How can I atomically "enqueue if free space OR dequeue then enqueue" for a Java queue / list?

Question

I've got a requirement for a list in Java with a fixed capacity but which always allows threads to add items to the start. If it's full it should remove an item from the end to make space. No other process will remove items, but other processes will wish to iterate over the items.

Is there something in the JDK which would allow me to do this atomically?

My current plan is just to use some existing threadsafe Collection (e.g. LinkedBlockingQueue) and further synchronise on it when I check capacity / add / remove. Would that work as well?

Thanks.

Answer 1

Your idea would work but would involve taking out multiple locks (see example below). Given you need to synchronize multiple operations when adding data you may as well wrap a LinkedList implementation of a Queue to avoid the overhead of additional locks.

// Create queue with fixed capacity.
Queue<Item> queue = new LinkedBlockingQueue<Item>(1000);

...

// Attempt to add item to queue, removing items if required.
synchronized(queue) { // First lock
  while (!queue.offer(item)) { // Second lock
    queue.take(); // Third lock
  }
}