package com.liuqiang.util;
/**
* Twitter_Snowflake<br>
* SnowFlake的結(jié)構(gòu)如下(每部分用-分開(kāi)):<br>
* 0 - 0000000000 0000000000 0000000000 0000000000 0 - 00000 - 00000 - 000000000000 <br>
* 1位標(biāo)識(shí)搂根，由于long基本類(lèi)型在Java中是帶符號(hào)的珍促，最高位是符號(hào)位，正數(shù)是0剩愧，負(fù)數(shù)是1猪叙，所以id一般是正數(shù)，最高位是0<br>
* 41位時(shí)間截(毫秒級(jí))仁卷，注意穴翩，41位時(shí)間截不是存儲(chǔ)當(dāng)前時(shí)間的時(shí)間截，而是存儲(chǔ)時(shí)間截的差值（當(dāng)前時(shí)間截 - 開(kāi)始時(shí)間截)
* 得到的值）锦积，這里的的開(kāi)始時(shí)間截芒帕，一般是我們的id生成器開(kāi)始使用的時(shí)間，由我們程序來(lái)指定的（如下下面程序IdWorker類(lèi)的startTime屬性）丰介。41位的時(shí)間截背蟆，可以使用69年，年T = (1L << 41) / (1000L * 60 * 60 * 24 * 365) = 69<br>
* 10位的數(shù)據(jù)機(jī)器位基矮，可以部署在1024個(gè)節(jié)點(diǎn)淆储，包括5位datacenterId和5位workerId<br>
* 12位序列冠场，毫秒內(nèi)的計(jì)數(shù)家浇，12位的計(jì)數(shù)順序號(hào)支持每個(gè)節(jié)點(diǎn)每毫秒(同一機(jī)器，同一時(shí)間截)產(chǎn)生4096個(gè)ID序號(hào)<br>
* 加起來(lái)剛好64位碴裙，為一個(gè)Long型钢悲。<br>
* SnowFlake的優(yōu)點(diǎn)是点额，整體上按照時(shí)間自增排序，并且整個(gè)分布式系統(tǒng)內(nèi)不會(huì)產(chǎn)生ID碰撞(由數(shù)據(jù)中心ID和機(jī)器ID作區(qū)分)莺琳，并且效率較高还棱，經(jīng)測(cè)試立润，SnowFlake每秒能夠產(chǎn)生26萬(wàn)ID左右缺前。
*/
public class SnowflakeIdWorker {
/**
* 開(kāi)始時(shí)間截 (2015-01-01)
*/
private final long twepoch = 1420041600000L;
/**
* 機(jī)器id所占的位數(shù)
*/
private final long workerIdBits = 5L;
/**
* 數(shù)據(jù)標(biāo)識(shí)id所占的位數(shù)
*/
private final long datacenterIdBits = 5L;
/**
* 支持的最大機(jī)器id封字，結(jié)果是31 (這個(gè)移位算法可以很快的計(jì)算出幾位二進(jìn)制數(shù)所能表示的最大十進(jìn)制數(shù))
*/
private final long maxWorkerId = -1L ^ (-1L << workerIdBits);
/**
* 支持的最大數(shù)據(jù)標(biāo)識(shí)id扭吁，結(jié)果是31
*/
private final long maxDatacenterId = -1L ^ (-1L << datacenterIdBits);
/**
* 序列在id中占的位數(shù)
*/
private final long sequenceBits = 12L;
/**
* 機(jī)器ID向左移12位
*/
private final long workerIdShift = sequenceBits;
/**
* 數(shù)據(jù)標(biāo)識(shí)id向左移17位(12+5)
*/
private final long datacenterIdShift = sequenceBits + workerIdBits;
/**
* 時(shí)間截向左移22位(5+5+12)
*/
private final long timestampLeftShift = sequenceBits + workerIdBits + datacenterIdBits;
/**
* 生成序列的掩碼烫沙，這里為4095 (0b111111111111=0xfff=4095)
*/
private final long sequenceMask = -1L ^ (-1L << sequenceBits);
/**
* 工作機(jī)器ID(0~31)
*/
private long workerId;
/**
* 數(shù)據(jù)中心ID(0~31)
*/
private long datacenterId;
/**
* 毫秒內(nèi)序列(0~4095)
*/
private long sequence = 0L;
/**
* 上次生成ID的時(shí)間截
*/
private long lastTimestamp = -1L;
//==============================Constructors=====================================
/**
* 構(gòu)造函數(shù)
*
* @param workerId 工作ID (0~31)
* @param datacenterId 數(shù)據(jù)中心ID (0~31)
*/
public SnowflakeIdWorker(long workerId, long datacenterId) {
if (workerId > maxWorkerId || workerId < 0) {
throw new IllegalArgumentException(String.format("worker Id can't be greater than %d or less than 0", maxWorkerId));
}
if (datacenterId > maxDatacenterId || datacenterId < 0) {
throw new IllegalArgumentException(String.format("datacenter Id can't be greater than %d or less than 0", maxDatacenterId));
}
this.workerId = workerId;
this.datacenterId = datacenterId;
}
// ==============================Methods==========================================
/**
* 獲得下一個(gè)ID (該方法是線程安全的)
*
* @return SnowflakeId
*/
public synchronized long nextId() {
long timestamp = timeGen();
//如果當(dāng)前時(shí)間小于上一次ID生成的時(shí)間戳厌小，說(shuō)明系統(tǒng)時(shí)鐘回退過(guò)這個(gè)時(shí)候應(yīng)當(dāng)拋出異常
if (timestamp < lastTimestamp) {
throw new RuntimeException(
String.format("Clock moved backwards. Refusing to generate id for %d milliseconds", lastTimestamp - timestamp));
}
//如果是同一時(shí)間生成的衫嵌，則進(jìn)行毫秒內(nèi)序列
if (lastTimestamp == timestamp) {
sequence = (sequence + 1) & sequenceMask;
//毫秒內(nèi)序列溢出
if (sequence == 0) {
//阻塞到下一個(gè)毫秒,獲得新的時(shí)間戳
timestamp = tilNextMillis(lastTimestamp);
}
}
//時(shí)間戳改變牍汹，毫秒內(nèi)序列重置
else {
sequence = 0L;
}
//上次生成ID的時(shí)間截
lastTimestamp = timestamp;
//移位并通過(guò)或運(yùn)算拼到一起組成64位的ID
return ((timestamp - twepoch) << timestampLeftShift) //
| (datacenterId << datacenterIdShift) //
| (workerId << workerIdShift) //
| sequence;
}
/**
* 阻塞到下一個(gè)毫秒秤茅，直到獲得新的時(shí)間戳
*
* @param lastTimestamp 上次生成ID的時(shí)間截
* @return 當(dāng)前時(shí)間戳
*/
protected long tilNextMillis(long lastTimestamp) {
long timestamp = timeGen();
while (timestamp <= lastTimestamp) {
timestamp = timeGen();
}
return timestamp;
}
/**
* 返回以毫秒為單位的當(dāng)前時(shí)間
*
* @return 當(dāng)前時(shí)間(毫秒)
*/
protected long timeGen() {
return System.currentTimeMillis();
}
}
public class SnowIdUtil {
private static SnowflakeIdWorker snowflakeIdWorker=new SnowflakeIdWorker(0,0);
private static long nextId(){
return snowflakeIdWorker.nextId();
}
}
