Java tutorial
/* * 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 {@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> * * <h2>Implementation Notes</h2> * * <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; // 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. */ 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; 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. */ @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. */ 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. */ @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. */ @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). */ 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. 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 {@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 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 + "]"; } }