Class J2SE14ReadWriteLock
- java.lang.Object
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- ca.odell.glazedlists.util.concurrent.J2SE14ReadWriteLock
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- All Implemented Interfaces:
ReadWriteLock
,java.io.Serializable
public class J2SE14ReadWriteLock extends java.lang.Object implements ReadWriteLock, java.io.Serializable
An implementation ofLockFactory
that has been derived from backport-util-concurrent.An implementation of
ReadWriteLock
supporting similar semantics toReentrantLock
.This class has the following properties:
- Acquisition order
The order of entry to the read and write lock is unspecified, subject to reentrancy constraints. A nonfair lock that is continously contended may indefinitely postpone one or more reader or writer threads, but will normally have higher throughput than a fair lock.
DEPARTURE FROM java.util.concurrent: this implementation impose a writer-preferrence and thus its acquisition order may be different than in java.util.concurrent.
- Reentrancy
This lock allows both readers and writers to reacquire read or write locks in the style of a
ReentrantLock
. Non-reentrant readers are not allowed until all write locks held by the writing thread have been released.Additionally, a writer can acquire the read lock, but not vice-versa. Among other applications, reentrancy can be useful when write locks are held during calls or callbacks to methods that perform reads under read locks. If a reader tries to acquire the write lock it will never succeed.
- Lock downgrading
Reentrancy also allows downgrading from the write lock to a read lock, by acquiring the write lock, then the read lock and then releasing the write lock. However, upgrading from a read lock to the write lock is not possible.
- Interruption of lock acquisition
The read lock and write lock both support interruption during lock acquisition.
Condition
supportThe write lock provides a
Condition
implementation that behaves in the same way, with respect to the write lock, as theCondition
implementation provided byReentrantLock#newCondition
does forReentrantLock
. ThisCondition
can, of course, only be used with the write lock.The read lock does not support a
Condition
andreadLock().newCondition()
throwsUnsupportedOperationException
.- Instrumentation
This class supports methods to determine whether locks are held or contended. These methods are designed for monitoring system state, not for synchronization control.
Serialization of this class behaves in the same way as built-in locks: a deserialized lock is in the unlocked state, regardless of its state when serialized.
Sample usages. Here is a code sketch showing how to exploit reentrancy to perform lock downgrading after updating a cache (exception handling is elided for simplicity):
class CachedData { Object data; volatile boolean cacheValid; ReentrantReadWriteLock rwl = new ReentrantReadWriteLock(); void processCachedData() { rwl.readLock().lock(); if (!cacheValid) { // Must release read lock before acquiring write lock rwl.readLock().unlock(); rwl.writeLock().lock(); // Recheck state because another thread might have acquired // write lock and changed state before we did. if (!cacheValid) { data = ... cacheValid = true; } // Downgrade by acquiring read lock before releasing write lock rwl.readLock().lock(); rwl.writeLock().unlock(); // Unlock write, still hold read } use(data); rwl.readLock().unlock(); } }
ReentrantReadWriteLocks can be used to improve concurrency in some uses of some kinds of Collections. This is typically worthwhile only when the collections are expected to be large, accessed by more reader threads than writer threads, and entail operations with overhead that outweighs synchronization overhead. For example, here is a class using a TreeMap that is expected to be large and concurrently accessed.class RWDictionary { private final Map<String, Data> m = new TreeMap<String, Data>(); private final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock(); private final Lock r = rwl.readLock(); private final Lock w = rwl.writeLock(); public Data get(String key) { r.lock(); try { return m.get(key); } finally { r.unlock(); } } public String[] allKeys() { r.lock(); try { return m.keySet().toArray(); } finally { r.unlock(); } } public Data put(String key, Data value) { w.lock(); try { return m.put(key, value); } finally { w.unlock(); } } public void clear() { w.lock(); try { m.clear(); } finally { w.unlock(); } } }
Implementation Notes
This lock supports a maximum of 65535 recursive write locks and 65535 read locks. Attempts to exceed these limits result in
Error
throws from locking methods.- Since:
- 1.5
- Author:
- Doug Lea
- See Also:
- Serialized Form
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Nested Class Summary
Nested Classes Modifier and Type Class Description static class
J2SE14ReadWriteLock.ReadLock
The lock returned by methodreadLock()
.static class
J2SE14ReadWriteLock.WriteLock
The lock returned by methodwriteLock()
.
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Constructor Summary
Constructors Constructor Description J2SE14ReadWriteLock()
Creates a newReentrantReadWriteLock
with default (nonfair) ordering properties.
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Method Summary
All Methods Instance Methods Concrete Methods Modifier and Type Method Description protected java.lang.Thread
getOwner()
Returns the thread that currently owns the write lock, ornull
if not owned.int
getQueueLength()
Returns an estimate of the number of threads waiting to acquire either the read or write lock.int
getReadHoldCount()
Queries the number of reentrant read holds on this lock by the current thread.int
getReadLockCount()
Queries the number of read locks held for this lock.int
getWriteHoldCount()
Queries the number of reentrant write holds on this lock by the current thread.boolean
hasQueuedThreads()
Queries whether any threads are waiting to acquire the read or write lock.boolean
isFair()
Returnstrue
if this lock has fairness set true.boolean
isWriteLocked()
Queries if the write lock is held by any thread.boolean
isWriteLockedByCurrentThread()
Queries if the write lock is held by the current thread.Lock
readLock()
Return the lock used for reading.java.lang.String
toString()
Returns a string identifying this lock, as well as its lock state.Lock
writeLock()
Return the lock used for writing.
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Method Detail
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writeLock
public Lock writeLock()
Description copied from interface:ReadWriteLock
Return the lock used for writing.- Specified by:
writeLock
in interfaceReadWriteLock
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readLock
public Lock readLock()
Description copied from interface:ReadWriteLock
Return the lock used for reading.- Specified by:
readLock
in interfaceReadWriteLock
-
isFair
public final boolean isFair()
Returnstrue
if this lock has fairness set true.- Returns:
true
if this lock has fairness set true
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getOwner
protected java.lang.Thread getOwner()
Returns the thread that currently owns the write lock, ornull
if not owned. When this method is called by a thread that is not the owner, the return value reflects a best-effort approximation of current lock status. For example, the owner may be momentarilynull
even if there are threads trying to acquire the lock but have not yet done so. This method is designed to facilitate construction of subclasses that provide more extensive lock monitoring facilities.- Returns:
- the owner, or
null
if not owned
-
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.
-
isWriteLocked
public boolean isWriteLocked()
Queries if the write lock is held by any thread. This method is designed for use in monitoring system state, not for synchronization control.- Returns:
true
if any thread holds the write lock andfalse
otherwise
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isWriteLockedByCurrentThread
public boolean isWriteLockedByCurrentThread()
Queries if the write lock is held by the current thread.- Returns:
true
if the current thread holds the write lock andfalse
otherwise
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getWriteHoldCount
public int getWriteHoldCount()
Queries the number of reentrant write holds on this lock by the current thread. A writer thread has a hold on a lock for each lock action that is not matched by an unlock action.- Returns:
- the number of holds on the write lock by the current thread, or zero if the write lock is not held by the current thread
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getReadHoldCount
public int getReadHoldCount()
Queries the number of reentrant read holds on this lock by the current thread. A reader thread has a hold on a lock for each lock action that is not matched by an unlock action.- Returns:
- the number of holds on the read lock by the current thread, or zero if the read lock is not held by the current thread
- Since:
- 1.6
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hasQueuedThreads
public final boolean hasQueuedThreads()
Queries whether any threads are waiting to acquire the read or write lock. Note that because cancellations may occur at any time, atrue
return does not guarantee that any other thread will ever acquire a lock. This method is designed primarily for use in monitoring of the system state.- Returns:
true
if there may be other threads waiting to acquire the lock
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getQueueLength
public final int getQueueLength()
Returns an estimate of the number of threads waiting to acquire either the read or write lock. The value is only an estimate because the number of threads may change dynamically while this method traverses internal data structures. This method is designed for use in monitoring of the system state, not for synchronization control.- Returns:
- the estimated number of threads waiting for this lock
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toString
public java.lang.String toString()
Returns a string identifying this lock, as well as its lock state. The state, in brackets, includes the String"Write locks ="
followed by the number of reentrantly held write locks, and the String"Read locks ="
followed by the number of held read locks.- Overrides:
toString
in classjava.lang.Object
- Returns:
- a string identifying this lock, as well as its lock state
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