java实现雪花算法生成8-4-4-4-12格式的UUID

要在Java中使用Snowflake算法生成8-4-4-4-12格式的UUID，可以稍微修改上面的示例代码。具体来说，我们需要将48位的数字分成5个部分，其中前8位表示时间戳的高位，接下来的4位表示数据中心标识，4位表示工作机器标识，接下来的4位表示时间戳的低位，最后的36位表示序列号。

下面是一个示例代码，可用于生成8-4-4-4-12格式的UUID：

import java.util.UUID;

public class SnowflakeIdGenerator {
    private final long datacenterId;
    private final long workerId;
    private long sequence = 0L;
    private final long twepoch = 1288834974657L;
    private final long datacenterIdBits = 4L;
    private final long workerIdBits = 4L;
    private final long maxDatacenterId = -1L ^ (-1L << datacenterIdBits);
    private final long maxWorkerId = -1L ^ (-1L << workerIdBits);
    private final long sequenceBits = 12L;
    private final long workerIdShift = sequenceBits;
    private final long datacenterIdShift = sequenceBits + workerIdBits;
    private final long timestampLeftShift = sequenceBits + workerIdBits + datacenterIdBits;
    private final long sequenceMask = -1L ^ (-1L << sequenceBits);
    private long lastTimestamp = -1L;

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

    public synchronized UUID nextId() {
        long timestamp = timeGen();

        if (timestamp < lastTimestamp) {
            throw new RuntimeException("Clock moved backwards. Refusing to generate id for " + (lastTimestamp - timestamp) + " milliseconds");
        }

        if (lastTimestamp == timestamp) {
            sequence = (sequence + 1) & sequenceMask;
            if (sequence == 0) {
                timestamp = tilNextMillis(lastTimestamp);
            }
        } else {
            sequence = 0L;
        }

        lastTimestamp = timestamp;

        long mostSigBits = (timestamp - twepoch) << timestampLeftShift;
        mostSigBits |= (datacenterId << datacenterIdShift);
        mostSigBits |= (workerId << workerIdShift);
        mostSigBits |= (sequence);

        // Split into 5 parts
        String uuidStr = String.format("%016x", mostSigBits);
        String part1 = uuidStr.substring(0, 8);
        String part2 = uuidStr.substring(8, 12);
        String part3 = uuidStr.substring(12, 16);
        String part4 = uuidStr.substring(16, 20);
        String part5 = uuidStr.substring(20, 32);

        // Combine the parts into UUID format
        String uuid = part1 + "-" + part2 + "-" + part3 + "-" + part4 + "-" + part5;

        return UUID.fromString(uuid);
    }

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

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

要使用此类生成UUID，只需实例化`SnowflakeIdGenerator`类并调用`nextId()`方法。例如：

SnowflakeIdGenerator idGenerator = new SnowflakeIdGenerator(1, 1);
UUID uuid = idGenerator.nextId();
System.out.println(uuid.toString());

这将生成一个唯一的UUID，格式为8-4-4-4-12。在此示例中，`datacenterId`和`workerId`都设置为1，可以根据需要更改它们。