/**
* Retrieve, but does not remove, the head of this queue, or returns <tt>null</tt> if this queue
* is empty. Unlike <tt>poll</tt>, if no expired elements are available in the queue, this method
* returns the element that will expire next, if one exists.
*
* @return the head of this queue, or <tt>null</tt> if this queue is empty.
*/
@Override
public QueueElement<E> peek() {
lock.lock();
try {
antiStarvation();
QueueElement<E> e = null;
QueueElement<E>[] seeks = new QueueElement[priorities];
boolean foundElement = false;
for (int i = priorities - 1; i > -1; i--) {
e = queues[i].peek();
debug("peek(): considering [{0}] from P[{1}]", e, i);
seeks[priorities - i - 1] = e;
foundElement |= e != null;
}
if (foundElement) {
e = null;
for (int i = 0; e == null && i < priorities; i++) {
if (seeks[i] != null && seeks[i].getDelay(TimeUnit.MILLISECONDS) > 0) {
debug("peek, ignoring [{0}]", seeks[i]);
} else {
e = seeks[i];
}
}
if (e != null) {
debug("peek(): choosing [{0}]", e);
}
if (e == null) {
int first;
for (first = 0; e == null && first < priorities; first++) {
e = seeks[first];
}
if (e != null) {
debug("peek(): initial choosing [{0}]", e);
}
for (int i = first; i < priorities; i++) {
QueueElement<E> ee = seeks[i];
if (ee != null
&& ee.getDelay(TimeUnit.MILLISECONDS) < e.getDelay(TimeUnit.MILLISECONDS)) {
debug("peek(): choosing [{0}] over [{1}]", ee, e);
e = ee;
}
}
}
}
if (e != null) {
debug("peek(): [{0}], from P[{1}]", e.getElement().toString(), e.getPriority());
} else {
debug("peek(): NULL");
}
return e;
} finally {
lock.unlock();
}
}
/**
* Insert the specified {@link QueueElement} element into the queue.
*
* @param queueElement the {@link QueueElement} element to add.
* @param ignoreSize if the queue is bound to a maximum size and the maximum size is reached, this
* parameter (if set to <tt>true</tt>) allows to ignore the maximum size and add the element
* to the queue.
* @return <tt>true</tt> if the element has been inserted, <tt>false</tt> if the element was not
* inserted (the queue has reached its maximum size).
* @throws NullPointerException if the specified element is null
*/
boolean offer(QueueElement<E> queueElement, boolean ignoreSize) {
if (queueElement == null) {
throw new NullPointerException("queueElement is NULL");
}
if (queueElement.getPriority() < 0 || queueElement.getPriority() >= priorities) {
throw new IllegalArgumentException("priority out of range: " + queueElement);
}
if (queueElement.inQueue) {
throw new IllegalStateException("queueElement already in a queue: " + queueElement);
}
if (!ignoreSize && currentSize != null && currentSize.get() >= maxSize) {
return false;
}
boolean accepted;
lock.lock();
try {
accepted = queues[queueElement.getPriority()].offer(queueElement);
debug(
"offer([{0}]), to P[{1}] delay[{2}ms] accepted[{3}]",
queueElement.getElement().toString(),
queueElement.getPriority(),
queueElement.getDelay(TimeUnit.MILLISECONDS),
accepted);
if (accepted) {
if (currentSize != null) {
currentSize.incrementAndGet();
}
queueElement.inQueue = true;
}
} finally {
lock.unlock();
}
return accepted;
}
/**
* Promote elements beyond max wait time from a lower priority sub-queue to a higher priority
* sub-queue.
*
* @param lowerQ lower priority sub-queue.
* @param higherQ higher priority sub-queue.
* @param msg sub-queues msg (from-to) for debugging purposes.
*/
private void antiStarvation(
DelayQueue<QueueElement<E>> lowerQ, DelayQueue<QueueElement<E>> higherQ, String msg) {
int moved = 0;
QueueElement<E> e = lowerQ.poll();
while (e != null && e.getDelay(TimeUnit.MILLISECONDS) < -maxWait) {
e.setDelay(0, TimeUnit.MILLISECONDS);
if (!higherQ.offer(e)) {
throw new IllegalStateException(
"Could not move element to higher sub-queue, element rejected");
}
e.priority++;
e = lowerQ.poll();
moved++;
}
if (e != null) {
if (!lowerQ.offer(e)) {
throw new IllegalStateException(
"Could not reinsert element to current sub-queue, element rejected");
}
}
debug("anti-starvation, moved {0} element(s) {1}", moved, msg);
}