高并发分布式系统中获取唯一ID

在高并发分布式系统中生成唯一ID的方法很多，可以根据项目的特性选择合适的方法。
1、如果分布式系统共用一个oracle数据库，可以用SYS_GUID() 生成32位的唯一ID，或者用sequence自增的方式生成唯一ID。
2、使用UUID,使用UUID有一个弊端：生成是序列过长，没有规律不方便记忆。
3、自己通过代码实现，设计思路是通过时间戳+主机编号+序列号，代码如下：
public class IdWorker {

private final long workerId;
private final static long twepoch = 1515290472631L;
private long sequence = 0L;
private final static long hostIdBits = 7L;//主机占7位，即最大值127
public final static long maxHostId = -1L ^ -1L << hostIdBits;//通过位移计算得到hostId的最大值
private final static long sequenceBits = 10L;//序列占10位，即最大值1023
private final static long hostIdLeftShift = sequenceBits;
private final static long timestampLeftShift = sequenceBits + hostIdBits;
public final static long maxSequence = -1L ^ -1L << sequenceBits;//通过位移计算得到序列的最大值
private long lastTimestamp = -1L;

public IdWorker(final long workerId) {
	super();
	if (workerId > this.maxHostId || workerId < 0) {
		throw new IllegalArgumentException(
				String.format("workerId大于 %d，或小于0", this.maxHostId));
	}
	this.workerId = workerId;
}

public synchronized long nextId() {
	long timestamp = this.timeGen();
	if (this.lastTimestamp == timestamp) {
		this.sequence = (this.sequence + 1) & this.maxSequence;
		if (this.sequence == 0) {
			System.out.println("sequence大于" + maxSequence);
			timestamp = this.tilNextMillis(this.lastTimestamp);
		}
	} else {
		this.sequence = 0;
	}
	if (timestamp < this.lastTimestamp) {
		try {
			throw new Exception("当前时间小于上次生成workerId的时间");
		} catch (Exception e) {
			e.printStackTrace();
		}
	}

	this.lastTimestamp = timestamp;
	long nextId = ((timestamp - twepoch << timestampLeftShift)) | (this.workerId << this.hostIdLeftShift)
			| (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) {
	final IdWorker worker2 = new IdWorker(2);
	for (int i = 0; i < 10; i++) {
		new Thread(new Runnable() {
			public void run() {
				while (true) {
					worker2.nextId();
				}
			}
		}).start();
	}
}

}