Back to index...

	/*
	* 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.TimeUnit;
	import java.util.Collection;

	/**
	* 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.)
	* <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 = getThreadId(Thread.currentThread());
	}

	/**
	* 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 != 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 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 != 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.
	*/
	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 != 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 != 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.
	*/
	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 != 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 == 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).
	*/
	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> {@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.
	*
	* <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> {@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.
	*
	* <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 + "]";
	}

	/**
	* Returns the thread id for the given thread. We must access
	* this directly rather than via method Thread.getId() because
	* getId() is not final, and has been known to be overridden in
	* ways that do not preserve unique mappings.
	*/
	static final long getThreadId(Thread thread) {
	return UNSAFE.getLongVolatile(thread, TID_OFFSET);
	}

	// Unsafe mechanics
	private static final sun.misc.Unsafe UNSAFE;
	private static final long TID_OFFSET;
	static {
	try {
	UNSAFE = sun.misc.Unsafe.getUnsafe();
	Class<?> tk = Thread.class;
	TID_OFFSET = UNSAFE.objectFieldOffset
	(tk.getDeclaredField("tid"));
	} catch (Exception e) {
	throw new Error(e);
	}
	}

	}

Back to index...