Except for the lack of support for specified fairness policies, * or {@link Condition} objects, a SequenceLock can be used in the * same way as {@link ReentrantLock}. It provides similar status and * monitoring methods, such as {@link #isHeldByCurrentThread}. * SequenceLocks may be preferable in contexts in which multiple * threads invoke short read-only methods much more frequently than * fully locked methods. * *
Methods {@code awaitAvailability} and {@code getSequence} can * be used together to define (partially) optimistic read-only methods * that are usually more efficient than ReadWriteLocks when they * apply. These methods should in general be structured as loops that * await lock availability, then read {@code volatile} fields into * local variables (and may further read other values derived from * these, for example the {@code length} of a {@code volatile} array), * and retry if the sequence number changed while doing so. * Alternatively, because {@code awaitAvailability} accommodates * reentrancy, a method can retry a bounded number of times before * switching to locking mode. While conceptually straightforward, * expressing these ideas can be verbose. For example: * *
{@code
* class Point {
* private volatile double x, y;
* private final SequenceLock sl = new SequenceLock();
*
* // an exclusively locked method
* void move(double deltaX, double deltaY) {
* sl.lock();
* try {
* x += deltaX;
* y += deltaY;
* } finally {
* sl.unlock();
* }
* }
*
* // A read-only method
* double distanceFromOriginV1() {
* double currentX, currentY;
* long seq;
* do {
* seq = sl.awaitAvailability();
* currentX = x;
* currentY = y;
* } while (sl.getSequence() != seq); // retry if sequence changed
* return Math.sqrt(currentX * currentX + currentY * currentY);
* }
*
* // Uses bounded retries before locking
* double distanceFromOriginV2() {
* double currentX, currentY;
* long seq;
* int retries = RETRIES_BEFORE_LOCKING; // for example 8
* try {
* do {
* if (--retries < 0)
* sl.lock();
* seq = sl.awaitAvailability();
* currentX = x;
* currentY = y;
* } while (sl.getSequence() != seq);
* } finally {
* if (retries < 0)
* sl.unlock();
* }
* return Math.sqrt(currentX * currentX + currentY * currentY);
* }
* }}
*
* @since 1.8
* @author Doug Lea
*/
public class SequenceLock implements Lock, java.io.Serializable {
private static final long serialVersionUID = 7373984872572414699L;
static final class Sync extends AbstractQueuedLongSynchronizer {
private static final long serialVersionUID = 2540673546047039555L;
/**
* The number of times to spin in lock() and awaitAvailability().
*/
final int spins;
/**
* The number of reentrant holds on this lock. Uses a long for
* compatibility with other AbstractQueuedLongSynchronizer
* operations. Accessed only by lock holder.
*/
long holds;
Sync(int spins) { this.spins = spins; }
// overrides of AQLS methods
public final boolean isHeldExclusively() {
return (getState() & 1L) != 0L &&
getExclusiveOwnerThread() == Thread.currentThread();
}
public final boolean tryAcquire(long acquires) {
Thread current = Thread.currentThread();
long c = getState();
if ((c & 1L) == 0L) {
if (compareAndSetState(c, c + 1L)) {
holds = acquires;
setExclusiveOwnerThread(current);
return true;
}
}
else if (current == getExclusiveOwnerThread()) {
holds += acquires;
return true;
}
return false;
}
public final boolean tryRelease(long releases) {
if (Thread.currentThread() != getExclusiveOwnerThread())
throw new IllegalMonitorStateException();
if ((holds -= releases) == 0L) {
setExclusiveOwnerThread(null);
setState(getState() + 1L);
return true;
}
return false;
}
public final long tryAcquireShared(long unused) {
return (((getState() & 1L) == 0L) ? 1L :
(getExclusiveOwnerThread() == Thread.currentThread()) ? 0L:
-1L);
}
public final boolean tryReleaseShared(long unused) {
return (getState() & 1L) == 0L;
}
public final Condition newCondition() {
throw new UnsupportedOperationException();
}
// Other methods in support of SequenceLock
final long getSequence() {
return getState();
}
final void lock() {
int k = spins;
while (!tryAcquire(1L)) {
if (k == 0) {
acquire(1L);
break;
}
--k;
}
}
final long awaitAvailability() {
long s;
while (((s = getState()) & 1L) != 0L &&
getExclusiveOwnerThread() != Thread.currentThread()) {
acquireShared(1L);
releaseShared(1L);
}
return s;
}
final long tryAwaitAvailability(long nanos)
throws InterruptedException, TimeoutException {
Thread current = Thread.currentThread();
for (;;) {
long s = getState();
if ((s & 1L) == 0L || getExclusiveOwnerThread() == current) {
releaseShared(1L);
return s;
}
if (!tryAcquireSharedNanos(1L, nanos))
throw new TimeoutException();
// since tryAcquireSharedNanos doesn't return seq
// retry with minimal wait time.
nanos = 1L;
}
}
final boolean isLocked() {
return (getState() & 1L) != 0L;
}
final Thread getOwner() {
return (getState() & 1L) == 0L ? null : getExclusiveOwnerThread();
}
final long getHoldCount() {
return isHeldExclusively() ? holds : 0;
}
private void readObject(ObjectInputStream s)
throws IOException, ClassNotFoundException {
s.defaultReadObject();
holds = 0L;
setState(0L); // reset to unlocked state
}
}
private final Sync sync;
/**
* The default spin value for constructor. Future versions of this
* class might choose platform-specific values. Currently, except
* on uniprocessors, it is set to a small value that overcomes near
* misses between releases and acquires.
*/
static final int DEFAULT_SPINS =
Runtime.getRuntime().availableProcessors() > 1 ? 64 : 0;
/**
* Creates an instance of {@code SequenceLock} with the default
* number of retry attempts to acquire the lock before blocking.
*/
public SequenceLock() { sync = new Sync(DEFAULT_SPINS); }
/**
* Creates an instance of {@code SequenceLock} that will retry
* attempts to acquire the lock at least the given number of times
* before blocking.
*
* @param spins the number of times before blocking
*/
public SequenceLock(int spins) { sync = new Sync(spins); }
/**
* Returns the current sequence number of this lock. The sequence
* number is advanced upon each acquire or release action. When
* this value is odd, the lock is held; when even, it is released.
*
* @return the current sequence number
*/
public long getSequence() { return sync.getSequence(); }
/**
* Returns the current sequence number when the lock is, or
* becomes, available. A lock is available if it is either
* released, or is held by the current thread. If the lock is not
* available, the current thread becomes disabled for thread
* scheduling purposes and lies dormant until the lock has been
* released by some other thread.
*
* @return the current sequence number
*/
public long awaitAvailability() { return sync.awaitAvailability(); }
/**
* Returns the current sequence number if the lock is, or
* becomes, available within the specified waiting time.
*
* If the lock is not available, the current thread becomes * disabled for thread scheduling purposes and lies dormant until * one of three things happens: * *
If the current thread already holds this lock then the hold count * is incremented by one and the method returns immediately without * incrementing the sequence number. * *
If this lock not held by another thread, this method * increments the sequence number (which thus becomes an odd * number), sets the lock hold count to one, and returns * immediately. * *
If the lock is held by another thread then the current * thread may retry acquiring this lock, depending on the {@code * spin} count established in constructor. If the lock is still * not acquired, the current thread becomes disabled for thread * scheduling purposes and lies dormant until enabled by * some other thread releasing the lock. */ public void lock() { sync.lock(); } /** * Acquires the lock unless the current thread is * {@linkplain Thread#interrupt interrupted}. * *
If the current thread already holds this lock then the hold count * is incremented by one and the method returns immediately without * incrementing the sequence number. * *
If this lock not held by another thread, this method * increments the sequence number (which thus becomes an odd * number), sets the lock hold count to one, and returns * immediately. * *
If the lock is held by another thread then the current * thread may retry acquiring this lock, depending on the {@code * spin} count established in constructor. If the lock is still * not acquired, the current thread becomes disabled for thread * scheduling purposes and lies dormant until one of two things * happens: * *
If the lock is acquired by the current thread then the lock hold * count is set to one and the sequence number is incremented. * *
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(1L); } /** * Acquires the lock only if it is not held by another thread at the time * of invocation. * *
If the current thread already holds this lock then the hold * count is incremented by one and the method returns {@code true} * without incrementing the sequence number. * *
If this lock not held by another thread, this method * increments the sequence number (which thus becomes an odd * number), sets the lock hold count to one, and returns {@code * true}. * *
If the lock is held by another thread then this method * returns {@code false}. * * @return {@code true} if the lock was free and was acquired by the * current thread, or the lock was already held by the current * thread; and {@code false} otherwise */ public boolean tryLock() { return sync.tryAcquire(1L); } /** * Acquires the 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}. * *
If the current thread already holds this lock then the hold count * is incremented by one and the method returns immediately without * incrementing the sequence number. * *
If this lock not held by another thread, this method * increments the sequence number (which thus becomes an odd * number), sets the lock hold count to one, and returns * immediately. * *
If the lock is held by another thread then the current * thread may retry acquiring this lock, depending on the {@code * spin} count established in constructor. If the lock is still * not acquired, the current thread becomes disabled for thread * scheduling purposes and lies dormant until one of three things * happens: * *
If the lock is acquired then the value {@code true} is returned and * the 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 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 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(1L, 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 sequence number is incremented (thus becoming an even * number) and 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); } /** * Throws UnsupportedOperationException. SequenceLocks * do not support Condition objects. * * @throws UnsupportedOperationException always */ public Condition newCondition() { throw new UnsupportedOperationException(); } /** * Queries the number of holds on this 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. * *
The hold count information is typically only used for testing and
* debugging purposes.
*
* @return the number of holds on this lock by the current thread,
* or zero if this lock is not held by the current thread
*/
public long getHoldCount() { return sync.getHoldCount(); }
/**
* Queries if this lock is held by the current thread.
*
* @return {@code true} if current thread holds this lock and
* {@code false} otherwise
*/
public boolean isHeldByCurrentThread() { return sync.isHeldExclusively(); }
/**
* Queries if this lock is held by any thread. This method is
* designed for use in monitoring of the system state,
* not for synchronization control.
*
* @return {@code true} if any thread holds this lock and
* {@code false} otherwise
*/
public boolean isLocked() { return sync.isLocked(); }
/**
* Returns the thread that currently owns this 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 whether any threads are waiting to acquire this lock. Note that
* because cancellations may occur at any time, a {@code true}
* return does not guarantee that any other thread will ever
* acquire this 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 this
* lock. Note that because cancellations may occur at any time, a
* {@code true} return does not guarantee that this thread
* will ever acquire this 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 this 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 this 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