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java.util.concurrent.locks
Class ReentrantReadWriteLock

java.lang.Object
  extended by 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:

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(); }
    }
 }

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

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

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
Method Detail

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

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