线程安全的三大不安全案例以及解决方法

1.不安全的买票：多个线程去抢票，如果不线程同步，那么就会出现问题，具体看代码：

/**
 * @author DB
 * @title: ThreadAnQuan
 * @projectName XianCheng
 * @description: 买票案例
 * @date 2021/11/13  16:53
 */
public class ThreadAnQuan1 {
    public static void main(String[] args) {
        BuyTicket buyTicket = new BuyTicket();
        new Thread(buyTicket, "小明").start();
        new Thread(buyTicket, "小红").start();
        new Thread(buyTicket, "小亮").start();
    }
}

class BuyTicket implements Runnable{
    //票数
    private int ticketNum = 10;
    boolean flag = true;   //外部停止方法，一个标志位

    @Override
    public void run() {
        while (true){
            try {
                buy();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    private void buy() throws InterruptedException {

        //判断是否有票
        if (ticketNum <= 0){
            flag = false;
            return;
        }

        //模拟延时
        Thread.sleep(100);

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


}

结果：

2.不安全的取钱

/**
 * @author DB
 * @title: ThreadAnQuan2
 * @projectName XianCheng
 * @description: TODO
 * @date 2021/11/14  17:33
 */
public class ThreadAnQuan2 {
    public static void main(String[] args) {
        Account account = new Account(100, "结婚基金");
        Drawing you = new Drawing(account,50, "你");
        Drawing girlFriend = new Drawing(account,100, "女朋友");

        you.start();
        girlFriend.start();
    }
}

//帐户
class Account{
    int money;   //帐户余额
    String name; //帐户名称

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

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

    //帐户
    Account account;
    //取了多少钱
    int drawingMoney;
    //手里有多少钱
    int nowMoney;

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

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

        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

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

        //打印帐户还有多少钱
        System.out.println(account.name + "余额为：" + account.money);
        //打印手里有多少钱 this.getName() == Thread.currentThread().getName()
        System.out.println(this.getName() + "手里的钱:" + nowMoney);

    }
}

3.线程不安全的集合

/**
 * @author DB
 * @title: ThreadAnQuan3
 * @projectName XianCheng
 * @description: 线程不安全的集合
 * @date 2021/11/14  18:20
 */
public class ThreadAnQuan3 {
    public static void main(String[] args) throws InterruptedException {
        List<String> list = new ArrayList<String>();
        for (int i = 0; i < 10000; i++) {
            new Thread(() -> {
                list.add(Thread.currentThread().getName());
            }).start();
        }

        Thread.sleep(3000);

        System.out.println(list.size());
    }
}

如何保证前程的安全：同步方法及代码块，使用synchronnized锁，注意：只有需要修改的内容才需要加锁

第一个案例：

第二个案例：

第三个案例：