多线程练习（卖票和转账）

买票练习

@Slf4j(topic = "c.ExerciseSell")
public class ExerciseSell {
    public static void main(String[] args) throws InterruptedException {
        // 模拟多人买票
        TicketWindow window = new TicketWindow(1000);

        // 所有线程的集合
        List<Thread> threadList = new ArrayList<>();
        // 卖出的票数统计
        List<Integer> amountList = new Vector<>();
        for (int i = 0; i < 2000; i++) {
            Thread thread = new Thread(() -> {
                // 买票
                int amount = window.sell(random(5));
                // 统计买票数
                amountList.add(amount);
            });
            threadList.add(thread);
            thread.start();
        }

        for (Thread thread : threadList) {
            thread.join();
        }

        // 统计卖出的票数和剩余票数
        log.debug("余票：{}", window.getCount());
        log.debug("卖出的票数：{}", amountList.stream().mapToInt(i -> i).sum());
    }

    // Random 为线程安全
    static Random random = new Random();

    // 随机 1~5
    public static int random(int amount) {
        return random.nextInt(amount) + 1;
    }
}

// 售票窗口
class TicketWindow {
    //此时 count是共享变量
    private int count;

    public TicketWindow(int count) {
        this.count = count;
    }

    // 获取余票数量
    public int getCount() {
        return count;
    }

    // 售票 加上synchronized 同一时刻只有一个线程可以访问此线程
    public synchronized int sell(int amount) {
        if (this.count >= amount) {
            this.count -= amount;
            return amount;
        } else {
            return 0;
        }
    }
}

将上面代码的Vector改为ArrayList，是否会出现线程安全问题？

输出如下

22:09:36.690 c.ExerciseSell [main] - 余票：0
Exception in thread "main" java.lang.NullPointerException
	at cn.itcast.n4.exercise.ExerciseSell.lambda$main$1(ExerciseSell.java:37)
	at java.util.stream.ReferencePipeline$4$1.accept(ReferencePipeline.java:210)
	at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1384)
	at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:482)
	at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:472)
	at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708)
	at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
	at java.util.stream.IntPipeline.reduce(IntPipeline.java:457)
	at java.util.stream.IntPipeline.sum(IntPipeline.java:415)
	at cn.itcast.n4.exercise.ExerciseSell.main(ExerciseSell.java:37)

由于arrayList并不是线程安全的，多个线程调用add方法会导致线程安全问题

Vector是线程安全的集合所以没有此问题

转账练习

public static void main(String[] args) throws InterruptedException {
        //注意有A B两个账户
        Account a = new Account(1000);
        Account b = new Account(1000);
        Thread t1 = new Thread(() -> {
            for (int i = 0; i < 1000; i++) {
                a.transfer(b, randomAmount());
            }
        }, "t1");
        Thread t2 = new Thread(() -> {
            for (int i = 0; i < 1000; i++) {
                b.transfer(a, randomAmount());
            }
        }, "t2");
        t1.start();
        t2.start();
        //等待两个线程执行完毕
        t1.join();
        t2.join();
        // 查看转账2000次后的总金额
        log.debug("total:{}", (a.getMoney() + b.getMoney()));
    }

    // Random 为线程安全
    static Random random = new Random();

    // 随机 1~100
    public static int randomAmount() {
        return random.nextInt(100) + 1;
    }
}

// 账户
class Account {
    //money是共享变量
    // 账户余额
    private int money;

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

    public int getMoney() {
        return money;
    }

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

    // 转账
    public void transfer(Account target, int amount) {
        //由于a,b是两个对象，两个对象都要加锁，所以要加类锁
        synchronized (Account.class) {
            if (this.money >= amount) {
                this.setMoney(this.getMoney() - amount);
                target.setMoney(target.getMoney() + amount);
            }
        }
    }

单个对象使用对象锁，多个对象并发修改的情况下使用类锁