为什么我的两个简单Java线程没有得到交织访问？

Question

public class X {
    public static void main(String[] args) {
        System.out.println("Creating threads");

        Thread t1 = (new Thread(new MyThread()));
        System.out.println(String.format("Thread t1 id = %d",
            t1.getId()));

        Thread t2 = (new Thread(new MyThread()));
        System.out.println(String.format("Thread t2 id = %d",
            t2.getId()));

        System.out.println("Starting threads");
        t1.start();
        t2.start();
    }
}

class MyThread implements Runnable {    
    @Override
    public void run () {
        for (int i = 0; i <= 5; i++) {
            Resource.access();
        }
        System.out.println(String.format("Thread %d completed",
            Thread.currentThread().getId()));
    }
}

class Resource {
    private static Object monitor = new Object();

    public static void access() {
        synchronized (monitor) {
            System.out.println(String.format(
                "Thread %d accessing resource", 
                Thread.currentThread().getId()));
            try {
                Thread.sleep(1000);
            } catch (InterruptedException ie) {
                ie.printStackTrace();
                System.exit(1);
            }
            System.out.println(String.format(
                "Thread %d leaving resource", 
                Thread.currentThread().getId()));
        }
    } 
}

Answer 1

将sleep语句移到synchronized块之外，以便其他线程有机会访问它。否则，在释放监视器时，您将依赖于一个线程访问该方法的非常小的机会窗口。

另一种方法是让您的睡眠保持原位，但将持续时间从1000毫秒改为100毫秒。然后将你的循环从5增加到5000。

底线是，线程资源竞争并不总是像预期的那样表现出来。线程并不总是协作来演示成功的交互。

class Resource {
    private static Object monitor = new Object();
    
    public static void access() {
        synchronized (monitor) {
            System.out.println(
                    String.format("Thread %d accessing resource",
                            Thread.currentThread().getId()));
            System.out.println(
                    String.format("Thread %d leaving resource",
                            Thread.currentThread().getId()));
        }
        try {
            Thread.sleep(1000);
        } catch (InterruptedException ie) {
            ie.printStackTrace();
            System.exit(1);
        }
    }