
x = _compareUnsigned(_machine, id._machine);
if (x != 0)
return x;
return _compareUnsigned(_inc, id._inc);
}
/**
* Gets the timestamp (number of seconds since the Unix epoch).
*
* @return the timestamp
*/
public int getTimestamp() {
return _time;
}
/**
* Gets the timestamp as a {@code Date} instance.
*
* @return the Date
*/
public Date getDate() {
return new Date(_time * 1000L);
}
/**
* Gets the current value of the auto-incrementing counter.
*
* @return the current counter value.
*/
public static int getCurrentCounter() {
return _nextInc.get();
}
static {
try {
// build a 2-byte machine piece based on NICs info
int machinePiece;
{
try {
StringBuilder sb = new StringBuilder();
Enumeration<NetworkInterface> e = NetworkInterface.getNetworkInterfaces();
while (e.hasMoreElements()) {
NetworkInterface ni = e.nextElement();
sb.append(ni.toString());
}
machinePiece = sb.toString().hashCode() << 16;
} catch (Throwable e) {
// exception sometimes happens with IBM JVM, use random
LOGGER.log(Level.WARNING, e.getMessage(), e);
machinePiece = (new Random().nextInt()) << 16;
}
LOGGER.fine("machine piece post: " + Integer.toHexString(machinePiece));
}
// add a 2 byte process piece. It must represent not only the JVM but the class loader.
// Since static var belong to class loader there could be collisions otherwise
final int processPiece;
{
int processId = new java.util.Random().nextInt();
try {
processId = java.lang.management.ManagementFactory.getRuntimeMXBean().getName().hashCode();
} catch (Throwable t) {
}
ClassLoader loader = ObjectId.class.getClassLoader();
int loaderId = loader != null ? System.identityHashCode(loader) : 0;
StringBuilder sb = new StringBuilder();
sb.append(Integer.toHexString(processId));
sb.append(Integer.toHexString(loaderId));
processPiece = sb.toString().hashCode() & 0xFFFF;
LOGGER.fine("process piece: " + Integer.toHexString(processPiece));
}
_genmachine = machinePiece | processPiece;
LOGGER.fine("machine : " + Integer.toHexString(_genmachine));
} catch (Exception e) {
throw new RuntimeException(e);
}
}
@Override
public boolean equals(Object o) {
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;
ObjectId that = (ObjectId) o;
return Objects.equal(this.serialVersionUID, that.serialVersionUID) &&
Objects.equal(this.LOGGER, that.LOGGER) &&
Objects.equal(this._time, that._time) &&
Objects.equal(this._machine, that._machine) &&
Objects.equal(this._inc, that._inc) &&
Objects.equal(this._new, that._new) &&
Objects.equal(this._nextInc, that._nextInc) &&
Objects.equal(this._genmachine, that._genmachine);
}
@Override
public int hashCode() {
return Objects.hashCode(serialVersionUID, LOGGER, _time, _machine, _inc, _new,
_nextInc, _genmachine);
}
public static void main(String[] args) {
System.out.println(new ObjectId().toHexString());
System.out.println(new ObjectId().toHexString());
System.out.println(new ObjectId().toHexString());
}
}参考资料：