雪花算法生成id工具类

雪花算法是一种分布式系统中生成唯一ID的算法，它可以生成一个64位的ID，其中包含了时间戳、机器ID和序列号等信息。下面是一个Java实现的雪花算法生成ID的工具类：

public class SnowflakeIdGenerator {

    // 起始的时间戳
    private final static long START_TIMESTAMP = 1609459200000L; //2021-01-01 00:00:00

    // 每一部分占用的位数
    private final static long SEQUENCE_BIT = 12; // 序列号占用的位数
    private final static long MACHINE_BIT = 5; // 机器标识占用的位数
    private final static long DATACENTER_BIT = 5; // 数据中心占用的位数

    // 每一部分的最大值
    private final static long MAX_DATACENTER_NUM = ~(-1L << DATACENTER_BIT);
    private final static long MAX_MACHINE_NUM = ~(-1L << MACHINE_BIT);
    private final static long MAX_SEQUENCE_NUM = ~(-1L << SEQUENCE_BIT);

    // 每一部分向左的位移
    private final static long MACHINE_LEFT = SEQUENCE_BIT;
    private final static long DATACENTER_LEFT = SEQUENCE_BIT + MACHINE_BIT;
    private final static long TIMESTAMP_LEFT = DATACENTER_LEFT + DATACENTER_BIT;

    private long datacenterId; // 数据中心ID
    private long machineId; // 机器ID
    private long sequence = 0L; // 序列号
    private long lastTimestamp = -1L; // 上一次生成ID的时间戳

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

    /**
     * 生成下一个唯一的ID
     *
     * @return Snowflake ID
     */
    public synchronized long nextId() {
        long timestamp = System.currentTimeMillis();

        // 时钟回拨，抛出异常
        if (timestamp < lastTimestamp) {
            throw new RuntimeException("Clock moved backwards. Refusing to generate id");
        }

        // 同一毫秒内生成ID
        if (lastTimestamp == timestamp) {
            sequence = (sequence + 1) & MAX_SEQUENCE_NUM;
            // 当前毫秒内的序列号已经达到最大值，等待下一毫秒再生成ID
            if (sequence == 0) {
                timestamp = tilNextMillis(lastTimestamp);
            }
        }
        // 不同毫秒内生成ID
        else {
            sequence = 0L;
        }

        lastTimestamp = timestamp;

        // 生成ID
        return ((timestamp - START_TIMESTAMP) << TIMESTAMP_LEFT)
                | (datacenterId << DATACENTER_LEFT)
                | (machineId << MACHINE_LEFT)
                | sequence;
    }

    /**
     * 等待下一个毫秒的到来
     *
     * @param lastTimestamp 上一次生成ID的时间戳
     * @return 当前毫秒的时间戳
     */
    private long tilNextMillis(long lastTimestamp) {
        long timestamp = System.currentTimeMillis();
        while (timestamp <= lastTimestamp) {
            timestamp = System.currentTimeMillis();
        }
        return timestamp;
    }

}

使用示例：

SnowflakeIdGenerator idGenerator = new SnowflakeIdGenerator(1, 1);
long id = idGenerator.nextId();

其中，`datacenterId`和`machineId`分别表示数据中心ID和机器ID，可以根据实际情况设置。`nextId()`方法用于生成下一个唯一的ID。