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import java.net.InetAddress;
import java.net.UnknownHostException;
import java.text.SimpleDateFormat;

public enum IdGenerator {

	INSTANCE;

	private long workerId;   			//用ip地址最后几个字节标示
	private long datacenterId = 0L; 	//可配置在properties中,启动时加载,此处默认先写成0
	private long sequence = 0L;
	private long workerIdBits = 8L; 	//节点ID长度
	private long sequenceBits = 12L; 	//序列号12位
	private long workerIdShift = sequenceBits; //机器节点左移12位
	private long datacenterIdShift = sequenceBits + workerIdBits; //数据中心节点左移14位
	private long sequenceMask = ~(-1L << sequenceBits); //4095
	private long lastTimestamp = -1L;

	IdGenerator() {
		workerId = 0x000000FF & getLastIP();
	}

	public synchronized String nextId() {
		long timestamp = timeGen(); //获取当前毫秒数
		//如果服务器时间有问题(时钟后退) 报错。
		if (timestamp < lastTimestamp) {
			throw new RuntimeException(String.format("Clock moved backwards.  Refusing to generate id for %d milliseconds", lastTimestamp - timestamp));
		}
		//如果上次生成时间和当前时间相同,在同一毫秒内
		if (lastTimestamp == timestamp) {
			//sequence自增，因为sequence只有12bit，所以和sequenceMask相与一下，去掉高位
			sequence = (sequence + 1) & sequenceMask;
			//判断是否溢出,也就是每毫秒内超过4095，当为4096时，与sequenceMask相与，sequence就等于0
			if (sequence == 0) {
				timestamp = tilNextMillis(lastTimestamp); //自旋等待到下一毫秒
			}
		} else {
			sequence = 0L; //如果和上次生成时间不同,重置sequence，就是下一毫秒开始，sequence计数重新从0开始累加
		}
		lastTimestamp = timestamp;

		long suffix = (datacenterId << datacenterIdShift) | (workerId << workerIdShift) | sequence;

		SimpleDateFormat sd = new SimpleDateFormat("yyyyMMddHHMMssSSS");
		String datePrefix = sd.format(timestamp);

		return datePrefix + suffix;
	}

	protected long tilNextMillis(long lastTimestamp) {
		long timestamp = timeGen();
		while (timestamp <= lastTimestamp) {
			timestamp = timeGen();
		}
		return timestamp;
	}

	protected long timeGen() {
		return System.currentTimeMillis();
	}

	private byte getLastIP() {
		byte lastip = 0;
		try {
			InetAddress ip = InetAddress.getLocalHost();
			byte[] ipByte = ip.getAddress();
			lastip = ipByte[ipByte.length - 1];
		} catch (UnknownHostException e) {
			e.printStackTrace();
		}
		return lastip;
	}
}

测试

import org.junit.Assert;
import org.junit.Test;

import java.util.Collection;
import java.util.Collections;
import java.util.HashSet;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;

public class IdGeneratorTest {

	@Test
	public void nextId() throws Exception {
		final IdGenerator idg = IdGenerator.INSTANCE;
		ExecutorService es = Executors.newFixedThreadPool(10);
		final HashSet idSet = new HashSet<String>();
		Collection collection = Collections.synchronizedCollection(idSet);
		long start = System.currentTimeMillis();
		System.out.println("***** start generate id ******");
		for (int i = 0; i < 10; i++)
			es.execute(() -> {
				for (int j = 0; j < 100000; j++) {
					String id = idg.nextId();
					synchronized (idSet) {
						idSet.add(id);
					}
				}
			});
		es.shutdown();
		es.awaitTermination(10, TimeUnit.SECONDS);
		long end = System.currentTimeMillis();
		System.out.println("***** end generate id *****");
		System.out.println("***** cost " + (end - start) + " ms!");
		Assert.assertEquals(10 * 100000, idSet.size());
		//idSet.forEach(o -> System.out.println(o));
	}
}

经测试，在1000000次的生成中未出现重复！！