/*

  * 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

  * or visit www.oracle.com if you need additional information or have any

  * questions.

  */

 /*

  * 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.concurrent.*;

 import java.util.concurrent.atomic.*;

 import java.util.*;

 /**

  * An implementation of {@link ReadWriteLock} supporting similar

  * semantics to {@link ReentrantLock}.

  * <p>This class has the following properties:

  *

  * <ul>

  * <li><b>Acquisition order</b>

  *

  * <p> This class does not impose a reader or writer preference

  * ordering for lock access.  However, it does support an optional

  * <em>fairness</em> policy.

  *

  * <dl>

  * <dt><b><i>Non-fair mode (default)</i></b>

  * <dd>When constructed as non-fair (the default), the order of entry

  * to the read and write lock is unspecified, subject to reentrancy

  * constraints.  A nonfair lock that is continuously contended may

  * indefinitely postpone one or more reader or writer threads, but

  * will normally have higher throughput than a fair lock.

  * <p>

  *

  * <dt><b><i>Fair mode</i></b>

  * <dd> When constructed as fair, threads contend for entry using an

  * approximately arrival-order policy. When the currently held lock

  * is released either the longest-waiting single writer thread will

  * be assigned the write lock, or if there is a group of reader threads

  * waiting longer than all waiting writer threads, that group will be

  * assigned the read lock.

  *

  * <p>A thread that tries to acquire a fair read lock (non-reentrantly)

  * will block if either the write lock is held, or there is a waiting

  * writer thread. The thread will not acquire the read lock until

  * after the oldest currently waiting writer thread has acquired and

  * released the write lock. Of course, if a waiting writer abandons

  * its wait, leaving one or more reader threads as the longest waiters

  * in the queue with the write lock free, then those readers will be

  * assigned the read lock.

  *

  * <p>A thread that tries to acquire a fair write lock (non-reentrantly)

  * will block unless both the read lock and write lock are free (which

  * implies there are no waiting threads).  (Note that the non-blocking

  * {@link ReadLock#tryLock()} and {@link WriteLock#tryLock()} methods

  * do not honor this fair setting and will acquire the lock if it is

  * possible, regardless of waiting threads.)

  * <p>

  * </dl>

  *

  * <li><b>Reentrancy</b>

  *

  * <p>This lock allows both readers and writers to reacquire read or

  * write locks in the style of a {@link ReentrantLock}. Non-reentrant

  * readers are not allowed until all write locks held by the writing

  * thread have been released.

  *

  * <p>Additionally, a writer can acquire the read lock, but not

  * vice-versa.  Among other applications, reentrancy can be useful

  * when write locks are held during calls or callbacks to methods that

  * perform reads under read locks.  If a reader tries to acquire the

  * write lock it will never succeed.

  *

  * <li><b>Lock downgrading</b>

  * <p>Reentrancy also allows downgrading from the write lock to a read lock,

  * by acquiring the write lock, then the read lock and then releasing the

  * write lock. However, upgrading from a read lock to the write lock is

  * <b>not</b> possible.

  *

  * <li><b>Interruption of lock acquisition</b>

  * <p>The read lock and write lock both support interruption during lock

  * acquisition.

  *

  * <li><b>{@link Condition} support</b>

  * <p>The write lock provides a {@link Condition} implementation that

  * behaves in the same way, with respect to the write lock, as the

  * {@link Condition} implementation provided by

  * {@link ReentrantLock#newCondition} does for {@link ReentrantLock}.

  * This {@link Condition} can, of course, only be used with the write lock.

  *

  * <p>The read lock does not support a {@link Condition} and

  * {@code readLock().newCondition()} throws

  * {@code UnsupportedOperationException}.

  *

  * <li><b>Instrumentation</b>

  * <p>This class supports methods to determine whether locks

  * are held or contended. These methods are designed for monitoring

  * system state, not for synchronization control.

  * </ul>

  *

  * <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><b>Sample usages</b>. Here is a code sketch showing how to perform

  * lock downgrading after updating a cache (exception handling is

  * particularly tricky when handling multiple locks in a non-nested

  * fashion):

  *

  * <pre> {@code

  * class CachedData {

  *   Object data;

  *   volatile boolean cacheValid;

  *   final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();

  *

  *   void processCachedData() {

  *     rwl.readLock().lock();

  *     if (!cacheValid) {

  *        // Must release read lock before acquiring write lock

  *        rwl.readLock().unlock();

  *        rwl.writeLock().lock();

  *        try {

  *          // Recheck state because another thread might have

  *          // acquired write lock and changed state before we did.

  *          if (!cacheValid) {

  *            data = ...

  *            cacheValid = true;

  *          }

  *          // Downgrade by acquiring read lock before releasing write lock

  *          rwl.readLock().lock();

  *        } finally {

  *          rwl.writeLock().unlock(); // Unlock write, still hold read

  *        }

  *     }

  *

  *     try {

  *       use(data);

  *     } finally {

  *       rwl.readLock().unlock();

  *     }

  *   }

  * }}</pre>

  *

  * ReentrantReadWriteLocks can be used to improve concurrency in some

  * uses of some kinds of Collections. This is typically worthwhile

  * only when the collections are expected to be large, accessed by

  * more reader threads than writer threads, and entail operations with

  * overhead that outweighs synchronization overhead. For example, here

  * is a class using a TreeMap that is expected to be large and

  * concurrently accessed.

  *

  * <pre>{@code

  * class RWDictionary {

  *    private final Map<String, Data> m = new TreeMap<String, Data>();

  *    private final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();

  *    private final Lock r = rwl.readLock();

  *    private final Lock w = rwl.writeLock();

  *

  *    public Data get(String key) {

  *        r.lock();

  *        try { return m.get(key); }

  *        finally { r.unlock(); }

  *    }

  *    public String[] allKeys() {

  *        r.lock();

  *        try { return m.keySet().toArray(); }

  *        finally { r.unlock(); }

  *    }

  *    public Data put(String key, Data value) {

  *        w.lock();

  *        try { return m.put(key, value); }

  *        finally { w.unlock(); }

  *    }

  *    public void clear() {

  *        w.lock();

  *        try { m.clear(); }

  *        finally { w.unlock(); }

  *    }

  * }}</pre>

  *

  * <h3>Implementation Notes</h3>

  *

  * <p>This lock supports a maximum of 65535 recursive write locks

  * and 65535 read locks. Attempts to exceed these limits result in

  * {@link Error} throws from locking methods.

  *

  * @since 1.5

  * @author Doug Lea

  *

  */

 public class ReentrantReadWriteLock

         implements ReadWriteLock, java.io.Serializable {

     private static final long serialVersionUID = -6992448646407690164L;

     /** Inner class providing readlock */

     private final ReentrantReadWriteLock.ReadLock readerLock;

     /** Inner class providing writelock */

     private final ReentrantReadWriteLock.WriteLock writerLock;

     /** Performs all synchronization mechanics */

     final Sync sync;

     /**

      * Creates a new {@code ReentrantReadWriteLock} with

      * default (nonfair) ordering properties.

      */

     public ReentrantReadWriteLock() {

         this(false);

     }

     /**

      * Creates a new {@code ReentrantReadWriteLock} with

      * the given fairness policy.

      *

      * @param fair {@code true} if this lock should use a fair ordering policy

      */

     public ReentrantReadWriteLock(boolean fair) {

         sync = fair ? new FairSync() : new NonfairSync();

         readerLock = new ReadLock(this);

         writerLock = new WriteLock(this);

     }

     public ReentrantReadWriteLock.WriteLock writeLock() { return writerLock; }

     public ReentrantReadWriteLock.ReadLock  readLock()  { return readerLock; }

     /**

      * Synchronization implementation for ReentrantReadWriteLock.

      * Subclassed into fair and nonfair versions.

      */

     abstract static class Sync extends AbstractQueuedSynchronizer {

         private static final long serialVersionUID = 6317671515068378041L;

         /*

          * Read vs write count extraction constants and functions.

          * Lock state is logically divided into two unsigned shorts:

          * The lower one representing the exclusive (writer) lock hold count,

          * and the upper the shared (reader) hold count.

          */

         static final int SHARED_SHIFT   = 16;

         static final int SHARED_UNIT    = (1 << SHARED_SHIFT);

         static final int MAX_COUNT      = (1 << SHARED_SHIFT) - 1;

         static final int EXCLUSIVE_MASK = (1 << SHARED_SHIFT) - 1;

         /** Returns the number of shared holds represented in count  */

         static int sharedCount(int c)    { return c >>> SHARED_SHIFT; }

         /** Returns the number of exclusive holds represented in count  */

         static int exclusiveCount(int c) { return c & EXCLUSIVE_MASK; }

         /**

          * A counter for per-thread read hold counts.

          * Maintained as a ThreadLocal; cached in cachedHoldCounter

          */

         static final class HoldCounter {

             int count = 0;

             // Use id, not reference, to avoid garbage retention

             final long tid = Thread.currentThread().getId();

         }

         /**

          * ThreadLocal subclass. Easiest to explicitly define for sake

          * of deserialization mechanics.

          */

         static final class ThreadLocalHoldCounter

             extends ThreadLocal<HoldCounter> {

             public HoldCounter initialValue() {

                 return new HoldCounter();

             }

         }

         /**

          * The number of reentrant read locks held by current thread.

          * Initialized only in constructor and readObject.

          * Removed whenever a thread's read hold count drops to 0.

          */

         private transient ThreadLocalHoldCounter readHolds;

         /**

          * The hold count of the last thread to successfully acquire

          * readLock. This saves ThreadLocal lookup in the common case

          * where the next thread to release is the last one to

          * acquire. This is non-volatile since it is just used

          * as a heuristic, and would be great for threads to cache.

          *

          * <p>Can outlive the Thread for which it is caching the read

          * hold count, but avoids garbage retention by not retaining a

          * reference to the Thread.

          *

          * <p>Accessed via a benign data race; relies on the memory

          * model's final field and out-of-thin-air guarantees.

          */

         private transient HoldCounter cachedHoldCounter;

         /**

          * firstReader is the first thread to have acquired the read lock.

          * firstReaderHoldCount is firstReader's hold count.

          *

          * <p>More precisely, firstReader is the unique thread that last

          * changed the shared count from 0 to 1, and has not released the

          * read lock since then; null if there is no such thread.

          *

          * <p>Cannot cause garbage retention unless the thread terminated

          * without relinquishing its read locks, since tryReleaseShared

          * sets it to null.

          *

          * <p>Accessed via a benign data race; relies on the memory

          * model's out-of-thin-air guarantees for references.

          *

          * <p>This allows tracking of read holds for uncontended read

          * locks to be very cheap.

          */

         private transient Thread firstReader = null;

         private transient int firstReaderHoldCount;

         Sync() {

             readHolds = new ThreadLocalHoldCounter();

             setState(getState()); // ensures visibility of readHolds

         }

         /*

          * Acquires and releases use the same code for fair and

          * nonfair locks, but differ in whether/how they allow barging

          * when queues are non-empty.

          */

         /**

          * Returns true if the current thread, when trying to acquire

          * the read lock, and otherwise eligible to do so, should block

          * because of policy for overtaking other waiting threads.

          */

         abstract boolean readerShouldBlock();

         /**

          * Returns true if the current thread, when trying to acquire

          * the write lock, and otherwise eligible to do so, should block

          * because of policy for overtaking other waiting threads.

          */

         abstract boolean writerShouldBlock();

         /*

          * Note that tryRelease and tryAcquire can be called by

          * Conditions. So it is possible that their arguments contain

          * both read and write holds that are all released during a

          * condition wait and re-established in tryAcquire.

          */

         protected final boolean tryRelease(int releases) {

             if (!isHeldExclusively())

                 throw new IllegalMonitorStateException();

             int nextc = getState() - releases;

             boolean free = exclusiveCount(nextc) == 0;

             if (free)

                 setExclusiveOwnerThread(null);

             setState(nextc);

             return free;

         }

         protected final boolean tryAcquire(int acquires) {

             /*

              * Walkthrough:

              * 1. If read count nonzero or write count nonzero

              *    and owner is a different thread, fail.

              * 2. If count would saturate, fail. (This can only

              *    happen if count is already nonzero.)

              * 3. Otherwise, this thread is eligible for lock if

              *    it is either a reentrant acquire or

              *    queue policy allows it. If so, update state

              *    and set owner.

              */

             Thread current = Thread.currentThread();

             int c = getState();

             int w = exclusiveCount(c);

             if (c != 0) {

                 // (Note: if c != 0 and w == 0 then shared count != 0)

                 if (w == 0 || current != getExclusiveOwnerThread())

                     return false;

                 if (w + exclusiveCount(acquires) > MAX_COUNT)

                     throw new Error("Maximum lock count exceeded");

                 // Reentrant acquire

                 setState(c + acquires);

                 return true;

             }

             if (writerShouldBlock() ||

                 !compareAndSetState(c, c + acquires))

                 return false;

             setExclusiveOwnerThread(current);

             return true;

         }

         protected final boolean tryReleaseShared(int unused) {

             Thread current = Thread.currentThread();

             if (firstReader == current) {

                 // assert firstReaderHoldCount > 0;

                 if (firstReaderHoldCount == 1)

                     firstReader = null;

                 else

                     firstReaderHoldCount--;

             } else {

                 HoldCounter rh = cachedHoldCounter;

                 if (rh == null || rh.tid != current.getId())

                     rh = readHolds.get();

                 int count = rh.count;

                 if (count <= 1) {

                     readHolds.remove();

                     if (count <= 0)

                         throw unmatchedUnlockException();

                 }

                 --rh.count;

             }

             for (;;) {

                 int c = getState();

                 int nextc = c - SHARED_UNIT;

                 if (compareAndSetState(c, nextc))

                     // Releasing the read lock has no effect on readers,

                     // but it may allow waiting writers to proceed if

                     // both read and write locks are now free.

                     return nextc == 0;

             }

         }

         private IllegalMonitorStateException unmatchedUnlockException() {

             return new IllegalMonitorStateException(

                 "attempt to unlock read lock, not locked by current thread");

         }

         protected final int tryAcquireShared(int unused) {

             /*

              * Walkthrough:

              * 1. If write lock held by another thread, fail.

              * 2. Otherwise, this thread is eligible for

              *    lock wrt state, so ask if it should block

              *    because of queue policy. If not, try

              *    to grant by CASing state and updating count.

              *    Note that step does not check for reentrant

              *    acquires, which is postponed to full version

              *    to avoid having to check hold count in

              *    the more typical non-reentrant case.

              * 3. If step 2 fails either because thread

              *    apparently not eligible or CAS fails or count

              *    saturated, chain to version with full retry loop.

              */

             Thread current = Thread.currentThread();

             int c = getState();

             if (exclusiveCount(c) != 0 &&

                 getExclusiveOwnerThread() != current)

                 return -1;

             int r = sharedCount(c);

             if (!readerShouldBlock() &&

                 r < MAX_COUNT &&

                 compareAndSetState(c, c + SHARED_UNIT)) {

                 if (r == 0) {

                     firstReader = current;

                     firstReaderHoldCount = 1;

                 } else if (firstReader == current) {

                     firstReaderHoldCount++;

                 } else {

                     HoldCounter rh = cachedHoldCounter;

                     if (rh == null || rh.tid != current.getId())

                         cachedHoldCounter = rh = readHolds.get();

                     else if (rh.count == 0)

                         readHolds.set(rh);

                     rh.count++;

                 }

                 return 1;

             }

             return fullTryAcquireShared(current);

         }

         /**

          * Full version of acquire for reads, that handles CAS misses

          * and reentrant reads not dealt with in tryAcquireShared.

          */

         final int fullTryAcquireShared(Thread current) {

             /*

              * This code is in part redundant with that in

              * tryAcquireShared but is simpler overall by not

              * complicating tryAcquireShared with interactions between

              * retries and lazily reading hold counts.

              */

             HoldCounter rh = null;

             for (;;) {

                 int c = getState();

                 if (exclusiveCount(c) != 0) {

                     if (getExclusiveOwnerThread() != current)

                         return -1;

                     // else we hold the exclusive lock; blocking here

                     // would cause deadlock.

                 } else if (readerShouldBlock()) {

                     // Make sure we're not acquiring read lock reentrantly

                     if (firstReader == current) {

                         // assert firstReaderHoldCount > 0;

                     } else {

                         if (rh == null) {

                             rh = cachedHoldCounter;

                             if (rh == null || rh.tid != current.getId()) {

                                 rh = readHolds.get();

                                 if (rh.count == 0)

                                     readHolds.remove();

                             }

                         }

                         if (rh.count == 0)

                             return -1;

                     }

                 }

                 if (sharedCount(c) == MAX_COUNT)

                     throw new Error("Maximum lock count exceeded");

                 if (compareAndSetState(c, c + SHARED_UNIT)) {

                     if (sharedCount(c) == 0) {

                         firstReader = current;

                         firstReaderHoldCount = 1;

                     } else if (firstReader == current) {

                         firstReaderHoldCount++;

                     } else {

                         if (rh == null)

                             rh = cachedHoldCounter;

                         if (rh == null || rh.tid != current.getId())

                             rh = readHolds.get();

                         else if (rh.count == 0)

                             readHolds.set(rh);

                         rh.count++;

                         cachedHoldCounter = rh; // cache for release

                     }

                     return 1;

                 }

             }

         }

         /**

          * Performs tryLock for write, enabling barging in both modes.

          * This is identical in effect to tryAcquire except for lack

          * of calls to writerShouldBlock.

          */

         final boolean tryWriteLock() {

             Thread current = Thread.currentThread();

             int c = getState();

             if (c != 0) {

                 int w = exclusiveCount(c);

                 if (w == 0 || current != getExclusiveOwnerThread())

                     return false;

                 if (w == MAX_COUNT)

                     throw new Error("Maximum lock count exceeded");

             }

             if (!compareAndSetState(c, c + 1))

                 return false;

             setExclusiveOwnerThread(current);

             return true;

         }

         /**

          * Performs tryLock for read, enabling barging in both modes.

          * This is identical in effect to tryAcquireShared except for

          * lack of calls to readerShouldBlock.

          */

         final boolean tryReadLock() {

             Thread current = Thread.currentThread();

             for (;;) {

                 int c = getState();

                 if (exclusiveCount(c) != 0 &&

                     getExclusiveOwnerThread() != current)

                     return false;

                 int r = sharedCount(c);

                 if (r == MAX_COUNT)

                     throw new Error("Maximum lock count exceeded");

                 if (compareAndSetState(c, c + SHARED_UNIT)) {

                     if (r == 0) {

                         firstReader = current;

                         firstReaderHoldCount = 1;

                     } else if (firstReader == current) {

                         firstReaderHoldCount++;

                     } else {

                         HoldCounter rh = cachedHoldCounter;

                         if (rh == null || rh.tid != current.getId())

                             cachedHoldCounter = rh = readHolds.get();

                         else if (rh.count == 0)

                             readHolds.set(rh);

                         rh.count++;

                     }

                     return true;

                 }

             }

         }

         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();

         }

         // Methods relayed to outer class

         final ConditionObject newCondition() {

             return new ConditionObject();

         }

         final Thread getOwner() {

             // Must read state before owner to ensure memory consistency

             return ((exclusiveCount(getState()) == 0) ?

                     null :

                     getExclusiveOwnerThread());

         }

         final int getReadLockCount() {

             return sharedCount(getState());

         }

         final boolean isWriteLocked() {

             return exclusiveCount(getState()) != 0;

         }

         final int getWriteHoldCount() {

             return isHeldExclusively() ? exclusiveCount(getState()) : 0;

         }

         final int getReadHoldCount() {

             if (getReadLockCount() == 0)

                 return 0;

             Thread current = Thread.currentThread();

             if (firstReader == current)

                 return firstReaderHoldCount;

             HoldCounter rh = cachedHoldCounter;

             if (rh != null && rh.tid == current.getId())

                 return rh.count;

             int count = readHolds.get().count;

             if (count == 0) readHolds.remove();

             return count;

         }

         /**

          * Reconstitute this lock instance from a stream

          * @param s the stream

          */

         private void readObject(java.io.ObjectInputStream s)

             throws java.io.IOException, ClassNotFoundException {

             s.defaultReadObject();

             readHolds = new ThreadLocalHoldCounter();

             setState(0); // reset to unlocked state

         }

         final int getCount() { return getState(); }

     }

     /**

      * Nonfair version of Sync

      */

     static final class NonfairSync extends Sync {

         private static final long serialVersionUID = -8159625535654395037L;

         final boolean writerShouldBlock() {

             return false; // writers can always barge

         }

         final boolean readerShouldBlock() {

             /* As a heuristic to avoid indefinite writer starvation,

              * block if the thread that momentarily appears to be head

              * of queue, if one exists, is a waiting writer.  This is

              * only a probabilistic effect since a new reader will not

              * block if there is a waiting writer behind other enabled

              * readers that have not yet drained from the queue.

              */

             return apparentlyFirstQueuedIsExclusive();

         }

     }

     /**

      * Fair version of Sync

      */

     static final class FairSync extends Sync {

         private static final long serialVersionUID = -2274990926593161451L;

         final boolean writerShouldBlock() {

             return hasQueuedPredecessors();

         }

         final boolean readerShouldBlock() {

             return hasQueuedPredecessors();

         }

     }

     /**

      * The lock returned by method {@link ReentrantReadWriteLock#readLock}.

      */

     public static class ReadLock implements Lock, java.io.Serializable {

         private static final long serialVersionUID = -5992448646407690164L;

         private final Sync sync;

         /**

          * Constructor for use by subclasses

          *

          * @param lock the outer lock object

          * @throws NullPointerException if the lock is null

          */

         protected ReadLock(ReentrantReadWriteLock lock) {

             sync = lock.sync;

         }

         /**

          * Acquires the read lock.

          *

          * <p>Acquires the read lock if the write lock is not held by

          * another thread and returns immediately.

          *

          * <p>If the write lock is held by another thread then

          * the current thread becomes disabled for thread scheduling

          * purposes and lies dormant until the read lock has been acquired.

          */

         public void lock() {

             sync.acquireShared(1);

         }

         /**

          * Acquires the read lock unless the current thread is

          * {@linkplain Thread#interrupt interrupted}.

          *

          * <p>Acquires the read lock if the write lock is not held

          * by another thread and returns immediately.

          *

          * <p>If the write 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 read lock is acquired by the current thread; or

          *

          * <li>Some other thread {@linkplain Thread#interrupt interrupts}

          * the current thread.

          *

          * </ul>

          *

          * <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 read 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.acquireSharedInterruptibly(1);

         }

         /**

          * Acquires the read lock only if the write lock is not held by

          * another thread at the time of invocation.

          *

          * <p>Acquires the read lock if the write lock is not held by

          * another thread and returns immediately with the value

          * {@code true}. Even when this lock has been set to use a

          * fair ordering policy, a call to {@code tryLock()}

          * <em>will</em> immediately acquire the read lock if it is

          * available, whether or not other threads are currently

          * waiting for the read 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 write lock is held by another thread then

          * this method will return immediately with the value

          * {@code false}.

          *

          * @return {@code true} if the read lock was acquired

          */

         public  boolean tryLock() {

             return sync.tryReadLock();

         }

         /**

          * Acquires the read lock if the write lock 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 read lock if the write lock is not held by

          * another thread and returns immediately with the value

          * {@code true}. 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 write 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 read 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 read lock is acquired then the value {@code true} is

          * returned.

          *

          * <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 read 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 read lock

          * @param unit the time unit of the timeout argument

          * @return {@code true} if the read lock was 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.tryAcquireSharedNanos(1, unit.toNanos(timeout));

         }

         /**

          * Attempts to release this lock.

          *

          * <p> If the number of readers is now zero then the lock

          * is made available for write lock attempts.

          */

         public  void unlock() {

             sync.releaseShared(1);

         }

         /**

          * Throws {@code UnsupportedOperationException} because

          * {@code ReadLocks} do not support conditions.

          *

          * @throws UnsupportedOperationException always

          */

         public Condition newCondition() {

             throw new UnsupportedOperationException();

         }

         /**

          * Returns a string identifying this lock, as well as its lock state.

          * The state, in brackets, includes the String {@code "Read locks ="}

          * followed by the number of held read locks.

          *

          * @return a string identifying this lock, as well as its lock state

          */

         public String toString() {

             int r = sync.getReadLockCount();

             return super.toString() +

                 "[Read locks = " + r + "]";

         }

     }

     /**

      * The lock returned by method {@link ReentrantReadWriteLock#writeLock}.

      */

     public static class WriteLock implements Lock, java.io.Serializable {

         private static final long serialVersionUID = -4992448646407690164L;

         private final Sync sync;

         /**

          * Constructor for use by subclasses

          *

          * @param lock the outer lock object

          * @throws NullPointerException if the lock is null

          */

         protected WriteLock(ReentrantReadWriteLock lock) {

             sync = lock.sync;

         }

         /**

          * Acquires the write lock.

          *

          * <p>Acquires the write lock if neither the read nor write lock

          * are held by another thread

          * and returns immediately, setting the write lock hold count to

          * one.

          *

          * <p>If the current thread already holds the write 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 write lock has been acquired, at which

          * time the write lock hold count is set to one.

          */

         public void lock() {

             sync.acquire(1);

         }

         /**

          * Acquires the write lock unless the current thread is

          * {@linkplain Thread#interrupt interrupted}.

          *

          * <p>Acquires the write lock if neither the read nor write lock

          * are held by another thread

          * and returns immediately, setting the write 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 write lock is acquired by the current thread; or

          *

          * <li>Some other thread {@linkplain Thread#interrupt interrupts}

          * the current thread.

          *

          * </ul>

          *

          * <p>If the write 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 write 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 write lock only if it is not held by another thread

          * at the time of invocation.

          *

          * <p>Acquires the write lock if neither the read nor write lock

          * are held by another thread

          * and returns immediately with the value {@code true},

          * setting the write 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 write 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 write lock was already held

          * by the current thread; and {@code false} otherwise.

          */

         public boolean tryLock( ) {

             return sync.tryWriteLock();

         }

         /**

          * Acquires the write 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 write lock if neither the read nor write lock

          * are held by another thread

          * and returns immediately with the value {@code true},

          * setting the write 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 write 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 write 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 write lock is acquired then the value {@code true} is

          * returned and the write 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 write 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 write 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 write 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 write lock is not held when any {@link

          * Condition} method is called then an {@link

          * IllegalMonitorStateException} is thrown.  (Read locks are

          * held independently of write locks, so are not checked or

          * affected. However it is essentially always an error to

          * invoke a condition waiting method when the current thread

          * has also acquired read locks, since other threads that

          * could unblock it will not be able to acquire the write

          * lock.)

          *

          * <li>When the condition {@linkplain Condition#await() waiting}

          * methods are called the write lock is released and, before

          * they return, the write 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();

         }

         /**

          * 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() + "]");

         }

         /**

          * Queries if this write lock is held by the current thread.

          * Identical in effect to {@link

          * ReentrantReadWriteLock#isWriteLockedByCurrentThread}.

          *

          * @return {@code true} if the current thread holds this lock and

          *         {@code false} otherwise

          * @since 1.6

          */

         public boolean isHeldByCurrentThread() {

             return sync.isHeldExclusively();

         }

         /**

          * Queries the number of holds on this write lock by the current

          * thread.  A thread has a hold on a lock for each lock action

          * that is not matched by an unlock action.  Identical in effect

          * to {@link ReentrantReadWriteLock#getWriteHoldCount}.

          *

          * @return the number of holds on this lock by the current thread,

          *         or zero if this lock is not held by the current thread

          * @since 1.6

          */

         public int getHoldCount() {

             return sync.getWriteHoldCount();

         }

     }

     // Instrumentation and status

     /**

      * 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 the write 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 the number of read locks held for this lock. This

      * method is designed for use in monitoring system state, not for

      * synchronization control.

      * @return the number of read locks held.

      */

     public int getReadLockCount() {

         return sync.getReadLockCount();

     }

     /**

      * Queries if the write lock is held by any thread. This method is

      * designed for use in monitoring system state, not for

      * synchronization control.

      *

      * @return {@code true} if any thread holds the write lock and

      *         {@code false} otherwise

      */

     public boolean isWriteLocked() {

         return sync.isWriteLocked();

     }

     /**

      * Queries if the write lock is held by the current thread.

      *

      * @return {@code true} if the current thread holds the write lock and

      *         {@code false} otherwise

      */

     public boolean isWriteLockedByCurrentThread() {

         return sync.isHeldExclusively();

     }

     /**

      * Queries the number of reentrant write holds on this lock by the

      * current thread.  A writer thread has a hold on a lock for

      * each lock action that is not matched by an unlock action.

      *

      * @return the number of holds on the write lock by the current thread,

      *         or zero if the write lock is not held by the current thread

      */

     public int getWriteHoldCount() {

         return sync.getWriteHoldCount();

     }

     /**

      * Queries the number of reentrant read holds on this lock by the

      * current thread.  A reader thread has a hold on a lock for

      * each lock action that is not matched by an unlock action.

      *

      * @return the number of holds on the read lock by the current thread,

      *         or zero if the read lock is not held by the current thread

      * @since 1.6

      */

     public int getReadHoldCount() {

         return sync.getReadHoldCount();

     }

     /**

      * Returns a collection containing threads that may be waiting to

      * acquire the write 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 lock monitoring facilities.

      *

      * @return the collection of threads

      */

     protected Collection<Thread> getQueuedWriterThreads() {

         return sync.getExclusiveQueuedThreads();

     }

     /**

      * Returns a collection containing threads that may be waiting to

      * acquire the read 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 lock monitoring facilities.

      *

      * @return the collection of threads

      */

     protected Collection<Thread> getQueuedReaderThreads() {

         return sync.getSharedQueuedThreads();

     }

     /**

      * Queries whether any threads are waiting to acquire the read or

      * write lock. Note that because cancellations may occur at any

      * time, a {@code true} return does not guarantee that any other

      * thread will ever acquire a 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 either

      * the read or write lock. Note that because cancellations may

      * occur at any time, a {@code true} return does not guarantee

      * that this thread will ever acquire a 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

      * either the read or write 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 either the read or write 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 the write 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 the write 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 the write 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 the String {@code "Write locks ="}

      * followed by the number of reentrantly held write locks, and the

      * String {@code "Read locks ="} followed by the number of held

      * read locks.

      *

      * @return a string identifying this lock, as well as its lock state

      */

     public String toString() {

         int c = sync.getCount();

         int w = Sync.exclusiveCount(c);

         int r = Sync.sharedCount(c);

         return super.toString() +

             "[Write locks = " + w + ", Read locks = " + r + "]";

     }

 }