Java-允许一个线程更新值，其他线程等待并跳过关键部分

Question

嗨，我遇到一种情况，我必须只允许一个线程更新变量。

有一个触发器，它可能会调用多个线程来更新此变量，但是更新只能由到达关键部分的第一个线程发生一次。

理想情况下，流程应如下所示:

线程1；调用 Thread-2 和 Thread-3 来更新由锁或互斥锁保护的临界区中的变量

使用此防护的临界区仅允许一个线程进入，Thread-2 和 Thread-3 在外面等待。

一旦该变量被 Thread-1 更新；线程 2 和线程 3 继续执行其他工作，而不会对变量造成影响。

我提出了以下实现，但我无法让其他线程等待并跳过更新:

public class Main {


    private static ReentrantLock lock = new ReentrantLock();
    private int counter = 0;

    public static void main(String[] args) {
        Main m = new Main();

        new Thread(m::doSomeOperation).start();

        new Thread(m::doSomeOperation).start();

        new Thread(m::doSomeOperation).start();


    }


    private void doSomeOperation() {

    try {
        System.out.println("Thread about to acquire lock: " + Thread.currentThread().getName());
        if (lock.tryLock()) {
            System.out.println("Lock held by " + Thread.currentThread().getName() + " " + lock.isHeldByCurrentThread());
            counter++;
            // Thread.sleep(3000);
            System.out.println("Counter value: " + counter + " worked by thread " + Thread.currentThread().getName());
        }

    } catch (Exception ex) {
        ex.printStackTrace();
    } finally {
        if (lock.isHeldByCurrentThread()) {
            lock.unlock();
            System.out.println("Unlocked: " + Thread.currentThread().getName());
        }
    }


}
}

最后，计数器值为 3，我希望计数器值为 1，并且我希望其他线程等待，直到第一个线程更新计数器。想法表示赞赏。我更喜欢使用锁/互斥体的解决方案，而不是等待和通知。

输出:

Thread about to acquire lock: Thread-0
Lock held by Thread-0 true
Thread about to acquire lock: Thread-1
Counter value: 1 worked by thread Thread-0
Unlocked: Thread-0
Thread about to acquire lock: Thread-2
Lock held by Thread-2 true
Counter value: 2 worked by thread Thread-2
Unlocked: Thread-2

Process finished with exit code 0

注意 我的用例是不同的 - 更新计数器的示例是为了简单起见。实际上我正在 doSomeOperation 方法中更新 session token 。

Answer 1

您可以在这里使用 AtomicInteger 并放弃担心形式锁:

public class Worker {
    private static AtomicInteger counter = new AtomicInteger(0);

    private void doSomeOperation() {
        counter.incrementAndGet();
        System.out.println("Counter value: " + counter + " worked by thread " + Thread.currentThread().getName());
    }
}

public class Main {
    public static void main(String[] args) {
        Worker w = new Worker();
        new Thread(w::doSomeOperation).start();
        new Thread(w::doSomeOperation).start();
        new Thread(w::doSomeOperation).start();
    }
}