In this implementation, this operation returns in + * constant time. + * + * @return {@code true} if there may be other threads waiting to acquire + */ + public final boolean hasQueuedThreads() { + return head != tail; + } + + /** + * 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() { + // handle only fast path, else relay + return (head == tail) ? null : fullGetFirstQueuedThread(); + } + + /** + * Version of getFirstQueuedThread called when fastpath fails + */ + private Thread fullGetFirstQueuedThread() { + /* + * The first node is normally head.next. Try to get its + * thread field, ensuring consistent reads: If thread + * field is nulled out or s.prev is no longer head, then + * some other thread(s) concurrently performed setHead in + * between some of our reads. We try this twice before + * resorting to traversal. + */ + Node h, s; + Thread st; + if (((h = head) != null && (s = h.next) != null && + s.prev == head && (st = s.thread) != null) || + ((h = head) != null && (s = h.next) != null && + s.prev == head && (st = s.thread) != null)) + return st; + + /* + * Head's next field might not have been set yet, or may have + * been unset after setHead. So we must check to see if tail + * is actually first node. If not, we continue on, safely + * traversing from tail back to head to find first, + * guaranteeing termination. + */ + + Node t = tail; + Thread firstThread = null; + while (t != null && t != head) { + Thread tt = t.thread; + if (tt != null) + firstThread = tt; + t = t.prev; + } + return firstThread; + } + + /** + * 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.thread == 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.isShared() && + s.thread != 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(int 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() {
+ // The correctness of this depends on head being initialized
+ // before tail and on head.next being accurate if the current
+ // thread is first in queue.
+ Node t = tail; // Read fields in reverse initialization order
+ Node h = head;
+ Node s;
+ return h != t &&
+ ((s = h.next) == null || s.thread != 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.thread != 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 CollectionAcquires the lock if it is not held by another thread and returns + * immediately, setting the lock hold count to one. + * + *
If the current thread already holds the lock then the hold + * count is incremented by one and the method returns immediately. + * + *
If the lock is held by another thread then the + * current thread becomes disabled for thread scheduling + * purposes and lies dormant until the lock has been acquired, + * at which time the lock hold count is set to one. + */ + public void lock() { + sync.lock(); + } + + /** + * Acquires the lock unless the current thread is + * {@linkplain Thread#interrupt interrupted}. + * + *
Acquires the lock if it is not held by another thread and returns + * immediately, setting the lock hold count to one. + * + *
If the current thread already holds this lock then the hold count + * is incremented by one and the method returns immediately. + * + *
If the lock is held by another thread then the + * current thread becomes disabled for thread scheduling + * purposes and lies dormant until one of two things happens: + * + *
If the lock is acquired by the current thread then the lock hold + * count is set to one. + * + *
If the current thread: + * + *
In this implementation, as this method is an explicit + * interruption point, preference is given to responding to the + * interrupt over normal or reentrant acquisition of the lock. + * + * @throws InterruptedException if the current thread is interrupted + */ + public void lockInterruptibly() throws InterruptedException { + sync.acquireInterruptibly(1); + } + + /** + * Acquires the lock only if it is not held by another thread at the time + * of invocation. + * + *
Acquires the lock if it is not held by another thread and + * returns immediately with the value {@code true}, setting the + * lock hold count to one. Even when this lock has been set to use a + * fair ordering policy, a call to {@code tryLock()} will + * immediately acquire the lock if it is available, whether or not + * other threads are currently waiting for the lock. + * This "barging" behavior can be useful in certain + * circumstances, even though it breaks fairness. If you want to honor + * the fairness setting for this lock, then use + * {@link #tryLock(long, TimeUnit) tryLock(0, TimeUnit.SECONDS) } + * which is almost equivalent (it also detects interruption). + * + *
If the current thread already holds this lock then the hold + * count is incremented by one and the method returns {@code true}. + * + *
If the lock is held by another thread then this method will return + * immediately with the value {@code false}. + * + * @return {@code true} if the lock was free and was acquired by the + * current thread, or the lock was already held by the current + * thread; and {@code false} otherwise + */ + public boolean tryLock() { + return sync.nonfairTryAcquire(1); + } + + /** + * Acquires the lock if it is not held by another thread within the given + * waiting time and the current thread has not been + * {@linkplain Thread#interrupt interrupted}. + * + *
Acquires the lock if it is not held by another thread and returns + * immediately with the value {@code true}, setting the lock hold count + * to one. If this lock has been set to use a fair ordering policy then + * an available lock will not be acquired if any other threads + * are waiting for the lock. This is in contrast to the {@link #tryLock()} + * method. If you want a timed {@code tryLock} that does permit barging on + * a fair lock then combine the timed and un-timed forms together: + * + *
{@code
+ * if (lock.tryLock() ||
+ * lock.tryLock(timeout, unit)) {
+ * ...
+ * }}
+ *
+ * If the current thread + * already holds this lock then the hold count is incremented by one and + * the method returns {@code true}. + * + *
If the lock is held by another thread then the + * current thread becomes disabled for thread scheduling + * purposes and lies dormant until one of three things happens: + * + *
If the lock is acquired then the value {@code true} is returned and + * the lock hold count is set to one. + * + *
If the current thread: + * + *
If the specified waiting time elapses then the value {@code false} + * is returned. If the time is less than or equal to zero, the method + * will not wait at all. + * + *
In this implementation, as this method is an explicit + * interruption point, preference is given to responding to the + * interrupt over normal or reentrant acquisition of the lock, and + * over reporting the elapse of the waiting time. + * + * @param timeout the time to wait for the lock + * @param unit the time unit of the timeout argument + * @return {@code true} if the lock was free and was acquired by the + * current thread, or the lock was already held by the current + * thread; and {@code false} if the waiting time elapsed before + * the lock could be acquired + * @throws InterruptedException if the current thread is interrupted + * @throws NullPointerException if the time unit is null + */ + public boolean tryLock(long timeout, TimeUnit unit) + throws InterruptedException { + return sync.tryAcquireNanos(1, unit.toNanos(timeout)); + } + + /** + * Attempts to release this lock. + * + *
If the current thread is the holder of this lock then the hold + * count is decremented. If the hold count is now zero then the lock + * is released. If the current thread is not the holder of this + * lock then {@link IllegalMonitorStateException} is thrown. + * + * @throws IllegalMonitorStateException if the current thread does not + * hold this lock + */ + public void unlock() { + sync.release(1); + } + + /** + * Returns a {@link Condition} instance for use with this + * {@link Lock} instance. + * + *
The returned {@link Condition} instance supports the same + * usages as do the {@link Object} monitor methods ({@link + * Object#wait() wait}, {@link Object#notify notify}, and {@link + * Object#notifyAll notifyAll}) when used with the built-in + * monitor lock. + * + *
A thread has a hold on a lock for each lock action that is not + * matched by an unlock action. + * + *
The hold count information is typically only used for testing and + * debugging purposes. For example, if a certain section of code should + * not be entered with the lock already held then we can assert that + * fact: + * + *
{@code
+ * class X {
+ * ReentrantLock lock = new ReentrantLock();
+ * // ...
+ * public void m() {
+ * assert lock.getHoldCount() == 0;
+ * lock.lock();
+ * try {
+ * // ... method body
+ * } finally {
+ * lock.unlock();
+ * }
+ * }
+ * }}
+ *
+ * @return the number of holds on this lock by the current thread,
+ * or zero if this lock is not held by the current thread
+ */
+ public int getHoldCount() {
+ return sync.getHoldCount();
+ }
+
+ /**
+ * Queries if this lock is held by the current thread.
+ *
+ * Analogous to the {@link Thread#holdsLock(Object)} method for + * built-in monitor locks, this method is typically used for + * debugging and testing. For example, a method that should only be + * called while a lock is held can assert that this is the case: + * + *
{@code
+ * class X {
+ * ReentrantLock lock = new ReentrantLock();
+ * // ...
+ *
+ * public void m() {
+ * assert lock.isHeldByCurrentThread();
+ * // ... method body
+ * }
+ * }}
+ *
+ * It can also be used to ensure that a reentrant lock is used + * in a non-reentrant manner, for example: + * + *
{@code
+ * class X {
+ * ReentrantLock lock = new ReentrantLock();
+ * // ...
+ *
+ * public void m() {
+ * assert !lock.isHeldByCurrentThread();
+ * lock.lock();
+ * try {
+ * // ... method body
+ * } finally {
+ * lock.unlock();
+ * }
+ * }
+ * }}
+ *
+ * @return {@code true} if current thread holds this lock and
+ * {@code false} otherwise
+ */
+ public boolean isHeldByCurrentThread() {
+ return sync.isHeldExclusively();
+ }
+
+ /**
+ * Queries if this lock is held by any thread. This method is
+ * designed for use in monitoring of the system state,
+ * not for synchronization control.
+ *
+ * @return {@code true} if any thread holds this lock and
+ * {@code false} otherwise
+ */
+ public boolean isLocked() {
+ return sync.isLocked();
+ }
+
+ /**
+ * Returns {@code true} if this lock has fairness set true.
+ *
+ * @return {@code true} if this lock has fairness set true
+ */
+ public final boolean isFair() {
+ return sync instanceof FairSync;
+ }
+
+ /**
+ * Returns the thread that currently owns this lock, or
+ * {@code null} if not owned. When this method is called by a
+ * thread that is not the owner, the return value reflects a
+ * best-effort approximation of current lock status. For example,
+ * the owner may be momentarily {@code null} even if there are
+ * threads trying to acquire the lock but have not yet done so.
+ * This method is designed to facilitate construction of
+ * subclasses that provide more extensive lock monitoring
+ * facilities.
+ *
+ * @return the owner, or {@code null} if not owned
+ */
+ protected Thread getOwner() {
+ return sync.getOwner();
+ }
+
+ /**
+ * Queries whether any threads are waiting to acquire this lock. Note that
+ * because cancellations may occur at any time, a {@code true}
+ * return does not guarantee that any other thread will ever
+ * acquire this lock. This method is designed primarily for use in
+ * monitoring of the system state.
+ *
+ * @return {@code true} if there may be other threads waiting to
+ * acquire the lock
+ */
+ public final boolean hasQueuedThreads() {
+ return sync.hasQueuedThreads();
+ }
+
+ /**
+ * Queries whether the given thread is waiting to acquire this
+ * lock. Note that because cancellations may occur at any time, a
+ * {@code true} return does not guarantee that this thread
+ * will ever acquire this lock. This method is designed primarily for use
+ * in monitoring of the system state.
+ *
+ * @param thread the thread
+ * @return {@code true} if the given thread is queued waiting for this lock
+ * @throws NullPointerException if the thread is null
+ */
+ public final boolean hasQueuedThread(Thread thread) {
+ return sync.isQueued(thread);
+ }
+
+ /**
+ * Returns an estimate of the number of threads waiting to
+ * acquire this lock. 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 of the system state, not for synchronization
+ * control.
+ *
+ * @return the estimated number of threads waiting for this lock
+ */
+ public final int getQueueLength() {
+ return sync.getQueueLength();
+ }
+
+ /**
+ * Returns a collection containing threads that may be waiting to
+ * acquire this lock. 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
+ */
+ protected Collection