java - 线程与单锁不同步

Question

我无法正确同步该程序，第二个 println 中的结果也应该为 0，因为两个线程各创建和弹出 10000 次。我必须以不同的方式同步吗？

import java.util.*;

public class Main00 {

Queue<Integer> q = new PriorityQueue<Integer>();
Random rand = new Random();

public static void main(String[] args) {
    new Main00().doStuff();
}

public void doStuff(){
    Thread t1=new Thread(new Runnable(){
        public void run(){
            for(int i=0;i<10000;i++)produce();
        }
    });
    Thread t2=new Thread(new Runnable(){
        public void run(){
            for(int i=0;i<10000;i++)consume();
        }
    });

    System.out.println("Starting threads, q size is : "+q.size());
    t1.start();
    t2.start();

    try{
        t1.join();
        t1.join();
    }catch(InterruptedException e){}

    System.out.println("Ending threads, q size is : "+q.size());
}

synchronized public void produce() {
    q.add(rand.nextInt(100));
}

synchronized public void consume() {
    q.poll();
}

}

Answer 1

您没有加入第二个线程:

    t1.join();
    t1.join();

应该是:

    t1.join();
    t2.join();

您还使用 poll不会阻塞:

Retrieves and removes the head of this queue, or returns null if this queue is empty.

您可能想使用PriorityBlockingQueue :

Multiple threads should not access a PriorityQueue instance concurrently if any of the threads modifies the queue. Instead, use the thread-safe PriorityBlockingQueue class.

您可以使用take避免方法busy-wait :

Retrieves and removes the head of this queue, waiting if necessary until an element becomes available.