jsr166e.extra.SequenceLock Maven / Gradle / Ivy
/*
* 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 jsr166e.extra;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.AbstractQueuedLongSynchronizer;
import java.util.Collection;
import java.io.ObjectOutputStream;
import java.io.ObjectInputStream;
import java.io.IOException;
/**
* A reentrant mutual exclusion {@link Lock} in which each lock
* acquisition or release advances a sequence number. When the
* sequence number (accessible using {@link #getSequence()}) is odd,
* the lock is held. When it is even (i.e., ({@code lock.getSequence()
* & 1L) == 0L}), the lock is released. Method {@link
* #awaitAvailability} can be used to await availability of the lock,
* returning its current sequence number. Sequence numbers (as well as
* reentrant hold counts) are of type {@code long} to ensure that they
* will not wrap around until hundreds of years of use under current
* processor rates. A SequenceLock can be created with a specified
* number of spins. Attempts to acquire the lock in method {@link
* #lock} will retry at least the given number of times before
* blocking. If not specified, a default, possibly platform-specific,
* value is used.
*
* 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:
*
*
*
* - The lock becomes available, in which case the current
* sequence number is returned.
*
*
- Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread, in which case this method throws
* {@link InterruptedException}.
*
*
- The specified waiting time elapses, in which case
* this method throws {@link TimeoutException}.
*
*
*
* @param timeout the time to wait for availability
* @param unit the time unit of the timeout argument
* @return the current sequence number if the lock is available
* upon return from this method
* @throws InterruptedException if the current thread is interrupted
* @throws TimeoutException if the lock was not available within
* the specified waiting time
* @throws NullPointerException if the time unit is null
*/
public long tryAwaitAvailability(long timeout, TimeUnit unit)
throws InterruptedException, TimeoutException {
return sync.tryAwaitAvailability(unit.toNanos(timeout));
}
/**
* Acquires the lock.
*
* 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:
*
*
*
* - The lock is acquired by the current thread; or
*
*
- Some other thread {@linkplain Thread#interrupt interrupts} the
* current thread.
*
*
*
* 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:
*
*
*
* - has its interrupted status set on entry to this method; or
*
*
- is {@linkplain Thread#interrupt interrupted} while acquiring
* the lock,
*
*
*
* then {@link InterruptedException} is thrown and the current thread's
* interrupted status is cleared.
*
* 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:
*
*
*
* - The lock is acquired by the current thread; or
*
*
- Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread; or
*
*
- The specified waiting time elapses
*
*
*
* 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:
*
*
*
* - has its interrupted status set on entry to this method; or
*
*
- is {@linkplain Thread#interrupt interrupted} while
* acquiring the lock,
*
*
* then {@link InterruptedException} is thrown and the current thread's
* interrupted status is cleared.
*
* 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 getQueuedThreads() {
return sync.getQueuedThreads();
}
/**
* 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() + "]");
}
}