java/12 : java.base/java/util/concurrent/locks/ReentrantReadWriteLock.java

ReentrantReadWriteLock
https://openjdk.java.net/
GPLv2 + Classpath Exception
/*
 * 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.Collection;
import java.util.concurrent.TimeUnit;
import jdk.internal.vm.annotation.ReservedStackAccess;

An implementation of ReadWriteLock supporting similar semantics to ReentrantLock. This class has the following properties:

Acquisition order
This class does not impose a reader or writer preference
ordering for lock access.  However, it does support an optional
fairness policy.

Non-fair mode (default)
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.
Fair mode
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.
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.
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 ReadLock.tryLock() and WriteLock.tryLock() methods do not honor this fair setting and will immediately acquire the lock if it is possible, regardless of waiting threads.) 
Reentrancy
This lock allows both readers and writers to reacquire read or write locks in the style of a ReentrantLock. Non-reentrant readers are not allowed until all write locks held by the writing thread have been released. 
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.
Lock downgrading
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
not possible.
Interruption of lock acquisition
The read lock and write lock both support interruption during lock
acquisition.
Condition support
The write lock provides a Condition implementation that behaves in the same way, with respect to the write lock, as the Condition implementation provided by ReentrantLock.newCondition does for ReentrantLock. This Condition can, of course, only be used with the write lock. 
The read lock does not support a Condition and readLock().newCondition() throws UnsupportedOperationException. 
Instrumentation
This class supports methods to determine whether locks
are held or contended. These methods are designed for monitoring
system state, not for synchronization control.

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.
Sample usages. 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):
 
class CachedData {
  Object data;
  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();
    }
  }
 }
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.
 
class RWDictionary {
  private final Map<String, Data> m = new TreeMap<>();
  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 List<String> allKeys() {
    r.lock();
    try { return new ArrayList<>(m.keySet()); }
    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(); }
  }
 }
Implementation Notes
This lock supports a maximum of 65535 recursive write locks and 65535 read locks. Attempts to exceed these limits result in Error throws from locking methods. 
Author: Doug Lea
Since: 1.5/**
 * 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.
 *
 * <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 immediately acquire the lock
 * if it is possible, regardless of waiting threads.)
 * </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;
 *   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<>();
 *   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 List<String> allKeys() {
 *     r.lock();
 *     try { return new ArrayList<>(m.keySet()); }
 *     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 /** Inner class providing readlock */
    private final ReentrantReadWriteLock.ReadLock readerLock;
    Inner class providing writelock /** Inner class providing writelock */
    private final ReentrantReadWriteLock.WriteLock writerLock;
    Performs all synchronization mechanics /** Performs all synchronization mechanics */
    final Sync sync;

    Creates a new ReentrantReadWriteLock with default (nonfair) ordering properties. /**
     * Creates a new {@code ReentrantReadWriteLock} with
     * default (nonfair) ordering properties.
     */
    public ReentrantReadWriteLock() {
        this(false);
    }

    Creates a new ReentrantReadWriteLock with the given fairness policy. 
Params: fair – true if this lock should use a fair ordering policy/**
     * 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.
/**
     * 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. /** 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. /** 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.
/**
         * A counter for per-thread read hold counts.
         * Maintained as a ThreadLocal; cached in cachedHoldCounter.
         */
        static final class HoldCounter {
            int count;          // initially 0
            // Use id, not reference, to avoid garbage retention
            final long tid = LockSupport.getThreadId(Thread.currentThread());
        }

        ThreadLocal subclass. Easiest to explicitly define for sake
of deserialization mechanics.
/**
         * 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.
/**
         * 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.
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.
Accessed via a benign data race; relies on the memory
model's final field and out-of-thin-air guarantees.
/**
         * 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.
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.
Cannot cause garbage retention unless the thread terminated
without relinquishing its read locks, since tryReleaseShared
sets it to null.
Accessed via a benign data race; relies on the memory
model's out-of-thin-air guarantees for references.
This allows tracking of read holds for uncontended read
locks to be very cheap.
/**
         * 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;
        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.
/**
         * 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.
/**
         * 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.
         */
        @ReservedStackAccess
        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;
        }

        @ReservedStackAccess
        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;
        }

        @ReservedStackAccess
        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 != LockSupport.getThreadId(current))
                    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 static IllegalMonitorStateException unmatchedUnlockException() {
            return new IllegalMonitorStateException(
                "attempt to unlock read lock, not locked by current thread");
        }

        @ReservedStackAccess
        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 != LockSupport.getThreadId(current))
                        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.
/**
         * 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 != LockSupport.getThreadId(current)) {
                                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 != LockSupport.getThreadId(current))
                            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.
/**
         * Performs tryLock for write, enabling barging in both modes.
         * This is identical in effect to tryAcquire except for lack
         * of calls to writerShouldBlock.
         */
        @ReservedStackAccess
        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.
/**
         * Performs tryLock for read, enabling barging in both modes.
         * This is identical in effect to tryAcquireShared except for
         * lack of calls to readerShouldBlock.
         */
        @ReservedStackAccess
        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 != LockSupport.getThreadId(current))
                            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 == LockSupport.getThreadId(current))
                return rh.count;

            int count = readHolds.get().count;
            if (count == 0) readHolds.remove();
            return count;
        }

        Reconstitutes the instance from a stream (that is, deserializes it).
/**
         * Reconstitutes the instance from a stream (that is, deserializes it).
         */
        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
/**
     * 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
/**
     * 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 ReentrantReadWriteLock.readLock. /**
     * 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.
Params: lock – the outer lock object
Throws: NullPointerException – if the lock is null/**
         * 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.
Acquires the read lock if the write lock is not held by
another thread and returns immediately.
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.
/**
         * 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 interrupted. Acquires the read lock if the write lock is not held
by another thread and returns immediately.
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:

The read lock is acquired by the current thread; or
Some other thread interrupts the current thread. 
If the current thread:

has its interrupted status set on entry to this method; or
is interrupted while acquiring the read lock, 
 then InterruptedException is thrown and the current thread's interrupted status is cleared. 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/**
         * 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.
Acquires the read lock if the write lock is not held by another thread and returns immediately with the value true. Even when this lock has been set to use a fair ordering policy, a call to tryLock() will 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 
tryLock(0, TimeUnit.SECONDS) which is almost equivalent (it also detects interruption). 
If the write lock is held by another thread then this method will return immediately with the value false. 
Returns: true if the read lock was acquired/**
         * 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 &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 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 
interrupted. Acquires the read lock if the write lock is not held by another thread and returns immediately with the value true. If this lock has been set to use a fair ordering policy then an available lock will not be acquired if any other threads are waiting for the lock. This is in contrast to the tryLock() method. If you want a timed tryLock that does permit barging on a fair lock then combine the timed and un-timed forms together: 
 
if (lock.tryLock() ||
    lock.tryLock(timeout, unit)) {
  ...
 }
If the 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:

The read lock is acquired by the current thread; or
Some other thread interrupts the current thread; or 
The specified waiting time elapses.

If the read lock is acquired then the value true is returned. 
If the current thread:

has its interrupted status set on entry to this method; or
is interrupted while acquiring the read lock, 
 then InterruptedException is thrown and the current thread's interrupted status is cleared. If the specified waiting time elapses then the value false is returned. If the time is less than or equal to zero, the method will not wait at all. 
In this implementation, as this method is an explicit
interruption point, preference is given to responding to
the interrupt over normal or reentrant acquisition of the
lock, and over reporting the elapse of the waiting time.
Params: timeout – the time to wait for the read lock
unit – the time unit of the timeout argument
Throws: InterruptedException – if the current thread is interrupted
NullPointerException – if the time unit is null
Returns: true if the read lock was acquired/**
         * 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> {@code
         * 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.
If the number of readers is now zero then the lock is made available for write lock attempts. If the current thread does not hold this lock then IllegalMonitorStateException is thrown. 
Throws: IllegalMonitorStateException – if the current thread
does not hold this lock/**
         * Attempts to release this lock.
         *
         * <p>If the number of readers is now zero then the lock
         * is made available for write lock attempts. If the current
         * thread does not hold this lock then {@link
         * IllegalMonitorStateException} is thrown.
         *
         * @throws IllegalMonitorStateException if the current thread
         * does not hold this lock
         */
        public void unlock() {
            sync.releaseShared(1);
        }

        Throws UnsupportedOperationException because ReadLocks do not support conditions. 
Throws: UnsupportedOperationException – always/**
         * 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 "Read locks =" followed by the number of held read locks. 
Returns: a string identifying this lock, as well as its lock state/**
         * 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 ReentrantReadWriteLock.writeLock. /**
     * 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.
Params: lock – the outer lock object
Throws: NullPointerException – if the lock is null/**
         * 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.
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.
If the current thread already holds the write lock then the
hold count is incremented by one and the method returns
immediately.
If the lock is held by another thread then the current
thread becomes disabled for thread scheduling purposes and
lies dormant until the write lock has been acquired, at which
time the write lock hold count is set to one.
/**
         * 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 interrupted. 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.
If the current thread already holds this lock then the
hold count is incremented by one and the method returns
immediately.
If the lock is held by another thread then the current
thread becomes disabled for thread scheduling purposes and
lies dormant until one of two things happens:

The write lock is acquired by the current thread; or
Some other thread interrupts the current thread. 
If the write lock is acquired by the current thread then the
lock hold count is set to one.
If the current thread:

has its interrupted status set on entry to this method;
or
is interrupted while acquiring the write lock, 
 then InterruptedException is thrown and the current thread's interrupted status is cleared. 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/**
         * 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.
Acquires the write lock if neither the read nor write lock are held by another thread and returns immediately with the value 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 tryLock() will 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 tryLock(0, TimeUnit.SECONDS) which is almost equivalent (it also detects interruption). 
If the current thread already holds this lock then the hold count is incremented by one and the method returns true. 
If the lock is held by another thread then this method will return immediately with the value false. 
Returns: 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 false otherwise./**
         * 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 &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 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 interrupted. Acquires the write lock if neither the read nor write lock are held by another thread and returns immediately with the value 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 will not be acquired if any other threads are waiting for the write lock. This is in contrast to the tryLock() method. If you want a timed tryLock that does permit barging on a fair lock then combine the timed and un-timed forms together: 
 
if (lock.tryLock() ||
    lock.tryLock(timeout, unit)) {
  ...
 }
If the current thread already holds this lock then the hold count is incremented by one and the method returns true. 
If the lock is held by another thread then the current
thread becomes disabled for thread scheduling purposes and
lies dormant until one of three things happens:

The write lock is acquired by the current thread; or
Some other thread interrupts the current thread; or 
The specified waiting time elapses

If the write lock is acquired then the value true is returned and the write lock hold count is set to one. 
If the current thread:

has its interrupted status set on entry to this method;
or
is interrupted while acquiring the write lock, 
 then InterruptedException is thrown and the current thread's interrupted status is cleared. If the specified waiting time elapses then the value false is returned. If the time is less than or equal to zero, the method will not wait at all. 
In this implementation, as this method is an explicit
interruption point, preference is given to responding to
the interrupt over normal or reentrant acquisition of the
lock, and over reporting the elapse of the waiting time.
Params: timeout – the time to wait for the write lock
unit – the time unit of the timeout argument
Throws: InterruptedException – if the current thread is interrupted
NullPointerException – if the time unit is null
Returns: 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 false if the waiting time elapsed before the lock could be acquired./**
         * 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> {@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 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.
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 IllegalMonitorStateException is thrown. 
Throws: IllegalMonitorStateException – if the current thread does not
hold this lock/**
         * 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 Condition instance for use with this Lock instance. The returned Condition instance supports the same usages as do the Object monitor methods (wait, notify, and notifyAll) when used with the built-in monitor lock. 

If this write lock is not held when any Condition method is called then an 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.) 
When the condition 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. 
If a thread is interrupted while waiting then the wait will terminate, an InterruptedException will be thrown, and the thread's interrupted status will be cleared. 
Waiting threads are signalled in FIFO order.
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 fair locks favors those threads that have been
waiting the longest.

Returns: the Condition object/**
         * 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 "Unlocked" or the String "Locked by" followed by the name of the owning thread. 
Returns: a string identifying this lock, as well as its lock state/**
         * 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 ReentrantReadWriteLock.isWriteLockedByCurrentThread. 
Returns: true if the current thread holds this lock and false otherwise
Since: 1.6/**
         * 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 ReentrantReadWriteLock.getWriteHoldCount. 
Returns: 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/**
         * 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 true if this lock has fairness set true. 
Returns: true if this lock has fairness set true/**
     * 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 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 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. 
Returns: the owner, or null if not owned/**
     * 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.
Returns: the number of read locks held/**
     * 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.
Returns: true if any thread holds the write lock and false otherwise/**
     * 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.
Returns: true if the current thread holds the write lock and false otherwise/**
     * 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.
Returns: 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/**
     * 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.
Returns: 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/**
     * 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.
Returns: the collection of threads/**
     * 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.
Returns: the collection of threads/**
     * 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 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. 
Returns: true if there may be other threads waiting to acquire the lock/**
     * 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 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. 
Params: thread – the thread
Throws: NullPointerException – if the thread is null
Returns: true if the given thread is queued waiting for this lock/**
     * 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 system state, not for
synchronization control.
Returns: the estimated number of threads waiting for this lock/**
     * 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 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.
Returns: the collection of threads/**
     * 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 true return does not guarantee that a future signal will awaken any threads. This method is designed primarily for use in monitoring of the system state. 
Params: condition – the condition
Throws: IllegalMonitorStateException – if this lock is not held
IllegalArgumentException – if the given condition is
        not associated with this lock
NullPointerException – if the condition is null
Returns: true if there are any waiting threads/**
     * 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.
Params: condition – the condition
Throws: IllegalMonitorStateException – if this lock is not held
IllegalArgumentException – if the given condition is
        not associated with this lock
NullPointerException – if the condition is null
Returns: the estimated number of waiting threads/**
     * 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.
Params: condition – the condition
Throws: IllegalMonitorStateException – if this lock is not held
IllegalArgumentException – if the given condition is
        not associated with this lock
NullPointerException – if the condition is null
Returns: the collection of threads/**
     * 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 "Write locks =" followed by the number of reentrantly held write locks, and the String "Read locks =" followed by the number of held read locks. 
Returns: a string identifying this lock, as well as its lock state/**
     * 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 + "]";
    }

}
Params:	lock – the outer lock object
Throws:	NullPointerException – if the lock is null
Params:	timeout – the time to wait for the read lock unit – the time unit of the timeout argument
Throws:	InterruptedException – if the current thread is interrupted NullPointerException – if the time unit is null
Returns:	`true` if the read lock was acquired
Params:	timeout – the time to wait for the write lock unit – the time unit of the timeout argument
Throws:	InterruptedException – if the current thread is interrupted NullPointerException – if the time unit is null
Returns:	`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 `false` if the waiting time elapsed before the lock could be acquired.
Returns:	`true` if the current thread holds this lock and `false` otherwise
Since:	1.6
Returns:	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
Returns:	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
Params:	thread – the thread
Throws:	NullPointerException – if the thread is null
Returns:	`true` if the given thread is queued waiting for this lock
Params:	condition – the condition
Throws:	IllegalMonitorStateException – if this lock is not held IllegalArgumentException – if the given condition is not associated with this lock NullPointerException – if the condition is null
Returns:	`true` if there are any waiting threads
/

java/ 12/ java.base/java/util/concurrent/locks/ReentrantReadWriteLock.java

Implementation Notes
