微信搶紅包簡單實現（隨機分配金額、並發控制）

package test.concurrent;

import java.util.ArrayList;
import java.util.List;
import java.util.Random;
import java.util.concurrent.Semaphore;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.ReentrantLock;

public class QiangHongbao {

    public static class Hongbao {
        private Double money;
        private Integer num;

        public Hongbao(Double money, int num) {
            this.money = money;
            this.num = num;
        }

        public Double getMoney() {
            return money;
        }

        public Integer getNum() {
            return num;
        }

        public void setMoney(Double money) {
            this.money = money;
        }

        public void setNum(Integer num) {
            this.num = num;
        }

    }


    /**
     * 如果紅包金額和數量用cas修改的化，會產生金額和數量不一致的問題
     * 當前的紅包數量可能被其他線程減了1，但是金額沒減
     * 分配算法參考：https://www.zhihu.com/question/22625187
     * @return
     */
    public static synchronized double getRandomMoney(Hongbao hongbao) throws RuntimeException {
        if (hongbao.getNum() <= 0) {
            return 0;
        }
        if (hongbao.getNum() == 1) {
            hongbao.setNum(hongbao.getNum() - 1);
            return (double) Math.round(hongbao.getMoney() * 100) / 100;
        }
        double rest = hongbao.getMoney();
        int restNum = hongbao.getNum();
        Random r = new Random();
        double min = 0.01;
        double max = rest / restNum * 2;
        double imoney = r.nextDouble() * max;
        double get = (double) Math.round((imoney <= min ? min : imoney) * 100) / 100;

        hongbao.setNum(hongbao.getNum() - 1);
        hongbao.setMoney(hongbao.getMoney() - get);

        return get;
    }


    public static void main(String[] args) {
        int threadNum = 100;
        double AllMoney = 200.00;
        int packageNum = 20;
        //200塊，20個包，100個人搶
        Hongbao hongbao = new Hongbao(AllMoney, packageNum);
        List<Double> collect = new ArrayList<>();
        Random r = new Random();
        Semaphore semaphore = new Semaphore(packageNum);
        for (int i = 0; i < threadNum; i++) {
            Thread th = new Thread(() -> {
                try {
                    //模擬網絡延遲
                    Thread.sleep(r.nextInt(10));
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }

                try {
                    long startTime = System.currentTimeMillis();
                    semaphore.tryAcquire(10, TimeUnit.MILLISECONDS);
                    if(r.nextInt(threadNum) == 1){
                        //隨機掛掉一個, 補給其他等待的人
                        throw new RuntimeException();
                    }
　　　　　　　　　　　　//獲取紅包
                    Double iGet = getRandomMoney(hongbao);
                    collect.add(iGet);
                    System.out.println(Thread.currentThread().getName()
                            + " i get money " + iGet + " cost " + (System.currentTimeMillis() - startTime) +"ms");
                } catch (Exception e) {
                    System.out.println(Thread.currentThread().getName() + " get money failed, cause " + e.getMessage());
                } finally {
                   semaphore.release();
                }

            });
            th.start();
        }
　　　　　//下面只是為了確認金額無誤
        try {
            Thread.sleep(1000L);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        double total = collect.stream().mapToDouble(Double::doubleValue).sum();
        System.out.println("total money is " + total);
    }
}


1、我看網上說紅包金額和個數扣減用cas，但是我發現2個屬性不是整體變動，會存在並發問題。我這里用了悲觀鎖。
2、線程控制用了信號量
歡迎留言