生成4位隨機數
阿新 • • 發佈:2021-06-11
本來是用隨機演算法的,但是會有重複,花了點時間研究了下,發現雪花演算法好似不會重複,
程式碼如下:
package com.excel.test; import java.util.concurrent.atomic.AtomicInteger; public class MinuteCounter { private static final int MASK = 0x7FFFFFFF; private final AtomicInteger atom; public MinuteCounter() { atom = new AtomicInteger(0); }public final int incrementAndGet() { return atom.incrementAndGet() & MASK; } public int get() { return atom.get() & MASK; } public void set(int newValue) { atom.set(newValue & MASK); } }
package com.excel.test; /** * 此方式只適用於一個單體應用,不適合分散式系統 * * 將產生的Id型別更改為Integer 32bit <br> * 把時間戳的單位改為分鐘,使用25個位元的時間戳(分鐘) <br> * 去掉機器ID和資料中心ID <br> * 7個位元作為自增值,即2的7次方等於128。 * */ public class SnowflakeIdWorker3rd { /** 開始時間戳 (2019-01-01) */ private final int twepoch = 25771200;// 1546272000000L/1000/60; /** 序列在id中佔的位數 */ private final long sequenceBits = 7L; /** 時間截向左移7位 */ private final long timestampLeftShift = sequenceBits; /** 生成序列的掩碼,這裡為127 */ private final int sequenceMask = -1 ^ (-1 << sequenceBits); /** 分鐘內序列(0~127) */ private int sequence = 0; private int laterSequence = 0; /** 上次生成ID的時間戳 */ private int lastTimestamp = -1; private final MinuteCounter counter = new MinuteCounter(); /** 預支時間標誌位 */ boolean isAdvance = false; // ==============================Constructors===================================== public SnowflakeIdWorker3rd() { } // ==============================Methods========================================== /** * 獲得下一個ID (該方法是執行緒安全的) * * @return SnowflakeId */ public synchronized int nextId() { int timestamp = timeGen(); // 如果當前時間小於上一次ID生成的時間戳,說明系統時鐘回退過這個時候應當丟擲異常 if (timestamp < lastTimestamp) { throw new RuntimeException(String.format( "Clock moved backwards. Refusing to generate id for %d milliseconds", lastTimestamp - timestamp)); } if(timestamp > counter.get()) { counter.set(timestamp); isAdvance = false; } // 如果是同一時間生成的,則進行分鐘內序列 if (lastTimestamp == timestamp || isAdvance) { if(!isAdvance) { sequence = (sequence + 1) & sequenceMask; } // 分鐘內自增列溢位 if (sequence == 0) { // 預支下一個分鐘,獲得新的時間戳 isAdvance = true; int laterTimestamp = counter.get(); if (laterSequence == 0) { laterTimestamp = counter.incrementAndGet(); } int nextId = ((laterTimestamp - twepoch) << timestampLeftShift) // | laterSequence; laterSequence = (laterSequence + 1) & sequenceMask; return nextId; } } // 時間戳改變,分鐘內序列重置 else { sequence = 0; laterSequence = 0; } // 上次生成ID的時間截 lastTimestamp = timestamp; // 移位並通過或運算拼到一起組成32位的ID return ((timestamp - twepoch) << timestampLeftShift) // | sequence; } /** * 返回以分鐘為單位的當前時間 * * @return 當前時間(分鐘) */ protected int timeGen() { String timestamp = String.valueOf(System.currentTimeMillis() / 1000 / 60); return Integer.valueOf(timestamp); } }
測試類
package com.excel.test; import java.util.HashSet; import java.util.Set; public class Main { public static void main(String[] args) throws InterruptedException { SnowflakeIdWorker3rd worker3rd = new SnowflakeIdWorker3rd(); Set<Integer> set = new HashSet<>();for (int j = 0; j < 50; j++) { for (int i = 0; i < 100; i++) { int l = worker3rd.nextId();//% 10000; Thread.sleep(10); set.add(l); } System.out.println(set.size()); } } }