This class also supports methods that conditionally provide * conversions across the three modes. For example, method {@link * #tryConvertToWriteLock} attempts to "upgrade" a mode, returning * a valid write stamp if (1) already in writing mode (2) in reading * mode and there are no other readers or (3) in optimistic read mode * and the lock is available. The forms of these methods are designed to * help reduce some of the code bloat that otherwise occurs in * retry-based designs. * *
StampedLocks are designed for use as internal utilities in the * development of thread-safe components. Their use relies on * knowledge of the internal properties of the data, objects, and * methods they are protecting. They are not reentrant, so locked * bodies should not call other unknown methods that may try to * re-acquire locks (although you may pass a stamp to other methods * that can use or convert it). The use of read lock modes relies on * the associated code sections being side-effect-free. Unvalidated * optimistic read sections cannot call methods that are not known to * tolerate potential inconsistencies. Stamps use finite * representations, and are not cryptographically secure (i.e., a * valid stamp may be guessable). Stamp values may recycle after (no * sooner than) one year of continuous operation. A stamp held without * use or validation for longer than this period may fail to validate * correctly. StampedLocks are serializable, but always deserialize * into initial unlocked state, so they are not useful for remote * locking. * *
Like {@link java.util.concurrent.Semaphore Semaphore}, but unlike most * {@link Lock} implementations, StampedLocks have no notion of ownership. * Locks acquired in one thread can be released or converted in another. * *
The scheduling policy of StampedLock does not consistently * prefer readers over writers or vice versa. All "try" methods are * best-effort and do not necessarily conform to any scheduling or * fairness policy. A zero return from any "try" method for acquiring * or converting locks does not carry any information about the state * of the lock; a subsequent invocation may succeed. * *
Because it supports coordinated usage across multiple lock * modes, this class does not directly implement the {@link Lock} or * {@link ReadWriteLock} interfaces. However, a StampedLock may be * viewed {@link #asReadLock()}, {@link #asWriteLock()}, or {@link * #asReadWriteLock()} in applications requiring only the associated * set of functionality. * *
Memory Synchronization. Methods with the effect of * successfully locking in any mode have the same memory * synchronization effects as a Lock action, as described in * Chapter 17 of The Java Language Specification. * Methods successfully unlocking in write mode have the same memory * synchronization effects as an Unlock action. In optimistic * read usages, actions prior to the most recent write mode unlock action * are guaranteed to happen-before those following a tryOptimisticRead * only if a later validate returns true; otherwise there is no guarantee * that the reads between tryOptimisticRead and validate obtain a * consistent snapshot. * *
Sample Usage. The following illustrates some usage idioms * in a class that maintains simple two-dimensional points. The sample * code illustrates some try/catch conventions even though they are * not strictly needed here because no exceptions can occur in their * bodies. * *
{@code
* class Point {
* private double x, y;
* private final StampedLock sl = new StampedLock();
*
* // an exclusively locked method
* void move(double deltaX, double deltaY) {
* long stamp = sl.writeLock();
* try {
* x += deltaX;
* y += deltaY;
* } finally {
* sl.unlockWrite(stamp);
* }
* }
*
* // a read-only method
* // upgrade from optimistic read to read lock
* double distanceFromOrigin() {
* long stamp = sl.tryOptimisticRead();
* try {
* retryHoldingLock: for (;; stamp = sl.readLock()) {
* if (stamp == 0L)
* continue retryHoldingLock;
* // possibly racy reads
* double currentX = x;
* double currentY = y;
* if (!sl.validate(stamp))
* continue retryHoldingLock;
* return Math.hypot(currentX, currentY);
* }
* } finally {
* if (StampedLock.isReadLockStamp(stamp))
* sl.unlockRead(stamp);
* }
* }
*
* // upgrade from optimistic read to write lock
* void moveIfAtOrigin(double newX, double newY) {
* long stamp = sl.tryOptimisticRead();
* try {
* retryHoldingLock: for (;; stamp = sl.writeLock()) {
* if (stamp == 0L)
* continue retryHoldingLock;
* // possibly racy reads
* double currentX = x;
* double currentY = y;
* if (!sl.validate(stamp))
* continue retryHoldingLock;
* if (currentX != 0.0 || currentY != 0.0)
* break;
* stamp = sl.tryConvertToWriteLock(stamp);
* if (stamp == 0L)
* continue retryHoldingLock;
* // exclusive access
* x = newX;
* y = newY;
* return;
* }
* } finally {
* if (StampedLock.isWriteLockStamp(stamp))
* sl.unlockWrite(stamp);
* }
* }
*
* // upgrade read lock to write lock
* void moveIfAtOrigin2(double newX, double newY) {
* long stamp = sl.readLock();
* try {
* while (x == 0.0 && y == 0.0) {
* long ws = sl.tryConvertToWriteLock(stamp);
* if (ws != 0L) {
* stamp = ws;
* x = newX;
* y = newY;
* break;
* }
* else {
* sl.unlockRead(stamp);
* stamp = sl.writeLock();
* }
* }
* } finally {
* sl.unlock(stamp);
* }
* }
* }}
*
* @jls 17.4 Memory Model
* @since 1.8
* @author Doug Lea
*/
public class StampedLock implements java.io.Serializable {
/*
* Algorithmic notes:
*
* The design employs elements of Sequence locks
* (as used in linux kernels; see Lameter's
* http://www.lameter.com/gelato2005.pdf
* and elsewhere; see
* Boehm's http://www.hpl.hp.com/techreports/2012/HPL-2012-68.html)
* and Ordered RW locks (see Shirako et al
* http://dl.acm.org/citation.cfm?id=2312015)
*
* Conceptually, the primary state of the lock includes a sequence
* number that is odd when write-locked and even otherwise.
* However, this is offset by a reader count that is non-zero when
* read-locked. The read count is ignored when validating
* "optimistic" seqlock-reader-style stamps. Because we must use
* a small finite number of bits (currently 7) for readers, a
* supplementary reader overflow word is used when the number of
* readers exceeds the count field. We do this by treating the max
* reader count value (RBITS) as a spinlock protecting overflow
* updates.
*
* Waiters use a modified form of CLH lock used in
* AbstractQueuedSynchronizer (AQS; see its internal documentation
* for a fuller account), where each node is either a ReaderNode
* or WriterNode. Implementation of queued Writer mode is
* identical to AQS except for lock-state operations. Sets of
* waiting readers are grouped (linked) under a common node (field
* cowaiters) so act as a single node with respect to most CLH
* mechanics. This simplifies the scheduling policy to a
* mainly-FIFO scheme that incorporates elements of Phase-Fair
* locks (see Brandenburg & Anderson, especially
* http://www.cs.unc.edu/~bbb/diss/). Method release does not
* itself wake up cowaiters. This is done by the primary thread,
* but helped by other cowaiters as they awaken.
*
* These rules apply to threads actually queued. Threads may also
* try to acquire locks before or in the process of enqueueing
* regardless of preference rules, and so may "barge" their way
* in. Methods writeLock and readLock (but not the other variants
* of each) first unconditionally try to CAS state, falling back
* to test-and-test-and-set retries on failure, slightly shrinking
* race windows on initial attempts, thus making success more
* likely. Also, when some threads cancel (via interrupt or
* timeout), phase-fairness is at best roughly approximated.
*
* Nearly all of these mechanics are carried out in methods
* acquireWrite and acquireRead, that, as typical of such code,
* sprawl out because actions and retries rely on consistent sets
* of locally cached reads.
*
* For an explanation of the use of acquireFence, see
* http://gee.cs.oswego.edu/dl/html/j9mm.html as well as Boehm's
* paper (above). Note that sequence validation (mainly method
* validate()) requires stricter ordering rules than apply to
* normal volatile reads (of "state"). To ensure that writeLock
* acquisitions strictly precede subsequent writes in cases where
* this is not already forced, we use a storeStoreFence.
*
* The memory layout keeps lock state and queue pointers together
* (normally on the same cache line). This usually works well for
* read-mostly loads. In most other cases, the natural tendency of
* CLH locks to reduce memory contention lessens motivation to
* further spread out contended locations, but might be subject to
* future improvements.
*/
private static final long serialVersionUID = -6001602636862214147L;
/** The number of bits to use for reader count before overflowing */
private static final int LG_READERS = 7; // 127 readers
// Values for lock state and stamp operations
private static final long RUNIT = 1L;
private static final long WBIT = 1L << LG_READERS;
private static final long RBITS = WBIT - 1L;
private static final long RFULL = RBITS - 1L;
private static final long ABITS = RBITS | WBIT;
private static final long SBITS = ~RBITS; // note overlap with ABITS
// not writing and conservatively non-overflowing
private static final long RSAFE = ~(3L << (LG_READERS - 1));
/*
* 3 stamp modes can be distinguished by examining (m = stamp & ABITS):
* write mode: m == WBIT
* optimistic read mode: m == 0L (even when read lock is held)
* read mode: m > 0L && m <= RFULL (the stamp is a copy of state, but the
* read hold count in the stamp is unused other than to determine mode)
*
* This differs slightly from the encoding of state:
* (state & ABITS) == 0L indicates the lock is currently unlocked.
* (state & ABITS) == RBITS is a special transient value
* indicating spin-locked to manipulate reader bits overflow.
*/
/** Initial value for lock state; avoids failure value zero. */
private static final long ORIGIN = WBIT << 1;
// Special value from cancelled acquire methods so caller can throw IE
private static final long INTERRUPTED = 1L;
// Bits for Node.status
static final int WAITING = 1;
static final int CANCELLED = 0x80000000; // must be negative
/** CLH nodes */
abstract static class Node {
volatile Node prev; // initially attached via casTail
volatile Node next; // visibly nonnull when signallable
Thread waiter; // visibly nonnull when enqueued
volatile int status; // written by owner, atomic bit ops by others
// methods for atomic operations
final boolean casPrev(Node c, Node v) { // for cleanQueue
return U.weakCompareAndSetReference(this, PREV, c, v);
}
final boolean casNext(Node c, Node v) { // for cleanQueue
return U.weakCompareAndSetReference(this, NEXT, c, v);
}
final int getAndUnsetStatus(int v) { // for signalling
return U.getAndBitwiseAndInt(this, STATUS, ~v);
}
final void setPrevRelaxed(Node p) { // for off-queue assignment
U.putReference(this, PREV, p);
}
final void setStatusRelaxed(int s) { // for off-queue assignment
U.putInt(this, STATUS, s);
}
final void clearStatus() { // for reducing unneeded signals
U.putIntOpaque(this, STATUS, 0);
}
private static final long STATUS
= U.objectFieldOffset(Node.class, "status");
private static final long NEXT
= U.objectFieldOffset(Node.class, "next");
private static final long PREV
= U.objectFieldOffset(Node.class, "prev");
}
static final class WriterNode extends Node { // node for writers
}
static final class ReaderNode extends Node { // node for readers
volatile ReaderNode cowaiters; // list of linked readers
final boolean casCowaiters(ReaderNode c, ReaderNode v) {
return U.weakCompareAndSetReference(this, COWAITERS, c, v);
}
final void setCowaitersRelaxed(ReaderNode p) {
U.putReference(this, COWAITERS, p);
}
private static final long COWAITERS
= U.objectFieldOffset(ReaderNode.class, "cowaiters");
}
/** Head of CLH queue */
private transient volatile Node head;
/** Tail (last) of CLH queue */
private transient volatile Node tail;
// views
transient ReadLockView readLockView;
transient WriteLockView writeLockView;
transient ReadWriteLockView readWriteLockView;
/** Lock sequence/state */
private transient volatile long state;
/** extra reader count when state read count saturated */
private transient int readerOverflow;
/**
* Creates a new lock, initially in unlocked state.
*/
public StampedLock() {
state = ORIGIN;
}
// internal lock methods
private boolean casState(long expect, long update) {
return U.compareAndSetLong(this, STATE, expect, update);
}
@ReservedStackAccess
private long tryAcquireWrite() {
long s, nextState;
if (((s = state) & ABITS) == 0L && casState(s, nextState = s | WBIT)) {
U.storeStoreFence();
return nextState;
}
return 0L;
}
@ReservedStackAccess
private long tryAcquireRead() {
for (long s, m, nextState;;) {
if ((m = (s = state) & ABITS) < RFULL) {
if (casState(s, nextState = s + RUNIT))
return nextState;
}
else if (m == WBIT)
return 0L;
else if ((nextState = tryIncReaderOverflow(s)) != 0L)
return nextState;
}
}
/**
* Returns an unlocked state, incrementing the version and
* avoiding special failure value 0L.
*
* @param s a write-locked state (or stamp)
*/
private static long unlockWriteState(long s) {
return ((s += WBIT) == 0L) ? ORIGIN : s;
}
private long releaseWrite(long s) {
long nextState = state = unlockWriteState(s);
signalNext(head);
return nextState;
}
/**
* Exclusively acquires the lock, blocking if necessary
* until available.
*
* @return a write stamp that can be used to unlock or convert mode
*/
@ReservedStackAccess
public long writeLock() {
// try unconditional CAS confirming weak read
long s = U.getLongOpaque(this, STATE) & ~ABITS, nextState;
if (casState(s, nextState = s | WBIT)) {
U.storeStoreFence();
return nextState;
}
return acquireWrite(false, false, 0L);
}
/**
* Exclusively acquires the lock if it is immediately available.
*
* @return a write stamp that can be used to unlock or convert mode,
* or zero if the lock is not available
*/
public long tryWriteLock() {
return tryAcquireWrite();
}
/**
* Exclusively acquires the lock if it is available within the
* given time and the current thread has not been interrupted.
* Behavior under timeout and interruption matches that specified
* for method {@link Lock#tryLock(long,TimeUnit)}.
*
* @param time the maximum time to wait for the lock
* @param unit the time unit of the {@code time} argument
* @return a write stamp that can be used to unlock or convert mode,
* or zero if the lock is not available
* @throws InterruptedException if the current thread is interrupted
* before acquiring the lock
*/
public long tryWriteLock(long time, TimeUnit unit)
throws InterruptedException {
long nanos = unit.toNanos(time);
if (!Thread.interrupted()) {
long nextState;
if ((nextState = tryAcquireWrite()) != 0L)
return nextState;
if (nanos <= 0L)
return 0L;
nextState = acquireWrite(true, true, System.nanoTime() + nanos);
if (nextState != INTERRUPTED)
return nextState;
}
throw new InterruptedException();
}
/**
* Exclusively acquires the lock, blocking if necessary
* until available or the current thread is interrupted.
* Behavior under interruption matches that specified
* for method {@link Lock#lockInterruptibly()}.
*
* @return a write stamp that can be used to unlock or convert mode
* @throws InterruptedException if the current thread is interrupted
* before acquiring the lock
*/
public long writeLockInterruptibly() throws InterruptedException {
long nextState;
if (!Thread.interrupted() &&
((nextState = tryAcquireWrite()) != 0L ||
(nextState = acquireWrite(true, false, 0L)) != INTERRUPTED))
return nextState;
throw new InterruptedException();
}
/**
* Non-exclusively acquires the lock, blocking if necessary
* until available.
*
* @return a read stamp that can be used to unlock or convert mode
*/
@ReservedStackAccess
public long readLock() {
// unconditionally optimistically try non-overflow case once
long s = U.getLongOpaque(this, STATE) & RSAFE, nextState;
if (casState(s, nextState = s + RUNIT))
return nextState;
else
return acquireRead(false, false, 0L);
}
/**
* Non-exclusively acquires the lock if it is immediately available.
*
* @return a read stamp that can be used to unlock or convert mode,
* or zero if the lock is not available
*/
public long tryReadLock() {
return tryAcquireRead();
}
/**
* Non-exclusively acquires the lock if it is available within the
* given time and the current thread has not been interrupted.
* Behavior under timeout and interruption matches that specified
* for method {@link Lock#tryLock(long,TimeUnit)}.
*
* @param time the maximum time to wait for the lock
* @param unit the time unit of the {@code time} argument
* @return a read stamp that can be used to unlock or convert mode,
* or zero if the lock is not available
* @throws InterruptedException if the current thread is interrupted
* before acquiring the lock
*/
public long tryReadLock(long time, TimeUnit unit)
throws InterruptedException {
long nanos = unit.toNanos(time);
if (!Thread.interrupted()) {
long nextState;
if (tail == head && (nextState = tryAcquireRead()) != 0L)
return nextState;
if (nanos <= 0L)
return 0L;
nextState = acquireRead(true, true, System.nanoTime() + nanos);
if (nextState != INTERRUPTED)
return nextState;
}
throw new InterruptedException();
}
/**
* Non-exclusively acquires the lock, blocking if necessary
* until available or the current thread is interrupted.
* Behavior under interruption matches that specified
* for method {@link Lock#lockInterruptibly()}.
*
* @return a read stamp that can be used to unlock or convert mode
* @throws InterruptedException if the current thread is interrupted
* before acquiring the lock
*/
public long readLockInterruptibly() throws InterruptedException {
long nextState;
if (!Thread.interrupted() &&
((nextState = tryAcquireRead()) != 0L ||
(nextState = acquireRead(true, false, 0L)) != INTERRUPTED))
return nextState;
throw new InterruptedException();
}
/**
* Returns a stamp that can later be validated, or zero
* if exclusively locked.
*
* @return a valid optimistic read stamp, or zero if exclusively locked
*/
public long tryOptimisticRead() {
long s;
return (((s = state) & WBIT) == 0L) ? (s & SBITS) : 0L;
}
/**
* Returns true if the lock has not been exclusively acquired
* since issuance of the given stamp. Always returns false if the
* stamp is zero. Always returns true if the stamp represents a
* currently held lock. Invoking this method with a value not
* obtained from {@link #tryOptimisticRead} or a locking method
* for this lock has no defined effect or result.
*
* @param stamp a stamp
* @return {@code true} if the lock has not been exclusively acquired
* since issuance of the given stamp; else false
*/
public boolean validate(long stamp) {
U.loadFence();
return (stamp & SBITS) == (state & SBITS);
}
/**
* If the lock state matches the given stamp, releases the
* exclusive lock.
*
* @param stamp a stamp returned by a write-lock operation
* @throws IllegalMonitorStateException if the stamp does
* not match the current state of this lock
*/
@ReservedStackAccess
public void unlockWrite(long stamp) {
if (state != stamp || (stamp & WBIT) == 0L)
throw new IllegalMonitorStateException();
releaseWrite(stamp);
}
/**
* If the lock state matches the given stamp, releases the
* non-exclusive lock.
*
* @param stamp a stamp returned by a read-lock operation
* @throws IllegalMonitorStateException if the stamp does
* not match the current state of this lock
*/
@ReservedStackAccess
public void unlockRead(long stamp) {
long s, m;
if ((stamp & RBITS) != 0L) {
while (((s = state) & SBITS) == (stamp & SBITS) &&
((m = s & RBITS) != 0L)) {
if (m < RFULL) {
if (casState(s, s - RUNIT)) {
if (m == RUNIT)
signalNext(head);
return;
}
}
else if (tryDecReaderOverflow(s) != 0L)
return;
}
}
throw new IllegalMonitorStateException();
}
/**
* If the lock state matches the given stamp, releases the
* corresponding mode of the lock.
*
* @param stamp a stamp returned by a lock operation
* @throws IllegalMonitorStateException if the stamp does
* not match the current state of this lock
*/
public void unlock(long stamp) {
if ((stamp & WBIT) != 0L)
unlockWrite(stamp);
else
unlockRead(stamp);
}
/**
* If the lock state matches the given stamp, atomically performs one of
* the following actions. If the stamp represents holding a write
* lock, returns it. Or, if a read lock, if the write lock is
* available, releases the read lock and returns a write stamp.
* Or, if an optimistic read, returns a write stamp only if
* immediately available. This method returns zero in all other
* cases.
*
* @param stamp a stamp
* @return a valid write stamp, or zero on failure
*/
public long tryConvertToWriteLock(long stamp) {
long a = stamp & ABITS, m, s, nextState;
while (((s = state) & SBITS) == (stamp & SBITS)) {
if ((m = s & ABITS) == 0L) {
if (a != 0L)
break;
if (casState(s, nextState = s | WBIT)) {
U.storeStoreFence();
return nextState;
}
} else if (m == WBIT) {
if (a != m)
break;
return stamp;
} else if (m == RUNIT && a != 0L) {
if (casState(s, nextState = s - RUNIT + WBIT))
return nextState;
} else
break;
}
return 0L;
}
/**
* If the lock state matches the given stamp, atomically performs one of
* the following actions. If the stamp represents holding a write
* lock, releases it and obtains a read lock. Or, if a read lock,
* returns it. Or, if an optimistic read, acquires a read lock and
* returns a read stamp only if immediately available. This method
* returns zero in all other cases.
*
* @param stamp a stamp
* @return a valid read stamp, or zero on failure
*/
public long tryConvertToReadLock(long stamp) {
long a, s, nextState;
while (((s = state) & SBITS) == (stamp & SBITS)) {
if ((a = stamp & ABITS) >= WBIT) {
if (s != stamp) // write stamp
break;
nextState = state = unlockWriteState(s) + RUNIT;
signalNext(head);
return nextState;
} else if (a == 0L) { // optimistic read stamp
if ((s & ABITS) < RFULL) {
if (casState(s, nextState = s + RUNIT))
return nextState;
} else if ((nextState = tryIncReaderOverflow(s)) != 0L)
return nextState;
} else { // already a read stamp
if ((s & ABITS) == 0L)
break;
return stamp;
}
}
return 0L;
}
/**
* If the lock state matches the given stamp then, atomically, if the stamp
* represents holding a lock, releases it and returns an
* observation stamp. Or, if an optimistic read, returns it if
* validated. This method returns zero in all other cases, and so
* may be useful as a form of "tryUnlock".
*
* @param stamp a stamp
* @return a valid optimistic read stamp, or zero on failure
*/
public long tryConvertToOptimisticRead(long stamp) {
long a, m, s, nextState;
U.loadFence();
while (((s = state) & SBITS) == (stamp & SBITS)) {
if ((a = stamp & ABITS) >= WBIT) {
if (s != stamp) // write stamp
break;
return releaseWrite(s);
} else if (a == 0L) { // already an optimistic read stamp
return stamp;
} else if ((m = s & ABITS) == 0L) { // invalid read stamp
break;
} else if (m < RFULL) {
if (casState(s, nextState = s - RUNIT)) {
if (m == RUNIT)
signalNext(head);
return nextState & SBITS;
}
} else if ((nextState = tryDecReaderOverflow(s)) != 0L)
return nextState & SBITS;
}
return 0L;
}
/**
* Releases the write lock if it is held, without requiring a
* stamp value. This method may be useful for recovery after
* errors.
*
* @return {@code true} if the lock was held, else false
*/
@ReservedStackAccess
public boolean tryUnlockWrite() {
long s;
if (((s = state) & WBIT) != 0L) {
releaseWrite(s);
return true;
}
return false;
}
/**
* Releases one hold of the read lock if it is held, without
* requiring a stamp value. This method may be useful for recovery
* after errors.
*
* @return {@code true} if the read lock was held, else false
*/
@ReservedStackAccess
public boolean tryUnlockRead() {
long s, m;
while ((m = (s = state) & ABITS) != 0L && m < WBIT) {
if (m < RFULL) {
if (casState(s, s - RUNIT)) {
if (m == RUNIT)
signalNext(head);
return true;
}
}
else if (tryDecReaderOverflow(s) != 0L)
return true;
}
return false;
}
// status monitoring methods
/**
* Returns combined state-held and overflow read count for given
* state s.
*/
private int getReadLockCount(long s) {
long readers;
if ((readers = s & RBITS) >= RFULL)
readers = RFULL + readerOverflow;
return (int) readers;
}
/**
* Returns {@code true} if the lock is currently held exclusively.
*
* @return {@code true} if the lock is currently held exclusively
*/
public boolean isWriteLocked() {
return (state & WBIT) != 0L;
}
/**
* Returns {@code true} if the lock is currently held non-exclusively.
*
* @return {@code true} if the lock is currently held non-exclusively
*/
public boolean isReadLocked() {
return (state & RBITS) != 0L;
}
/**
* Tells whether a stamp represents holding a lock exclusively.
* This method may be useful in conjunction with
* {@link #tryConvertToWriteLock}, for example: {@code
* long stamp = sl.tryOptimisticRead();
* try {
* ...
* stamp = sl.tryConvertToWriteLock(stamp);
* ...
* } finally {
* if (StampedLock.isWriteLockStamp(stamp))
* sl.unlockWrite(stamp);
* }}
*
* @param stamp a stamp returned by a previous StampedLock operation
* @return {@code true} if the stamp was returned by a successful
* write-lock operation
* @since 10
*/
public static boolean isWriteLockStamp(long stamp) {
return (stamp & ABITS) == WBIT;
}
/**
* Tells whether a stamp represents holding a lock non-exclusively.
* This method may be useful in conjunction with
* {@link #tryConvertToReadLock}, for example: {@code
* long stamp = sl.tryOptimisticRead();
* try {
* ...
* stamp = sl.tryConvertToReadLock(stamp);
* ...
* } finally {
* if (StampedLock.isReadLockStamp(stamp))
* sl.unlockRead(stamp);
* }}
*
* @param stamp a stamp returned by a previous StampedLock operation
* @return {@code true} if the stamp was returned by a successful
* read-lock operation
* @since 10
*/
public static boolean isReadLockStamp(long stamp) {
return (stamp & RBITS) != 0L;
}
/**
* Tells whether a stamp represents holding a lock.
* This method may be useful in conjunction with
* {@link #tryConvertToReadLock} and {@link #tryConvertToWriteLock},
* for example: {@code
* long stamp = sl.tryOptimisticRead();
* try {
* ...
* stamp = sl.tryConvertToReadLock(stamp);
* ...
* stamp = sl.tryConvertToWriteLock(stamp);
* ...
* } finally {
* if (StampedLock.isLockStamp(stamp))
* sl.unlock(stamp);
* }}
*
* @param stamp a stamp returned by a previous StampedLock operation
* @return {@code true} if the stamp was returned by a successful
* read-lock or write-lock operation
* @since 10
*/
public static boolean isLockStamp(long stamp) {
return (stamp & ABITS) != 0L;
}
/**
* Tells whether a stamp represents a successful optimistic read.
*
* @param stamp a stamp returned by a previous StampedLock operation
* @return {@code true} if the stamp was returned by a successful
* optimistic read operation, that is, a non-zero return from
* {@link #tryOptimisticRead()} or
* {@link #tryConvertToOptimisticRead(long)}
* @since 10
*/
public static boolean isOptimisticReadStamp(long stamp) {
return (stamp & ABITS) == 0L && stamp != 0L;
}
/**
* 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 getReadLockCount(state);
}
/**
* Returns a string identifying this lock, as well as its lock
* state. The state, in brackets, includes the String {@code
* "Unlocked"} or the String {@code "Write-locked"} or the String
* {@code "Read-locks:"} followed by the current number of
* read-locks held.
*
* @return a string identifying this lock, as well as its lock state
*/
public String toString() {
long s = state;
return super.toString() +
((s & ABITS) == 0L ? "[Unlocked]" :
(s & WBIT) != 0L ? "[Write-locked]" :
"[Read-locks:" + getReadLockCount(s) + "]");
}
// views
/**
* Returns a plain {@link Lock} view of this StampedLock in which
* the {@link Lock#lock} method is mapped to {@link #readLock},
* and similarly for other methods. The returned Lock does not
* support a {@link Condition}; method {@link Lock#newCondition()}
* throws {@code UnsupportedOperationException}.
*
* @return the lock
*/
public Lock asReadLock() {
ReadLockView v;
if ((v = readLockView) != null) return v;
return readLockView = new ReadLockView();
}
/**
* Returns a plain {@link Lock} view of this StampedLock in which
* the {@link Lock#lock} method is mapped to {@link #writeLock},
* and similarly for other methods. The returned Lock does not
* support a {@link Condition}; method {@link Lock#newCondition()}
* throws {@code UnsupportedOperationException}.
*
* @return the lock
*/
public Lock asWriteLock() {
WriteLockView v;
if ((v = writeLockView) != null) return v;
return writeLockView = new WriteLockView();
}
/**
* Returns a {@link ReadWriteLock} view of this StampedLock in
* which the {@link ReadWriteLock#readLock()} method is mapped to
* {@link #asReadLock()}, and {@link ReadWriteLock#writeLock()} to
* {@link #asWriteLock()}.
*
* @return the lock
*/
public ReadWriteLock asReadWriteLock() {
ReadWriteLockView v;
if ((v = readWriteLockView) != null) return v;
return readWriteLockView = new ReadWriteLockView();
}
// view classes
final class ReadLockView implements Lock {
public void lock() { readLock(); }
public void lockInterruptibly() throws InterruptedException {
readLockInterruptibly();
}
public boolean tryLock() { return tryReadLock() != 0L; }
public boolean tryLock(long time, TimeUnit unit)
throws InterruptedException {
return tryReadLock(time, unit) != 0L;
}
public void unlock() { unstampedUnlockRead(); }
public Condition newCondition() {
throw new UnsupportedOperationException();
}
}
final class WriteLockView implements Lock {
public void lock() { writeLock(); }
public void lockInterruptibly() throws InterruptedException {
writeLockInterruptibly();
}
public boolean tryLock() { return tryWriteLock() != 0L; }
public boolean tryLock(long time, TimeUnit unit)
throws InterruptedException {
return tryWriteLock(time, unit) != 0L;
}
public void unlock() { unstampedUnlockWrite(); }
public Condition newCondition() {
throw new UnsupportedOperationException();
}
}
final class ReadWriteLockView implements ReadWriteLock {
public Lock readLock() { return asReadLock(); }
public Lock writeLock() { return asWriteLock(); }
}
// Unlock methods without stamp argument checks for view classes.
// Needed because view-class lock methods throw away stamps.
final void unstampedUnlockWrite() {
long s;
if (((s = state) & WBIT) == 0L)
throw new IllegalMonitorStateException();
releaseWrite(s);
}
final void unstampedUnlockRead() {
long s, m;
while ((m = (s = state) & RBITS) > 0L) {
if (m < RFULL) {
if (casState(s, s - RUNIT)) {
if (m == RUNIT)
signalNext(head);
return;
}
}
else if (tryDecReaderOverflow(s) != 0L)
return;
}
throw new IllegalMonitorStateException();
}
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
s.defaultReadObject();
state = ORIGIN; // reset to unlocked state
}
// overflow handling methods
/**
* Tries to increment readerOverflow by first setting state
* access bits value to RBITS, indicating hold of spinlock,
* then updating, then releasing.
*
* @param s a reader overflow stamp: (s & ABITS) >= RFULL
* @return new stamp on success, else zero
*/
private long tryIncReaderOverflow(long s) {
// assert (s & ABITS) >= RFULL;
if ((s & ABITS) != RFULL)
Thread.onSpinWait();
else if (casState(s, s | RBITS)) {
++readerOverflow;
return state = s;
}
return 0L;
}
/**
* Tries to decrement readerOverflow.
*
* @param s a reader overflow stamp: (s & ABITS) >= RFULL
* @return new stamp on success, else zero
*/
private long tryDecReaderOverflow(long s) {
// assert (s & ABITS) >= RFULL;
if ((s & ABITS) != RFULL)
Thread.onSpinWait();
else if (casState(s, s | RBITS)) {
int r; long nextState;
if ((r = readerOverflow) > 0) {
readerOverflow = r - 1;
nextState = s;
}
else
nextState = s - RUNIT;
return state = nextState;
}
return 0L;
}
// release methods
/**
* Wakes up the successor of given node, if one exists, and unsets its
* WAITING status to avoid park race. This may fail to wake up an
* eligible thread when one or more have been cancelled, but
* cancelAcquire ensures liveness.
*/
static final void signalNext(Node h) {
Node s;
if (h != null && (s = h.next) != null && s.status > 0) {
s.getAndUnsetStatus(WAITING);
LockSupport.unpark(s.waiter);
}
}
/**
* Removes and unparks all cowaiters of node, if it exists.
*/
private static void signalCowaiters(ReaderNode node) {
if (node != null) {
for (ReaderNode c; (c = node.cowaiters) != null; ) {
if (node.casCowaiters(c, c.cowaiters))
LockSupport.unpark(c.waiter);
}
}
}
// queue link methods
private boolean casTail(Node c, Node v) {
return U.compareAndSetReference(this, TAIL, c, v);
}
/** tries once to CAS a new dummy node for head */
private void tryInitializeHead() {
Node h = new WriterNode();
if (U.compareAndSetReference(this, HEAD, null, h))
tail = h;
}
/**
* For explanation, see above and AbstractQueuedSynchronizer
* internal documentation.
*
* @param interruptible true if should check interrupts and if so
* return INTERRUPTED
* @param timed if true use timed waits
* @param time the System.nanoTime value to timeout at (and return zero)
* @return next state, or INTERRUPTED
*/
private long acquireWrite(boolean interruptible, boolean timed, long time) {
byte spins = 0, postSpins = 0; // retries upon unpark of first thread
boolean interrupted = false, first = false;
WriterNode node = null;
Node pred = null;
for (long s, nextState;;) {
if (!first && (pred = (node == null) ? null : node.prev) != null &&
!(first = (head == pred))) {
if (pred.status < 0) {
cleanQueue(); // predecessor cancelled
continue;
} else if (pred.prev == null) {
Thread.onSpinWait(); // ensure serialization
continue;
}
}
if ((first || pred == null) && ((s = state) & ABITS) == 0L &&
casState(s, nextState = s | WBIT)) {
U.storeStoreFence();
if (first) {
node.prev = null;
head = node;
pred.next = null;
node.waiter = null;
if (interrupted)
Thread.currentThread().interrupt();
}
return nextState;
} else if (node == null) { // retry before enqueuing
node = new WriterNode();
} else if (pred == null) { // try to enqueue
Node t = tail;
node.setPrevRelaxed(t);
if (t == null)
tryInitializeHead();
else if (!casTail(t, node))
node.setPrevRelaxed(null); // back out
else
t.next = node;
} else if (first && spins != 0) { // reduce unfairness
--spins;
Thread.onSpinWait();
} else if (node.status == 0) { // enable signal
if (node.waiter == null)
node.waiter = Thread.currentThread();
node.status = WAITING;
} else {
long nanos;
spins = postSpins = (byte)((postSpins << 1) | 1);
if (!timed)
LockSupport.park(this);
else if ((nanos = time - System.nanoTime()) > 0L)
LockSupport.parkNanos(this, nanos);
else
break;
node.clearStatus();
if ((interrupted |= Thread.interrupted()) && interruptible)
break;
}
}
return cancelAcquire(node, interrupted);
}
/**
* See above for explanation.
*
* @param interruptible true if should check interrupts and if so
* return INTERRUPTED
* @param timed if true use timed waits
* @param time the System.nanoTime value to timeout at (and return zero)
* @return next state, or INTERRUPTED
*/
private long acquireRead(boolean interruptible, boolean timed, long time) {
boolean interrupted = false;
ReaderNode node = null;
/*
* Loop:
* if empty, try to acquire
* if tail is Reader, try to cowait; restart if leader stale or cancels
* else try to create and enqueue node, and wait in 2nd loop below
*/
for (;;) {
ReaderNode leader; long nextState;
Node tailPred = null, t = tail;
if ((t == null || (tailPred = t.prev) == null) &&
(nextState = tryAcquireRead()) != 0L) // try now if empty
return nextState;
else if (t == null)
tryInitializeHead();
else if (tailPred == null || !(t instanceof ReaderNode)) {
if (node == null)
node = new ReaderNode();
if (tail == t) {
node.setPrevRelaxed(t);
if (casTail(t, node)) {
t.next = node;
break; // node is leader; wait in loop below
}
node.setPrevRelaxed(null);
}
} else if ((leader = (ReaderNode)t) == tail) { // try to cowait
for (boolean attached = false;;) {
if (leader.status < 0 || leader.prev == null)
break;
else if (node == null)
node = new ReaderNode();
else if (node.waiter == null)
node.waiter = Thread.currentThread();
else if (!attached) {
ReaderNode c = leader.cowaiters;
node.setCowaitersRelaxed(c);
attached = leader.casCowaiters(c, node);
if (!attached)
node.setCowaitersRelaxed(null);
} else {
long nanos = 0L;
if (!timed)
LockSupport.park(this);
else if ((nanos = time - System.nanoTime()) > 0L)
LockSupport.parkNanos(this, nanos);
interrupted |= Thread.interrupted();
if ((interrupted && interruptible) ||
(timed && nanos <= 0L))
return cancelCowaiter(node, leader, interrupted);
}
}
if (node != null)
node.waiter = null;
long ns = tryAcquireRead();
signalCowaiters(leader);
if (interrupted)
Thread.currentThread().interrupt();
if (ns != 0L)
return ns;
else
node = null; // restart if stale, missed, or leader cancelled
}
}
// node is leader of a cowait group; almost same as acquireWrite
byte spins = 0, postSpins = 0; // retries upon unpark of first thread
boolean first = false;
Node pred = null;
for (long nextState;;) {
if (!first && (pred = node.prev) != null &&
!(first = (head == pred))) {
if (pred.status < 0) {
cleanQueue(); // predecessor cancelled
continue;
} else if (pred.prev == null) {
Thread.onSpinWait(); // ensure serialization
continue;
}
}
if ((first || pred == null) &&
(nextState = tryAcquireRead()) != 0L) {
if (first) {
node.prev = null;
head = node;
pred.next = null;
node.waiter = null;
}
signalCowaiters(node);
if (interrupted)
Thread.currentThread().interrupt();
return nextState;
} else if (first && spins != 0) {
--spins;
Thread.onSpinWait();
} else if (node.status == 0) {
if (node.waiter == null)
node.waiter = Thread.currentThread();
node.status = WAITING;
} else {
long nanos;
spins = postSpins = (byte)((postSpins << 1) | 1);
if (!timed)
LockSupport.park(this);
else if ((nanos = time - System.nanoTime()) > 0L)
LockSupport.parkNanos(this, nanos);
else
break;
node.clearStatus();
if ((interrupted |= Thread.interrupted()) && interruptible)
break;
}
}
return cancelAcquire(node, interrupted);
}
// Cancellation support
/**
* Possibly repeatedly traverses from tail, unsplicing cancelled
* nodes until none are found. Unparks nodes that may have been
* relinked to be next eligible acquirer.
*/
private void cleanQueue() {
for (;;) { // restart point
for (Node q = tail, s = null, p, n;;) { // (p, q, s) triples
if (q == null || (p = q.prev) == null)
return; // end of list
if (s == null ? tail != q : (s.prev != q || s.status < 0))
break; // inconsistent
if (q.status < 0) { // cancelled
if ((s == null ? casTail(q, p) : s.casPrev(q, p)) &&
q.prev == p) {
p.casNext(q, s); // OK if fails
if (p.prev == null)
signalNext(p);
}
break;
}
if ((n = p.next) != q) { // help finish
if (n != null && q.prev == p && q.status >= 0) {
p.casNext(n, q);
if (p.prev == null)
signalNext(p);
}
break;
}
s = q;
q = q.prev;
}
}
}
/**
* If leader exists, possibly repeatedly traverses cowaiters,
* unsplicing the given cancelled node until not found.
*/
private void unlinkCowaiter(ReaderNode node, ReaderNode leader) {
if (leader != null) {
while (leader.prev != null && leader.status >= 0) {
for (ReaderNode p = leader, q; ; p = q) {
if ((q = p.cowaiters) == null)
return;
if (q == node) {
p.casCowaiters(q, q.cowaiters);
break; // recheck even if succeeded
}
}
}
}
}
/**
* If node non-null, forces cancel status and unsplices it from
* queue, wakes up any cowaiters, and possibly wakes up successor
* to recheck status.
*
* @param node the waiter (may be null if not yet enqueued)
* @param interrupted if already interrupted
* @return INTERRUPTED if interrupted or Thread.interrupted, else zero
*/
private long cancelAcquire(Node node, boolean interrupted) {
if (node != null) {
node.waiter = null;
node.status = CANCELLED;
cleanQueue();
if (node instanceof ReaderNode)
signalCowaiters((ReaderNode)node);
}
return (interrupted || Thread.interrupted()) ? INTERRUPTED : 0L;
}
/**
* If node non-null, forces cancel status and unsplices from
* leader's cowaiters list unless/until it is also cancelled.
*
* @param node if non-null, the waiter
* @param leader if non-null, the node heading cowaiters list
* @param interrupted if already interrupted
* @return INTERRUPTED if interrupted or Thread.interrupted, else zero
*/
private long cancelCowaiter(ReaderNode node, ReaderNode leader,
boolean interrupted) {
if (node != null) {
node.waiter = null;
node.status = CANCELLED;
unlinkCowaiter(node, leader);
}
return (interrupted || Thread.interrupted()) ? INTERRUPTED : 0L;
}
// Unsafe
private static final Unsafe U = Unsafe.getUnsafe();
private static final long STATE
= U.objectFieldOffset(StampedLock.class, "state");
private static final long HEAD
= U.objectFieldOffset(StampedLock.class, "head");
private static final long TAIL
= U.objectFieldOffset(StampedLock.class, "tail");
static {
Class ensureLoaded = LockSupport.class;
}
}