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在这段代码中，我使用 10 个线程更新 AtomicInteger 变量。我希望Counter.getInstance().holder.n的最终结果是 1000000，但它会打印出像 991591 这样的随机数。我的代码有什么问题？

public class Test {


    public static void main(String[] args) {

        List<Thread> list = new ArrayList<Thread>();
        for (int i = 0; i < 10; i++) {
            list.add(new Thread() {

                public void run() {
                    for (int i = 0; i < 100000; i++) {
                        Counter.getInstance().holder.n.incrementAndGet();
                    }
                }
            });
        }
        for (Thread thread : list) {
            thread.start();
        }

        try {
            Thread.sleep(10000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        System.out.println(Counter.getInstance().holder.n);


    } 
} 

class Counter {
    private static Counter counter;
    Holder holder = new Holder();


    public static Counter getInstance() {
        if (counter == null) {
            counter = new Counter();
        }
        return counter;
    }
    class Holder {
        AtomicInteger n = new AtomicInteger(0);
    } 
}
4

1 回答 1

2

您在这里有两个主要的并发问题：

  1. 您不必等待每个Thread人都正确完成工作。有多种方法可以实现这一点，最简单的是使用Thread.join().
  2. 您的单例实现似乎不正确。我想你打算用一个内部类来实现它。似乎这个答案可以帮助理解这里发生了什么。

这是似乎或多或少正确的实现。

class Test {
    public static void main(String[] args) throws InterruptedException {

        List<Thread> list = new ArrayList<Thread>();
        for (int i = 0; i < 10; i++) {
            list.add(new Thread() {

                public void run() {
                    for (int i = 0; i < 100000; i++) {
                        Counter.getInstance().n.incrementAndGet();
                    }
                }
            });
        }
        for (Thread thread : list) {
            thread.start();
        }

        for (Thread thread : list) {
            thread.join();
        }

        System.out.println(Counter.getInstance().n);
    }
}

class Counter {
    public AtomicInteger n = new AtomicInteger(0);

    public static Counter getInstance() {
        return Holder.instance;
    }
    private static class Holder {
        private static final Counter instance = new Counter();
    }
}

你也可以使用类似的东西CountDownLatch。例如：

final int count = 10;
CountDownLatch latch = new CountDownLatch(count);
List<Thread> list = new ArrayList<Thread>();
for (int i = 0; i < count; i++) {
    list.add(new Thread() {

        public void run() {
            for (int i = 0; i < 100000; i++) {
                Counter.getInstance().n.incrementAndGet();
            }
            latch.countDown();
        }
    });
}
for (Thread thread : list) {
    thread.start();
}

latch.await();

System.out.println(Counter.getInstance().n);
于 2019-11-29T13:39:44.190 回答