| JavaTM 2 Platform Std. Ed. v1.6.0
java.util.concurrent.locks
Class ReentrantReadWriteLock
java.lang.Object
 java.util.concurrent.locks.ReentrantReadWriteLock
- All Implemented Interfaces:
- Serializable, ReadWriteLock
public class ReentrantReadWriteLock - extends Object
- implements ReadWriteLock, Serializable
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 continously 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() and ReentrantReadWriteLock.WriteLock.tryLock() methods
do not honor this fair setting and will 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 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.
- 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 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) {
// Must release read lock before acquiring write lock
rwl.readLock().unlock();
rwl.writeLock().lock();
// 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();
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(); }
}
}
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
|
Constructor Summary |
ReentrantReadWriteLock()
Creates a new ReentrantReadWriteLock with
default (nonfair) ordering properties. |
ReentrantReadWriteLock(boolean fair)
Creates a new ReentrantReadWriteLock with
the given fairness policy. |
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 - true if this lock should use a fair ordering policy
writeLock
public ReentrantReadWriteLock.WriteLock writeLock()
- Description copied from interface:
ReadWriteLock
- Returns the lock used for writing.
- Specified by:
writeLock in interface ReadWriteLock
- Returns:
- the lock used for writing.
readLock
public ReentrantReadWriteLock.ReadLock readLock()
- Description copied from interface:
ReadWriteLock
- Returns the lock used for reading.
- Specified by:
readLock in interface ReadWriteLock
- Returns:
- the lock used for reading.
isFair
public final boolean isFair()
- Returns
true if this lock has fairness set true.
- Returns:
true if this lock has fairness set true
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:
- 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 and
false 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 and
false 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
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:
- the collection of threads
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:
- 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, 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:
true if there may be other threads waiting to
acquire the lock
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 - 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 of the system state, not for
synchronization control.
- Returns:
- the estimated number of threads waiting for this lock
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:
- the collection of threads
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 - the condition
- Returns:
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
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 - the condition
- Returns:
- 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
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 - the condition
- Returns:
- 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
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.
- Overrides:
toString in class Object
- Returns:
- a string identifying this lock, as well as its lock state
Copyright 2003 Sun Microsystems, Inc. All rights reserved
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