java - 从其他线程更新 AtomicInteger

Question

我有一个类，它在各自的线程上创建许多新对象，我想保持跨线程的运行计数。我想要一个 AtomicInteger 但它没有达到我的预期，而只是得到了一个较小的版本。我假设这是一个竞争条件错误 - 但我不完全确定。

A 创建了这个测试示例，它重新创建了我想做的事情。

public class Test {

    public static void main(String args[]) {
        AtomicInteger total = new AtomicInteger(0);
        for (int i = 0; i < 10; i++) {

            DoThing doThing = new DoThing();

            Thread thread = new Thread(doThing);
            thread.start();
            total.addAndGet(doThing.getTally());
        }

        System.out.println(total.get());
    }
}

class DoThing implements Runnable {

    int tally = 0;
    @Override
    public void run() {

        for(int i = 0; i< 100; i++) {
            tally++;
        }

        System.out.println("Tally is " + tally);

    }

    public int getTally() {
        return tally;
    }
}

但是，这会输出:

Tally is 100
Tally is 100
Tally is 100
Tally is 100
Tally is 100
Tally is 100
Tally is 100
Tally is 100
0
Tally is 100
Tally is 100

当我希望最终输出为 1000 时。如何跨线程递增？

提前致谢。

Answer 1

试试这个:

public static void main(String args[]) {
    AtomicInteger tally = new AtomicInteger(0);
    List<Thread> threadList = new ArrayList<Thread>();
    for (int i = 0; i < 10; i++) {
        Thread t = new Thread(new DoThing(tally));
        t.start();
        threadList.add(t);
    }
    for (Thread t : threadList) {
        try { t.join(); } catch (Exception e){}
    }
    System.out.println("Total tally: " + tally.get());
}

public static class DoThing implements Runnable {
    private static final Random rand = new Random();
    private final AtomicInteger tally;

    public DoThing(AtomicInteger tally) {
        this.tally = tally;
    }

    @Override public void run() {
        for (int i = 0; i < 100; i++) {
            int currTally  = tally.incrementAndGet();
            System.out.println("Thread " + Thread.currentThread().getName() + ": " + currTally);
            // Random sleep to show that your threads are properly concurrently incrementing
            try { Thread.sleep(rand.nextInt(10)); } catch (Exception e) {}
        }
    }
}

问题的根源在于您误解了如何使用 AtomicInteger，您将其视为普通的 int 并且根本没有同时访问它.

此外，getTally() 也是一个竞争条件，直到您通过调用 Thread.join() 确保线程已完成为止。

因此，您可以通过让线程中的所有 Runnable 更新相同的 AtomicInteger 实例来保持最新的计数，并且您可以确保您拥有通过在获取计数之前通过 join() 等待所有线程完成计数，可以得到正确的总数。