public class ReentrantReadWriteLock extends Object implements ReadWriteLock, Serializable
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
ReentrantReadWriteLock.ReadLock.tryLock() and ReentrantReadWriteLock.WriteLock.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
ReentrantLock.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<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(); }
}
}
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.
| Modifier and Type | Class and Description |
|---|---|
static class |
ReentrantReadWriteLock.ReadLock
The lock returned by method
readLock(). |
static class |
ReentrantReadWriteLock.WriteLock
The lock returned by method
writeLock(). |
| Constructor and Description |
|---|
ReentrantReadWriteLock()
Creates a new
ReentrantReadWriteLock with
default (nonfair) ordering properties. |
ReentrantReadWriteLock(boolean fair)
Creates a new
ReentrantReadWriteLock with
the given fairness policy. |
| Modifier and Type | Method and Description |
|---|---|
protected Thread |
getOwner()
Returns the thread that currently owns the write lock, or
null if not owned. |
protected Collection<Thread> |
getQueuedReaderThreads()
Returns a collection containing threads that may be waiting to
acquire the read lock.
|
protected Collection<Thread> |
getQueuedThreads()
Returns a collection containing threads that may be waiting to
acquire either the read or write lock.
|
protected Collection<Thread> |
getQueuedWriterThreads()
Returns a collection containing threads that may be waiting to
acquire the write lock.
|
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.
|
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 |
getWriteHoldCount()
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 |
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.
|
boolean |
isFair()
Returns
true 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.
|
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.
|
public ReentrantReadWriteLock()
ReentrantReadWriteLock with
default (nonfair) ordering properties.public ReentrantReadWriteLock(boolean fair)
ReentrantReadWriteLock with
the given fairness policy.fair - true if this lock should use a fair ordering policypublic ReentrantReadWriteLock.WriteLock writeLock()
ReadWriteLockwriteLock in interface ReadWriteLockpublic ReentrantReadWriteLock.ReadLock readLock()
ReadWriteLockreadLock in interface ReadWriteLockpublic final boolean isFair()
true if this lock has fairness set true.true if this lock has fairness set trueprotected Thread getOwner()
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 ownedpublic int getReadLockCount()
public boolean isWriteLocked()
true if any thread holds the write lock and
false otherwisepublic boolean isWriteLockedByCurrentThread()
true if the current thread holds the write lock and
false otherwisepublic int getWriteHoldCount()
public int getReadHoldCount()
protected Collection<Thread> getQueuedWriterThreads()
protected Collection<Thread> getQueuedReaderThreads()
public final boolean hasQueuedThreads()
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 lockpublic final boolean hasQueuedThread(Thread thread)
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 threadtrue if the given thread is queued waiting for this lockNullPointerException - if the thread is nullpublic final int getQueueLength()
protected Collection<Thread> getQueuedThreads()
public boolean hasWaiters(Condition condition)
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 conditiontrue if there are any waiting threadsIllegalMonitorStateException - if this lock is not heldIllegalArgumentException - if the given condition is
not associated with this lockNullPointerException - if the condition is nullpublic int getWaitQueueLength(Condition condition)
condition - the conditionIllegalMonitorStateException - if this lock is not heldIllegalArgumentException - if the given condition is
not associated with this lockNullPointerException - if the condition is nullprotected Collection<Thread> getWaitingThreads(Condition condition)
condition - the conditionIllegalMonitorStateException - if this lock is not heldIllegalArgumentException - if the given condition is
not associated with this lockNullPointerException - if the condition is nullpublic String toString()
"Write locks ="
followed by the number of reentrantly held write locks, and the
String "Read locks =" followed by the number of held
read locks.Dracula Javadoc Sample, Made with Allan Im.