private void moveNodes() {
boolean remainsActive;
this.lockNodes = true;
QNode node;
Iterator<QNode> simNodes = this.nodesQueue.iterator();
while (simNodes.hasNext()) {
node = simNodes.next();
remainsActive = node.doSimStep(time);
if (!remainsActive) simNodes.remove();
}
this.lockNodes = false;
}
/** Gets rid of cancelled node s with original predecessor pred. */
void clean(QNode pred, QNode s) {
s.waiter = null; // forget thread
/*
* At any given time, exactly one node on list cannot be
* deleted -- the last inserted node. To accommodate this,
* if we cannot delete s, we save its predecessor as
* "cleanMe", deleting the previously saved version
* first. At least one of node s or the node previously
* saved can always be deleted, so this always terminates.
*/
while (pred.next == s) { // Return early if already unlinked
QNode h = head;
QNode hn = h.next; // Absorb cancelled first node as head
if (hn != null && hn.isCancelled()) {
advanceHead(h, hn);
continue;
}
QNode t = tail; // Ensure consistent read for tail
if (t == h) return;
QNode tn = t.next;
if (t != tail) continue;
if (tn != null) {
advanceTail(t, tn);
continue;
}
if (s != t) { // If not tail, try to unsplice
QNode sn = s.next;
if (sn == s || pred.casNext(s, sn)) return;
}
QNode dp = cleanMe;
if (dp != null) { // Try unlinking previous cancelled node
QNode d = dp.next;
QNode dn;
if (d == null
|| // d is gone or
d == dp
|| // d is off list or
!d.isCancelled()
|| // d not cancelled or
(d != t
&& // d not tail and
(dn = d.next) != null
&& // has successor
dn != d
&& // that is on list
dp.casNext(d, dn))) // d unspliced
casCleanMe(dp, null);
if (dp == pred) return; // s is already saved node
} else if (casCleanMe(null, pred)) return; // Postpone cleaning s
}
}
/**
* Spins/blocks until node s is fulfilled.
*
* @param s the waiting node
* @param e the comparison value for checking match
* @param timed true if timed wait
* @param nanos timeout value
* @return matched item, or s if cancelled
*/
Object awaitFulfill(QNode s, Object e, boolean timed, long nanos) {
/* Same idea as TransferStack.awaitFulfill */
long lastTime = timed ? System.nanoTime() : 0;
Thread w = Thread.currentThread();
int spins = ((head.next == s) ? (timed ? maxTimedSpins : maxUntimedSpins) : 0);
for (; ; ) {
if (w.isInterrupted()) s.tryCancel(e);
Object x = s.item;
if (x != e) return x;
if (timed) {
long now = System.nanoTime();
nanos -= now - lastTime;
lastTime = now;
if (nanos <= 0) {
s.tryCancel(e);
continue;
}
}
if (spins > 0) --spins;
else if (s.waiter == null) s.waiter = w;
else if (!timed) LockSupport.park(this);
else if (nanos > spinForTimeoutThreshold) LockSupport.parkNanos(this, nanos);
}
}
/** Puts or takes an item. */
Object transfer(Object e, boolean timed, long nanos) {
/* Basic algorithm is to loop trying to take either of
* two actions:
*
* 1. If queue apparently empty or holding same-mode nodes,
* try to add node to queue of waiters, wait to be
* fulfilled (or cancelled) and return matching item.
*
* 2. If queue apparently contains waiting items, and this
* call is of complementary mode, try to fulfill by CAS'ing
* item field of waiting node and dequeuing it, and then
* returning matching item.
*
* In each case, along the way, check for and try to help
* advance head and tail on behalf of other stalled/slow
* threads.
*
* The loop starts off with a null check guarding against
* seeing uninitialized head or tail values. This never
* happens in current SynchronousQueue, but could if
* callers held non-volatile/final ref to the
* transferer. The check is here anyway because it places
* null checks at top of loop, which is usually faster
* than having them implicitly interspersed.
*/
QNode s = null; // constructed/reused as needed
boolean isData = (e != null);
for (; ; ) {
QNode t = tail;
QNode h = head;
if (t == null || h == null) // saw uninitialized value
continue; // spin
if (h == t || t.isData == isData) { // empty or same-mode
QNode tn = t.next;
if (t != tail) // inconsistent read
continue;
if (tn != null) { // lagging tail
advanceTail(t, tn);
continue;
}
if (timed && nanos <= 0) // can't wait
return null;
if (s == null) s = new QNode(e, isData);
if (!t.casNext(null, s)) // failed to link in
continue;
advanceTail(t, s); // swing tail and wait
Object x = awaitFulfill(s, e, timed, nanos);
if (x == s) { // wait was cancelled
clean(t, s);
return null;
}
if (!s.isOffList()) { // not already unlinked
advanceHead(t, s); // unlink if head
if (x != null) // and forget fields
s.item = s;
s.waiter = null;
}
return (x != null) ? x : e;
} else { // complementary-mode
QNode m = h.next; // node to fulfill
if (t != tail || m == null || h != head) continue; // inconsistent read
Object x = m.item;
if (isData == (x != null)
|| // m already fulfilled
x == m
|| // m cancelled
!m.casItem(x, e)) { // lost CAS
advanceHead(h, m); // dequeue and retry
continue;
}
advanceHead(h, m); // successfully fulfilled
LockSupport.unpark(m.waiter);
return (x != null) ? x : e;
}
}
}
/**
* Tries to cas nh as new head; if successful, unlink old head's next node to avoid garbage
* retention.
*/
void advanceHead(QNode h, QNode nh) {
if (h == head && UNSAFE.compareAndSwapObject(this, headOffset, h, nh))
h.next = h; // forget old next
}