分布式ID生成器

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：

• https://github.com/mongodb/mongo-java-driver/blob/master/src/main/org/bson/types/ObjectId.java
• http://docs.mongodb.org/manual/reference/object-id/