java.lang.Objectjava.util.concurrent.locks.ReentrantReadWriteLock
public class 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 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.)
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.
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
The read lock does not support a
Condition
and
readLock().newCondition()
throws
UnsupportedOperationException.
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) { // 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(); } } }
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 Class Summary | |
---|---|
static class |
ReentrantReadWriteLock.ReadLock
The lock returned by method readLock() . |
static class |
ReentrantReadWriteLock.WriteLock
The lock returned by method writeLock() . |
Constructor Summary | |
---|---|
ReentrantReadWriteLock
() Creates a new ReentrantReadWriteLock with default (nonfair) ordering properties. |
|
ReentrantReadWriteLock
(boolean fair) Creates a new ReentrantReadWriteLock with the given fairness policy. |
Method Summary | |
---|---|
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. |
Methods inherited from class java.lang. Object |
---|
clone , equals , finalize , getClass , hashCode , notify , notifyAll , wait , wait , wait |
Constructor Detail |
---|
public ReentrantReadWriteLock()
public ReentrantReadWriteLock(boolean fair)
Method Detail |
---|
public ReentrantReadWriteLock.WriteLock writeLock()
public ReentrantReadWriteLock.ReadLock readLock()
public final boolean isFair()
protected Thread getOwner()
public int getReadLockCount()
public boolean isWriteLocked()
public boolean isWriteLockedByCurrentThread()
public int getWriteHoldCount()
public int getReadHoldCount()
protected Collection<Thread> getQueuedWriterThreads()
protected Collection<Thread> getQueuedReaderThreads()
public final boolean hasQueuedThreads()
public final boolean hasQueuedThread(Thread thread)
public final int getQueueLength()
protected Collection<Thread> getQueuedThreads()
public boolean hasWaiters(Condition condition)
public int getWaitQueueLength(Condition condition)
protected Collection<Thread> getWaitingThreads(Condition condition)
public String toString()