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. 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.

• Reentrancy

  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.

• 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

  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 readLock().newCondition() throws UnsupportedOperationException .

• Interruption of lock acquisition

  The read lock and write lock both support interruption during lock acquisition. And both view the interruptible lock methods as explicit interruption points and give preference to responding to the interrupt over normal or reentrant acquisition of the lock, or over reporting the elapse of the waiting time, as applicable.

• Instrumentation Condition support

  This class supports methods to determine whether locks are held or contended. These methods are designed for monitoring system state, not for synchronization control. 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 .

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 usage . 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();
   }
 }
 

 
 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 Considerations

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. In addition to the above, this reference implementation has the following property:

• Ownership

  While not exposing a means to query the owner, the write lock does internally define an owner and so the write lock can only be released by the thread that acquired it.

  In contrast, the read lock has no concept of ownership. Consequently, while not a particularly good practice, it is possible to acquire a read lock in one thread, and release it in another. This can occasionally be useful.

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.

Since:

1.5

See Also:

Serialized Form

Nested Class Summary
static class	ReentrantReadWriteLock.ReadLock ReentrantReadWriteLock.WriterConditionObject           The lock returned by method readLock()           Condition implementation for use with ReentrantReadWriteLock .
static class	ReentrantReadWriteLock.WriteLock           The lock returned by method writeLock()

Method Summary
protected   Thread	getOwner ()           Returns the thread that currently owns the exclusive 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. acquire this 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. acquire this lock.
int	getReadLockCount getReadLocks ()           Queries the number of read locks held for this lock.
protected   Collection < Thread	getWaitingThreads ( Condition           Returns a collection containing those threads that may be waiting on the given condition associated with the write lock.
int	getWaitQueueLength ( 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           Queries whether the given thread is waiting to acquire this lock.
protected   Thread	getWriter ()           Returns the thread that currently owns the write lock, or null if not owned.
boolean	hasQueuedThreads isFair ()           Queries whether any threads are waiting to acquire.           Return true if this lock has fairness set true.
boolean	hasWaiters ( 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 Lock	readLock ()           Returns           Return the lock used for reading.
String Lock	toString writeLock ()           Returns a string identifying this lock, as well as its           Return the lock state. used for writing.
ReentrantReadWriteLock.WriteLock	writeLock ()           Returns the lock used for writing.

Methods inherited from class java.lang. Object
clone , equals , finalize , getClass , hashCode , notify , notifyAll , toString , wait , wait , wait

ReentrantReadWriteLock

public ReentrantReadWriteLock()

Creates a new ReentrantReadWriteLock with default 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.WriteLockLockwriteLock()

Description copied from interface: ReadWriteLock

Returns Return the lock used for writing.

Specified by:

writeLock in interface ReadWriteLock

Returns:

the lock used for writing.

readLock

public ReentrantReadWriteLock.ReadLockLockreadLock()

Description copied from interface: ReadWriteLock

Returns Return the lock used for reading.

Specified by:

readLock in interface ReadWriteLock

Returns:

the lock used for reading.

isFair

public final boolean isFair()

Returns Return true if this lock has fairness set true.

Returns:

true if this lock has fairness set true.

getOwner getReadLocks

protected Thread
public int getOwner getReadLocks ()

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. Queries the number of read locks held for this lock. This method is designed for use in monitoring of the system state, not for synchronization control.

Returns:

the owner, or null if not owned. the number of read locks held.

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 of the system state, not for synchronization control.

Returns:

true if any thread holds write lock and false otherwise.

isWriteLockedByCurrentThread

public boolean isWriteLockedByCurrentThread()

Queries if the write lock is held by the current thread.

Returns:

true if current thread holds this 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 this lock by the current thread, or zero if this lock is not held by the current thread.

getQueuedWriterThreads getQueueLength

protected Collection< Thread> 
public int getQueuedWriterThreads getQueueLength ()

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 an estimate of the number of threads waiting to acquire this 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 collection of threads the estimated number of threads waiting for this lock

getQueuedReaderThreads getWriter

protected CollectionThread< Thread> getQueuedReaderThreads getWriter ()

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. Returns the thread that currently owns the write 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:

the collection of threads the owner, or null if not owned.

hasQueuedThreads

 
public final boolean hasQueuedThreads () 

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.

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 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.

Parameters:

thread - the thread

Returns:

true if the given thread is queued waiting for this lock.

Throws:

NullPointerException - if thread is null

getQueueLength

 
public final int getQueueLength () 

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.

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. acquire this 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

hasWaiters getQueuedWriterThreads

public boolean 
protected Collection< Thread> hasWaiters getQueuedWriterThreads ( 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. 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.

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 condition null the collection of threads

getWaitQueueLength getQueuedReaderThreads

public int 
protected Collection< Thread> getWaitQueueLength getQueuedReaderThreads ( 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. 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.

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 condition null the collection of threads

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 condition null

toString

 
public StringtoString () 

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.

Overrides:

toString in class Object

Returns:

a string identifying this lock, as well as its lock state.