/*

  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

  *

  * This code is free software; you can redistribute it and/or modify it

  * under the terms of the GNU General Public License version 2 only, as

  * published by the Free Software Foundation.  Oracle designates this

  * particular file as subject to the "Classpath" exception as provided

  * by Oracle in the LICENSE file that accompanied this code.

  *

  * This code is distributed in the hope that it will be useful, but WITHOUT

  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

  * version 2 for more details (a copy is included in the LICENSE file that

  * accompanied this code).

  *

  * You should have received a copy of the GNU General Public License version

  * 2 along with this work; if not, write to the Free Software Foundation,

  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

  *

  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

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  */

 /*

  * This file is available under and governed by the GNU General Public

  * License version 2 only, as published by the Free Software Foundation.

  * However, the following notice accompanied the original version of this

  * file:

  *

  * 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.*;

 import java.util.concurrent.*;

 import java.util.concurrent.atomic.*;

 /**

  * 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() 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>

  * 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 methods {@code isLocked} and

  * {@code getLockQueueLength}, as well as some associated

  * {@code protected} access methods that may be 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 {@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.

      *

      * <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.lock();

     }

     /**

      * 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 "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).

      *

      * <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>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>

      * 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} 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>

      * 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>

      * 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 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<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() + "]");

     }

 }