Question

我正在刷我的多线程基础知识。要了解我在以下程序中创建的同步 -

package thread;

public class SynchronizedCounter implements Runnable{

    private static int counter = 0;

    public void run() {

        while(counter < 10) {
            synchronized (SynchronizedCounter.class) {
                System.out.println(Thread.currentThread().getName() + "  :: reads ## " + counter);
                counter++;
                System.out.println(Thread.currentThread().getName() + "  :: updated value ## " + counter);
            }

        }   
    }

    public static void main(String[] args) {

        Thread[] threads = new Thread[5];

        for(int i = 0; i < threads.length;  i++) {
            threads[i] = new Thread(new SynchronizedCounter(), "Thread-" + i);
            threads[i].start();
        }

        for(int i = 0; i < threads.length;  i++) {
            try {
                threads[i].join();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

现在当我执行时，得到了这个结果：

Thread-0  :: reads --> 0
Thread-0  :: updated value --> 1
Thread-3  :: reads --> 1
Thread-3  :: updated value --> 2
Thread-0  :: reads --> 2
Thread-0  :: updated value --> 3
Thread-0  :: reads --> 3
Thread-0  :: updated value --> 4
Thread-0  :: reads --> 4
Thread-0  :: updated value --> 5
Thread-0  :: reads --> 5
Thread-0  :: updated value --> 6
Thread-0  :: reads --> 6
Thread-0  :: updated value --> 7
Thread-0  :: reads --> 7
Thread-0  :: updated value --> 8
Thread-0  :: reads --> 8
Thread-0  :: updated value --> 9
Thread-0  :: reads --> 9
Thread-0  :: updated value --> 10
Thread-1  :: reads --> 10
Thread-1  :: updated value --> 11
Thread-2  :: reads --> 11
Thread-2  :: updated value --> 12
Thread-4  :: reads --> 12
Thread-4  :: updated value --> 13
Thread-3  :: reads --> 13
Thread-3  :: updated value --> 14

一切看起来都不错，但是我无法理解为什么“计数器”的值超过10。虽然循环传递虽然值是13或14超过10.我理解当控制试图进入时它可能读取不同的值，当它到达第一次打印时，即“:: reads - ＆gt;”。但理想情况下，任何线程都应该将此值增加到10.请帮助我了解我缺少的位置。

谢谢!!

Answer 1

因为synchronized（SynchronizedCounter.class）在while循环中。想象计数器命中9.所有线程仍然可以进入while块，然后一个将运行，将计数器递增到10.它将退出synchronized块，允许另一个线程运行，将计数器递增到11.这样每个都继续线程，所以计数器将命中14.然后下一次通过while循环，计数器＆lt; 10测试将失败，他们将全部从run（）返回，从而退出。

我猜另一个计数器＆lt;同步块内的10个测试将修复它。

同步奇怪的行为

1 个答案: