多线程静态变量累加求和 , 计算结果是对的,但是不明白为什么是对的.

Question

package thread.worker;

/**
 * Created by huli3 on 2017/8/16.
 */
public class ExecutorTest {
    static int sum = 0;

    public static void main(String[] args) throws InterruptedException {
        ThreadGroup hl = new ThreadGroup("hl");
        for (int i = 0; i < 10; i++) {
            Thread thread = new Thread(hl, new MyThread(hl));
            thread.start();
        }
        while (hl.activeCount() > 0) {
            Thread.currentThread().yield();
        }
        System.out.println(sum);
    }

    static class MyThread implements Runnable {
        ThreadGroup threadGroup;

        public MyThread(ThreadGroup threadGroup) {
            this.threadGroup = threadGroup;
        }

        public void run() {
            for (int j = 0; j < 4; j++) {
                sum++;
                System.out.println(threadGroup.activeCount() + " sum " + sum + "  and current thread is :" + "id:"+Thread.currentThread().getId());
            }
//            System.out.println(threadGroup.activeCount() + " sum " + sum + "  and current thread is :" + Thread.currentThread().getId());

        }
    }
}

Answer 1

因为你用了volatile 修饰变量啊，所以线程里的每次取值都是新的，但是你的System.out.println()打印比较低效，所以会有看起来乱的现象