| 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. When constructed as fair, threads contend for entry using an approximately arrival-order policy. When the write lock is released either the longest-waiting single writer will be assigned the write lock, or if there is a reader waiting longer than any writer, the set of readers will be assigned the read lock. When constructed as non-fair, the order of entry to the lock need not be in arrival order. In either case, if readers are active and a writer enters the lock then no subsequent readers will be granted the read lock until after that writer has acquired and released the write lock.
This lock allows both readers and writers to reacquire read or write locks in the style of a ReentrantLock. 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.
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 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) {
// upgrade lock manually
rwl.readLock().unlock(); // must unlock first to obtain writelock
rwl.writeLock().lock();
if (!cacheValid) { // recheck
data = ...
cacheValid = true;
}
// downgrade lock
rwl.readLock().lock(); // reacquire read without giving up write 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(); }
}
}
A reentrant write lock intrinsically defines an owner and can only be released by the thread that acquired it. In contrast, in this implementation, the read lock has no concept of ownership, and there is no requirement that the thread releasing a read lock is the same as the one that acquired it. However, this property is not guaranteed to hold in future implementations of this class.
This lock supports a maximum of 65536 recursive write locks and 65536 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 ordering properties.
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Creates a new ReentrantReadWriteLock with
the given fairness policy.
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| Public Methods | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
Returns an estimate of the number of threads waiting to
acquire.
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Queries the number of read locks held for this lock.
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Returns an estimate of the number of threads waiting on the
given condition associated with the write lock.
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Queries the number of reentrant write holds on this lock by the
current thread.
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Queries whether the given thread is waiting to acquire this
lock.
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Queries whether any threads are waiting to acquire.
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Queries whether any threads are waiting on the given condition
associated with the write lock.
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Returns true if this lock has fairness set true.
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Queries if the write lock is held by any thread.
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Queries if the write lock is held by the current thread.
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Returns the lock used for reading.
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Returns a string identifying this lock, as well as its lock state.
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Returns the lock used for writing.
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| Protected Methods | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
Returns the thread that currently owns the exclusive lock, or
null if not owned.
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Returns a collection containing threads that may be waiting to
acquire the read lock.
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Returns a collection containing threads that may be waiting to
acquire.
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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
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Creates a new ReentrantReadWriteLock with default 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. 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 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 condition 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 this lock. Note that because cancellations may occur at any time, a true return does not guarantee that this thread will ever acquire. This method is designed primarily for use in monitoring of the system state.
| thread | the thread |
|---|
| NullPointerException | if thread is null |
|---|
Queries whether any threads are waiting to acquire. Note that because cancellations may occur at any time, a true return does not guarantee that any other thread will ever acquire. This method is designed primarily for use in monitoring of the system state.
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 |
|---|
| IllegalMonitorStateException | if this lock is not held |
|---|---|
| IllegalArgumentException | if the given condition is not associated with this lock |
| NullPointerException | if condition null |
Returns 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.
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. The state, in brackets, includes the String "Write locks =" follwed 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 exclusive lock, or null if not owned. Note that 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 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. 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 condition null |