- All Implemented Interfaces:
Serializable
,ReadWriteLock
public class ReentrantReadWriteLock extends Object implements ReadWriteLock, Serializable
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
ReentrantReadWriteLock.ReadLock.tryLock()
andReentrantReadWriteLock.WriteLock.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();
}
}
}
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<>();
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.
- Since:
- 1.5
- See Also:
- Serialized Form
-
Nested Class Summary
Modifier and TypeClassDescriptionstatic class
The lock returned by methodreadLock()
.static class
The lock returned by methodwriteLock()
. -
Constructor Summary
ConstructorDescriptionCreates a newReentrantReadWriteLock
with default (nonfair) ordering properties.ReentrantReadWriteLock(boolean fair)
Creates a newReentrantReadWriteLock
with the given fairness policy. -
Method Summary
Modifier and TypeMethodDescriptionprotected Thread
getOwner()
Returns the thread that currently owns the write lock, ornull
if not owned.protected Collection<Thread>
Returns a collection containing threads that may be waiting to acquire the read lock.protected Collection<Thread>
Returns a collection containing threads that may be waiting to acquire either the read or write lock.protected Collection<Thread>
Returns a collection containing threads that may be waiting to acquire the write lock.int
Returns an estimate of the number of threads waiting to acquire either the read or write lock.int
Queries the number of reentrant read holds on this lock by the current thread.int
Queries the number of read locks held for this lock.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.int
getWaitQueueLength(Condition condition)
Returns an estimate of the number of threads waiting on the given condition associated with the write lock.int
Queries the number of reentrant write holds on this lock by the current thread.boolean
hasQueuedThread(Thread thread)
Queries whether the given thread is waiting to acquire either the read or write lock.boolean
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.boolean
isFair()
Returnstrue
if this lock has fairness set true.boolean
Queries if the write lock is held by any thread.boolean
Queries if the write lock is held by the current thread.readLock()
Returns the lock used for reading.toString()
Returns a string identifying this lock, as well as its lock state.Returns the lock used for writing.
-
Constructor Details
-
ReentrantReadWriteLock
public ReentrantReadWriteLock()Creates a newReentrantReadWriteLock
with default (nonfair) ordering properties. -
ReentrantReadWriteLock
public ReentrantReadWriteLock(boolean fair)Creates a newReentrantReadWriteLock
with the given fairness policy.- Parameters:
fair
-true
if this lock should use a fair ordering policy
-
-
Method Details
-
writeLock
Description copied from interface:ReadWriteLock
Returns the lock used for writing.- Specified by:
writeLock
in interfaceReadWriteLock
- Returns:
- the lock used for writing
-
readLock
Description copied from interface:ReadWriteLock
Returns the lock used for reading.- Specified by:
readLock
in interfaceReadWriteLock
- Returns:
- the lock used for reading
-
isFair
public final boolean isFair()Returnstrue
if this lock has fairness set true.- Returns:
true
if this lock has fairness set true
-
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
-
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
-
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
-
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
-
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:
- the collection of threads
-
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:
- the collection of threads
-
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
-
hasQueuedThread
Queries whether the given thread is waiting to acquire either the read or write lock. Note that because cancellations may occur at any time, atrue
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
- the thread- Returns:
true
if the given thread is queued waiting for this lock- Throws:
NullPointerException
- if the thread is null
-
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:
- the estimated number of threads waiting for this lock
-
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:
- the collection of threads
-
hasWaiters
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, atrue
return does not guarantee that a futuresignal
will awaken any threads. This method is designed primarily for use in monitoring of the system state.- Parameters:
condition
- the condition- Returns:
true
if there are any waiting threads- Throws:
IllegalMonitorStateException
- if this lock is not heldIllegalArgumentException
- if the given condition is not associated with this lockNullPointerException
- if the condition is null
-
getWaitQueueLength
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
- the condition- Returns:
- the estimated number of waiting threads
- Throws:
IllegalMonitorStateException
- if this lock is not heldIllegalArgumentException
- if the given condition is not associated with this lockNullPointerException
- if the condition is null
-
getWaitingThreads
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
- the condition- Returns:
- the collection of threads
- Throws:
IllegalMonitorStateException
- if this lock is not heldIllegalArgumentException
- if the given condition is not associated with this lockNullPointerException
- if the condition is null
-
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.
-