java - 为什么同步在此代码中无法正常工作？

Question

在过去的几天里，我一直在阅读有关多线程的文章，并且遇到了一个使用多线程的简单任务。这是任务:

创建一个模拟 50 米运行比赛的应用程序(在我的代码中它们是 10，无所谓)。运行者的数量应该是 5，你应该为每个运行者线程命名。打印获胜者。所有其他线程也应该完成比赛。打印出每位参赛者完成比赛所用的时间，并突出显示获胜者的时间。

这是我写的代码:

public class Racer implements Runnable {
    public static String winner;
    public static int time = 0;

    public void incrementTime() {
        synchronized (Racer.class) {
            time++;
        }
    }
    public void race() {
        for (int distance = 1; distance <= 10; distance++) {
            incrementTime();
            System.out.println("Distance covered by " + Thread.currentThread().getName() + " is " + distance + " meters.");
            boolean finalDest = this.isTheRaceOver(distance);
            if (finalDest) {
                break;
            }
        }
    }
    private boolean isTheRaceOver(int finalDistance) {
        boolean isRaceOver = false;
        if (Racer.winner == null && finalDistance == 10) {
            String winnerName = Thread.currentThread().getName();
            Racer.winner = winnerName;
            System.out.println("The winner is : " + Racer.winner + " with time " + time);
            isRaceOver = true;
        } else if (Racer.winner == null) {
            isRaceOver = false;
        } else if (finalDistance != 10) {
            isRaceOver = false;
        } else if (finalDistance == 10) {
            System.out.println(Thread.currentThread().getName() + " is with time " + time);
        }
        return isRaceOver;
    }
    @Override
    public void run() {
        this.race();
    }
}

public class RacerDemo {
    public static void main(String[] args) {
        Racer racer = new Racer();
        Thread a = new Thread(racer, "A");
        Thread b = new Thread(racer, "B");
        Thread c = new Thread(racer, "C");
        Thread d = new Thread(racer, "D");
        Thread e = new Thread(racer, "E");
        a.start();
        b.start();
        c.start();
        d.start();
        e.start();
    }
}

一个输出是:

Distance covered by A is 1 meters. Distance covered by C is 1 meters. Distance covered by C is 2 meters. Distance covered by C is 3 meters.Distance covered by C is 4 meters.Distance covered by C is 5 meters.Distance covered by C is 6 meters.Distance covered by C is 7 meters.Distance covered by C is 8 meters.Distance covered by C is 9 meters.Distance covered by C is 10 meters.The winner is : C with time 12  // should be 11 ?Distance covered by B is 1 meters.Distance covered by B is 2 meters. ...... and so on

令我困扰的是，当它打印出每个赛车手(线程)跑完一段距离所花费的时间时，它并没有显示正确的时间。我使 incrementTime() 同步，但程序也不能正常工作。你能告诉我哪里出了问题吗？我的错误在哪里？

Answer 1

每个运行者都会增加时间，导致不一致的状态。您应该将赛车手与实际比赛分开，这可能应该在一个单独的类别中。

您的问题出现在 Racer Runnable 的 race 方法中，其中每个运行者都会增加 static time 字段，从而导致意外行为。