public class StampedLock extends Object implements Serializable
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. 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. 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.
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);
}
}
}| Constructor and Description |
|---|
StampedLock()
Creates a new lock, initially in unlocked state.
|
| Modifier and Type | Method and Description |
|---|---|
int |
getReadLockCount()
Queries the number of read locks held for this lock.
|
boolean |
isReadLocked()
Returns
true if the lock is currently held non-exclusively. |
boolean |
isWriteLocked()
Returns
true if the lock is currently held exclusively. |
long |
readLock()
Non-exclusively acquires the lock, blocking if necessary
until available.
|
long |
readLockInterruptibly()
Non-exclusively acquires the lock, blocking if necessary
until available or the current thread is interrupted.
|
String |
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, 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, performs one of
the following actions.
|
long |
tryConvertToWriteLock(long stamp)
If the lock state matches the given stamp, performs one of
the following actions.
|
long |
tryOptimisticRead()
Returns a stamp that can later be validated, or zero
if exclusively locked.
|
long |
tryReadLock()
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 |
tryUnlockRead()
Releases one hold of the read lock if it is held, without
requiring a stamp value.
|
boolean |
tryUnlockWrite()
Releases the write lock if it is held, without requiring a
stamp value.
|
long |
tryWriteLock()
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 |
writeLock()
Exclusively acquires the lock, blocking if necessary
until available.
|
long |
writeLockInterruptibly()
Exclusively acquires the lock, blocking if necessary
until available or the current thread is interrupted.
|
public StampedLock()
public long writeLock()
public long tryWriteLock()
public long tryWriteLock(long time,
TimeUnit unit)
throws InterruptedException
Lock.tryLock(long,TimeUnit).time - the maximum time to wait for the lockunit - the time unit of the time argumentInterruptedException - if the current thread is interrupted
before acquiring the lockpublic long writeLockInterruptibly()
throws InterruptedException
Lock.lockInterruptibly().InterruptedException - if the current thread is interrupted
before acquiring the lockpublic long readLock()
public long tryReadLock()
public long tryReadLock(long time,
TimeUnit unit)
throws InterruptedException
Lock.tryLock(long,TimeUnit).time - the maximum time to wait for the lockunit - the time unit of the time argumentInterruptedException - if the current thread is interrupted
before acquiring the lockpublic long readLockInterruptibly()
throws InterruptedException
Lock.lockInterruptibly().InterruptedException - if the current thread is interrupted
before acquiring the lockpublic long tryOptimisticRead()
public boolean validate(long stamp)
tryOptimisticRead() or a locking method
for this lock has no defined effect or result.stamp - a stamptrue if the lock has not been exclusively acquired
since issuance of the given stamp; else falsepublic void unlockWrite(long stamp)
stamp - a stamp returned by a write-lock operationIllegalMonitorStateException - if the stamp does
not match the current state of this lockpublic void unlockRead(long stamp)
stamp - a stamp returned by a read-lock operationIllegalMonitorStateException - if the stamp does
not match the current state of this lockpublic void unlock(long stamp)
stamp - a stamp returned by a lock operationIllegalMonitorStateException - if the stamp does
not match the current state of this lockpublic long tryConvertToWriteLock(long stamp)
stamp - a stamppublic long tryConvertToReadLock(long stamp)
stamp - a stamppublic long tryConvertToOptimisticRead(long stamp)
stamp - a stamppublic boolean tryUnlockWrite()
true if the lock was held, else falsepublic boolean tryUnlockRead()
true if the read lock was held, else falsepublic boolean isWriteLocked()
true if the lock is currently held exclusively.true if the lock is currently held exclusivelypublic boolean isReadLocked()
true if the lock is currently held non-exclusively.true if the lock is currently held non-exclusivelypublic int getReadLockCount()
public String toString()
"Unlocked" or the String "Write-locked" or the String
"Read-locks:" followed by the current number of
read-locks held.Copyright © 2002–2015 The Neo4j Graph Database Project. All rights reserved.