java - 多线程java中的一进一出要求与统一服务员要求

Question

在解释多线程中的有界缓冲区示例时，以下术语是什么:

1.一进一出要求

2.服务员统一要求

我在书中看到以下说法java concurrency in practice

BoundedBuffer meets the one-in, one-out requirement, but does not meet the uniform waiters requirement because waiting threads might be waiting for either the “not full” and “not empty” condition.

有人可以向我解释一下这两个术语及其区别吗？

谢谢

Answer 1

这些术语指的是在任意监视器上 wait() 的线程:

• 统一等待者:WAITING线程是“平等的”，即它们等待相同的条件变为真。
• 一进一出:有关条件的通知最多允许一个线程继续。

只有在满足两个属性时才应使用 notify()，否则调用 notifyAll()(尽管效率较低)。

Goetz 在第 303 页对此进行了解释，并给出了两个具体示例:

BoundedBuffer meets the one-in, one-out requirement, but does not meet the uniform waiters requirement because waiting threads might be waiting for either the "not full" [producers block on put] and "not empty" [consumers block on take] condition". A "starting gate" latch […] in which a single event releases a set of threads, does not meet the one-in, one-out requirement because opening the starting gate lets multiple threads proceed.

您还可以使用 java.util.concurrent 中的类来演示这一点:

BlockingQueue<String> queue = new ArrayBlockingQueue<>(1);

new Thread(() -> {
    try {
        TimeUnit.SECONDS.sleep(1L);
        System.out.println("Producer starts.");
        queue.put("foo");
    } catch (InterruptedException e) { /* NOP */ }
}).start();

try {
    System.out.println("Consumer starts.");
    System.out.println(queue.take());
} catch (InterruptedException e) { /* NOP */ }

消费者必须等待生产者将“foo”放入队列。两者等待不同的条件，因此， BlockingQueue不符合服务员统一要求。

另一方面，CountDownLatch满足服务员统一要求，但不满足一进一出要求。一旦锁存器达到零，所有等待线程都会被释放:

CountDownLatch latch = new CountDownLatch(1);
Runnable r = () -> {
    try {
        latch.await();
        System.out.println("Thread proceeds.");
    } catch (InterruptedException e) { /* NOP */ }
};

new Thread(r).start();
new Thread(r).start();

try {
    TimeUnit.SECONDS.sleep(1L);
    System.out.println("Count down.");
    latch.countDown();
} catch (InterruptedException e) { /* NOP */ }