Class StampedLock

public class StampedLock
extends Object
implements Serializable

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(long) 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(long) 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(long) returns true if the lock has not been acquired in write mode since obtaining a given stamp. 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 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 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(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 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.

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.

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();

   void move(double deltaX, double deltaY) { // an exclusively locked method
     long stamp = sl.writeLock();
     try {
       x += deltaX;
       y += deltaY;
     } finally {
       sl.unlockWrite(stamp);
     }
   }

   double distanceFromOrigin() { // A read-only method
     long stamp = sl.tryOptimisticRead();
     double currentX = x, currentY = y;
     if (!sl.validate(stamp)) {
        stamp = sl.readLock();
        try {
          currentX = x;
          currentY = y;
        } finally {
           sl.unlockRead(stamp);
        }
     }
     return Math.sqrt(currentX * currentX + currentY * currentY);
   }

   void moveIfAtOrigin(double newX, double newY) { // upgrade
     // Could instead start with optimistic, not read mode
     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);
     }
   }
 }

Constructors

StampedLock

public StampedLock()

Creates a new lock, initially in unlocked state.

Methods

writeLock

public long writeLock()

Exclusively acquires the lock, blocking if necessary until available.

Returns:

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

tryWriteLock

public long tryWriteLock(long time,
                         TimeUnit unit)
                  throws InterruptedException

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 - the maximum time to wait for the lock

unit - the time unit of the time argument

Returns:

a 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

public long writeLockInterruptibly()
                            throws InterruptedException

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().

Returns:

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

tryReadLock

public long tryReadLock(long time,
                        TimeUnit unit)
                 throws InterruptedException

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 - the maximum time to wait for the lock

unit - the time unit of the time argument

Returns:

a 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

public long readLockInterruptibly()
                           throws 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 Lock.lockInterruptibly().

Returns:

a 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 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 from tryOptimisticRead() 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, 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, 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, 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()

Returns true if the lock is currently held exclusively.

Returns:

true if the lock is currently held exclusively

isReadLocked

public boolean isReadLocked()

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

Returns:

true if the lock is currently held non-exclusively

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

public String 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.

Overrides:

toString in class Object

Returns:

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

asReadLock

public Lock asReadLock()

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.

Returns:

the lock

asWriteLock

public Lock asWriteLock()

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.

Returns:

the lock

asReadWriteLock

public ReadWriteLock asReadWriteLock()

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

Returns:

the lock