A {@code ReentrantLock} is owned by the thread last + * successfully locking, but not yet unlocking it. A thread invoking + * {@code lock} will return, successfully acquiring the lock, when + * the lock is not owned by another thread. The method will return + * immediately if the current thread already owns the lock. This can + * be checked using methods {@link #isHeldByCurrentThread}, and {@link + * #getHoldCount}. + * + *
The constructor for this class accepts an optional + * fairness parameter. When set {@code true}, under + * contention, locks favor granting access to the longest-waiting + * thread. Otherwise this lock does not guarantee any particular + * access order. Programs using fair locks accessed by many threads + * may display lower overall throughput (i.e., are slower; often much + * slower) than those using the default setting, but have smaller + * variances in times to obtain locks and guarantee lack of + * starvation. Note however, that fairness of locks does not guarantee + * fairness of thread scheduling. Thus, one of many threads using a + * fair lock may obtain it multiple times in succession while other + * active threads are not progressing and not currently holding the + * lock. + * Also note that the untimed {@link #tryLock() tryLock} method does not + * honor the fairness setting. It will succeed if the lock + * is available even if other threads are waiting. + * + *
It is recommended practice to always immediately + * follow a call to {@code lock} with a {@code try} block, most + * typically in a before/after construction such as: + * + *
+ * class X {
+ * private final ReentrantLock lock = new ReentrantLock();
+ * // ...
+ *
+ * public void m() {
+ * lock.lock(); // block until condition holds
+ * try {
+ * // ... method body
+ * } finally {
+ * lock.unlock()
+ * }
+ * }
+ * }
+ *
+ *
+ * In addition to implementing the {@link Lock} interface, this + * class defines methods {@code isLocked} and + * {@code getLockQueueLength}, as well as some associated + * {@code protected} access methods that may be useful for + * instrumentation and monitoring. + * + *
Serialization of this class behaves in the same way as built-in + * locks: a deserialized lock is in the unlocked state, regardless of + * its state when serialized. + * + *
This lock supports a maximum of 2147483647 recursive locks by + * the same thread. Attempts to exceed this limit result in + * {@link Error} throws from locking methods. + * + * @since 1.5 + * @author Doug Lea + */ +public class ReentrantLock implements Lock, java.io.Serializable { + private static final long serialVersionUID = 7373984872572414699L; + /** Synchronizer providing all implementation mechanics */ + private final Sync sync; + + /** + * Base of synchronization control for this lock. Subclassed + * into fair and nonfair versions below. Uses AQS state to + * represent the number of holds on the lock. + */ + abstract static class Sync extends AbstractQueuedSynchronizer { + private static final long serialVersionUID = -5179523762034025860L; + + /** + * Performs {@link Lock#lock}. The main reason for subclassing + * is to allow fast path for nonfair version. + */ + abstract void lock(); + + /** + * Performs non-fair tryLock. tryAcquire is + * implemented in subclasses, but both need nonfair + * try for trylock method. + */ + final boolean nonfairTryAcquire(int acquires) { + final Thread current = Thread.currentThread(); + int c = getState(); + if (c == 0) { + if (compareAndSetState(0, acquires)) { + setExclusiveOwnerThread(current); + return true; + } + } + else if (current == getExclusiveOwnerThread()) { + int nextc = c + acquires; + if (nextc < 0) // overflow + throw new Error("Maximum lock count exceeded"); + setState(nextc); + return true; + } + return false; + } + + protected final boolean tryRelease(int releases) { + int c = getState() - releases; + if (Thread.currentThread() != getExclusiveOwnerThread()) + throw new IllegalMonitorStateException(); + boolean free = false; + if (c == 0) { + free = true; + setExclusiveOwnerThread(null); + } + setState(c); + return free; + } + + protected final boolean isHeldExclusively() { + // While we must in general read state before owner, + // we don't need to do so to check if current thread is owner + return getExclusiveOwnerThread() == Thread.currentThread(); + } + + final ConditionObject newCondition() { + return new ConditionObject(); + } + + // Methods relayed from outer class + + final Thread getOwner() { + return getState() == 0 ? null : getExclusiveOwnerThread(); + } + + final int getHoldCount() { + return isHeldExclusively() ? getState() : 0; + } + + final boolean isLocked() { + return getState() != 0; + } + + /** + * Reconstitutes this lock instance from a stream. + * @param s the stream + */ + private void readObject(java.io.ObjectInputStream s) + throws java.io.IOException, ClassNotFoundException { + s.defaultReadObject(); + setState(0); // reset to unlocked state + } + } + + /** + * Sync object for non-fair locks + */ + static final class NonfairSync extends Sync { + private static final long serialVersionUID = 7316153563782823691L; + + /** + * Performs lock. Try immediate barge, backing up to normal + * acquire on failure. + */ + final void lock() { + if (compareAndSetState(0, 1)) + setExclusiveOwnerThread(Thread.currentThread()); + else + acquire(1); + } + + protected final boolean tryAcquire(int acquires) { + return nonfairTryAcquire(acquires); + } + } + + /** + * Sync object for fair locks + */ + static final class FairSync extends Sync { + private static final long serialVersionUID = -3000897897090466540L; + + final void lock() { + acquire(1); + } + + /** + * Fair version of tryAcquire. Don't grant access unless + * recursive call or no waiters or is first. + */ + protected final boolean tryAcquire(int acquires) { + final Thread current = Thread.currentThread(); + int c = getState(); + if (c == 0) { + if (!hasQueuedPredecessors() && + compareAndSetState(0, acquires)) { + setExclusiveOwnerThread(current); + return true; + } + } + else if (current == getExclusiveOwnerThread()) { + int nextc = c + acquires; + if (nextc < 0) + throw new Error("Maximum lock count exceeded"); + setState(nextc); + return true; + } + return false; + } + } + + /** + * Creates an instance of {@code ReentrantLock}. + * This is equivalent to using {@code ReentrantLock(false)}. + */ + public ReentrantLock() { + sync = new NonfairSync(); + } + + /** + * Creates an instance of {@code ReentrantLock} with the + * given fairness policy. + * + * @param fair {@code true} if this lock should use a fair ordering policy + */ + public ReentrantLock(boolean fair) { + sync = fair ? new FairSync() : new NonfairSync(); + } + + /** + * Acquires the lock. + * + *
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 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: + * + *
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: + * + *
+ * 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} 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: + * + *
+ * 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: + * + *
+ * 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