ReentrantReadWriteLock
public
class
ReentrantReadWriteLock
extends Object
implements
ReadWriteLock,
Serializable
java.lang.Object | |
↳ | java.util.concurrent.locks.ReentrantReadWriteLock |
An implementation of ReadWriteLock
supporting similar
semantics to ReentrantLock
.
This class has the following properties:
- Acquisition order
This class does not impose a reader or writer preference ordering for lock access. However, it does support an optional fairness policy.
- Non-fair mode (default)
- When constructed as non-fair (the default), the order of entry to the read and write lock is unspecified, subject to reentrancy constraints. A nonfair lock that is continuously contended may indefinitely postpone one or more reader or writer threads, but will normally have higher throughput than a fair lock.
- Fair mode
- When constructed as fair, threads contend for entry using an
approximately arrival-order policy. When the currently held lock
is released, either the longest-waiting single writer thread will
be assigned the write lock, or if there is a group of reader threads
waiting longer than all waiting writer threads, that group will be
assigned the read lock.
A thread that tries to acquire a fair read lock (non-reentrantly) will block if either the write lock is held, or there is a waiting writer thread. The thread will not acquire the read lock until after the oldest currently waiting writer thread has acquired and released the write lock. Of course, if a waiting writer abandons its wait, leaving one or more reader threads as the longest waiters in the queue with the write lock free, then those readers will be assigned the read lock.
A thread that tries to acquire a fair write lock (non-reentrantly) will block unless both the read lock and write lock are free (which implies there are no waiting threads). (Note that the non-blocking
ReadLock#tryLock()
andWriteLock#tryLock()
methods do not honor this fair setting and will immediately acquire the lock if it is possible, regardless of waiting threads.)
- 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 perform lock downgrading after updating a cache (exception handling is particularly tricky when handling multiple locks in a non-nested fashion):
class CachedData {
Object data;
boolean cacheValid;
final 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();
try {
// 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();
} finally {
rwl.writeLock().unlock(); // Unlock write, still hold read
}
}
try {
use(data);
} finally {
rwl.readLock().unlock();
}
}
}
class RWDictionary {
private final Map<String, Data> m = new TreeMap<>();
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 List<String> allKeys() {
r.lock();
try { return new ArrayList<>(m.keySet()); }
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.
Summary
Nested classes | |
---|---|
class |
ReentrantReadWriteLock.ReadLock
The lock returned by method |
class |
ReentrantReadWriteLock.WriteLock
The lock returned by method |
Public constructors | |
---|---|
ReentrantReadWriteLock()
Creates a new |
|
ReentrantReadWriteLock(boolean fair)
Creates a new |
Public methods | |
---|---|
final
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
|
getWaitQueueLength(Condition condition)
Returns an estimate of the number of threads waiting on the given condition associated with the write lock. |
int
|
getWriteHoldCount()
Queries the number of reentrant write holds on this lock by the current thread. |
final
boolean
|
hasQueuedThread(Thread thread)
Queries whether the given thread is waiting to acquire either the read or write lock. |
final
boolean
|
hasQueuedThreads()
Queries whether any threads are waiting to acquire the read or write lock. |
boolean
|
hasWaiters(Condition condition)
Queries whether any threads are waiting on the given condition associated with the write lock. |
final
boolean
|
isFair()
Returns |
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. |
ReentrantReadWriteLock.ReadLock
|
readLock()
Returns the lock used for reading. |
String
|
toString()
Returns a string identifying this lock, as well as its lock state. |
ReentrantReadWriteLock.WriteLock
|
writeLock()
Returns the lock used for writing. |
Protected methods | |
---|---|
Thread
|
getOwner()
Returns the thread that currently owns the write lock, or
|
Collection<Thread>
|
getQueuedReaderThreads()
Returns a collection containing threads that may be waiting to acquire the read lock. |
Collection<Thread>
|
getQueuedThreads()
Returns a collection containing threads that may be waiting to acquire either the read or write lock. |
Collection<Thread>
|
getQueuedWriterThreads()
Returns a collection containing threads that may be waiting to acquire the write lock. |
Collection<Thread>
|
getWaitingThreads(Condition condition)
Returns a collection containing those threads that may be waiting on the given condition associated with the write lock. |
Inherited methods | |
---|---|
Public constructors
ReentrantReadWriteLock
public ReentrantReadWriteLock ()
Creates a new ReentrantReadWriteLock
with
default (nonfair) ordering properties.
ReentrantReadWriteLock
public ReentrantReadWriteLock (boolean fair)
Creates a new ReentrantReadWriteLock
with
the given fairness policy.
Parameters | |
---|---|
fair |
boolean : true if this lock should use a fair ordering policy |
Public methods
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 system state, not for synchronization control.
Returns | |
---|---|
int |
the estimated number of threads waiting for this lock |
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 | |
---|---|
int |
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 |
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 | |
---|---|
int |
the number of read locks held |
getWaitQueueLength
public int getWaitQueueLength (Condition condition)
Returns an estimate of the number of threads waiting on the given condition associated with the write lock. Note that because timeouts and interrupts may occur at any time, the estimate serves only as an upper bound on the actual number of waiters. This method is designed for use in monitoring of the system state, not for synchronization control.
Parameters | |
---|---|
condition |
Condition : the condition |
Returns | |
---|---|
int |
the estimated number of waiting threads |
Throws | |
---|---|
IllegalMonitorStateException |
if this lock is not held |
IllegalArgumentException |
if the given condition is not associated with this lock |
NullPointerException |
if the condition is null |
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 | |
---|---|
int |
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 |
hasQueuedThread
public final boolean hasQueuedThread (Thread thread)
Queries whether the given thread is waiting to acquire either
the read or write lock. Note that because cancellations may
occur at any time, a true
return does not guarantee
that this thread will ever acquire a lock. This method is
designed primarily for use in monitoring of the system state.
Parameters | |
---|---|
thread |
Thread : the thread |
Returns | |
---|---|
boolean |
true if the given thread is queued waiting for this lock |
Throws | |
---|---|
NullPointerException |
if the thread is null |
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, a true
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 | |
---|---|
boolean |
true if there may be other threads waiting to
acquire the lock |
hasWaiters
public boolean hasWaiters (Condition condition)
Queries whether any threads are waiting on the given condition
associated with the write lock. Note that because timeouts and
interrupts may occur at any time, a true
return does
not guarantee that a future signal
will awaken any
threads. This method is designed primarily for use in
monitoring of the system state.
Parameters | |
---|---|
condition |
Condition : the condition |
Returns | |
---|---|
boolean |
true if there are any waiting threads |
Throws | |
---|---|
IllegalMonitorStateException |
if this lock is not held |
IllegalArgumentException |
if the given condition is not associated with this lock |
NullPointerException |
if the condition is null |
isFair
public final boolean isFair ()
Returns true
if this lock has fairness set true.
Returns | |
---|---|
boolean |
true if this lock has fairness set true |
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 | |
---|---|
boolean |
true if any thread holds the write lock and
false otherwise |
isWriteLockedByCurrentThread
public boolean isWriteLockedByCurrentThread ()
Queries if the write lock is held by the current thread.
Returns | |
---|---|
boolean |
true if the current thread holds the write lock and
false otherwise |
readLock
public ReentrantReadWriteLock.ReadLock readLock ()
Returns the lock used for reading.
Returns | |
---|---|
ReentrantReadWriteLock.ReadLock |
the lock used for reading |
toString
public 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.
Returns | |
---|---|
String |
a string identifying this lock, as well as its lock state |
writeLock
public ReentrantReadWriteLock.WriteLock writeLock ()
Returns the lock used for writing.
Returns | |
---|---|
ReentrantReadWriteLock.WriteLock |
the lock used for writing |
Protected methods
getOwner
protected Thread getOwner ()
Returns the thread that currently owns the write lock, or
null
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 momentarily null
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 | |
---|---|
Thread |
the owner, or null if not owned |
getQueuedReaderThreads
protected Collection<Thread> getQueuedReaderThreads ()
Returns a collection containing threads that may be waiting to acquire the read lock. Because the actual set of threads may change dynamically while constructing this result, the returned collection is only a best-effort estimate. The elements of the returned collection are in no particular order. This method is designed to facilitate construction of subclasses that provide more extensive lock monitoring facilities.
Returns | |
---|---|
Collection<Thread> |
the collection of threads |
getQueuedThreads
protected Collection<Thread> getQueuedThreads ()
Returns a collection containing threads that may be waiting to acquire either the read or write lock. Because the actual set of threads may change dynamically while constructing this result, the returned collection is only a best-effort estimate. The elements of the returned collection are in no particular order. This method is designed to facilitate construction of subclasses that provide more extensive monitoring facilities.
Returns | |
---|---|
Collection<Thread> |
the collection of threads |
getQueuedWriterThreads
protected Collection<Thread> getQueuedWriterThreads ()
Returns a collection containing threads that may be waiting to acquire the write lock. Because the actual set of threads may change dynamically while constructing this result, the returned collection is only a best-effort estimate. The elements of the returned collection are in no particular order. This method is designed to facilitate construction of subclasses that provide more extensive lock monitoring facilities.
Returns | |
---|---|
Collection<Thread> |
the collection of threads |
getWaitingThreads
protected Collection<Thread> getWaitingThreads (Condition condition)
Returns a collection containing those threads that may be waiting on the given condition associated with the write lock. Because the actual set of threads may change dynamically while constructing this result, the returned collection is only a best-effort estimate. The elements of the returned collection are in no particular order. This method is designed to facilitate construction of subclasses that provide more extensive condition monitoring facilities.
Parameters | |
---|---|
condition |
Condition : the condition |
Returns | |
---|---|
Collection<Thread> |
the collection of threads |
Throws | |
---|---|
IllegalMonitorStateException |
if this lock is not held |
IllegalArgumentException |
if the given condition is not associated with this lock |
NullPointerException |
if the condition is null |
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Last updated 2023-02-08 UTC.