java 实现雪花算法

雪花算法（Snowflake）是一种分布式唯一ID生成算法，主要用于解决分布式系统中生成全局唯一ID的需求。下面是一个简单的Java实现雪花算法的示例：

public class SnowflakeIdGenerator {
    // 定义起始的时间戳，这里使用的是2022-01-01 00:00:00
    private static final long START_TIMESTAMP = 1640995200000L;

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

    // 各个部分最大值
    private static final long MAX_SEQUENCE = ~(-1L << SEQUENCE_BIT);
    private static final long MAX_MACHINE_NUM = ~(-1L << MACHINE_BIT);
    private static final long MAX_DATACENTER_NUM = ~(-1L << DATACENTER_BIT);

    // 工作节点、数据中心和序列号
    private long machineId;
    private long datacenterId;
    private long sequence = 0;
    private long lastTimestamp = -1;

    public SnowflakeIdGenerator(long machineId, long datacenterId) {
        if (machineId > MAX_MACHINE_NUM || machineId < 0) {
            throw new IllegalArgumentException("Invalid machine ID");
        }
        if (datacenterId > MAX_DATACENTER_NUM || datacenterId < 0) {
            throw new IllegalArgumentException("Invalid datacenter ID");
        }
        this.machineId = machineId;
        this.datacenterId = datacenterId;
    }

    public synchronized long nextId() {
        long currentTimestamp = System.currentTimeMillis();
        if (currentTimestamp < lastTimestamp) {
            throw new RuntimeException("Clock moved backwards. Refusing to generate id.");
        }

        if (currentTimestamp == lastTimestamp) {
            sequence = (sequence + 1) & MAX_SEQUENCE;
            if (sequence == 0) {
                // 当前毫秒的序列号已经用光，等待下一毫秒重新生成
                currentTimestamp = getNextMillis();
            }
        } else {
            sequence = 0;
        }

        lastTimestamp = currentTimestamp;

        return ((currentTimestamp - START_TIMESTAMP) << (MACHINE_BIT + SEQUENCE_BIT))
                | (datacenterId << SEQUENCE_BIT)
                | (machineId << SEQUENCE_BIT)
                | sequence;
    }

    protected long getNextMillis() {
        long timestamp = System.currentTimeMillis();
        while (timestamp <= lastTimestamp) {
            timestamp = System.currentTimeMillis();
        }
        return timestamp;
    }
}

使用示例：

public class Main {
    public static void main(String[] args) {
        SnowflakeIdGenerator generator = new SnowflakeIdGenerator(1, 1);
        long id = generator.nextId();
        System.out.println("Generated ID: " + id);
    }
}

以上代码是一个简单的雪花算法实现，可根据需要进行调整和扩展。注意，这里的实现是单机版的，如果要在分布式环境中使用，需要保证每个节点的 `machineId` 和 `datacenterId` 是唯一的。