线程未与单锁同步

Question

I cannot properly synchronize this program, the result should be 0 in the second println too, because the two threads create and pop 10000 times each. Do i have to synchronize in a different way ?

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

You're not joining the second thread:

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

Should be:

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

You're also using poll which doesn't block:

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

You may want to use 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.

You can utilize the take method to avoid busy-wait :

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

Answer 2

A call to poll does not necessarily consume an element. If there is no element in the queue, it just returns null.

To make sure you effectively consume an element, you would write:

  while(q.poll() == null);

Moreover, the class PriorityQueue is not threadsafe according to http://docs.oracle.com/javase/7/docs/api/java/util/PriorityQueue.html . You shoule use the thread-safe PriorityBlockingQueue class, which has a poll method that blocks with a timeout.