Java 实现的SnowFlake生成UUID (Java代码实战-007)
SnowFlake所生成的ID一共分成四部分:
1.第一位
占用1bit,其值始终是0,没有实际作用。
2.时间戳
占用41bit,精确到毫秒,总共可以容纳约69 年的时间。
3.工作机器id
占用10bit,其中高位5bit是数据中心ID(datacenterId),低位5bit是工作节点ID(workerId),做多可以容纳1024个节点。
4.序列号
占用12bit,这个值在同一毫秒同一节点上从0开始不断累加,最多可以累加到4095。
SnowFlake算法在同一毫秒内最多可以生成多少个全局唯一ID呢?只需要做一个简单的乘法:
同一毫秒的ID数量 = 1024 X 4096 = 4194304
这个数字在绝大多数并发场景下都是够用的。
import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; /** * Created by xfyou 2018/6/8 14:01. */ public class SnowFlakeIdGenerator { // The initial time(2017-01-01) private static final long INITIAL_TIME_STAMP = 1483200000000L; // Machine ID bits private static final long WORK_ID_BITS = 5L; // DataCenter ID bits private static final long DATACENTER_ID_BITS = 5L; // The maximum machine ID supported is 31, which can quickly calculate the maximum decimal number that a few binary numbers can represent. private static final long MAX_WORKER_ID = ~(-1L << WORK_ID_BITS); // The maximum datacenter ID supported is 31 private static final long MAX_DATACENTER_ID = ~(-1L << DATACENTER_ID_BITS); // Sequence ID bits private static final long SEQUENCE_BITS = 12L; // The machine ID offset,12 private static final long WORKERID_OFFSET = SEQUENCE_BITS; // The datacent ID offset,12+5 private static final long DATACENTERID_OFFSET = WORK_ID_BITS + SEQUENCE_BITS; // The timestape offset, 5+5+12 private static final long TIMESTAMP_OFFSET = DATACENTER_ID_BITS + WORK_ID_BITS + SEQUENCE_BITS; // Sequence mask,4095 private static final long SEQUENCE_MASK = ~(-1L << SEQUENCE_BITS); // Worker ID ,0~31 private long workerId; // Datacenter ID,0~31 private long datacenterId; // Sequence,0~4095 private long sequence = 0L; // Last timestamp private long lastTimestamp = -1L; public SnowFlakeIdGenerator(long workerId, long datacenterId) { if (workerId > MAX_WORKER_ID || workerId < 0) throw new IllegalArgumentException(String.format("WorkerID can't be greater than %d or less than 0", MAX_WORKER_ID)); if (datacenterId > MAX_DATACENTER_ID || datacenterId < 0) throw new IllegalArgumentException(String.format("DataCenterID can't be greater than %d or less than 0", MAX_WORKER_ID)); this.workerId = workerId; this.datacenterId = datacenterId; } public synchronized long nextId() { long timeStamp = System.currentTimeMillis(); if (timeStamp < lastTimestamp) throw new RuntimeException("The current time less than last time"); if (timeStamp == lastTimestamp) { sequence = (sequence + 1) & SEQUENCE_MASK; if (0 == sequence) timeStamp = tillNextMillis(lastTimestamp); } else { sequence = 0L; } lastTimestamp = timeStamp; return (timeStamp - INITIAL_TIME_STAMP) << TIMESTAMP_OFFSET | (datacenterId << DATACENTERID_OFFSET) | (workerId << WORKERID_OFFSET) | sequence; } private long tillNextMillis(long lastTimestamp) { long timestamp = System.currentTimeMillis(); while (timestamp <= lastTimestamp) timestamp = System.currentTimeMillis(); return timestamp; } public static void main(String[] args) { SnowFlakeIdGenerator generator = new SnowFlakeIdGenerator(1, 1); ExecutorService executorService = Executors.newCachedThreadPool(); for (int i = 0; i < 5; i++) { executorService.execute(new Runnable() { @Override public void run() { long id = generator.nextId(); System.out.println(id); } }); } executorService.shutdown(); } }
一个可能的输出为:
189783462515970048 189783462515970049 189783462515970050 189783462515970051 189783462520164352