The scene of students choosing courses is similar to that of buying train tickets. Baidu summed up four ways

Based on Redis

import java.util.List;

import java.util.Random;

import java.util.concurrent.ExecutorService;

import java.util.concurrent.Executors;

import redis.clients.jedis.Jedis;
import redis.clients.jedis.Transaction;

public class MyRunnable implements Runnable {
    String watchkeys = "total"; //Monitoring keys
    int total; //Total items
    Jedis redis = new Jedis("127.0.0.1");
    String userinfo;

    public MyRunnable() {
    }


    public MyRunnable(String userinfo, int count) {
        this.userinfo = userinfo;
        this.total = count;
    }

    public static void main(String[] args) {
        int total = 50;
        final Jedis redis = new Jedis("127.0.0.1");
        redis.set("total", String.valueOf(total)); //Set the total number of items
        //redis.del("setsucc", "setfail");
        redis.close();
        //Create users, set the total number of items, and then start to rush
        ExecutorService executor = Executors.newFixedThreadPool(20); //Maximum concurrency (⊙ o ⊙)
        for (int i = 0; i < 1000; i++) { //Number of people to be snapped up
            executor.execute(new MyRunnable("user" + getRandomString(6),
                                            total));
        }
        executor.shutdown();
    }

    @Override
    public void run() {
        redis.watch(watchkeys);
        int num = Integer.valueOf(redis.get(watchkeys));
        if (num >= 1 && num <= total) {
            Transaction tx = redis.multi(); //Open transaction
            tx.incrBy(watchkeys, -1); //Increment to - 1
            List list = tx.exec(); //Commit the transaction and return null if the watchkeys are changed at this time
            if (list == null || list.size() == 0) {
                String failUser = "fail" + userinfo;
                String failInfo = "User:" + failUser + "Failed to rush";
                System.out.println(failUser);
                redis.setnx(failUser, failInfo); //Business logic of failed flash purchase
            } else {
                for (Object obj : list) {
                    String succUser = "succ" + obj.toString() + userinfo;
                    String succInfo =
                        "User:" + succUser + "The number of people who have been snapped up is now successful" + (1 - (num - total));
                    System.out.println(succInfo);
                    redis.setnx(succUser, succInfo); //Business logic of successful flash purchase
                }
            }
        } else {
            String failUser = "kcfail" + userinfo;
            String failInfo = "User:" + failUser + "Product rush failed";
            System.out.println(failUser);
            redis.setnx(failUser, failInfo); //Business logic of failed flash purchase
            return;
        }
        redis.close();
    }

    private static String getRandomString(int length) {
        String base = "abcdefghijklmnopqrstuvwxyz0123456789";
        Random random = new Random();
        StringBuffer sb = new StringBuffer();
        for (int i = 0; i < length; i++) {
            int number = random.nextInt(base.length());
            sb.append(base.charAt(number));
        }
        return sb.toString();
    }
}