/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
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/*
 *
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 *
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent.locks;

import java.util.Collection;
import java.util.concurrent.TimeUnit;
import jdk.internal.vm.annotation.ReservedStackAccess;

/**
 * A reentrant mutual exclusion {@link Lock} with the same basic
 * behavior and semantics as the implicit monitor lock accessed using
 * {@code synchronized} methods and statements, but with extended
 * capabilities.
 *
 * <p>A {@code ReentrantLock} is <em>owned</em> 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}.
 *
 * <p>The constructor for this class accepts an optional
 * <em>fairness</em> 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()} method does not
 * honor the fairness setting. It will succeed if the lock
 * is available even if other threads are waiting.
 *
 * <p>It is recommended practice to <em>always</em> immediately
 * follow a call to {@code lock} with a {@code try} block, most
 * typically in a before/after construction such as:
 *
 * <pre> {@code
 * class X {
 * private final ReentrantLock lock = new ReentrantLock();
 * // ...
 *
 * public void m() {
 * lock.lock(); // block until condition holds
 * try {
 * // ... method body
 * } finally {
 * lock.unlock()
 * }
 * }
 * }}</pre>
 *
 * <p>In addition to implementing the {@link Lock} interface, this
 * class defines a number of {@code public} and {@code protected}
 * methods for inspecting the state of the lock. Some of these
 * methods are only useful for instrumentation and monitoring.
 *
 * <p>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.
 *
 * <p>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 non-fair tryLock. tryAcquire is implemented in
 * subclasses, but both need nonfair try for trylock method.
 */
 @ReservedStackAccess
 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;
 }

 @ReservedStackAccess
 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 the instance from a stream (that is, deserializes it).
 */
 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;
 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;
 /**
 * Fair version of tryAcquire. Don't grant access unless
 * recursive call or no waiters or is first.
 */
 @ReservedStackAccess
 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.
 *
 * <p>Acquires the lock if it is not held by another thread and returns
 * immediately, setting the lock hold count to one.
 *
 * <p>If the current thread already holds the lock then the hold
 * count is incremented by one and the method returns immediately.
 *
 * <p>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.acquire(1);
 }

 /**
 * Acquires the lock unless the current thread is
 * {@linkplain Thread#interrupt interrupted}.
 *
 * <p>Acquires the lock if it is not held by another thread and returns
 * immediately, setting the lock hold count to one.
 *
 * <p>If the current thread already holds this lock then the hold count
 * is incremented by one and the method returns immediately.
 *
 * <p>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:
 *
 * <ul>
 *
 * <li>The lock is acquired by the current thread; or
 *
 * <li>Some other thread {@linkplain Thread#interrupt interrupts} the
 * current thread.
 *
 * </ul>
 *
 * <p>If the lock is acquired by the current thread then the lock hold
 * count is set to one.
 *
 * <p>If the current thread:
 *
 * <ul>
 *
 * <li>has its interrupted status set on entry to this method; or
 *
 * <li>is {@linkplain Thread#interrupt interrupted} while acquiring
 * the lock,
 *
 * </ul>
 *
 * then {@link InterruptedException} is thrown and the current thread's
 * interrupted status is cleared.
 *
 * <p>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.
 *
 * <p>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()} <em>will</em>
 * immediately acquire the lock if it is available, whether or not
 * other threads are currently waiting for the lock.
 * This &quot;barging&quot; 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).
 *
 * <p>If the current thread already holds this lock then the hold
 * count is incremented by one and the method returns {@code true}.
 *
 * <p>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}.
 *
 * <p>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 <em>will not</em> 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:
 *
 * <pre> {@code
 * if (lock.tryLock() ||
 * lock.tryLock(timeout, unit)) {
 * ...
 * }}</pre>
 *
 * <p>If the current thread
 * already holds this lock then the hold count is incremented by one and
 * the method returns {@code true}.
 *
 * <p>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:
 *
 * <ul>
 *
 * <li>The lock is acquired by the current thread; or
 *
 * <li>Some other thread {@linkplain Thread#interrupt interrupts}
 * the current thread; or
 *
 * <li>The specified waiting time elapses
 *
 * </ul>
 *
 * <p>If the lock is acquired then the value {@code true} is returned and
 * the lock hold count is set to one.
 *
 * <p>If the current thread:
 *
 * <ul>
 *
 * <li>has its interrupted status set on entry to this method; or
 *
 * <li>is {@linkplain Thread#interrupt interrupted} while
 * acquiring the lock,
 *
 * </ul>
 * then {@link InterruptedException} is thrown and the current thread's
 * interrupted status is cleared.
 *
 * <p>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.
 *
 * <p>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.
 *
 * <p>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.
 *
 * <p>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.
 *
 * <ul>
 *
 * <li>If this lock is not held when any of the {@link Condition}
 * {@linkplain Condition#await() waiting} or {@linkplain
 * Condition#signal signalling} methods are called, then an {@link
 * IllegalMonitorStateException} is thrown.
 *
 * <li>When the condition {@linkplain Condition#await() waiting}
 * methods are called the lock is released and, before they
 * return, the lock is reacquired and the lock hold count restored
 * to what it was when the method was called.
 *
 * <li>If a thread is {@linkplain Thread#interrupt interrupted}
 * while waiting then the wait will terminate, an {@link
 * InterruptedException} will be thrown, and the thread's
 * interrupted status will be cleared.
 *
 * <li>Waiting threads are signalled in FIFO order.
 *
 * <li>The ordering of lock reacquisition for threads returning
 * from waiting methods is the same as for threads initially
 * acquiring the lock, which is in the default case not specified,
 * but for <em>fair</em> locks favors those threads that have been
 * waiting the longest.
 *
 * </ul>
 *
 * @return the Condition object
 */
 public Condition newCondition() {
 return sync.newCondition();
 }

 /**
 * Queries the number of holds on this lock by the current thread.
 *
 * <p>A thread has a hold on a lock for each lock action that is not
 * matched by an unlock action.
 *
 * <p>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:
 *
 * <pre> {@code
 * class X {
 * ReentrantLock lock = new ReentrantLock();
 * // ...
 * public void m() {
 * assert lock.getHoldCount() == 0;
 * lock.lock();
 * try {
 * // ... method body
 * } finally {
 * lock.unlock();
 * }
 * }
 * }}</pre>
 *
 * @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.
 *
 * <p>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:
 *
 * <pre> {@code
 * class X {
 * ReentrantLock lock = new ReentrantLock();
 * // ...
 *
 * public void m() {
 * assert lock.isHeldByCurrentThread();
 * // ... method body
 * }
 * }}</pre>
 *
 * <p>It can also be used to ensure that a reentrant lock is used
 * in a non-reentrant manner, for example:
 *
 * <pre> {@code
 * class X {
 * ReentrantLock lock = new ReentrantLock();
 * // ...
 *
 * public void m() {
 * assert !lock.isHeldByCurrentThread();
 * lock.lock();
 * try {
 * // ... method body
 * } finally {
 * lock.unlock();
 * }
 * }
 * }}</pre>
 *
 * @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 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<Thread> getQueuedThreads() {
 return sync.getQueuedThreads();
 }

 /**
 * Queries whether any threads are waiting on the given condition
 * associated with this lock. Note that because timeouts and
 * interrupts may occur at any time, a {@code true} return does
 * not guarantee that a future {@code signal} will awaken any
 * threads. This method is designed primarily for use in
 * monitoring of the system state.
 *
 * @param condition the condition
 * @return {@code true} if there are any waiting threads
 * @throws IllegalMonitorStateException if this lock is not held
 * @throws IllegalArgumentException if the given condition is
 * not associated with this lock
 * @throws NullPointerException if the condition is null
 */
 public boolean hasWaiters(Condition condition) {
 if (condition == null)
 throw new NullPointerException();
 if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
 throw new IllegalArgumentException("not owner");
 return sync.hasWaiters((AbstractQueuedSynchronizer.ConditionObject)condition);
 }

 /**
 * Returns an estimate of the number of threads waiting on the
 * given condition associated with this lock. Note that because
 * timeouts and interrupts may occur at any time, the estimate
 * serves only as an upper bound on the actual number of waiters.
 * This method is designed for use in monitoring of the system
 * state, not for synchronization control.
 *
 * @param condition the condition
 * @return the estimated number of waiting threads
 * @throws IllegalMonitorStateException if this lock is not held
 * @throws IllegalArgumentException if the given condition is
 * not associated with this lock
 * @throws NullPointerException if the condition is null
 */
 public int getWaitQueueLength(Condition condition) {
 if (condition == null)
 throw new NullPointerException();
 if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
 throw new IllegalArgumentException("not owner");
 return sync.getWaitQueueLength((AbstractQueuedSynchronizer.ConditionObject)condition);
 }

 /**
 * Returns a collection containing those threads that may be
 * waiting on the given condition associated with 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 condition monitoring facilities.
 *
 * @param condition the condition
 * @return the collection of threads
 * @throws IllegalMonitorStateException if this lock is not held
 * @throws IllegalArgumentException if the given condition is
 * not associated with this lock
 * @throws NullPointerException if the condition is null
 */
 protected Collection<Thread> getWaitingThreads(Condition condition) {
 if (condition == null)
 throw new NullPointerException();
 if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
 throw new IllegalArgumentException("not owner");
 return sync.getWaitingThreads((AbstractQueuedSynchronizer.ConditionObject)condition);
 }

 /**
 * Returns a string identifying this lock, as well as its lock state.
 * The state, in brackets, includes either the String {@code "Unlocked"}
 * or the String {@code "Locked by"} followed by the
 * {@linkplain Thread#getName name} of the owning thread.
 *
 * @return a string identifying this lock, as well as its lock state
 */
 public String toString() {
 Thread o = sync.getOwner();
 return super.toString() + ((o == null) ?
 "[Unlocked]" :
 "[Locked by thread " + o.getName() + "]");
 }
}