IdGenerator(雪花)

/**
 * tweeter的snowflake 移植到Java.参考资料：https://github.com/twitter/snowflake
 * id构成: 42位的时间前缀 + 10位的节点标识 + 12位的sequence避免并发的数字(12位不够用时强制得到新的时间前缀)
 * id单调递增，长整型
 * 注意这里进行了小改动: snowkflake是5位的datacenter加5位的机器id; 这里变成使用10位的机器id
 * snowkflake当时间调整时将拒绝分配ID，这里改成分配UUID的tMostSignificantBits
 * 通过取服务器名中的编号来分配机器id，实现了去中心化【仅适用于包含编号的机器】
 * 使用：long id = IdGenerator.nextId();
 */
@Log4j2
public final class IdGenerator {
    /**
     * 每台机器分配不同的id
     */
    private static final long WORKER_ID;

    /**
     * 时间起始标记点，作为基准，一般取系统的最近时间
     */
    private static final long EPOCH_NUM = 1568017964000L;

    /**
     * 机器标识位数
     */
    private static final long WORKER_IDBITS = 10L;

    /**
     * 机器ID最大值: 1023
     */
    private static final long MAXWORKER_ID = -1L ^ -1L << WORKER_IDBITS;

    /**
     * 毫秒内自增位
     */
    private static final long SEQUENCE_BITS = 12L;

    /**
     * 12
     */
    private static final long WORKER_IDSHIFT = SEQUENCE_BITS;

    /**
     * 22
     */
    private static final long TIMESTAMP_LEFTSHIFT = WORKER_IDBITS + SEQUENCE_BITS;

    /**
     * 4095,111111111111,12位
     */
    private static final long SEQUENCE_MASK = -1L ^ -1L << SEQUENCE_BITS;

    /**
     * 0，并发控制
     */
    private static long sequenceSe = 0L;

    private static long lastTimestamp = -1L;

    static {
        String hostName = null;
        try {
            InetAddress netAddress = InetAddress.getLocalHost();
            hostName = netAddress.getHostName();
        } catch (UnknownHostException e) {
            log.error(e);
        }
        if (null != hostName && !"".equals(hostName)) {
            String hostNo = "";
            for (int i = 0; i < hostName.length(); i++) {
                if (hostName.charAt(i) > 48 && hostName.charAt(i) <= 57) {
                    // 取最后一组数字
                    if ("".equals(hostNo)) {
                        hostNo += hostName.charAt(i);
                    } else {
                        hostNo = "";
                        hostNo += hostName.charAt(i);
                    }
                }
            }
            if (!"".equals(hostNo)) {
                WORKER_ID = Integer.parseInt(hostNo) % MAXWORKER_ID;
            } else {
                WORKER_ID = new SecureRandom().nextInt(1000) + 1;
            }

        } else {
            WORKER_ID = new SecureRandom().nextInt(1000) + 1;
        }
    }

    /**
     * Next id long.
     *
     * @return the long
     */
    public static synchronized long nextId() {
        long timestamp = timeGen();
        // 如果上一个timestamp与新产生的相等，则sequence加一(0-4095循环); 对新的timestamp，sequence从0开始
        if (lastTimestamp == timestamp) {
            sequenceSe = sequenceSe + 1 & SEQUENCE_MASK;
            if (sequenceSe == 0) {
                // 重新生成timestamp
                timestamp = tilNextMillis(lastTimestamp);
            }
        } else {
            sequenceSe = 0;
        }

        if (timestamp < lastTimestamp) {
            UUID uuid = UUID.randomUUID();
            return uuid.getMostSignificantBits();
        }

        lastTimestamp = timestamp;
        return timestamp - EPOCH_NUM << TIMESTAMP_LEFTSHIFT | WORKER_ID << WORKER_IDSHIFT | sequenceSe;
    }

    /**
     *
     */
    /**
     * 等待下一个毫秒的到来, 保证返回的毫秒数在参数lastTimestamp之后
     *
     * @param lastTimestamp the last Timestamp
     * @return the long
     */
    private static long tilNextMillis(long lastTimestamp) {
        long timestamp = timeGen();
        while (timestamp <= lastTimestamp) {
            timestamp = timeGen();
        }
        return timestamp;
    }

    /**
     * 获得系统当前毫秒数
     *
     * @return the long
     */
    public static long timeGen() {
        return System.currentTimeMillis();
    }

}