Class StampedLock
- All Implemented Interfaces:
Serializable
- Writing. Method
writeLock()
possibly blocks waiting for exclusive access, returning a stamp that can be used in methodunlockWrite(long)
to release the lock. Untimed and timed versions oftryWriteLock
are also provided. When the lock is held in write mode, no read locks may be obtained, and all optimistic read validations will fail. - Reading. Method
readLock()
possibly blocks waiting for non-exclusive access, returning a stamp that can be used in methodunlockRead(long)
to release the lock. Untimed and timed versions oftryReadLock
are also provided. - Optimistic Reading. Method
tryOptimisticRead()
returns a non-zero stamp only if the lock is not currently held in write mode. Methodvalidate(long)
returns true if the lock has not been acquired in write mode since obtaining a given stamp, in which case all actions prior to the most recent write lock release happen-before actions following the call totryOptimisticRead
. This mode can be thought of as an extremely weak version of a read-lock, that can be broken by a writer at any time. The use of optimistic read mode for short read-only code segments often reduces contention and improves throughput. However, its use is inherently fragile. Optimistic read sections should only read fields and hold them in local variables for later use after validation. Fields read while in optimistic read mode may be wildly inconsistent, so usage applies only when you are familiar enough with data representations to check consistency and/or repeatedly invoke methodvalidate()
. For example, such steps are typically required when first reading an object or array reference, and then accessing one of its fields, elements or methods.
This class also supports methods that conditionally provide
conversions across the three modes. For example, method tryConvertToWriteLock(long)
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 Semaphore
, but unlike most
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 Lock
or
ReadWriteLock
interfaces. However, a StampedLock may be
viewed asReadLock()
, asWriteLock()
, or 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.
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);
}
}
}
- See Java Language Specification:
-
17.4 Memory Model
- Since:
- 1.8
- See Also:
-
Constructor Summary
-
Method Summary
Modifier and TypeMethodDescriptionReturns a plainLock
view of this StampedLock in which theLock.lock()
method is mapped toreadLock()
, and similarly for other methods.Returns aReadWriteLock
view of this StampedLock in which theReadWriteLock.readLock()
method is mapped toasReadLock()
, andReadWriteLock.writeLock()
toasWriteLock()
.Returns a plainLock
view of this StampedLock in which theLock.lock()
method is mapped towriteLock()
, and similarly for other methods.int
Queries the number of read locks held for this lock.static boolean
isLockStamp
(long stamp) Tells whether a stamp represents holding a lock.static boolean
isOptimisticReadStamp
(long stamp) Tells whether a stamp represents a successful optimistic read.boolean
Returnstrue
if the lock is currently held non-exclusively.static boolean
isReadLockStamp
(long stamp) Tells whether a stamp represents holding a lock non-exclusively.boolean
Returnstrue
if the lock is currently held exclusively.static boolean
isWriteLockStamp
(long stamp) Tells whether a stamp represents holding a lock exclusively.long
readLock()
Non-exclusively acquires the lock, blocking if necessary until available.long
Non-exclusively acquires the lock, blocking if necessary until available or the current thread is interrupted.toString()
Returns a string identifying this lock, as well as its lock state.long
tryConvertToOptimisticRead
(long stamp) If the lock state matches the given stamp then, atomically, if the stamp represents holding a lock, releases it and returns an observation stamp.long
tryConvertToReadLock
(long stamp) If the lock state matches the given stamp, atomically performs one of the following actions.long
tryConvertToWriteLock
(long stamp) If the lock state matches the given stamp, atomically performs one of the following actions.long
Returns a stamp that can later be validated, or zero if exclusively locked.long
Non-exclusively acquires the lock if it is immediately available.long
tryReadLock
(long time, TimeUnit unit) Non-exclusively acquires the lock if it is available within the given time and the current thread has not been interrupted.boolean
Releases one hold of the read lock if it is held, without requiring a stamp value.boolean
Releases the write lock if it is held, without requiring a stamp value.long
Exclusively acquires the lock if it is immediately available.long
tryWriteLock
(long time, TimeUnit unit) Exclusively acquires the lock if it is available within the given time and the current thread has not been interrupted.void
unlock
(long stamp) If the lock state matches the given stamp, releases the corresponding mode of the lock.void
unlockRead
(long stamp) If the lock state matches the given stamp, releases the non-exclusive lock.void
unlockWrite
(long stamp) If the lock state matches the given stamp, releases the exclusive lock.boolean
validate
(long stamp) Returns true if the lock has not been exclusively acquired since issuance of the given stamp.long
Exclusively acquires the lock, blocking if necessary until available.long
Exclusively acquires the lock, blocking if necessary until available or the current thread is interrupted.
-
Constructor Details
-
StampedLock
public StampedLock()Creates a new lock, initially in unlocked state.
-
-
Method Details
-
writeLock
public long writeLock()Exclusively acquires the lock, blocking if necessary until available.- Returns:
- a write stamp that can be used to unlock or convert mode
-
tryWriteLock
public long tryWriteLock()Exclusively acquires the lock if it is immediately available.- Returns:
- a write stamp that can be used to unlock or convert mode, or zero if the lock is not available
-
tryWriteLock
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 methodLock.tryLock(long,TimeUnit)
.- Parameters:
time
- the maximum time to wait for the lockunit
- the time unit of thetime
argument- Returns:
- 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
-
writeLockInterruptibly
Exclusively acquires the lock, blocking if necessary until available or the current thread is interrupted. Behavior under interruption matches that specified for methodLock.lockInterruptibly()
.- Returns:
- a write stamp that can be used to unlock or convert mode
- Throws:
InterruptedException
- if the current thread is interrupted before acquiring the lock
-
readLock
public long readLock()Non-exclusively acquires the lock, blocking if necessary until available.- Returns:
- a read stamp that can be used to unlock or convert mode
-
tryReadLock
public long tryReadLock()Non-exclusively acquires the lock if it is immediately available.- Returns:
- a read stamp that can be used to unlock or convert mode, or zero if the lock is not available
-
tryReadLock
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 methodLock.tryLock(long,TimeUnit)
.- Parameters:
time
- the maximum time to wait for the lockunit
- the time unit of thetime
argument- Returns:
- 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
-
readLockInterruptibly
Non-exclusively acquires the lock, blocking if necessary until available or the current thread is interrupted. Behavior under interruption matches that specified for methodLock.lockInterruptibly()
.- Returns:
- a read stamp that can be used to unlock or convert mode
- Throws:
InterruptedException
- if the current thread is interrupted before acquiring the lock
-
tryOptimisticRead
public long tryOptimisticRead()Returns a stamp that can later be validated, or zero if exclusively locked.- Returns:
- a valid optimistic read stamp, or zero if exclusively locked
-
validate
public boolean validate(long stamp) 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 fromtryOptimisticRead()
or a locking method for this lock has no defined effect or result.- Parameters:
stamp
- a stamp- Returns:
true
if the lock has not been exclusively acquired since issuance of the given stamp; else false
-
unlockWrite
public void unlockWrite(long stamp) If the lock state matches the given stamp, releases the exclusive lock.- Parameters:
stamp
- a stamp returned by a write-lock operation- Throws:
IllegalMonitorStateException
- if the stamp does not match the current state of this lock
-
unlockRead
public void unlockRead(long stamp) If the lock state matches the given stamp, releases the non-exclusive lock.- Parameters:
stamp
- a stamp returned by a read-lock operation- Throws:
IllegalMonitorStateException
- if the stamp does not match the current state of this lock
-
unlock
public void unlock(long stamp) If the lock state matches the given stamp, releases the corresponding mode of the lock.- Parameters:
stamp
- a stamp returned by a lock operation- Throws:
IllegalMonitorStateException
- if the stamp does not match the current state of this lock
-
tryConvertToWriteLock
public long tryConvertToWriteLock(long 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.- Parameters:
stamp
- a stamp- Returns:
- a valid write stamp, or zero on failure
-
tryConvertToReadLock
public long tryConvertToReadLock(long stamp) 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.- Parameters:
stamp
- a stamp- Returns:
- a valid read stamp, or zero on failure
-
tryConvertToOptimisticRead
public long tryConvertToOptimisticRead(long stamp) 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".- Parameters:
stamp
- a stamp- Returns:
- a valid optimistic read stamp, or zero on failure
-
tryUnlockWrite
public boolean tryUnlockWrite()Releases the write lock if it is held, without requiring a stamp value. This method may be useful for recovery after errors.- Returns:
true
if the lock was held, else false
-
tryUnlockRead
public boolean tryUnlockRead()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.- Returns:
true
if the read lock was held, else false
-
isWriteLocked
public boolean isWriteLocked()Returnstrue
if the lock is currently held exclusively.- Returns:
true
if the lock is currently held exclusively
-
isReadLocked
public boolean isReadLocked()Returnstrue
if the lock is currently held non-exclusively.- Returns:
true
if the lock is currently held non-exclusively
-
isWriteLockStamp
public static boolean isWriteLockStamp(long stamp) Tells whether a stamp represents holding a lock exclusively. This method may be useful in conjunction withtryConvertToWriteLock(long)
, for example:long stamp = sl.tryOptimisticRead(); try { ... stamp = sl.tryConvertToWriteLock(stamp); ... } finally { if (StampedLock.isWriteLockStamp(stamp)) sl.unlockWrite(stamp); }
- Parameters:
stamp
- a stamp returned by a previous StampedLock operation- Returns:
true
if the stamp was returned by a successful write-lock operation- Since:
- 10
-
isReadLockStamp
public static boolean isReadLockStamp(long stamp) Tells whether a stamp represents holding a lock non-exclusively. This method may be useful in conjunction withtryConvertToReadLock(long)
, for example:long stamp = sl.tryOptimisticRead(); try { ... stamp = sl.tryConvertToReadLock(stamp); ... } finally { if (StampedLock.isReadLockStamp(stamp)) sl.unlockRead(stamp); }
- Parameters:
stamp
- a stamp returned by a previous StampedLock operation- Returns:
true
if the stamp was returned by a successful read-lock operation- Since:
- 10
-
isLockStamp
public static boolean isLockStamp(long stamp) Tells whether a stamp represents holding a lock. This method may be useful in conjunction withtryConvertToReadLock(long)
andtryConvertToWriteLock(long)
, for example:long stamp = sl.tryOptimisticRead(); try { ... stamp = sl.tryConvertToReadLock(stamp); ... stamp = sl.tryConvertToWriteLock(stamp); ... } finally { if (StampedLock.isLockStamp(stamp)) sl.unlock(stamp); }
- Parameters:
stamp
- a stamp returned by a previous StampedLock operation- Returns:
true
if the stamp was returned by a successful read-lock or write-lock operation- Since:
- 10
-
isOptimisticReadStamp
public static boolean isOptimisticReadStamp(long stamp) Tells whether a stamp represents a successful optimistic read.- Parameters:
stamp
- a stamp returned by a previous StampedLock operation- Returns:
true
if the stamp was returned by a successful optimistic read operation, that is, a non-zero return fromtryOptimisticRead()
ortryConvertToOptimisticRead(long)
- Since:
- 10
-
getReadLockCount
public int getReadLockCount()Queries the number of read locks held for this lock. This method is designed for use in monitoring system state, not for synchronization control.- Returns:
- the number of read locks held
-
toString
Returns a string identifying this lock, as well as its lock state. The state, in brackets, includes the String"Unlocked"
or the String"Write-locked"
or the String"Read-locks:"
followed by the current number of read-locks held. -
asReadLock
Returns a plainLock
view of this StampedLock in which theLock.lock()
method is mapped toreadLock()
, and similarly for other methods. The returned Lock does not support aCondition
; methodLock.newCondition()
throwsUnsupportedOperationException
.- Returns:
- the lock
-
asWriteLock
Returns a plainLock
view of this StampedLock in which theLock.lock()
method is mapped towriteLock()
, and similarly for other methods. The returned Lock does not support aCondition
; methodLock.newCondition()
throwsUnsupportedOperationException
.- Returns:
- the lock
-
asReadWriteLock
Returns aReadWriteLock
view of this StampedLock in which theReadWriteLock.readLock()
method is mapped toasReadLock()
, andReadWriteLock.writeLock()
toasWriteLock()
.- Returns:
- the lock
-