Twitter的分布式自增ID算法Snowflake
原文地址：http://www.cppblog.com/tx7do/archive/2014/06/10/207248.html
在分布式系统中，需要生成全局UID的场合还是比较多的，twitter的snowflake解决了这种需求，实现也还是很简单的，除去配置信息，核心代码就是毫秒级时间41位+机器ID 10位+毫秒内序列12位。
该项目地址为：https://github.com/twitter/snowflake是用Scala实现的
一个Java实现，貌似为淘宝的朋友写的。
public class IdWorker {
private final long workerId;
private final static long twepoch = 1361753741828L;
private long sequence = 0L;
private final static long workerIdBits = 4L;
public final static long maxWorkerId = -1L ^ -1L << workerIdBits;
private final static long sequenceBits = 10L;
private final static long workerIdShift = sequenceBits;
private final static long timestampLeftShift = sequenceBits + workerIdBits;
public final static long sequenceMask = -1L ^ -1L << sequenceBits;
private long lastTimestamp = -1L;
public IdWorker(final long workerId) {
super();
if (workerId > this.maxWorkerId || workerId < 0) {
throw new IllegalArgumentException(String.format(
"worker Id can't be greater than %d or less than 0",
this.maxWorkerId));
}
this.workerId = workerId;
}
public synchronized long nextId() {
long timestamp = this.timeGen();
if (this.lastTimestamp == timestamp) {
this.sequence = (this.sequence + 1) & this.sequenceMask;
if (this.sequence == 0) {
System.out.println("###########" + sequenceMask);
timestamp = this.tilNextMillis(this.lastTimestamp);
}
} else {
this.sequence = 0;
}
if (timestamp < this.lastTimestamp) {
try {
throw new Exception(
String.format(
"Clock moved backwards. Refusing to generate id for %d milliseconds",
this.lastTimestamp - timestamp));
} catch (Exception e) {
e.printStackTrace();
}
}
this.lastTimestamp = timestamp;
long nextId = ((timestamp - twepoch << timestampLeftShift))
| (this.workerId << this.workerIdShift) | (this.sequence);
// System.out.println("timestamp:" + timestamp + ",timestampLeftShift:"
// + timestampLeftShift + ",nextId:" + nextId + ",workerId:"
// + workerId + ",sequence:" + sequence);
return nextId;
}
private long tilNextMillis(final long lastTimestamp) {
long timestamp = this.timeGen();
while (timestamp <= lastTimestamp) {
timestamp = this.timeGen();
}
return timestamp;
}
private long timeGen() {
return System.currentTimeMillis();
}
public static void main(String[] args){
IdWorker worker2 = new IdWorker(2);
System.out.println(worker2.nextId());
}
}
这个方法里面需要提供一个workerId(工作机器id).机器级的话你可以使用MAC地址来唯一标示工作机器，工作进程级可以使用IP+Path来区分工作进程,这个就看你的需求了。下面这段代码是根据网卡设备+进程ID生成工作机器id
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.error(" IdWorker error. ", e);
machinePiece = new Random().nextInt() << 16;
}
logger.debug("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 = IdWorker.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.debug("process piece: " + Integer.toHexString(processPiece));
}
_genmachine = machinePiece | processPiece;
logger.debug("machine : " + Integer.toHexString(_genmachine));
} catch ( Exception e ) {
throw new RuntimeException(e);
}
}