java中常用的雪花算法有哪些

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mob64ca12db3721 2024-04-11 07:02:07 ©著作权

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雪花算法在Java中的应用

雪花算法（Snowflake）是Twitter开源的一种分布式ID生成算法，可以保证在分布式环境中生成唯一的ID。在Java开发中，雪花算法被广泛应用于分布式系统中，用来生成全局唯一的ID，例如订单号、用户ID等。本文将介绍几种在Java中常用的雪花算法，并提供代码示例。

1. Twitter的雪花算法

Twitter的雪花算法是最为经典的雪花算法之一，采用64位的long型数据结构来存储生成的ID。其中，高41位表示时间戳，中间10位表示机器ID，最后13位表示序列号。下面是Twitter的雪花算法Java实现示例：

public class SnowflakeIdWorker {
    private final long workerId;
    private final static long START_TIMESTAMP = 1480166465631L;
    private long sequence = 0L;
    private final static long SEQUENCE_BIT = 12;
    private final static long WORKER_BIT = 5;
    private final static long MAX_WORKER_ID = -1L ^ (-1L << WORKER_BIT);
    private final static long WORKER_SHIFT = SEQUENCE_BIT;
    private final static long TIMESTAMP_SHIFT = SEQUENCE_BIT + WORKER_BIT;
    private final static long SEQUENCE_MASK = -1L ^ (-1L << SEQUENCE_BIT);
    private long lastTimestamp = -1L;

    public SnowflakeIdWorker(long workerId) {
        if (workerId > MAX_WORKER_ID || workerId < 0) {
            throw new IllegalArgumentException("worker Id can't be greater than " + MAX_WORKER_ID + " or less than 0");
        }
        this.workerId = workerId;
    }

    public synchronized long nextId() {
        long timestamp = System.currentTimeMillis();
        if (timestamp < lastTimestamp) {
            throw new RuntimeException("Clock moved backwards.  Refusing to generate id");
        }
        if (lastTimestamp == timestamp) {
            sequence = (sequence + 1) & SEQUENCE_MASK;
            if (sequence == 0) {
                timestamp = tilNextMillis(lastTimestamp);
            }
        } else {
            sequence = 0L;
        }
        lastTimestamp = timestamp;
        return ((timestamp - START_TIMESTAMP) << TIMESTAMP_SHIFT) |
                (workerId << WORKER_SHIFT) |
                sequence;
    }

    private long tilNextMillis(long lastTimestamp) {
        long timestamp = System.currentTimeMillis();
        while (timestamp <= lastTimestamp) {
            timestamp = System.currentTimeMillis();
        }
        return timestamp;
    }
}

2. 百度的UidGenerator

百度的UidGenerator是一种基于雪花算法的分布式ID生成器，使用了更加严格的并发控制来保证生成的ID唯一性。下面是UidGenerator的Java实现示例：

public class UidGenerator {
    private final long baseEpoch = 1288834974657L;
    private final long workerIdBits = 5L;
    private final long sequenceBits = 12L;
    private final long workerIdShift = sequenceBits;
    private final long timestampShift = sequenceBits + workerIdBits;
    private final long sequenceMask = ~(-1L << sequenceBits);
    private long lastTimestamp = -1L;
    private long sequence = 0L;
    private final long workerId;

    public UidGenerator(long workerId) {
        this.workerId = workerId;
    }

    public synchronized long nextId() {
        long timestamp = System.currentTimeMillis();
        if (timestamp < lastTimestamp) {
            throw new RuntimeException("Clock moved backwards. Refusing to generate id");
        }
        if (timestamp == lastTimestamp) {
            sequence = (sequence + 1) & sequenceMask;
            if (sequence == 0) {
                timestamp = tilNextMillis(lastTimestamp);
            }
        } else {
            sequence = 0L;
        }
        lastTimestamp = timestamp;
        return ((timestamp - baseEpoch) << timestampShift) |
                (workerId << workerIdShift) |
                sequence;
    }

    private long tilNextMillis(long lastTimestamp) {
        long timestamp = System.currentTimeMillis();
        while (timestamp <= lastTimestamp) {
            timestamp = System.currentTimeMillis();
        }
        return timestamp;
    }
}