Question

Consider the following code snipet:

public class A {

    private final Executor executor = Executors.newCachedThreadPool();
    private final Queue<Object> messageQueue = new ConcurrentLinkedQueue<M>();

    public void sendMessage(Object message) {
        messageQueue.offer(message);
        executor.execute(new Runnable() {
            @Override
            public void run() {
                final Object message = messageQueue.poll();

                // Can message == null? 
            }
        });
    }
}

Is it guaranteed that messageQueue contains the message by the time when the Runnable instance will try to retrieve it? Or to phraise it a little bit more general: can two function calls be reordered by JIT/JVM according to JMM?

Answer 1

Yes, if there are not other producers/consumers.

Executor.execute() establish a happens-before relationship. Therefore everything in offer() happens-before poll(). poll() sees the effect of offer(). Although not formally specified, by any common sense, poll() should then return the object just added to the queue.