This class has the following properties: + * + *
This class does not impose a reader or writer preference + * ordering for lock access. However, it does support an optional + * fairness policy. + * + *
+ * + *
A thread that tries to acquire a fair read lock (non-reentrantly) + * will block if either the write lock is held, or there is a waiting + * writer thread. The thread will not acquire the read lock until + * after the oldest currently waiting writer thread has acquired and + * released the write lock. Of course, if a waiting writer abandons + * its wait, leaving one or more reader threads as the longest waiters + * in the queue with the write lock free, then those readers will be + * assigned the read lock. + * + *
A thread that tries to acquire a fair write lock (non-reentrantly) + * will block unless both the read lock and write lock are free (which + * implies there are no waiting threads). (Note that the non-blocking + * {@link ReadLock#tryLock()} and {@link WriteLock#tryLock()} methods + * do not honor this fair setting and will acquire the lock if it is + * possible, regardless of waiting threads.) + *
+ *
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. + * + *
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. + * + *
Reentrancy also allows downgrading from the write lock to a read lock, + * by acquiring the write lock, then the read lock and then releasing the + * write lock. However, upgrading from a read lock to the write lock is + * not possible. + * + *
The read lock and write lock both support interruption during lock + * acquisition. + * + *
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. + * + *
The read lock does not support a {@link Condition} and + * {@code readLock().newCondition()} throws + * {@code UnsupportedOperationException}. + * + *
This class supports methods to determine whether locks + * are held or contended. These methods are designed for monitoring + * system state, not for synchronization control. + *
Serialization of this class behaves in the same way as built-in + * locks: a deserialized lock is in the unlocked state, regardless of + * its state when serialized. + * + *
Sample usages. Here is a code sketch showing how to perform + * lock downgrading after updating a cache (exception handling is + * particularly tricky when handling multiple locks in a non-nested + * fashion): + * + *
{@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();
+ * }
+ * }
+ * }}
+ *
+ * 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.
+ *
+ * {@code
+ * class RWDictionary {
+ * private final Map 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 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(); }
+ * }
+ * }}
+ *
+ * This lock supports a maximum of 65535 recursive write locks
+ * and 65535 read locks. Attempts to exceed these limits result in
+ * {@link Error} throws from locking methods.
+ *
+ * @since 1.5
+ * @author Doug Lea
+ *
+ */
+public class ReentrantReadWriteLock
+ implements ReadWriteLock, java.io.Serializable {
+ private static final long serialVersionUID = -6992448646407690164L;
+ /** Inner class providing readlock */
+ private final ReentrantReadWriteLock.ReadLock readerLock;
+ /** Inner class providing writelock */
+ private final ReentrantReadWriteLock.WriteLock writerLock;
+ /** Performs all synchronization mechanics */
+ final Sync sync;
+
+ /**
+ * Creates a new {@code ReentrantReadWriteLock} with
+ * default (nonfair) ordering properties.
+ */
+ public ReentrantReadWriteLock() {
+ this(false);
+ }
+
+ /**
+ * Creates a new {@code ReentrantReadWriteLock} with
+ * the given fairness policy.
+ *
+ * @param fair {@code true} if this lock should use a fair ordering policy
+ */
+ public ReentrantReadWriteLock(boolean fair) {
+ sync = fair ? new FairSync() : new NonfairSync();
+ readerLock = new ReadLock(this);
+ writerLock = new WriteLock(this);
+ }
+
+ public ReentrantReadWriteLock.WriteLock writeLock() { return writerLock; }
+ public ReentrantReadWriteLock.ReadLock readLock() { return readerLock; }
+
+ /**
+ * Synchronization implementation for ReentrantReadWriteLock.
+ * Subclassed into fair and nonfair versions.
+ */
+ abstract static class Sync extends AbstractQueuedSynchronizer {
+ private static final long serialVersionUID = 6317671515068378041L;
+
+ /*
+ * Read vs write count extraction constants and functions.
+ * Lock state is logically divided into two unsigned shorts:
+ * The lower one representing the exclusive (writer) lock hold count,
+ * and the upper the shared (reader) hold count.
+ */
+
+ static final int SHARED_SHIFT = 16;
+ static final int SHARED_UNIT = (1 << SHARED_SHIFT);
+ static final int MAX_COUNT = (1 << SHARED_SHIFT) - 1;
+ static final int EXCLUSIVE_MASK = (1 << SHARED_SHIFT) - 1;
+
+ /** Returns the number of shared holds represented in count */
+ static int sharedCount(int c) { return c >>> SHARED_SHIFT; }
+ /** Returns the number of exclusive holds represented in count */
+ static int exclusiveCount(int c) { return c & EXCLUSIVE_MASK; }
+
+ /**
+ * A counter for per-thread read hold counts.
+ * Maintained as a ThreadLocal; cached in cachedHoldCounter
+ */
+ static final class HoldCounter {
+ int count = 0;
+ // Use id, not reference, to avoid garbage retention
+ final long tid = Thread.currentThread().getId();
+ }
+
+ /**
+ * ThreadLocal subclass. Easiest to explicitly define for sake
+ * of deserialization mechanics.
+ */
+ static final class ThreadLocalHoldCounter
+ extends ThreadLocal Can outlive the Thread for which it is caching the read
+ * hold count, but avoids garbage retention by not retaining a
+ * reference to the Thread.
+ *
+ * Accessed via a benign data race; relies on the memory
+ * model's final field and out-of-thin-air guarantees.
+ */
+ private transient HoldCounter cachedHoldCounter;
+
+ /**
+ * firstReader is the first thread to have acquired the read lock.
+ * firstReaderHoldCount is firstReader's hold count.
+ *
+ * More precisely, firstReader is the unique thread that last
+ * changed the shared count from 0 to 1, and has not released the
+ * read lock since then; null if there is no such thread.
+ *
+ * Cannot cause garbage retention unless the thread terminated
+ * without relinquishing its read locks, since tryReleaseShared
+ * sets it to null.
+ *
+ * Accessed via a benign data race; relies on the memory
+ * model's out-of-thin-air guarantees for references.
+ *
+ * This allows tracking of read holds for uncontended read
+ * locks to be very cheap.
+ */
+ private transient Thread firstReader = null;
+ private transient int firstReaderHoldCount;
+
+ Sync() {
+ readHolds = new ThreadLocalHoldCounter();
+ setState(getState()); // ensures visibility of readHolds
+ }
+
+ /*
+ * Acquires and releases use the same code for fair and
+ * nonfair locks, but differ in whether/how they allow barging
+ * when queues are non-empty.
+ */
+
+ /**
+ * Returns true if the current thread, when trying to acquire
+ * the read lock, and otherwise eligible to do so, should block
+ * because of policy for overtaking other waiting threads.
+ */
+ abstract boolean readerShouldBlock();
+
+ /**
+ * Returns true if the current thread, when trying to acquire
+ * the write lock, and otherwise eligible to do so, should block
+ * because of policy for overtaking other waiting threads.
+ */
+ abstract boolean writerShouldBlock();
+
+ /*
+ * Note that tryRelease and tryAcquire can be called by
+ * Conditions. So it is possible that their arguments contain
+ * both read and write holds that are all released during a
+ * condition wait and re-established in tryAcquire.
+ */
+
+ protected final boolean tryRelease(int releases) {
+ if (!isHeldExclusively())
+ throw new IllegalMonitorStateException();
+ int nextc = getState() - releases;
+ boolean free = exclusiveCount(nextc) == 0;
+ if (free)
+ setExclusiveOwnerThread(null);
+ setState(nextc);
+ return free;
+ }
+
+ protected final boolean tryAcquire(int acquires) {
+ /*
+ * Walkthrough:
+ * 1. If read count nonzero or write count nonzero
+ * and owner is a different thread, fail.
+ * 2. If count would saturate, fail. (This can only
+ * happen if count is already nonzero.)
+ * 3. Otherwise, this thread is eligible for lock if
+ * it is either a reentrant acquire or
+ * queue policy allows it. If so, update state
+ * and set owner.
+ */
+ Thread current = Thread.currentThread();
+ int c = getState();
+ int w = exclusiveCount(c);
+ if (c != 0) {
+ // (Note: if c != 0 and w == 0 then shared count != 0)
+ if (w == 0 || current != getExclusiveOwnerThread())
+ return false;
+ if (w + exclusiveCount(acquires) > MAX_COUNT)
+ throw new Error("Maximum lock count exceeded");
+ // Reentrant acquire
+ setState(c + acquires);
+ return true;
+ }
+ if (writerShouldBlock() ||
+ !compareAndSetState(c, c + acquires))
+ return false;
+ setExclusiveOwnerThread(current);
+ return true;
+ }
+
+ protected final boolean tryReleaseShared(int unused) {
+ Thread current = Thread.currentThread();
+ if (firstReader == current) {
+ // assert firstReaderHoldCount > 0;
+ if (firstReaderHoldCount == 1)
+ firstReader = null;
+ else
+ firstReaderHoldCount--;
+ } else {
+ HoldCounter rh = cachedHoldCounter;
+ if (rh == null || rh.tid != current.getId())
+ rh = readHolds.get();
+ int count = rh.count;
+ if (count <= 1) {
+ readHolds.remove();
+ if (count <= 0)
+ throw unmatchedUnlockException();
+ }
+ --rh.count;
+ }
+ for (;;) {
+ int c = getState();
+ int nextc = c - SHARED_UNIT;
+ if (compareAndSetState(c, nextc))
+ // Releasing the read lock has no effect on readers,
+ // but it may allow waiting writers to proceed if
+ // both read and write locks are now free.
+ return nextc == 0;
+ }
+ }
+
+ private IllegalMonitorStateException unmatchedUnlockException() {
+ return new IllegalMonitorStateException(
+ "attempt to unlock read lock, not locked by current thread");
+ }
+
+ protected final int tryAcquireShared(int unused) {
+ /*
+ * Walkthrough:
+ * 1. If write lock held by another thread, fail.
+ * 2. Otherwise, this thread is eligible for
+ * lock wrt state, so ask if it should block
+ * because of queue policy. If not, try
+ * to grant by CASing state and updating count.
+ * Note that step does not check for reentrant
+ * acquires, which is postponed to full version
+ * to avoid having to check hold count in
+ * the more typical non-reentrant case.
+ * 3. If step 2 fails either because thread
+ * apparently not eligible or CAS fails or count
+ * saturated, chain to version with full retry loop.
+ */
+ Thread current = Thread.currentThread();
+ int c = getState();
+ if (exclusiveCount(c) != 0 &&
+ getExclusiveOwnerThread() != current)
+ return -1;
+ int r = sharedCount(c);
+ if (!readerShouldBlock() &&
+ r < MAX_COUNT &&
+ compareAndSetState(c, c + SHARED_UNIT)) {
+ if (r == 0) {
+ firstReader = current;
+ firstReaderHoldCount = 1;
+ } else if (firstReader == current) {
+ firstReaderHoldCount++;
+ } else {
+ HoldCounter rh = cachedHoldCounter;
+ if (rh == null || rh.tid != current.getId())
+ cachedHoldCounter = rh = readHolds.get();
+ else if (rh.count == 0)
+ readHolds.set(rh);
+ rh.count++;
+ }
+ return 1;
+ }
+ return fullTryAcquireShared(current);
+ }
+
+ /**
+ * Full version of acquire for reads, that handles CAS misses
+ * and reentrant reads not dealt with in tryAcquireShared.
+ */
+ final int fullTryAcquireShared(Thread current) {
+ /*
+ * This code is in part redundant with that in
+ * tryAcquireShared but is simpler overall by not
+ * complicating tryAcquireShared with interactions between
+ * retries and lazily reading hold counts.
+ */
+ HoldCounter rh = null;
+ for (;;) {
+ int c = getState();
+ if (exclusiveCount(c) != 0) {
+ if (getExclusiveOwnerThread() != current)
+ return -1;
+ // else we hold the exclusive lock; blocking here
+ // would cause deadlock.
+ } else if (readerShouldBlock()) {
+ // Make sure we're not acquiring read lock reentrantly
+ if (firstReader == current) {
+ // assert firstReaderHoldCount > 0;
+ } else {
+ if (rh == null) {
+ rh = cachedHoldCounter;
+ if (rh == null || rh.tid != current.getId()) {
+ rh = readHolds.get();
+ if (rh.count == 0)
+ readHolds.remove();
+ }
+ }
+ if (rh.count == 0)
+ return -1;
+ }
+ }
+ if (sharedCount(c) == MAX_COUNT)
+ throw new Error("Maximum lock count exceeded");
+ if (compareAndSetState(c, c + SHARED_UNIT)) {
+ if (sharedCount(c) == 0) {
+ firstReader = current;
+ firstReaderHoldCount = 1;
+ } else if (firstReader == current) {
+ firstReaderHoldCount++;
+ } else {
+ if (rh == null)
+ rh = cachedHoldCounter;
+ if (rh == null || rh.tid != current.getId())
+ rh = readHolds.get();
+ else if (rh.count == 0)
+ readHolds.set(rh);
+ rh.count++;
+ cachedHoldCounter = rh; // cache for release
+ }
+ return 1;
+ }
+ }
+ }
+
+ /**
+ * Performs tryLock for write, enabling barging in both modes.
+ * This is identical in effect to tryAcquire except for lack
+ * of calls to writerShouldBlock.
+ */
+ final boolean tryWriteLock() {
+ Thread current = Thread.currentThread();
+ int c = getState();
+ if (c != 0) {
+ int w = exclusiveCount(c);
+ if (w == 0 || current != getExclusiveOwnerThread())
+ return false;
+ if (w == MAX_COUNT)
+ throw new Error("Maximum lock count exceeded");
+ }
+ if (!compareAndSetState(c, c + 1))
+ return false;
+ setExclusiveOwnerThread(current);
+ return true;
+ }
+
+ /**
+ * Performs tryLock for read, enabling barging in both modes.
+ * This is identical in effect to tryAcquireShared except for
+ * lack of calls to readerShouldBlock.
+ */
+ final boolean tryReadLock() {
+ Thread current = Thread.currentThread();
+ for (;;) {
+ int c = getState();
+ if (exclusiveCount(c) != 0 &&
+ getExclusiveOwnerThread() != current)
+ return false;
+ int r = sharedCount(c);
+ if (r == MAX_COUNT)
+ throw new Error("Maximum lock count exceeded");
+ if (compareAndSetState(c, c + SHARED_UNIT)) {
+ if (r == 0) {
+ firstReader = current;
+ firstReaderHoldCount = 1;
+ } else if (firstReader == current) {
+ firstReaderHoldCount++;
+ } else {
+ HoldCounter rh = cachedHoldCounter;
+ if (rh == null || rh.tid != current.getId())
+ cachedHoldCounter = rh = readHolds.get();
+ else if (rh.count == 0)
+ readHolds.set(rh);
+ rh.count++;
+ }
+ return true;
+ }
+ }
+ }
+
+ protected final boolean isHeldExclusively() {
+ // While we must in general read state before owner,
+ // we don't need to do so to check if current thread is owner
+ return getExclusiveOwnerThread() == Thread.currentThread();
+ }
+
+ // Methods relayed to outer class
+
+ final ConditionObject newCondition() {
+ return new ConditionObject();
+ }
+
+ final Thread getOwner() {
+ // Must read state before owner to ensure memory consistency
+ return ((exclusiveCount(getState()) == 0) ?
+ null :
+ getExclusiveOwnerThread());
+ }
+
+ final int getReadLockCount() {
+ return sharedCount(getState());
+ }
+
+ final boolean isWriteLocked() {
+ return exclusiveCount(getState()) != 0;
+ }
+
+ final int getWriteHoldCount() {
+ return isHeldExclusively() ? exclusiveCount(getState()) : 0;
+ }
+
+ final int getReadHoldCount() {
+ if (getReadLockCount() == 0)
+ return 0;
+
+ Thread current = Thread.currentThread();
+ if (firstReader == current)
+ return firstReaderHoldCount;
+
+ HoldCounter rh = cachedHoldCounter;
+ if (rh != null && rh.tid == current.getId())
+ return rh.count;
+
+ int count = readHolds.get().count;
+ if (count == 0) readHolds.remove();
+ return count;
+ }
+
+ /**
+ * Reconstitute this lock instance from a stream
+ * @param s the stream
+ */
+ private void readObject(java.io.ObjectInputStream s)
+ throws java.io.IOException, ClassNotFoundException {
+ s.defaultReadObject();
+ readHolds = new ThreadLocalHoldCounter();
+ setState(0); // reset to unlocked state
+ }
+
+ final int getCount() { return getState(); }
+ }
+
+ /**
+ * Nonfair version of Sync
+ */
+ static final class NonfairSync extends Sync {
+ private static final long serialVersionUID = -8159625535654395037L;
+ final boolean writerShouldBlock() {
+ return false; // writers can always barge
+ }
+ final boolean readerShouldBlock() {
+ /* As a heuristic to avoid indefinite writer starvation,
+ * block if the thread that momentarily appears to be head
+ * of queue, if one exists, is a waiting writer. This is
+ * only a probabilistic effect since a new reader will not
+ * block if there is a waiting writer behind other enabled
+ * readers that have not yet drained from the queue.
+ */
+ return apparentlyFirstQueuedIsExclusive();
+ }
+ }
+
+ /**
+ * Fair version of Sync
+ */
+ static final class FairSync extends Sync {
+ private static final long serialVersionUID = -2274990926593161451L;
+ final boolean writerShouldBlock() {
+ return hasQueuedPredecessors();
+ }
+ final boolean readerShouldBlock() {
+ return hasQueuedPredecessors();
+ }
+ }
+
+ /**
+ * The lock returned by method {@link ReentrantReadWriteLock#readLock}.
+ */
+ public static class ReadLock implements Lock, java.io.Serializable {
+ private static final long serialVersionUID = -5992448646407690164L;
+ private final Sync sync;
+
+ /**
+ * Constructor for use by subclasses
+ *
+ * @param lock the outer lock object
+ * @throws NullPointerException if the lock is null
+ */
+ protected ReadLock(ReentrantReadWriteLock lock) {
+ sync = lock.sync;
+ }
+
+ /**
+ * Acquires the read lock.
+ *
+ * Acquires the read lock if the write lock is not held by
+ * another thread and returns immediately.
+ *
+ * If the write lock is held by another thread then
+ * the current thread becomes disabled for thread scheduling
+ * purposes and lies dormant until the read lock has been acquired.
+ */
+ public void lock() {
+ sync.acquireShared(1);
+ }
+
+ /**
+ * Acquires the read lock unless the current thread is
+ * {@linkplain Thread#interrupt interrupted}.
+ *
+ * Acquires the read lock if the write lock is not held
+ * by another thread and returns immediately.
+ *
+ * If the write lock is held by another thread then the
+ * current thread becomes disabled for thread scheduling
+ * purposes and lies dormant until one of two things happens:
+ *
+ * If the current thread:
+ *
+ * In this implementation, as this method is an explicit
+ * interruption point, preference is given to responding to
+ * the interrupt over normal or reentrant acquisition of the
+ * lock.
+ *
+ * @throws InterruptedException if the current thread is interrupted
+ */
+ public void lockInterruptibly() throws InterruptedException {
+ sync.acquireSharedInterruptibly(1);
+ }
+
+ /**
+ * Acquires the read lock only if the write lock is not held by
+ * another thread at the time of invocation.
+ *
+ * Acquires the read lock if the write lock is not held by
+ * another thread and returns immediately with the value
+ * {@code true}. Even when this lock has been set to use a
+ * fair ordering policy, a call to {@code tryLock()}
+ * will immediately acquire the read lock if it is
+ * available, whether or not other threads are currently
+ * waiting for the read lock. This "barging" behavior
+ * can be useful in certain circumstances, even though it
+ * breaks fairness. If you want to honor the fairness setting
+ * for this lock, then use {@link #tryLock(long, TimeUnit)
+ * tryLock(0, TimeUnit.SECONDS) } which is almost equivalent
+ * (it also detects interruption).
+ *
+ * If the write lock is held by another thread then
+ * this method will return immediately with the value
+ * {@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}.
+ *
+ * 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 will not 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:
+ *
+ * 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:
+ *
+ * If the read lock is acquired then the value {@code true} is
+ * returned.
+ *
+ * If the current thread:
+ *
+ * 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.
+ *
+ * In this implementation, as this method is an explicit
+ * interruption point, preference is given to responding to
+ * the interrupt over normal or reentrant acquisition of the
+ * lock, and over reporting the elapse of the waiting time.
+ *
+ * @param timeout the time to wait for the read lock
+ * @param unit the time unit of the timeout argument
+ * @return {@code true} if the read lock was acquired
+ * @throws InterruptedException if the current thread is interrupted
+ * @throws NullPointerException if the time unit is null
+ *
+ */
+ public boolean tryLock(long timeout, TimeUnit unit)
+ throws InterruptedException {
+ return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
+ }
+
+ /**
+ * Attempts to release this lock.
+ *
+ * If the number of readers is now zero then the lock
+ * is made available for write lock attempts.
+ */
+ 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.
+ *
+ * Acquires the write lock if neither the read nor write lock
+ * are held by another thread
+ * and returns immediately, setting the write lock hold count to
+ * one.
+ *
+ * If the current thread already holds the write lock then the
+ * hold count is incremented by one and the method returns
+ * immediately.
+ *
+ * If the lock is held by another thread then the current
+ * thread becomes disabled for thread scheduling purposes and
+ * lies dormant until the write lock has been acquired, at which
+ * time the write lock hold count is set to one.
+ */
+ public void lock() {
+ sync.acquire(1);
+ }
+
+ /**
+ * Acquires the write lock unless the current thread is
+ * {@linkplain Thread#interrupt interrupted}.
+ *
+ * Acquires the write lock if neither the read nor write lock
+ * are held by another thread
+ * and returns immediately, setting the write lock hold count to
+ * one.
+ *
+ * If the current thread already holds this lock then the
+ * hold count is incremented by one and the method returns
+ * immediately.
+ *
+ * If the lock is held by another thread then the current
+ * thread becomes disabled for thread scheduling purposes and
+ * lies dormant until one of two things happens:
+ *
+ * If the write lock is acquired by the current thread then the
+ * lock hold count is set to one.
+ *
+ * If the current thread:
+ *
+ * In this implementation, as this method is an explicit
+ * interruption point, preference is given to responding to
+ * the interrupt over normal or reentrant acquisition of the
+ * lock.
+ *
+ * @throws InterruptedException if the current thread is interrupted
+ */
+ public void lockInterruptibly() throws InterruptedException {
+ sync.acquireInterruptibly(1);
+ }
+
+ /**
+ * Acquires the write lock only if it is not held by another thread
+ * at the time of invocation.
+ *
+ * Acquires the write lock if neither the read nor write lock
+ * are held by another thread
+ * and returns immediately with the value {@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()} will immediately acquire the
+ * lock if it is available, whether or not other threads are
+ * currently waiting for the write lock. This "barging"
+ * behavior can be useful in certain circumstances, even
+ * though it breaks fairness. If you want to honor the
+ * fairness setting for this lock, then use {@link
+ * #tryLock(long, TimeUnit) tryLock(0, TimeUnit.SECONDS) }
+ * which is almost equivalent (it also detects interruption).
+ *
+ * If the current thread already holds this lock then the
+ * hold count is incremented by one and the method returns
+ * {@code true}.
+ *
+ * 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}.
+ *
+ * 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
+ * will not 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:
+ *
+ * If the current thread already holds this lock then the
+ * hold count is incremented by one and the method returns
+ * {@code true}.
+ *
+ * If the lock is held by another thread then the current
+ * thread becomes disabled for thread scheduling purposes and
+ * lies dormant until one of three things happens:
+ *
+ * If the write lock is acquired then the value {@code true} is
+ * returned and the write lock hold count is set to one.
+ *
+ * If the current thread:
+ *
+ * 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.
+ *
+ * In this implementation, as this method is an explicit
+ * interruption point, preference is given to responding to
+ * the interrupt over normal or reentrant acquisition of the
+ * lock, and over reporting the elapse of the waiting time.
+ *
+ * @param timeout the time to wait for the write lock
+ * @param unit the time unit of the timeout argument
+ *
+ * @return {@code true} if the lock was free and was acquired
+ * by the current thread, or the write lock was already held by the
+ * current thread; and {@code false} if the waiting time
+ * elapsed before the lock could be acquired.
+ *
+ * @throws InterruptedException if the current thread is interrupted
+ * @throws NullPointerException if the time unit is null
+ *
+ */
+ public boolean tryLock(long timeout, TimeUnit unit)
+ throws InterruptedException {
+ return sync.tryAcquireNanos(1, unit.toNanos(timeout));
+ }
+
+ /**
+ * Attempts to release this lock.
+ *
+ * If the current thread is the holder of this lock then
+ * the hold count is decremented. If the hold count is now
+ * zero then the lock is released. If the current thread is
+ * not the holder of this lock then {@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.
+ * 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.
+ *
+ *
+ *
+ *
+ *
+ *
+ *
+ *
+ *
+ * then {@link InterruptedException} is thrown and the current
+ * thread's interrupted status is cleared.
+ *
+ * if (lock.tryLock() || lock.tryLock(timeout, unit) ) { ... }
+ *
+ *
+ *
+ *
+ *
+ *
+ *
+ *
+ *
then {@link InterruptedException} is thrown and the
+ * current thread's interrupted status is cleared.
+ *
+ *
+ *
+ *
+ *
+ *
+ *
+ *
+ *
+ * then {@link InterruptedException} is thrown and the current
+ * thread's interrupted status is cleared.
+ *
+ * if (lock.tryLock() || lock.tryLock(timeout, unit) ) { ... }
+ *
+ *
+ *
+ *
+ *
+ *
+ *
+ *
+ *
+ *
+ * then {@link InterruptedException} is thrown and the current
+ * thread's interrupted status is cleared.
+ *
+ *
+ *
+ *
+ *
+ * @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