全局不重复ID生成算法，已应用在实际项目中

/** 
* 64位ID (42(毫秒)+5(机器ID)+5(业务编码)+12(重复累加)) 
* @author Polim 
*/ 
public class IdWorker { 
private final static long twepoch = 1288834974657L; 
// 机器标识位数 
private final static long workerIdBits = 5L; 
// 数据中心标识位数 
private final static long datacenterIdBits = 5L; 
// 机器ID最大值 
private final static long maxWorkerId = -1L ^ (-1L << workerIdBits); 
// 数据中心ID最大值 
private final static long maxDatacenterId = -1L ^ (-1L << datacenterIdBits); 
// 毫秒内自增位 
private final static long sequenceBits = 12L; 
// 机器ID偏左移12位 
private final static long workerIdShift = sequenceBits; 
// 数据中心ID左移17位 
private final static long datacenterIdShift = sequenceBits + workerIdBits; 
// 时间毫秒左移22位 
private final static long timestampLeftShift = sequenceBits + workerIdBits + datacenterIdBits; 

private final static long sequenceMask = -1L ^ (-1L << sequenceBits); 

private static long lastTimestamp = -1L; 

private long sequence = 0L; 
private final long workerId; 
private final long datacenterId; 

public IdWorker(long workerId, long datacenterId) { 
if (workerId > maxWorkerId || workerId < 0) { 
throw new IllegalArgumentException("worker Id can't be greater than %d or less than 0"); 
} 
if (datacenterId > maxDatacenterId || datacenterId < 0) { 
throw new IllegalArgumentException("datacenter Id can't be greater than %d or less than 0"); 
} 
this.workerId = workerId; 
this.datacenterId = datacenterId; 
} 

public synchronized long nextId() { 
long timestamp = timeGen(); 
if (timestamp < lastTimestamp) { 
try { 
throw new Exception("Clock moved backwards.  Refusing to generate id for "+ (lastTimestamp - timestamp) + " milliseconds"); 
} catch (Exception e) { 
e.printStackTrace(); 
} 
} 

if (lastTimestamp == timestamp) { 
// 当前毫秒内，则+1 
sequence = (sequence + 1) & sequenceMask; 
if (sequence == 0) { 
// 当前毫秒内计数满了，则等待下一秒 
timestamp = tilNextMillis(lastTimestamp); 
} 
} else { 
sequence = 0; 
} 
lastTimestamp = timestamp; 
// ID偏移组合生成最终的ID，并返回ID 
long nextId = ((timestamp - twepoch) << timestampLeftShift) 
| (datacenterId << datacenterIdShift) 
| (workerId << workerIdShift) | 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(); 
} 
} 

这种方式是一种比较高效的方式。也是twitter使用的一种方式。 

测试类:---------------------------------------------------------- 
import java.util.concurrent.BrokenBarrierException; 
import java.util.concurrent.CountDownLatch; 
import java.util.concurrent.CyclicBarrier; 
import java.util.concurrent.TimeUnit; 

public class IdWorkerTest { 
    public static void main(String []args){ 
        IdWorkerTest test = new IdWorkerTest(); 
        test.test2(); 
    } 

    public void test2(){ 
        final IdWorker w = new IdWorker(1,2); 
        final CyclicBarrier cdl = new CyclicBarrier(100); 

        for(int i = 0; i < 100; i++){ 
            new Thread(new Runnable() { 
                @Override 
                public void run() { 
                try { 
                    cdl.await(); 
                } catch (InterruptedException e) { 
                    e.printStackTrace(); 
                } catch (BrokenBarrierException e) { 
                    e.printStackTrace(); 
                } 
                System.out.println(w.nextId());} 
             }).start(); 
        } 
        try { 
            TimeUnit.SECONDS.sleep(5); 
        } catch (InterruptedException e) { 
           e.printStackTrace(); 
        } 

    } 
}