java 雪花算法工具类

雪花算法是Twitter开源的分布式ID生成算法，它可以保证在分布式环境中生成全局唯一的ID。Java中也有相应的雪花算法工具类，常见的有Snowflake和Leaf。

其中，Snowflake是一个比较常用的Java雪花算法工具类，其生成的ID有64位，其中第1个bit是无用的，接下来的41个bit表示时间戳（精确到毫秒级别），5个bit表示数据中心ID，5个bit表示机器ID，最后12个bit表示序列号。Snowflake算法工具类需要传入两个参数：数据中心ID和机器ID，这两个参数需要根据具体的业务场景进行设置。

以下是Snowflake算法工具类的示例代码：

public class SnowflakeIdGenerator {
    // 起始的时间戳
    private final static long START_STAMP = 1480166465631L;
    // 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 ^ (-1L << DATACENTER_BIT);
    private final static long MAX_MACHINE_NUM = -1L ^ (-1L << MACHINE_BIT);
    private final static long MAX_SEQUENCE = -1L ^ (-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
    private long machineId; // 机器标识
    private long sequence = 0L; // 序列号
    private long lastStamp = -1L; // 上一次时间戳
    
    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;
    }
    
    public synchronized long nextId() {
        long currStamp = getNewStamp();
        if (currStamp < lastStamp) {
            throw new RuntimeException("Clock moved backwards. Refusing to generate id");
        }
        
        if (currStamp == lastStamp) {
            sequence = (sequence + 1) & MAX_SEQUENCE;
            if (sequence == 0L) {
                currStamp = getNextMill();
            }
        } else {
            sequence = 0L;
        }
        
        lastStamp = currStamp;
        
        return (currStamp - START_STAMP) << TIMESTAMP_LEFT
                | datacenterId << DATACENTER_LEFT
                | machineId << MACHINE_LEFT
                | sequence;
    }
    
    private long getNextMill() {
        long mill = getNewStamp();
        while (mill <= lastStamp) {
            mill = getNewStamp();
        }
        return mill;
    }
    
    private long getNewStamp() {
        return System.currentTimeMillis();
    }
}