See {@link AbstractQueuedSynchronizer} for usage * notes and examples. * * @since 1.6 * @author Doug Lea */ public abstract class AbstractQueuedLongSynchronizer extends AbstractOwnableSynchronizer implements java.io.Serializable { private static final long serialVersionUID = 7373984972572414692L; /** * Constructor for subclasses to call. */ public AbstractQueuedLongSynchronizer() {} /* * To keep sources in sync, the remainder of this source file is * exactly cloned from AbstractQueuedSynchronizer, replacing class * name and changing ints related with sync state to longs. Please * keep it that way. */ // Node status bits, also used as argument and return values static final int WAITING = 1; // must be 1 static final int CANCELLED = 0x80000000; // must be negative static final int COND = 2; // in a condition wait /** CLH Nodes */ abstract static class Node { volatile Node prev; // initially attached via casTail volatile Node next; // visibly nonnull when signallable Thread waiter; // visibly nonnull when enqueued volatile int status; // written by owner, atomic bit ops by others // methods for atomic operations final boolean casPrev(Node c, Node v) { // for cleanQueue return U.weakCompareAndSetReference(this, PREV, c, v); } final boolean casNext(Node c, Node v) { // for cleanQueue return U.weakCompareAndSetReference(this, NEXT, c, v); } final int getAndUnsetStatus(int v) { // for signalling return U.getAndBitwiseAndInt(this, STATUS, ~v); } final void setPrevRelaxed(Node p) { // for off-queue assignment U.putReference(this, PREV, p); } final void setStatusRelaxed(int s) { // for off-queue assignment U.putInt(this, STATUS, s); } final void clearStatus() { // for reducing unneeded signals U.putIntOpaque(this, STATUS, 0); } private static final long STATUS = U.objectFieldOffset(Node.class, "status"); private static final long NEXT = U.objectFieldOffset(Node.class, "next"); private static final long PREV = U.objectFieldOffset(Node.class, "prev"); } // Concrete classes tagged by type static final class ExclusiveNode extends Node { } static final class SharedNode extends Node { } static final class ConditionNode extends Node implements ForkJoinPool.ManagedBlocker { ConditionNode nextWaiter; // link to next waiting node /** * Allows Conditions to be used in ForkJoinPools without * risking fixed pool exhaustion. This is usable only for * untimed Condition waits, not timed versions. */ public final boolean isReleasable() { return status <= 1 || Thread.currentThread().isInterrupted(); } public final boolean block() { while (!isReleasable()) LockSupport.park(); return true; } } /** * Head of the wait queue, lazily initialized. */ private transient volatile Node head; /** * Tail of the wait queue. After initialization, modified only via casTail. */ private transient volatile Node tail; /** * The synchronization state. */ private volatile long state; /** * Returns the current value of synchronization state. * This operation has memory semantics of a {@code volatile} read. * @return current state value */ protected final long getState() { return state; } /** * Sets the value of synchronization state. * This operation has memory semantics of a {@code volatile} write. * @param newState the new state value */ protected final void setState(long newState) { state = newState; } /** * Atomically sets synchronization state to the given updated * value if the current state value equals the expected value. * This operation has memory semantics of a {@code volatile} read * and write. * * @param expect the expected value * @param update the new value * @return {@code true} if successful. False return indicates that the actual * value was not equal to the expected value. */ protected final boolean compareAndSetState(long expect, long update) { return U.compareAndSetLong(this, STATE, expect, update); } // Queuing utilities private boolean casTail(Node c, Node v) { return U.compareAndSetReference(this, TAIL, c, v); } /** tries once to CAS a new dummy node for head */ private void tryInitializeHead() { Node h = new ExclusiveNode(); if (U.compareAndSetReference(this, HEAD, null, h)) tail = h; } /** * Enqueues the node unless null. (Currently used only for * ConditionNodes; other cases are interleaved with acquires.) */ final void enqueue(Node node) { if (node != null) { for (;;) { Node t = tail; node.setPrevRelaxed(t); // avoid unnecessary fence if (t == null) // initialize tryInitializeHead(); else if (casTail(t, node)) { t.next = node; if (t.status < 0) // wake up to clean link LockSupport.unpark(node.waiter); break; } } } } /** Returns true if node is found in traversal from tail */ final boolean isEnqueued(Node node) { for (Node t = tail; t != null; t = t.prev) if (t == node) return true; return false; } /** * Wakes up the successor of given node, if one exists, and unsets its * WAITING status to avoid park race. This may fail to wake up an * eligible thread when one or more have been cancelled, but * cancelAcquire ensures liveness. */ private static void signalNext(Node h) { Node s; if (h != null && (s = h.next) != null && s.status != 0) { s.getAndUnsetStatus(WAITING); LockSupport.unpark(s.waiter); } } /** Wakes up the given node if in shared mode */ private static void signalNextIfShared(Node h) { Node s; if (h != null && (s = h.next) != null && (s instanceof SharedNode) && s.status != 0) { s.getAndUnsetStatus(WAITING); LockSupport.unpark(s.waiter); } } /** * Main acquire method, invoked by all exported acquire methods. * * @param node null unless a reacquiring Condition * @param arg the acquire argument * @param shared true if shared mode else exclusive * @param interruptible if abort and return negative on interrupt * @param timed if true use timed waits * @param time if timed, the System.nanoTime value to timeout * @return positive if acquired, 0 if timed out, negative if interrupted */ final int acquire(Node node, long arg, boolean shared, boolean interruptible, boolean timed, long time) { Thread current = Thread.currentThread(); byte spins = 0, postSpins = 0; // retries upon unpark of first thread boolean interrupted = false, first = false; Node pred = null; // predecessor of node when enqueued /* * Repeatedly: * Check if node now first * if so, ensure head stable, else ensure valid predecessor * if node is first or not yet enqueued, try acquiring * else if node not yet created, create it * else if not yet enqueued, try once to enqueue * else if woken from park, retry (up to postSpins times) * else if WAITING status not set, set and retry * else park and clear WAITING status, and check cancellation */ for (;;) { if (!first && (pred = (node == null) ? null : node.prev) != null && !(first = (head == pred))) { if (pred.status < 0) { cleanQueue(); // predecessor cancelled continue; } else if (pred.prev == null) { Thread.onSpinWait(); // ensure serialization continue; } } if (first || pred == null) { boolean acquired; try { if (shared) acquired = (tryAcquireShared(arg) >= 0); else acquired = tryAcquire(arg); } catch (Throwable ex) { cancelAcquire(node, interrupted, false); throw ex; } if (acquired) { if (first) { node.prev = null; head = node; pred.next = null; node.waiter = null; if (shared) signalNextIfShared(node); if (interrupted) current.interrupt(); } return 1; } } if (node == null) { // allocate; retry before enqueue if (shared) node = new SharedNode(); else node = new ExclusiveNode(); } else if (pred == null) { // try to enqueue node.waiter = current; Node t = tail; node.setPrevRelaxed(t); // avoid unnecessary fence if (t == null) tryInitializeHead(); else if (!casTail(t, node)) node.setPrevRelaxed(null); // back out else t.next = node; } else if (first && spins != 0) { --spins; // reduce unfairness on rewaits Thread.onSpinWait(); } else if (node.status == 0) { node.status = WAITING; // enable signal and recheck } else { long nanos; spins = postSpins = (byte)((postSpins << 1) | 1); if (!timed) LockSupport.park(this); else if ((nanos = time - System.nanoTime()) > 0L) LockSupport.parkNanos(this, nanos); else break; node.clearStatus(); if ((interrupted |= Thread.interrupted()) && interruptible) break; } } return cancelAcquire(node, interrupted, interruptible); } /** * Possibly repeatedly traverses from tail, unsplicing cancelled * nodes until none are found. */ private void cleanQueue() { for (;;) { // restart point for (Node q = tail, s = null, p, n;;) { // (p, q, s) triples if (q == null || (p = q.prev) == null) return; // end of list if (s == null ? tail != q : (s.prev != q || s.status < 0)) break; // inconsistent if (q.status < 0) { // cancelled if ((s == null ? casTail(q, p) : s.casPrev(q, p)) && q.prev == p) { p.casNext(q, s); // OK if fails if (p.prev == null) signalNext(p); } break; } if ((n = p.next) != q) { // help finish if (n != null && q.prev == p) { p.casNext(n, q); if (p.prev == null) signalNext(p); } break; } s = q; q = q.prev; } } } /** * Cancels an ongoing attempt to acquire. * * @param node the node (may be null if cancelled before enqueuing) * @param interrupted true if thread interrupted * @param interruptible if should report interruption vs reset */ private int cancelAcquire(Node node, boolean interrupted, boolean interruptible) { if (node != null) { node.waiter = null; node.status = CANCELLED; if (node.prev != null) cleanQueue(); } if (interrupted) { if (interruptible) return CANCELLED; else Thread.currentThread().interrupt(); } return 0; } // Main exported methods /** * Attempts to acquire in exclusive mode. This method should query * if the state of the object permits it to be acquired in the * exclusive mode, and if so to acquire it. * *
This method is always invoked by the thread performing * acquire. If this method reports failure, the acquire method * may queue the thread, if it is not already queued, until it is * signalled by a release from some other thread. This can be used * to implement method {@link Lock#tryLock()}. * *
The default * implementation throws {@link UnsupportedOperationException}. * * @param arg the acquire argument. This value is always the one * passed to an acquire method, or is the value saved on entry * to a condition wait. The value is otherwise uninterpreted * and can represent anything you like. * @return {@code true} if successful. Upon success, this object has * been acquired. * @throws IllegalMonitorStateException if acquiring would place this * synchronizer in an illegal state. This exception must be * thrown in a consistent fashion for synchronization to work * correctly. * @throws UnsupportedOperationException if exclusive mode is not supported */ protected boolean tryAcquire(long arg) { throw new UnsupportedOperationException(); } /** * Attempts to set the state to reflect a release in exclusive * mode. * *
This method is always invoked by the thread performing release. * *
The default implementation throws * {@link UnsupportedOperationException}. * * @param arg the release argument. This value is always the one * passed to a release method, or the current state value upon * entry to a condition wait. The value is otherwise * uninterpreted and can represent anything you like. * @return {@code true} if this object is now in a fully released * state, so that any waiting threads may attempt to acquire; * and {@code false} otherwise. * @throws IllegalMonitorStateException if releasing would place this * synchronizer in an illegal state. This exception must be * thrown in a consistent fashion for synchronization to work * correctly. * @throws UnsupportedOperationException if exclusive mode is not supported */ protected boolean tryRelease(long arg) { throw new UnsupportedOperationException(); } /** * Attempts to acquire in shared mode. This method should query if * the state of the object permits it to be acquired in the shared * mode, and if so to acquire it. * *
This method is always invoked by the thread performing * acquire. If this method reports failure, the acquire method * may queue the thread, if it is not already queued, until it is * signalled by a release from some other thread. * *
The default implementation throws {@link * UnsupportedOperationException}. * * @param arg the acquire argument. This value is always the one * passed to an acquire method, or is the value saved on entry * to a condition wait. The value is otherwise uninterpreted * and can represent anything you like. * @return a negative value on failure; zero if acquisition in shared * mode succeeded but no subsequent shared-mode acquire can * succeed; and a positive value if acquisition in shared * mode succeeded and subsequent shared-mode acquires might * also succeed, in which case a subsequent waiting thread * must check availability. (Support for three different * return values enables this method to be used in contexts * where acquires only sometimes act exclusively.) Upon * success, this object has been acquired. * @throws IllegalMonitorStateException if acquiring would place this * synchronizer in an illegal state. This exception must be * thrown in a consistent fashion for synchronization to work * correctly. * @throws UnsupportedOperationException if shared mode is not supported */ protected long tryAcquireShared(long arg) { throw new UnsupportedOperationException(); } /** * Attempts to set the state to reflect a release in shared mode. * *
This method is always invoked by the thread performing release. * *
The default implementation throws * {@link UnsupportedOperationException}. * * @param arg the release argument. This value is always the one * passed to a release method, or the current state value upon * entry to a condition wait. The value is otherwise * uninterpreted and can represent anything you like. * @return {@code true} if this release of shared mode may permit a * waiting acquire (shared or exclusive) to succeed; and * {@code false} otherwise * @throws IllegalMonitorStateException if releasing would place this * synchronizer in an illegal state. This exception must be * thrown in a consistent fashion for synchronization to work * correctly. * @throws UnsupportedOperationException if shared mode is not supported */ protected boolean tryReleaseShared(long arg) { throw new UnsupportedOperationException(); } /** * Returns {@code true} if synchronization is held exclusively with * respect to the current (calling) thread. This method is invoked * upon each call to a {@link ConditionObject} method. * *
The default implementation throws {@link * UnsupportedOperationException}. This method is invoked * internally only within {@link ConditionObject} methods, so need * not be defined if conditions are not used. * * @return {@code true} if synchronization is held exclusively; * {@code false} otherwise * @throws UnsupportedOperationException if conditions are not supported */ protected boolean isHeldExclusively() { throw new UnsupportedOperationException(); } /** * Acquires in exclusive mode, ignoring interrupts. Implemented * by invoking at least once {@link #tryAcquire}, * returning on success. Otherwise the thread is queued, possibly * repeatedly blocking and unblocking, invoking {@link * #tryAcquire} until success. This method can be used * to implement method {@link Lock#lock}. * * @param arg the acquire argument. This value is conveyed to * {@link #tryAcquire} but is otherwise uninterpreted and * can represent anything you like. */ public final void acquire(long arg) { if (!tryAcquire(arg)) acquire(null, arg, false, false, false, 0L); } /** * Acquires in exclusive mode, aborting if interrupted. * Implemented by first checking interrupt status, then invoking * at least once {@link #tryAcquire}, returning on * success. Otherwise the thread is queued, possibly repeatedly * blocking and unblocking, invoking {@link #tryAcquire} * until success or the thread is interrupted. This method can be * used to implement method {@link Lock#lockInterruptibly}. * * @param arg the acquire argument. This value is conveyed to * {@link #tryAcquire} but is otherwise uninterpreted and * can represent anything you like. * @throws InterruptedException if the current thread is interrupted */ public final void acquireInterruptibly(long arg) throws InterruptedException { if (Thread.interrupted() || (!tryAcquire(arg) && acquire(null, arg, false, true, false, 0L) < 0)) throw new InterruptedException(); } /** * Attempts to acquire in exclusive mode, aborting if interrupted, * and failing if the given timeout elapses. Implemented by first * checking interrupt status, then invoking at least once {@link * #tryAcquire}, returning on success. Otherwise, the thread is * queued, possibly repeatedly blocking and unblocking, invoking * {@link #tryAcquire} until success or the thread is interrupted * or the timeout elapses. This method can be used to implement * method {@link Lock#tryLock(long, TimeUnit)}. * * @param arg the acquire argument. This value is conveyed to * {@link #tryAcquire} but is otherwise uninterpreted and * can represent anything you like. * @param nanosTimeout the maximum number of nanoseconds to wait * @return {@code true} if acquired; {@code false} if timed out * @throws InterruptedException if the current thread is interrupted */ public final boolean tryAcquireNanos(long arg, long nanosTimeout) throws InterruptedException { if (!Thread.interrupted()) { if (tryAcquire(arg)) return true; if (nanosTimeout <= 0L) return false; int stat = acquire(null, arg, false, true, true, System.nanoTime() + nanosTimeout); if (stat > 0) return true; if (stat == 0) return false; } throw new InterruptedException(); } /** * Releases in exclusive mode. Implemented by unblocking one or * more threads if {@link #tryRelease} returns true. * This method can be used to implement method {@link Lock#unlock}. * * @param arg the release argument. This value is conveyed to * {@link #tryRelease} but is otherwise uninterpreted and * can represent anything you like. * @return the value returned from {@link #tryRelease} */ public final boolean release(long arg) { if (tryRelease(arg)) { signalNext(head); return true; } return false; } /** * Acquires in shared mode, ignoring interrupts. Implemented by * first invoking at least once {@link #tryAcquireShared}, * returning on success. Otherwise the thread is queued, possibly * repeatedly blocking and unblocking, invoking {@link * #tryAcquireShared} until success. * * @param arg the acquire argument. This value is conveyed to * {@link #tryAcquireShared} but is otherwise uninterpreted * and can represent anything you like. */ public final void acquireShared(long arg) { if (tryAcquireShared(arg) < 0) acquire(null, arg, true, false, false, 0L); } /** * Acquires in shared mode, aborting if interrupted. Implemented * by first checking interrupt status, then invoking at least once * {@link #tryAcquireShared}, returning on success. Otherwise the * thread is queued, possibly repeatedly blocking and unblocking, * invoking {@link #tryAcquireShared} until success or the thread * is interrupted. * @param arg the acquire argument. * This value is conveyed to {@link #tryAcquireShared} but is * otherwise uninterpreted and can represent anything * you like. * @throws InterruptedException if the current thread is interrupted */ public final void acquireSharedInterruptibly(long arg) throws InterruptedException { if (Thread.interrupted() || (tryAcquireShared(arg) < 0 && acquire(null, arg, true, true, false, 0L) < 0)) throw new InterruptedException(); } /** * Attempts to acquire in shared mode, aborting if interrupted, and * failing if the given timeout elapses. Implemented by first * checking interrupt status, then invoking at least once {@link * #tryAcquireShared}, returning on success. Otherwise, the * thread is queued, possibly repeatedly blocking and unblocking, * invoking {@link #tryAcquireShared} until success or the thread * is interrupted or the timeout elapses. * * @param arg the acquire argument. This value is conveyed to * {@link #tryAcquireShared} but is otherwise uninterpreted * and can represent anything you like. * @param nanosTimeout the maximum number of nanoseconds to wait * @return {@code true} if acquired; {@code false} if timed out * @throws InterruptedException if the current thread is interrupted */ public final boolean tryAcquireSharedNanos(long arg, long nanosTimeout) throws InterruptedException { if (!Thread.interrupted()) { if (tryAcquireShared(arg) >= 0) return true; if (nanosTimeout <= 0L) return false; int stat = acquire(null, arg, true, true, true, System.nanoTime() + nanosTimeout); if (stat > 0) return true; if (stat == 0) return false; } throw new InterruptedException(); } /** * Releases in shared mode. Implemented by unblocking one or more * threads if {@link #tryReleaseShared} returns true. * * @param arg the release argument. This value is conveyed to * {@link #tryReleaseShared} but is otherwise uninterpreted * and can represent anything you like. * @return the value returned from {@link #tryReleaseShared} */ public final boolean releaseShared(long arg) { if (tryReleaseShared(arg)) { signalNext(head); return true; } return false; } // Queue inspection methods /** * Queries whether any threads are waiting to acquire. Note that * because cancellations due to interrupts and timeouts may occur * at any time, a {@code true} return does not guarantee that any * other thread will ever acquire. * * @return {@code true} if there may be other threads waiting to acquire */ public final boolean hasQueuedThreads() { for (Node p = tail, h = head; p != h && p != null; p = p.prev) if (p.status >= 0) return true; return false; } /** * Queries whether any threads have ever contended to acquire this * synchronizer; that is, if an acquire method has ever blocked. * *
In this implementation, this operation returns in * constant time. * * @return {@code true} if there has ever been contention */ public final boolean hasContended() { return head != null; } /** * Returns the first (longest-waiting) thread in the queue, or * {@code null} if no threads are currently queued. * *
In this implementation, this operation normally returns in * constant time, but may iterate upon contention if other threads are * concurrently modifying the queue. * * @return the first (longest-waiting) thread in the queue, or * {@code null} if no threads are currently queued */ public final Thread getFirstQueuedThread() { Thread first = null, w; Node h, s; if ((h = head) != null && ((s = h.next) == null || (first = s.waiter) == null || s.prev == null)) { // traverse from tail on stale reads for (Node p = tail, q; p != null && (q = p.prev) != null; p = q) if ((w = p.waiter) != null) first = w; } return first; } /** * Returns true if the given thread is currently queued. * *
This implementation traverses the queue to determine * presence of the given thread. * * @param thread the thread * @return {@code true} if the given thread is on the queue * @throws NullPointerException if the thread is null */ public final boolean isQueued(Thread thread) { if (thread == null) throw new NullPointerException(); for (Node p = tail; p != null; p = p.prev) if (p.waiter == thread) return true; return false; } /** * Returns {@code true} if the apparent first queued thread, if one * exists, is waiting in exclusive mode. If this method returns * {@code true}, and the current thread is attempting to acquire in * shared mode (that is, this method is invoked from {@link * #tryAcquireShared}) then it is guaranteed that the current thread * is not the first queued thread. Used only as a heuristic in * ReentrantReadWriteLock. */ final boolean apparentlyFirstQueuedIsExclusive() { Node h, s; return (h = head) != null && (s = h.next) != null && !(s instanceof SharedNode) && s.waiter != null; } /** * Queries whether any threads have been waiting to acquire longer * than the current thread. * *
An invocation of this method is equivalent to (but may be * more efficient than): *
{@code
* getFirstQueuedThread() != Thread.currentThread()
* && hasQueuedThreads()}
*
* Note that because cancellations due to interrupts and * timeouts may occur at any time, a {@code true} return does not * guarantee that some other thread will acquire before the current * thread. Likewise, it is possible for another thread to win a * race to enqueue after this method has returned {@code false}, * due to the queue being empty. * *
This method is designed to be used by a fair synchronizer to * avoid barging. * Such a synchronizer's {@link #tryAcquire} method should return * {@code false}, and its {@link #tryAcquireShared} method should * return a negative value, if this method returns {@code true} * (unless this is a reentrant acquire). For example, the {@code * tryAcquire} method for a fair, reentrant, exclusive mode * synchronizer might look like this: * *
{@code
* protected boolean tryAcquire(long arg) {
* if (isHeldExclusively()) {
* // A reentrant acquire; increment hold count
* return true;
* } else if (hasQueuedPredecessors()) {
* return false;
* } else {
* // try to acquire normally
* }
* }}
*
* @return {@code true} if there is a queued thread preceding the
* current thread, and {@code false} if the current thread
* is at the head of the queue or the queue is empty
* @since 1.7
*/
public final boolean hasQueuedPredecessors() {
Thread first = null; Node h, s;
if ((h = head) != null && ((s = h.next) == null ||
(first = s.waiter) == null ||
s.prev == null))
first = getFirstQueuedThread(); // retry via getFirstQueuedThread
return first != null && first != Thread.currentThread();
}
// Instrumentation and monitoring methods
/**
* Returns an estimate of the number of threads waiting to
* acquire. The value is only an estimate because the number of
* threads may change dynamically while this method traverses
* internal data structures. This method is designed for use in
* monitoring system state, not for synchronization control.
*
* @return the estimated number of threads waiting to acquire
*/
public final int getQueueLength() {
int n = 0;
for (Node p = tail; p != null; p = p.prev) {
if (p.waiter != null)
++n;
}
return n;
}
/**
* Returns a collection containing threads that may be waiting to
* acquire. Because the actual set of threads may change
* dynamically while constructing this result, the returned
* collection is only a best-effort estimate. The elements of the
* returned collection are in no particular order. This method is
* designed to facilitate construction of subclasses that provide
* more extensive monitoring facilities.
*
* @return the collection of threads
*/
public final CollectionMethod documentation for this class describes mechanics, * not behavioral specifications from the point of view of Lock * and Condition users. Exported versions of this class will in * general need to be accompanied by documentation describing * condition semantics that rely on those of the associated * {@code AbstractQueuedLongSynchronizer}. * *
This class is Serializable, but all fields are transient, * so deserialized conditions have no waiters. */ public class ConditionObject implements Condition, java.io.Serializable { private static final long serialVersionUID = 1173984872572414699L; /** First node of condition queue. */ private transient ConditionNode firstWaiter; /** Last node of condition queue. */ private transient ConditionNode lastWaiter; /** * Creates a new {@code ConditionObject} instance. */ public ConditionObject() { } // Signalling methods /** * Removes and transfers one or all waiters to sync queue. */ private void doSignal(ConditionNode first, boolean all) { while (first != null) { ConditionNode next = first.nextWaiter; if ((firstWaiter = next) == null) lastWaiter = null; if ((first.getAndUnsetStatus(COND) & COND) != 0) { enqueue(first); if (!all) break; } first = next; } } /** * Moves the longest-waiting thread, if one exists, from the * wait queue for this condition to the wait queue for the * owning lock. * * @throws IllegalMonitorStateException if {@link #isHeldExclusively} * returns {@code false} */ public final void signal() { ConditionNode first = firstWaiter; if (!isHeldExclusively()) throw new IllegalMonitorStateException(); if (first != null) doSignal(first, false); } /** * Moves all threads from the wait queue for this condition to * the wait queue for the owning lock. * * @throws IllegalMonitorStateException if {@link #isHeldExclusively} * returns {@code false} */ public final void signalAll() { ConditionNode first = firstWaiter; if (!isHeldExclusively()) throw new IllegalMonitorStateException(); if (first != null) doSignal(first, true); } // Waiting methods /** * Adds node to condition list and releases lock. * * @param node the node * @return savedState to reacquire after wait */ private long enableWait(ConditionNode node) { if (isHeldExclusively()) { node.waiter = Thread.currentThread(); node.setStatusRelaxed(COND | WAITING); ConditionNode last = lastWaiter; if (last == null) firstWaiter = node; else last.nextWaiter = node; lastWaiter = node; long savedState = getState(); if (release(savedState)) return savedState; } node.status = CANCELLED; // lock not held or inconsistent throw new IllegalMonitorStateException(); } /** * Returns true if a node that was initially placed on a condition * queue is now ready to reacquire on sync queue. * @param node the node * @return true if is reacquiring */ private boolean canReacquire(ConditionNode node) { // check links, not status to avoid enqueue race return node != null && node.prev != null && isEnqueued(node); } /** * Unlinks the given node and other non-waiting nodes from * condition queue unless already unlinked. */ private void unlinkCancelledWaiters(ConditionNode node) { if (node == null || node.nextWaiter != null || node == lastWaiter) { ConditionNode w = firstWaiter, trail = null; while (w != null) { ConditionNode next = w.nextWaiter; if ((w.status & COND) == 0) { w.nextWaiter = null; if (trail == null) firstWaiter = next; else trail.nextWaiter = next; if (next == null) lastWaiter = trail; } else trail = w; w = next; } } } /** * Implements uninterruptible condition wait. *