Question

我正在尝试在Java中实现有限缓冲区，以了解有关并发的更多信息。几个线程将使用add（）和remove（）方法访问此缓冲区。我正在使用三个信号量来实现这一点（一个用于数量可利用的项目，一个用于空间可利用的空间，以及用于关键部分的二进制信号量）。当缓冲区已满/空时，我想分别打印“ buffer full”或“ buffer empty”。

在调用信号量中的wait（）之前，我已经将print语句设置为full / empty。但是，这似乎在错误的时间打印了满/空的数量，远远超过了应有的数量。我认为这是在错误的时间进行上下文切换。我应该更改打印声明的位置吗？

我的Semaphore类可以使用prtstmt参数打印完整/空白

public class Semaphore {

  private String prtstmt;
  private int count;

  public semaphore(String prtstmt, int count) {
    this.count = count;
    this.prtstmt = prtstmt;
  }

  public synchronized void V(){
    count = count + 1;
    notify();
  }

  public synchronized void P(){
    count = count - 1;
    if (count < 0) {
      System.out.println(prtstmt);
      wait();
    }
  }

}

有界缓冲区

public class BoundedBufferUsingSemaphore {

private static final int SIZE = 10;

public static void main(String[] args) {

    Semaphore full = new Semaphore(0, "Buffer full!");
    Semaphore empty = new Semaphore(SIZE, "Buffer Empty!");
    Semaphore mutex = new Semaphore(1, null);
    Vector<Integer> sQueue = new Vector<Integer>();

    Thread producerThread = new Thread(new Runnable() {

        @Override
        public void run() {

            for (int i = 0; i < 5000; i++) {
                empty.p();
                mutex.p();
                System.out.println(Thread.currentThread().getName() + " is trying to insert item " + i);
                sQueue.add(i);
                mutex.v();
                full.v();
            }
        }
    });

    Thread consumerThread = new Thread(new Runnable() {

        @Override
        public void run() {
            while (true) {
                full.p();
                mutex.p();
                System.out.println(Thread.currentThread().getName() + " consuming item " + sQueue.remove(0));
                mutex.v();
                empty.v();
            }
        }
    });

    producerThread.setName("Producer");
    consumerThread.setName("Consumer");

    consumerThread.start();
    producerThread.start();

}
}

示例输出如下

Consumer consuming item 5
Buffer empty!
Producer is trying to insert item 6
Producer is trying to insert item 7
Consumer consuming item 6
Producer is trying to insert item 7
Consumer consuming item 6
Buffer empty!
Consumer consuming item 5
Buffer empty!
Producer is trying to insert item 6
Producer is trying to insert item 7
Consumer consuming item 6

Answer 1

本地信号量会执行以下操作！

boolean acquired = semaphore.tryAcquire();
if (acquired) {
    System.out.println("Got it!");
    semaphore.release();
} else {
    System.out.println("Returned immediately, would have blocked");
}

来源：How to know if a semaphore is going to block my process

在调用信号量中的wait（）之前打印

1 个答案: