无锁实现的雪花算法

gistfile1.txt

class SnowflakeIdGenerator {
    // 起始时间戳 (2024-01-01 00:00:00)
    private final long START_TIMESTAMP = 1704038400000L;
    
    // 每部分占用位数
    private final long SEQUENCE_BIT = 12;    // 序列号占12位
    private final long MACHINE_BIT = 5;      // 机器ID占5位
    private final long DATACENTER_BIT = 5;   // 数据中心占5位
    
    // 最大值
    private final long MAX_SEQUENCE = -1L ^ (-1L << SEQUENCE_BIT);
    private final long MAX_MACHINE_NUM = -1L ^ (-1L << MACHINE_BIT);
    private final long MAX_DATACENTER_NUM = -1L ^ (-1L << DATACENTER_BIT);
    
    // 每部分向左的位移
    private final long MACHINE_LEFT = SEQUENCE_BIT;
    private final long DATACENTER_LEFT = SEQUENCE_BIT + MACHINE_BIT;
    private final long TIMESTAMP_LEFT = DATACENTER_LEFT + DATACENTER_BIT;
    
    private long datacenterId;  // 数据中心ID
    private long machineId;     // 机器ID
    private final AtomicLong sequence = new AtomicLong(0L); // 改用 AtomicLong
    private volatile long lastTimestamp = -1L; // 使用 volatile 保证可见性
    
    public SnowflakeIdGenerator(long datacenterId, long machineId) {
        if (datacenterId > MAX_DATACENTER_NUM || datacenterId < 0) {
            throw new IllegalArgumentException("数据中心ID不能大于" + MAX_DATACENTER_NUM + "或小于0");
        }
        if (machineId > MAX_MACHINE_NUM || machineId < 0) {
            throw new IllegalArgumentException("机器ID不能大于" + MAX_MACHINE_NUM + "或小于0");
        }
        this.datacenterId = datacenterId;
        this.machineId = machineId;
    }
    
    // 生成下一个ID
    public long nextId() {
        long timestamp = System.currentTimeMillis();
        long currentSequence;
        
        while (true) {
            long lastTime = lastTimestamp;
            
            if (timestamp < lastTime) {
                throw new RuntimeException("系统时钟回退，拒绝生成ID");
            }
            
            if (lastTime == timestamp) {
                // CAS 更新序列号
                currentSequence = sequence.incrementAndGet() & MAX_SEQUENCE;
                if (currentSequence == 0) {
                    // 序列号用完，获取下一毫秒
                    timestamp = getNextMillis(lastTime);
                    continue;
                }
            } else {
                // 不同毫秒，序列号重置为0
                currentSequence = sequence.getAndSet(0);
                lastTimestamp = timestamp;
            }
            break;
        }
        
        return ((timestamp - START_TIMESTAMP) << TIMESTAMP_LEFT)
                | (datacenterId << DATACENTER_LEFT)
                | (machineId << MACHINE_LEFT)
                | currentSequence;
    }
    
    // 获取下一毫秒时间戳
    private long getNextMillis(long lastTimestamp) {
    
        long timestamp = System.currentTimeMillis();
        while (timestamp <= lastTimestamp) {
            timestamp = System.currentTimeMillis();
        }
        return timestamp;
    }
}