- 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 oftryWriteLockare 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 oftryReadLockare 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 SummaryConstructors
- 
Method SummaryModifier and TypeMethodDescriptionReturns a plainLockview of this StampedLock in which theLock.lock()method is mapped toreadLock(), and similarly for other methods.Returns aReadWriteLockview of this StampedLock in which theReadWriteLock.readLock()method is mapped toasReadLock(), andReadWriteLock.writeLock()toasWriteLock().Returns a plainLockview of this StampedLock in which theLock.lock()method is mapped towriteLock(), and similarly for other methods.intQueries the number of read locks held for this lock.static booleanisLockStamp(long stamp) Tells whether a stamp represents holding a lock.static booleanisOptimisticReadStamp(long stamp) Tells whether a stamp represents a successful optimistic read.booleanReturnstrueif the lock is currently held non-exclusively.static booleanisReadLockStamp(long stamp) Tells whether a stamp represents holding a lock non-exclusively.booleanReturnstrueif the lock is currently held exclusively.static booleanisWriteLockStamp(long stamp) Tells whether a stamp represents holding a lock exclusively.longreadLock()Non-exclusively acquires the lock, blocking if necessary until available.longNon-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.longtryConvertToOptimisticRead(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.longtryConvertToReadLock(long stamp) If the lock state matches the given stamp, atomically performs one of the following actions.longtryConvertToWriteLock(long stamp) If the lock state matches the given stamp, atomically performs one of the following actions.longReturns a stamp that can later be validated, or zero if exclusively locked.longNon-exclusively acquires the lock if it is immediately available.longtryReadLock(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.booleanReleases one hold of the read lock if it is held, without requiring a stamp value.booleanReleases the write lock if it is held, without requiring a stamp value.longExclusively acquires the lock if it is immediately available.longtryWriteLock(long time, TimeUnit unit) Exclusively acquires the lock if it is available within the given time and the current thread has not been interrupted.voidunlock(long stamp) If the lock state matches the given stamp, releases the corresponding mode of the lock.voidunlockRead(long stamp) If the lock state matches the given stamp, releases the non-exclusive lock.voidunlockWrite(long stamp) If the lock state matches the given stamp, releases the exclusive lock.booleanvalidate(long stamp) Returns true if the lock has not been exclusively acquired since issuance of the given stamp.longExclusively acquires the lock, blocking if necessary until available.longExclusively acquires the lock, blocking if necessary until available or the current thread is interrupted.
- 
Constructor Details- 
StampedLockpublic StampedLock()Creates a new lock, initially in unlocked state.
 
- 
- 
Method Details- 
writeLockpublic long writeLock()Exclusively acquires the lock, blocking if necessary until available.- Returns:
- a write stamp that can be used to unlock or convert mode
 
- 
tryWriteLockpublic 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
 
- 
tryWriteLockExclusively 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 lock
- unit- the time unit of the- timeargument
- 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
 
- 
writeLockInterruptiblyExclusively 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
 
- 
readLockpublic 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
 
- 
tryReadLockpublic 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
 
- 
tryReadLockNon-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 lock
- unit- the time unit of the- timeargument
- 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
 
- 
readLockInterruptiblyNon-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
 
- 
tryOptimisticReadpublic 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
 
- 
validatepublic 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:
- trueif the lock has not been exclusively acquired since issuance of the given stamp; else false
 
- 
unlockWritepublic 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
 
- 
unlockReadpublic 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
 
- 
unlockpublic 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
 
- 
tryConvertToWriteLockpublic 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
 
- 
tryConvertToReadLockpublic 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
 
- 
tryConvertToOptimisticReadpublic 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
 
- 
tryUnlockWritepublic 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:
- trueif the lock was held, else false
 
- 
tryUnlockReadpublic 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:
- trueif the read lock was held, else false
 
- 
isWriteLockedpublic boolean isWriteLocked()Returnstrueif the lock is currently held exclusively.- Returns:
- trueif the lock is currently held exclusively
 
- 
isReadLockedpublic boolean isReadLocked()Returnstrueif the lock is currently held non-exclusively.- Returns:
- trueif the lock is currently held non-exclusively
 
- 
isWriteLockStamppublic 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:
- trueif the stamp was returned by a successful write-lock operation
- Since:
- 10
 
- 
isReadLockStamppublic 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:
- trueif the stamp was returned by a successful read-lock operation
- Since:
- 10
 
- 
isLockStamppublic 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:
- trueif the stamp was returned by a successful read-lock or write-lock operation
- Since:
- 10
 
- 
isOptimisticReadStamppublic 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:
- trueif the stamp was returned by a successful optimistic read operation, that is, a non-zero return from- tryOptimisticRead()or- tryConvertToOptimisticRead(long)
- Since:
- 10
 
- 
getReadLockCountpublic 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
 
- 
toStringReturns 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.
- 
asReadLockReturns a plainLockview 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
 
- 
asWriteLockReturns a plainLockview 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
 
- 
asReadWriteLockReturns aReadWriteLockview of this StampedLock in which theReadWriteLock.readLock()method is mapped toasReadLock(), andReadWriteLock.writeLock()toasWriteLock().- Returns:
- the lock
 
 
-