/*
* @(#)ReentrantReadWriteLock.java 1.7 04/07/14
*
* Copyright 2004 Sun Microsystems, Inc. All rights reserved.
* SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*/
package java.util.concurrent.locks;
import java.util.concurrent.*;
import java.util.concurrent.atomic.*;
import java.util.*;
/**
* An implementation of {@link ReadWriteLock} supporting similar
* semantics to {@link ReentrantLock}.
* <p>This class has the following properties:
*
* <ul>
* <li><b>Acquisition order</b>
*
* <p> This class does not impose a reader or writer preference
* ordering for lock access. However, it does support an optional
* <em>fairness</em> policy. When constructed as fair, threads
* contend for entry using an approximately arrival-order policy. When
* the write lock is released either the longest-waiting single writer
* will be assigned the write lock, or if there is a reader waiting
* longer than any writer, the set of readers will be assigned the
* read lock. When constructed as non-fair, the order of entry to the
* lock need not be in arrival order. In either case, if readers are
* active and a writer enters the lock then no subsequent readers will
* be granted the read lock until after that writer has acquired and
* released the write lock.
*
* <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}. 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
* <tt>readLock().newCondition()</tt> throws
* <tt>UnsupportedOperationException</tt>.
*
* <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 exploit
* reentrancy to perform lock downgrading after updating a cache (exception
* handling is elided for simplicity):
* <pre>
* class CachedData {
* Object data;
* volatile boolean cacheValid;
* ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();
*
* void processCachedData() {
* rwl.readLock().lock();
* if (!cacheValid) {
* // upgrade lock manually
* rwl.readLock().unlock(); // must unlock first to obtain writelock
* rwl.writeLock().lock();
* if (!cacheValid) { // recheck
* data = ...
* cacheValid = true;
* }
* // downgrade lock
* rwl.readLock().lock(); // reacquire read without giving up write lock
* rwl.writeLock().unlock(); // unlock write, still hold read
* }
*
* use(data);
* 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>
* 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>A reentrant write lock intrinsically defines an owner and can
* only be released by the thread that acquired it. In contrast, in
* this implementation, the read lock has no concept of ownership, and
* there is no requirement that the thread releasing a read lock is
* the same as the one that acquired it. However, this property is
* not guaranteed to hold in future implementations of this class.
*
* <p> This lock supports a maximum of 65536 recursive write locks
* and 65536 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 */
private final Sync sync;
/**
* Creates a new <tt>ReentrantReadWriteLock</tt> with
* default ordering properties.
*/
public ReentrantReadWriteLock() {
sync = new NonfairSync();
readerLock = new ReadLock(this);
writerLock = new WriteLock(this);
}
/**
* Creates a new <tt>ReentrantReadWriteLock</tt> with
* the given fairness policy.
*
* @param fair 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; }
/*
* Read vs write count extraction constants and functions.
* Lock state is logically divided into two 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 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; }
/**
* Synchronization implementation for ReentrantReadWriteLock.
* Subclassed into fair and nonfair versions.
*/
abstract static class Sync extends AbstractQueuedSynchronizer {
/** Current (exclusive) owner thread */
transient Thread owner;
/**
* Perform write lock. Allows fast path in non-fair version.
*/
abstract void wlock();
/**
* Perform non-fair tryLock for write. tryAcquire is
* implemented in subclasses, but both versions need nonfair
* try for trylock method
*/
final boolean nonfairTryAcquire(int acquires) {
// mask out readlocks if called from condition methods
acquires = exclusiveCount(acquires);
Thread current = Thread.currentThread();
int c = getState();
int w = exclusiveCount(c);
if (w + acquires >= SHARED_UNIT)
throw new Error("Maximum lock count exceeded");
if (c != 0 && (w == 0 || current != owner))
return false;
if (!compareAndSetState(c, c + acquires))
return false;
owner = current;
return true;
}
/**
* Perform nonfair tryLock for read.
*/
final int nonfairTryAcquireShared(int acquires) {
for (;;) {
int c = getState();
int nextc = c + (acquires << SHARED_SHIFT);
if (nextc < c)
throw new Error("Maximum lock count exceeded");
if (exclusiveCount(c) != 0 &&
owner != Thread.currentThread())
return -1;
if (compareAndSetState(c, nextc))
return 1;
// Recheck count if lost CAS
}
}
protected final boolean tryRelease(int releases) {
Thread current = Thread.currentThread();
int c = getState();
if (owner != current)
throw new IllegalMonitorStateException();
int nextc = c - releases;
boolean free = false;
if (exclusiveCount(c) == releases) {
free = true;
owner = null;
}
setState(nextc);
return free;
}
protected final boolean tryReleaseShared(int releases) {
for (;;) {
int c = getState();
int nextc = c - (releases << SHARED_SHIFT);
if (nextc < 0)
throw new IllegalMonitorStateException();
if (compareAndSetState(c, nextc))
return nextc == 0;
}
}
protected final boolean isHeldExclusively() {
return exclusiveCount(getState()) != 0 &&
owner == Thread.currentThread();
}
// Methods relayed to outer class
final ConditionObject newCondition() {
return new ConditionObject();
}
final Thread getOwner() {
int c = exclusiveCount(getState());
Thread o = owner;
return (c == 0)? null : o;
}
final int getReadLockCount() {
return sharedCount(getState());
}
final boolean isWriteLocked() {
return exclusiveCount(getState()) != 0;
}
final int getWriteHoldCount() {
int c = exclusiveCount(getState());
Thread o = owner;
return (o == Thread.currentThread())? c : 0;
}
/**
* 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();
setState(0); // reset to unlocked state
}
final int getCount() { return getState(); }
}
/**
* Nonfair version of Sync
*/
final static class NonfairSync extends Sync {
protected final boolean tryAcquire(int acquires) {
return nonfairTryAcquire(acquires);
}
protected final int tryAcquireShared(int acquires) {
return nonfairTryAcquireShared(acquires);
}
// Use fastpath for main write lock method
final void wlock() {
if (compareAndSetState(0, 1))
owner = Thread.currentThread();
else
acquire(1);
}
}
/**
* Fair version of Sync
*/
final static class FairSync extends Sync {
protected final boolean tryAcquire(int acquires) {
// mask out readlocks if called from condition methods
acquires = exclusiveCount(acquires);
Thread current = Thread.currentThread();
Thread first;
int c = getState();
int w = exclusiveCount(c);
if (w + acquires >= SHARED_UNIT)
throw new Error("Maximum lock count exceeded");
if ((w == 0 || current != owner) &&
(c != 0 ||
((first = getFirstQueuedThread()) != null &&
first != current)))
return false;
if (!compareAndSetState(c, c + acquires))
return false;
owner = current;
return true;
}
protected final int tryAcquireShared(int acquires) {
Thread current = Thread.currentThread();
for (;;) {
int c = getState();
if (exclusiveCount(c) != 0) {
if (owner != current)
return -1;
} else {
Thread first = getFirstQueuedThread();
if (first != null && first != current)
return -1;
}
int nextc = c + (acquires << SHARED_SHIFT);
if (nextc < c)
throw new Error("Maximum lock count exceeded");
if (compareAndSetState(c, nextc))
return 1;
// Recheck count if lost CAS
}
}
final void wlock() { // no fast path
acquire(1);
}
}
/**
* 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 lock 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
* {@link 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 {@link 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 {@link 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
* <tt>true</tt>. Even when this lock has been set to use a
* fair ordering policy, a call to <tt>tryLock()</tt>
* <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
* <tt>false</tt>.
*
* @return <tt>true</tt> if the read lock was acquired.
*/
public boolean tryLock() {
return sync.nonfairTryAcquireShared(1) >= 0;
}
/**
* 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 {@link Thread#interrupt
* interrupted}.
*
* <p>Acquires the read lock if the write lock is not held by
* another thread and returns immediately with the value
* <tt>true</tt>. 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 <tt>tryLock</tt> that does
* permit barging on a fair lock then combine the timed and
* un-timed forms together:
*
* <pre>if (lock.tryLock() || lock.tryLock(timeout, unit) ) { ... }
* </pre>
*
* <p>If the write lock is held by another thread then the
* current thread becomes disabled for thread scheduling
* purposes and lies dormant until one of three things happens:
*
* <ul>
*
* <li>The read lock is acquired by the current thread; or
*
* <li>Some other thread {@link Thread#interrupt interrupts} the current
* thread; or
*
* <li>The specified waiting time elapses
*
* </ul>
*
* <p>If the read lock is acquired then the value <tt>true</tt> is
* returned.
*
* <p>If the current thread:
*
* <ul>
*
* <li>has its interrupted status set on entry to this method; or
*
* <li>is {@link 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
* <tt>false</tt> 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 <tt>true</tt> if the read lock was acquired.
*
* @throws InterruptedException if the current thread is interrupted
* @throws NullPointerException if 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 <tt>UnsupportedOperationException</tt> because
* <tt>ReadLocks</tt> do not support conditions.
* @throws UnsupportedOperationException always
*/
public Condition newCondition() {
throw new UnsupportedOperationException();
}
/**
* Returns a string identifying this lock, as well as its lock state.
* The state, in brackets, includes the String
* "Read locks =" followed by the number of held
* read locks.
* @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 lock null
*/
protected WriteLock(ReentrantReadWriteLock lock) {
sync = lock.sync;
}
/**
* Acquire 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.wlock();
}
/**
* Acquires the write lock unless the current thread is {@link
* 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 {@link 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 {@link 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 <tt>true</tt>,
* setting the write lock hold count to one. Even when this lock has
* been set to use a fair ordering policy, a call to
* <tt>tryLock()</tt> <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
* <tt>true</tt>.
*
* <p>If the lock is held by another thread then this method
* will return immediately with the value <tt>false</tt>.
*
* @return <tt>true</tt> if the lock was free and was acquired
* by the current thread, or the write lock was already held
* by the current thread; and <tt>false</tt> otherwise.
*/
public boolean tryLock( ) {
return sync.nonfairTryAcquire(1);
}
/**
* Acquires the write lock if it is not held by another thread
* within the given waiting time and the current thread has
* not been {@link 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 <tt>true</tt>,
* 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 <tt>tryLock</tt>
* that does permit barging on a fair lock then combine the
* timed and un-timed forms together:
*
* <pre>if (lock.tryLock() || lock.tryLock(timeout, unit) ) { ... }
* </pre>
*
* <p>If the current thread already holds this lock then the
* hold count is incremented by one and the method returns
* <tt>true</tt>.
*
* <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 {@link Thread#interrupt interrupts}
* the current thread; or
*
* <li>The specified waiting time elapses
*
* </ul>
*
* <p>If the write lock is acquired then the value <tt>true</tt> 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 {@link 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
* <tt>false</tt> 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 <tt>true</tt> if the lock was free and was acquired
* by the current thread, or the write lock was already held by the
* current thread; and <tt>false</tt> if the waiting time
* elapsed before the lock could be acquired.
*
* @throws InterruptedException if the current thread is interrupted
* @throws NullPointerException if 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 {@link 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 {@link Thread#interrupt interrupted} while
* waiting then the wait will terminate, an {@link
* InterruptedException} will be thrown, and the thread's
* interrupted status will be cleared.
*
* <li> Waiting threads are signalled in FIFO order.
*
* <li>The ordering of lock reacquisition for threads returning
* from waiting methods is the same as for threads initially
* acquiring the lock, which is in the default case not specified,
* but for <em>fair</em> locks favors those threads that have been
* waiting the longest.
*
* </ul>
* @return the Condition object
*/
public Condition newCondition() {
return sync.newCondition();
}
/**
* Returns a string identifying this lock, as well as its lock
* state. The state, in brackets includes either the String
* "Unlocked" or the String "Locked by"
* followed by the {@link Thread#getName} of the owning thread.
* @return a string identifying this lock, as well as its lock state.
*/
public String toString() {
Thread owner = sync.getOwner();
return super.toString() + ((owner == null) ?
"[Unlocked]" :
"[Locked by thread " + owner.getName() + "]");
}
}
// Instrumentation and status
/**
* Returns true if this lock has fairness set true.
* @return 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
* <tt>null</tt> if not owned. Note that the owner may be
* momentarily <tt>null</tt> 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 <tt>null</tt> 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 <tt>true</tt> if any thread holds the write lock and
* <tt>false</tt> otherwise.
*/
public boolean isWriteLocked() {
return sync.isWriteLocked();
}
/**
* Queries if the write lock is held by the current thread.
* @return <tt>true</tt> if the current thread holds the write lock and
* <tt>false</tt> 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();
}
/**
* 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 <tt>true</tt> 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 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 <tt>true</tt> 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 true if the given thread is queued waiting for this lock.
* @throws NullPointerException if 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 <tt>true</tt> return does
* not guarantee that a future <tt>signal</tt> will awaken any
* threads. This method is designed primarily for use in
* monitoring of the system state.
* @param condition the condition
* @return <tt>true</tt> 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 condition 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 condition 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 condition null
*/
protected Collection<Thread> getWaitingThreads(Condition condition) {
if (condition == null)
throw new NullPointerException();
if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
throw new IllegalArgumentException("not owner");
return sync.getWaitingThreads((AbstractQueuedSynchronizer.ConditionObject)condition);
}
/**
* Returns a string identifying this lock, as well as its lock state.
* The state, in brackets, includes the String "Write locks ="
* followed by the number of reentrantly held write locks, and the
* String "Read locks =" followed by the number of held
* read locks.
* @return a string identifying this lock, as well as its lock state.
*/
public String toString() {
int c = sync.getCount();
int w = exclusiveCount(c);
int r = sharedCount(c);
return super.toString() +
"[Write locks = " + w + ", Read locks = " + r + "]";
}
}
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