Added in API level 24

StampedLock

open class StampedLock : Serializable
kotlin.Any
   ↳ java.util.concurrent.locks.StampedLock

A capability-based lock with three modes for controlling read/write access. The state of a StampedLock consists of a version and mode. Lock acquisition methods return a stamp that represents and controls access with respect to a lock state; "try" versions of these methods may instead return the special value zero to represent failure to acquire access. Lock release and conversion methods require stamps as arguments, and fail if they do not match the state of the lock. The three modes are:

  • Writing. Method writeLock possibly blocks waiting for exclusive access, returning a stamp that can be used in method unlockWrite to release the lock. Untimed and timed versions of tryWriteLock 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 method unlockRead to release the lock. Untimed and timed versions of tryReadLock are also provided.
  • Optimistic Reading. Method tryOptimisticRead returns a non-zero stamp only if the lock is not currently held in write mode. Method validate 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 to tryOptimisticRead. 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 method validate(). 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 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.

<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 &amp;&amp; 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);
      }
    }
  }</code>

Summary

Public constructors

Creates a new lock, initially in unlocked state.

Public methods
open Lock!

Returns a plain Lock view of this StampedLock in which the Lock#lock method is mapped to readLock, and similarly for other methods.

open ReadWriteLock!

Returns a ReadWriteLock view of this StampedLock in which the ReadWriteLock#readLock() method is mapped to asReadLock(), and ReadWriteLock#writeLock() to asWriteLock().

open Lock!

Returns a plain Lock view of this StampedLock in which the Lock#lock method is mapped to writeLock, and similarly for other methods.

open Int

Queries the number of read locks held for this lock.

open static Boolean
isLockStamp(stamp: Long)

Tells whether a stamp represents holding a lock.

open static Boolean

Tells whether a stamp represents a successful optimistic read.

open static Boolean

Tells whether a stamp represents holding a lock non-exclusively.

open Boolean

Returns true if the lock is currently held non-exclusively.

open static Boolean

Tells whether a stamp represents holding a lock exclusively.

open Boolean

Returns true if the lock is currently held exclusively.

open Long

Non-exclusively acquires the lock, blocking if necessary until available.

open Long

Non-exclusively acquires the lock, blocking if necessary until available or the current thread is interrupted.

open String

Returns a string identifying this lock, as well as its lock state.

open Long

If the lock state matches the given stamp then, atomically, if the stamp represents holding a lock, releases it and returns an observation stamp.

open Long

If the lock state matches the given stamp, atomically performs one of the following actions.

open Long

If the lock state matches the given stamp, atomically performs one of the following actions.

open Long

Returns a stamp that can later be validated, or zero if exclusively locked.

open Long

Non-exclusively acquires the lock if it is immediately available.

open Long
tryReadLock(time: Long, unit: TimeUnit!)

Non-exclusively acquires the lock if it is available within the given time and the current thread has not been interrupted.

open Boolean

Releases one hold of the read lock if it is held, without requiring a stamp value.

open Boolean

Releases the write lock if it is held, without requiring a stamp value.

open Long

Exclusively acquires the lock if it is immediately available.

open Long
tryWriteLock(time: Long, unit: TimeUnit!)

Exclusively acquires the lock if it is available within the given time and the current thread has not been interrupted.

open Unit
unlock(stamp: Long)

If the lock state matches the given stamp, releases the corresponding mode of the lock.

open Unit
unlockRead(stamp: Long)

If the lock state matches the given stamp, releases the non-exclusive lock.

open Unit
unlockWrite(stamp: Long)

If the lock state matches the given stamp, releases the exclusive lock.

open Boolean
validate(stamp: Long)

Returns true if the lock has not been exclusively acquired since issuance of the given stamp.

open Long

Exclusively acquires the lock, blocking if necessary until available.

open Long

Exclusively acquires the lock, blocking if necessary until available or the current thread is interrupted.

Public constructors

StampedLock

Added in API level 24
StampedLock()

Creates a new lock, initially in unlocked state.

Public methods

asReadLock

Added in API level 24
open fun asReadLock(): Lock!

Returns a plain Lock view of this StampedLock in which the Lock#lock method is mapped to readLock, and similarly for other methods. The returned Lock does not support a Condition; method Lock#newCondition() throws UnsupportedOperationException.

Return
Lock! the lock

asReadWriteLock

Added in API level 24
open fun asReadWriteLock(): ReadWriteLock!

Returns a ReadWriteLock view of this StampedLock in which the ReadWriteLock#readLock() method is mapped to asReadLock(), and ReadWriteLock#writeLock() to asWriteLock().

Return
ReadWriteLock! the lock

asWriteLock

Added in API level 24
open fun asWriteLock(): Lock!

Returns a plain Lock view of this StampedLock in which the Lock#lock method is mapped to writeLock, and similarly for other methods. The returned Lock does not support a Condition; method Lock#newCondition() throws UnsupportedOperationException.

Return
Lock! the lock

getReadLockCount

Added in API level 24
open fun getReadLockCount(): Int

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
Int the number of read locks held

isLockStamp

Added in API level 33
open static fun isLockStamp(stamp: Long): Boolean

Tells whether a stamp represents holding a lock. This method may be useful in conjunction with tryConvertToReadLock and 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);
  }</code>

Parameters
stamp Long: a stamp returned by a previous StampedLock operation
Return
Boolean true if the stamp was returned by a successful read-lock or write-lock operation

isOptimisticReadStamp

Added in API level 33
open static fun isOptimisticReadStamp(stamp: Long): Boolean

Tells whether a stamp represents a successful optimistic read.

Parameters
stamp Long: a stamp returned by a previous StampedLock operation
Return
Boolean true if the stamp was returned by a successful optimistic read operation, that is, a non-zero return from tryOptimisticRead() or tryConvertToOptimisticRead(long)

isReadLockStamp

Added in API level 33
open static fun isReadLockStamp(stamp: Long): Boolean

Tells whether a stamp represents holding a lock non-exclusively. This method may be useful in conjunction with tryConvertToReadLock, for example:

<code>long stamp = sl.tryOptimisticRead();
  try {
    ...
    stamp = sl.tryConvertToReadLock(stamp);
    ...
  } finally {
    if (StampedLock.isReadLockStamp(stamp))
      sl.unlockRead(stamp);
  }</code>

Parameters
stamp Long: a stamp returned by a previous StampedLock operation
Return
Boolean true if the stamp was returned by a successful read-lock operation

isReadLocked

Added in API level 24
open fun isReadLocked(): Boolean

Returns true if the lock is currently held non-exclusively.

Return
Boolean true if the lock is currently held non-exclusively

isWriteLockStamp

Added in API level 33
open static fun isWriteLockStamp(stamp: Long): Boolean

Tells whether a stamp represents holding a lock exclusively. This method may be useful in conjunction with tryConvertToWriteLock, for example:

<code>long stamp = sl.tryOptimisticRead();
  try {
    ...
    stamp = sl.tryConvertToWriteLock(stamp);
    ...
  } finally {
    if (StampedLock.isWriteLockStamp(stamp))
      sl.unlockWrite(stamp);
  }</code>

Parameters
stamp Long: a stamp returned by a previous StampedLock operation
Return
Boolean true if the stamp was returned by a successful write-lock operation

isWriteLocked

Added in API level 24
open fun isWriteLocked(): Boolean

Returns true if the lock is currently held exclusively.

Return
Boolean true if the lock is currently held exclusively

readLock

Added in API level 24
open fun readLock(): Long

Non-exclusively acquires the lock, blocking if necessary until available.

Return
Long a read stamp that can be used to unlock or convert mode

readLockInterruptibly

Added in API level 24
open fun readLockInterruptibly(): Long

Non-exclusively acquires the lock, blocking if necessary until available or the current thread is interrupted. Behavior under interruption matches that specified for method Lock#lockInterruptibly().

Return
Long a read stamp that can be used to unlock or convert mode
Exceptions
java.lang.InterruptedException if the current thread is interrupted before acquiring the lock

toString

Added in API level 24
open fun toString(): String

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.

Return
String a string identifying this lock, as well as its lock state

tryConvertToOptimisticRead

Added in API level 24
open fun tryConvertToOptimisticRead(stamp: Long): Long

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 Long: a stamp
Return
Long a valid optimistic read stamp, or zero on failure

tryConvertToReadLock

Added in API level 24
open fun tryConvertToReadLock(stamp: Long): Long

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 Long: a stamp
Return
Long a valid read stamp, or zero on failure

tryConvertToWriteLock

Added in API level 24
open fun tryConvertToWriteLock(stamp: Long): Long

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 Long: a stamp
Return
Long a valid write stamp, or zero on failure

tryOptimisticRead

Added in API level 24
open fun tryOptimisticRead(): Long

Returns a stamp that can later be validated, or zero if exclusively locked.

Return
Long a valid optimistic read stamp, or zero if exclusively locked

tryReadLock

Added in API level 24
open fun tryReadLock(): Long

Non-exclusively acquires the lock if it is immediately available.

Return
Long a read stamp that can be used to unlock or convert mode, or zero if the lock is not available

tryReadLock

Added in API level 24
open fun tryReadLock(
    time: Long,
    unit: TimeUnit!
): Long

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 Lock#tryLock(long,TimeUnit).

Parameters
time Long: the maximum time to wait for the lock
unit TimeUnit!: the time unit of the time argument
Return
Long a read stamp that can be used to unlock or convert mode, or zero if the lock is not available
Exceptions
java.lang.InterruptedException if the current thread is interrupted before acquiring the lock

tryUnlockRead

Added in API level 24
open fun tryUnlockRead(): Boolean

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
Boolean true if the read lock was held, else false

tryUnlockWrite

Added in API level 24
open fun tryUnlockWrite(): Boolean

Releases the write lock if it is held, without requiring a stamp value. This method may be useful for recovery after errors.

Return
Boolean true if the lock was held, else false

tryWriteLock

Added in API level 24
open fun tryWriteLock(): Long

Exclusively acquires the lock if it is immediately available.

Return
Long a write stamp that can be used to unlock or convert mode, or zero if the lock is not available

tryWriteLock

Added in API level 24
open fun tryWriteLock(
    time: Long,
    unit: TimeUnit!
): Long

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 Lock#tryLock(long,TimeUnit).

Parameters
time Long: the maximum time to wait for the lock
unit TimeUnit!: the time unit of the time argument
Return
Long a write stamp that can be used to unlock or convert mode, or zero if the lock is not available
Exceptions
java.lang.InterruptedException if the current thread is interrupted before acquiring the lock

unlock

Added in API level 24
open fun unlock(stamp: Long): Unit

If the lock state matches the given stamp, releases the corresponding mode of the lock.

Parameters
stamp Long: a stamp returned by a lock operation
Exceptions
java.lang.IllegalMonitorStateException if the stamp does not match the current state of this lock

unlockRead

Added in API level 24
open fun unlockRead(stamp: Long): Unit

If the lock state matches the given stamp, releases the non-exclusive lock.

Parameters
stamp Long: a stamp returned by a read-lock operation
Exceptions
java.lang.IllegalMonitorStateException if the stamp does not match the current state of this lock

unlockWrite

Added in API level 24
open fun unlockWrite(stamp: Long): Unit

If the lock state matches the given stamp, releases the exclusive lock.

Parameters
stamp Long: a stamp returned by a write-lock operation
Exceptions
java.lang.IllegalMonitorStateException if the stamp does not match the current state of this lock

validate

Added in API level 24
open fun validate(stamp: Long): Boolean

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 tryOptimisticRead or a locking method for this lock has no defined effect or result.

Parameters
stamp Long: a stamp
Return
Boolean true if the lock has not been exclusively acquired since issuance of the given stamp; else false

writeLock

Added in API level 24
open fun writeLock(): Long

Exclusively acquires the lock, blocking if necessary until available.

Return
Long a write stamp that can be used to unlock or convert mode

writeLockInterruptibly

Added in API level 24
open fun writeLockInterruptibly(): Long

Exclusively acquires the lock, blocking if necessary until available or the current thread is interrupted. Behavior under interruption matches that specified for method Lock#lockInterruptibly().

Return
Long a write stamp that can be used to unlock or convert mode
Exceptions
java.lang.InterruptedException if the current thread is interrupted before acquiring the lock