多线程的三大不安全案例

1.买票案例

多个线程同时执行一个程序时，会存在重复执行的情况

 

package com.syn;

//不安全的买票
public class UnsafeBuyTicket {
    public static void main(String[] args) {
        BuyTicket buyTicket = new BuyTicket();

        new Thread(buyTicket,"叶子").start();
        new Thread(buyTicket,"双笙").start();
        new Thread(buyTicket,"黄牛").start();
    }
}

class BuyTicket implements Runnable{
    //票
    private int ticketNums = 10;
    boolean flag = true;  //外部停止方式

    @Override
    public void run() {
        //买票
        while (flag){
            try {
                buy();
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }
        }
    }

    private void buy() throws InterruptedException {
        //判断是否有票
        if (ticketNums<=0){
            flag = false;
            return;
        }

        Thread.sleep(10);

        //买票
        System.out.println(Thread.currentThread().getName()+"拿到"+ticketNums--);
    }
}

 

 

2.银行取钱案例
当模拟两个或者以上人同时去进行取钱操作时，会导致取出钱的总数多于账户的总余额，真实情况中不能存在

 

 

package com.syn;

public class UnsafeBank {
    public static void main(String[] args) {
        Account account = new Account(100,"钱的");

        Drawing you = new Drawing(account,50,"你");
        Drawing ss = new Drawing(account,100,"ss");

        you.start();
        ss.start();
    }
}

class Account{
    int money;  //余额
    String name;  //卡名

    public Account(int money, String name) {
        this.money = money;
        this.name = name;
    }
}

//银行：模拟取款
class Drawing extends Thread{

    Account account;//账户
    //取了多少钱
    int drawingMoney;
    //现在手里有多少钱
    int nowMoney;

    public Drawing(Account account,int drawingMoney,String name){
        super(name);
        this.account = account;
        this.drawingMoney = drawingMoney;
    }

    @Override
    public void run() {
        //判断有没有钱
        if (account.money-drawingMoney<0){
            System.out.println(Thread.currentThread().getName()+"钱不够，取不了");
            return;
        }

        try {
            Thread.sleep(5000);
        } catch (InterruptedException e) {
            throw new RuntimeException(e);
        }

        //卡内余额 = 余额 - 你取的钱
        account.money = account.money - drawingMoney;
        //你手里的钱
        nowMoney = nowMoney + drawingMoney;

        System.out.println(account.name+"余额为："+account.money);
        //Thread.currentThread().getName() = this.getName()
        System.out.println(this.getName()+"手里的钱："+nowMoney);

    }
}

 

3.线程集合案例

虽然开启了10000个线程往ArrayList里加数据，但有可能出现：某两个线程往ArrayList添加数据的时候，添加在了ArrayList的同一位置（ 比如ArrayList[5666] ），这样ArrayList的大小自然就不足10000了

 

package com.syn;

import java.util.ArrayList;
import java.util.List;

public class UnsafeList {
    public static void main(String[] args) throws InterruptedException {
        List<String> list = new ArrayList<String>();
        for (int i = 0; i < 100000; i++) {
            new Thread(()->{
                list.add(Thread.currentThread().getName());
            }).start();
        }
        System.out.println(list.size());
    }
}