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在多个线程中更新AtomicInteger变量时未获得预期结果

atomicinteger thread-safety concurrency multithreading java

zonyang · 技术社区 · 6 年前

在这段代码中,我使用10个线程更新一个AtomicInteger变量。我希望最后的结果 Counter.getInstance().holder.n 是1000000,但它会打印出像991591这样的随机数。我的密码怎么了?

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);
    } 
}

0 回复 | 直到 6 年前

Vladimir Pligin 6 年前

这里有两个主要的并发问题:

你不会等每一个 Thread 正确完成工作。实现这一目标有多种方法,最简单的是 Thread.join() .
你的单例实现似乎不正确。我想您打算用一个内部类来实现它。看来这个 answer 有助于了解这里发生了什么。

下面是看起来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();
    }
}

你可以用一些像 CountDownLatch 也。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);