Java 互斥体实现

Question

我正在用Java实现Mutex，代码如下所示，我不知道为什么结果与我的预期不符， 正如您在我显示的结果中看到的，它的运行是不可预测的

    enum MutexStatus {
    FREE, BUSY
};

enum FunctionTypes {
    INCREASE, DECREASE
};

class Mutex {
    private MutexStatus mutexStatus;

    public Mutex() {
        mutexStatus = MutexStatus.FREE;
    }

    public void acquire() {
        synchronized (this) {
            while (mutexStatus == MutexStatus.BUSY)
                ;
            mutexStatus = MutexStatus.BUSY;
        }
    }

    public void release() {
        mutexStatus = MutexStatus.FREE;
    }
}

class MyThread extends Thread {
    private static Mutex mutex = new Mutex();
    private static Integer sharedCounter = 0;
    private FunctionTypes funcType;

    public MyThread(String name, FunctionTypes functionType) {
        super.setName(name);
        funcType = functionType;
    }

    @Override
    public void run() {
        for (int i = 0; i < 100; i++) {
            mutex.acquire();
            switch (funcType) {
            case INCREASE:
                sharedCounter++;
                System.out.println(Thread.currentThread().getName() + " -> "
                        + sharedCounter);
                break;

            case DECREASE:
                sharedCounter--;
                System.out.println(Thread.currentThread().getName() + " -> "
                        + sharedCounter);
                break;
            default:
                break;
            }
            mutex.release();
        }
    }

}

public class MutexImplement {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        MyThread IncreaseThread = new MyThread("Increase thread",
                FunctionTypes.INCREASE);
        IncreaseThread.start();

        MyThread DecreaseThread = new MyThread("Decrease thread",
                FunctionTypes.DECREASE);
        DecreaseThread.start();
    }

}

<小时/>

结果是

>Increase thread -> 1
>Decrease thread -> 0
>Increase thread -> 1
>Decrease thread -> 0
>Decrease thread -> 1
>Increase thread -> 1
>...........

Answer 1

看看 http://docs.oracle.com/javase/specs/jls/se7/html/jls-17.html 中的 17.4 。线程将拥有它们自己共享(或实际上不共享)的变量的本地副本。正如已经建议的那样，使共享变量不稳定或使用一些同步/并发控制。您可能还想看看自旋循环(while 循环)并使用一些信号。

现在 Java 有很多构造可以为您完成这一切，尽管我怀疑您已被要求自己创建互斥体。为了记录，请尝试避免使用自己的自制软件并尝试使用提供的并发结构。