StampedLock Class
Definition
Important
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A capability-based lock with three modes for controlling read/write access.
[Android.Runtime.Register("java/util/concurrent/locks/StampedLock", ApiSince=24, DoNotGenerateAcw=true)]
public class StampedLock : Java.Lang.Object, IDisposable, Java.Interop.IJavaPeerable, Java.IO.ISerializable[<Android.Runtime.Register("java/util/concurrent/locks/StampedLock", ApiSince=24, DoNotGenerateAcw=true)>]
type StampedLock = class
inherit Object
interface ISerializable
interface IJavaObject
interface IDisposable
interface IJavaPeerable- Inheritance
- Attributes
- Implements
Remarks
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:
<ul>
<li><b>Writing.</b> 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.
<li><b>Reading.</b> 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.
<li><b>Optimistic Reading.</b> 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.
</ul>
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 java.util.concurrent.Semaphore 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.
<b>Memory Synchronization.</b> Methods with the effect of successfully locking in any mode have the same memory synchronization effects as a <em>Lock</em> action, as described in Chapter 17 of <cite>The Java Language Specification</cite>. Methods successfully unlocking in write mode have the same memory synchronization effects as an <em>Unlock</em> 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.
<b>Sample Usage.</b> 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 && 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);
}
}
}}
Added in 1.8.
Java documentation for java.util.concurrent.locks.StampedLock.
Portions of this page are modifications based on work created and shared by the Android Open Source Project and used according to terms described in the Creative Commons 2.5 Attribution License.
Constructors
| StampedLock() |
Creates a new lock, initially in unlocked state. |
| StampedLock(IntPtr, JniHandleOwnership) | |
Properties
| Class |
Returns the runtime class of this |
| Handle |
The handle to the underlying Android instance. (Inherited from Object) |
| IsReadLocked |
Returns |
| IsWriteLocked |
Returns |
| JniIdentityHashCode | (Inherited from Object) |
| JniPeerMembers | |
| PeerReference | (Inherited from Object) |
| ReadLockCount |
Queries the number of read locks held for this lock. |
| ThresholdClass | |
| ThresholdType | |
Methods
| AsReadLock() |
Returns a plain |
| AsReadWriteLock() |
Returns a |
| AsWriteLock() |
Returns a plain |
| Clone() |
Creates and returns a copy of this object. (Inherited from Object) |
| Dispose() | (Inherited from Object) |
| Dispose(Boolean) | (Inherited from Object) |
| Equals(Object) |
Indicates whether some other object is "equal to" this one. (Inherited from Object) |
| GetHashCode() |
Returns a hash code value for the object. (Inherited from Object) |
| IsLockStamp(Int64) |
Tells whether a stamp represents holding a lock. |
| IsOptimisticReadStamp(Int64) |
Tells whether a stamp represents a successful optimistic read. |
| IsReadLockStamp(Int64) |
Tells whether a stamp represents holding a lock non-exclusively. |
| IsWriteLockStamp(Int64) |
Tells whether a stamp represents holding a lock exclusively. |
| JavaFinalize() |
Called by the garbage collector on an object when garbage collection determines that there are no more references to the object. (Inherited from Object) |
| Notify() |
Wakes up a single thread that is waiting on this object's monitor. (Inherited from Object) |
| NotifyAll() |
Wakes up all threads that are waiting on this object's monitor. (Inherited from Object) |
| ReadLock() |
Non-exclusively acquires the lock, blocking if necessary until available. |
| ReadLockInterruptibly() |
Non-exclusively acquires the lock, blocking if necessary until available or the current thread is interrupted. |
| SetHandle(IntPtr, JniHandleOwnership) |
Sets the Handle property. (Inherited from Object) |
| ToArray<T>() | (Inherited from Object) |
| ToString() |
Returns a string representation of the object. (Inherited from Object) |
| TryConvertToOptimisticRead(Int64) |
If the lock state matches the given stamp then, atomically, if the stamp represents holding a lock, releases it and returns an observation stamp. |
| TryConvertToReadLock(Int64) |
If the lock state matches the given stamp, atomically performs one of the following actions. |
| TryConvertToWriteLock(Int64) |
If the lock state matches the given stamp, atomically performs one of the following actions. |
| TryOptimisticRead() |
Returns a stamp that can later be validated, or zero if exclusively locked. |
| TryReadLock() |
Non-exclusively acquires the lock if it is immediately available. |
| TryReadLock(Int64, TimeUnit) |
Non-exclusively acquires the lock if it is available within the given time and the current thread has not been interrupted. |
| TryUnlockRead() |
Releases one hold of the read lock if it is held, without requiring a stamp value. |
| TryUnlockWrite() |
Releases the write lock if it is held, without requiring a stamp value. |
| TryWriteLock() |
Exclusively acquires the lock if it is immediately available. |
| TryWriteLock(Int64, TimeUnit) |
Exclusively acquires the lock if it is available within the given time and the current thread has not been interrupted. |
| Unlock(Int64) |
If the lock state matches the given stamp, releases the corresponding mode of the lock. |
| UnlockRead(Int64) |
If the lock state matches the given stamp, releases the non-exclusive lock. |
| UnlockWrite(Int64) |
If the lock state matches the given stamp, releases the exclusive lock. |
| UnregisterFromRuntime() | (Inherited from Object) |
| Validate(Int64) |
Returns true if the lock has not been exclusively acquired since issuance of the given stamp. |
| Wait() |
Causes the current thread to wait until it is awakened, typically by being <em>notified</em> or <em>interrupted</em>. (Inherited from Object) |
| Wait(Int64, Int32) |
Causes the current thread to wait until it is awakened, typically by being <em>notified</em> or <em>interrupted</em>, or until a certain amount of real time has elapsed. (Inherited from Object) |
| Wait(Int64) |
Causes the current thread to wait until it is awakened, typically by being <em>notified</em> or <em>interrupted</em>, or until a certain amount of real time has elapsed. (Inherited from Object) |
| WriteLock() |
Exclusively acquires the lock, blocking if necessary until available. |
| WriteLockInterruptibly() |
Exclusively acquires the lock, blocking if necessary until available or the current thread is interrupted. |
Explicit Interface Implementations
| IJavaPeerable.Disposed() | (Inherited from Object) |
| IJavaPeerable.DisposeUnlessReferenced() | (Inherited from Object) |
| IJavaPeerable.Finalized() | (Inherited from Object) |
| IJavaPeerable.JniManagedPeerState | (Inherited from Object) |
| IJavaPeerable.SetJniIdentityHashCode(Int32) | (Inherited from Object) |
| IJavaPeerable.SetJniManagedPeerState(JniManagedPeerStates) | (Inherited from Object) |
| IJavaPeerable.SetPeerReference(JniObjectReference) | (Inherited from Object) |
Extension Methods
| JavaCast<TResult>(IJavaObject) |
Performs an Android runtime-checked type conversion. |
| JavaCast<TResult>(IJavaObject) | |
| GetJniTypeName(IJavaPeerable) | |