java.lang.Object | |
↳ | java.util.concurrent.locks.ReentrantReadWriteLock |
An implementation of ReadWriteLock
supporting similar
semantics to ReentrantLock
.
This class has the following properties:
This class does not impose a reader or writer preference ordering for lock access. However, it does support an optional fairness policy.
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
tryLock()
and tryLock()
methods
do not honor this fair setting and will immediately acquire the lock
if it is possible, regardless of waiting threads.)
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.
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.
The read lock and write lock both support interruption during lock acquisition.
Condition
support
The write lock provides a Condition
implementation that
behaves in the same way, with respect to the write lock, as the
Condition
implementation provided by
newCondition()
does for ReentrantLock
.
This Condition
can, of course, only be used with the write lock.
The read lock does not support a Condition
and
readLock().newCondition()
throws
UnsupportedOperationException
.
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;
volatile 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 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 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(); }
}
}}
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.
Nested Classes | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
ReentrantReadWriteLock.ReadLock | The lock returned by method readLock() . |
||||||||||
ReentrantReadWriteLock.WriteLock | The lock returned by method writeLock() . |
Public Constructors | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
Creates a new
ReentrantReadWriteLock with
default (nonfair) ordering properties. | |||||||||||
Creates a new
ReentrantReadWriteLock with
the given fairness policy. |
Public Methods | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
Returns an estimate of the number of threads waiting to acquire
either the read or write lock.
| |||||||||||
Queries the number of reentrant read holds on this lock by the
current thread.
| |||||||||||
Queries the number of read locks held for this lock.
| |||||||||||
Returns an estimate of the number of threads waiting on the
given condition associated with the write lock.
| |||||||||||
Queries the number of reentrant write holds on this lock by the
current thread.
| |||||||||||
Queries whether the given thread is waiting to acquire either
the read or write lock.
| |||||||||||
Queries whether any threads are waiting to acquire the read or
write lock.
| |||||||||||
Queries whether any threads are waiting on the given condition
associated with the write lock.
| |||||||||||
Returns
true if this lock has fairness set true. | |||||||||||
Queries if the write lock is held by any thread.
| |||||||||||
Queries if the write lock is held by the current thread.
| |||||||||||
Returns the lock used for reading.
| |||||||||||
Returns a string identifying this lock, as well as its lock state.
| |||||||||||
Returns the lock used for writing.
|
Protected Methods | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
Returns the thread that currently owns the write lock, or
null if not owned. | |||||||||||
Returns a collection containing threads that may be waiting to
acquire the read lock.
| |||||||||||
Returns a collection containing threads that may be waiting to
acquire either the read or write lock.
| |||||||||||
Returns a collection containing threads that may be waiting to
acquire the write lock.
| |||||||||||
Returns a collection containing those threads that may be
waiting on the given condition associated with the write lock.
|
[Expand]
Inherited Methods | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
From class
java.lang.Object
| |||||||||||
From interface
java.util.concurrent.locks.ReadWriteLock
|
Creates a new ReentrantReadWriteLock
with
default (nonfair) ordering properties.
Creates a new ReentrantReadWriteLock
with
the given fairness policy.
fair | true if this lock should use a fair ordering policy
|
---|
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.
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.
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 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.
condition | the condition |
---|
IllegalMonitorStateException | if this lock is not held |
---|---|
IllegalArgumentException | if the given condition is not associated with this lock |
NullPointerException | if the condition is null |
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.
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.
thread | the thread |
---|
true
if the given thread is queued waiting for this lockNullPointerException | if the thread is null |
---|
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.
true
if there may be other threads waiting to
acquire the lock
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.
condition | the condition |
---|
true
if there are any waiting threadsIllegalMonitorStateException | if this lock is not held |
---|---|
IllegalArgumentException | if the given condition is not associated with this lock |
NullPointerException | if the condition is null |
Returns true
if this lock has fairness set true.
true
if this lock has fairness set true
Queries if the write lock is held by any thread. This method is designed for use in monitoring system state, not for synchronization control.
true
if any thread holds the write lock and
false
otherwise
Queries if the write lock is held by the current thread.
true
if the current thread holds the write lock and
false
otherwise
Returns the lock used for reading.
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 the lock used for writing.
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.
null
if not owned
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 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 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 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.
condition | the condition |
---|
IllegalMonitorStateException | if this lock is not held |
---|---|
IllegalArgumentException | if the given condition is not associated with this lock |
NullPointerException | if the condition is null |