twitter ID生成算法

分布式系统中，有一些需要使用全局唯一ID的场景，这种时候为了防止ID冲突可以使用36位的UUID，但是UUID有一些缺点，首先他相对比较长，另外UUID一般是无序的。

为了满足Twitter每秒上万条消息的请求，每条消息都必须分配一条唯一的id，这些id还需要一些大致的顺序（方便客户端排序），并且在分布式系统中不同机器产生的id必须不同。

public class IdSeqGenerator {    private final static long twepoch            = 1463108596098L;             //日期起始点    private final long        workerId;    private long              sequence           = 0L;    private long              lastTimestamp      = -1L;    private final static long workerIdBits       = 10L;                        //机器ID占用10bits    private final static long sequenceBits       = 12L;                        //序列占用12bits    public final static long  maxWorkerId        = -1L ^ -1L << workerIdBits;  //机器ID 最大值    private final static long timestampLeftShift = sequenceBits + workerIdBits;//时间偏移位    private final static long workerIdShift      = sequenceBits;               //机器ID偏移位    public final static long  sequenceMask       = -1L ^ -1L << sequenceBits;  //序列掩码    public IdSeqGenerator(final long workerId) {        super();        if (workerId > maxWorkerId || workerId < 0) {            throw new IllegalArgumentException(String.format("worker Id can't be greater than %d or less than 0", maxWorkerId));        }        LogKit.log().info(String.format("worker Id : greater than %d or less than 0", maxWorkerId));        this.workerId = workerId;    }    //生成ID    public synchronized long nextId() {        long timestamp = this.timeGen();        //如果是同一时间生成的，则自增        if (this.lastTimestamp == timestamp) {            this.sequence = (this.sequence + 1) & sequenceMask;            if (this.sequence == 0) {                //生成下一个毫秒级的序列                timestamp = this.tilNextMillis(this.lastTimestamp);            }        } else {            //如果发现是下一个时间单位，则自增序列回0，重新自增             this.sequence = 0;        }        if (timestamp < this.lastTimestamp) {            try {                throw new Exception(String.format("Clock moved backwards.  Refusing to generate id for %d milliseconds", this.lastTimestamp - timestamp));            } catch (Exception e) {                e.printStackTrace();            }        }        this.lastTimestamp = timestamp;        long nextId = ((timestamp - twepoch << timestampLeftShift)) | (this.workerId << workerIdShift) | (this.sequence);        return nextId;    }    private long tilNextMillis(final long lastTimestamp) {        long timestamp = this.timeGen();        while (timestamp <= lastTimestamp) {            timestamp = this.timeGen();        }        return timestamp;    }    private long timeGen() {        return System.currentTimeMillis();    }    public static void main(String[] args) {        Map<Long, Long> store = new ConcurrentHashMap<>();        final int MAX_POOL_SIZE = 4;        ExecutorService service = Executors.newFixedThreadPool(MAX_POOL_SIZE);        for (int i = 0; i < MAX_POOL_SIZE; i++) {            final int x = i;            service.execute(() -> {                IdSeqGenerator seqKit = new IdSeqGenerator(x);                for (int j = 0; j < 100; j++) {                    long id = seqKit.nextId();                    if (store.containsKey(id)) {                        System.out.println("dublicate:" + id);                    } else {                        store.put(id, id);                        System.out.println(id);                    }                }                System.out.println("thread" + x + " end.");            });        }        service.shutdown();    }}