blocking queue implementation

Question

I've just implemented a custom blocking queue with an semaphore.

for a reason i cant find, my queue isn't getting blocked by the semaphore when my queue is empty.

here's my implementation:

package poolThread;

import java.util.LinkedList;
import java.util.Queue;
import java.util.concurrent.Semaphore;

public class MyQueue<E> {
Semaphore s = new Semaphore(0, true);
private Queue<E> queue = new LinkedList<E>(); 


public boolean isEmpty(){
    return this.queue.isEmpty();
}
public void enqueue(E e){
    queue.add(e);
    s.release();
}
public E dequeue(){
    E e = null;
    try {
        s.acquire();
    } catch (InterruptedException e1) {
        // TODO Auto-generated catch block
        e1.printStackTrace();
    }
    e = queue.remove();
    return e;

}

}

could you help me find the fault in my code?

Answer 1

The problem here is the LinkedList - which isn't thread-safe. So even if the permits are acquired properly, the remove() operation on the LinkedList can (and will) cause troubles. Here's a simple "test case" to show the behavior:

MyQueue<String> x = new MyQueue<>();

ExecutorService es = Executors.newFixedThreadPool(2);
for (int j = 0; j < 2; j++)
    es.submit(() -> {
        String tn = Thread.currentThread().getName();
        for (int i = 0; i < 2; i++)
            x.enqueue("v" + i);

        for (int i = 0; i < 2; i++) 
            System.out.println(tn + " deq: " + x.dequeue());
    });

The output will be something like (you will see null s due to NoSuchElementException s on the remove method):

pool-1-thread-2 deq: v0
pool-1-thread-1 deq: null

The simplest solution for this would be to replace your LinkedList with a java.util.concurrent.ConcurrentLinkedQueue .